Spark-ignition (SI) engine efficiency can be increased by operating lean and with increased compression ratio (CR), but both of these measures tend to increase the propensity for undesirable acoustic knock generation. It is well known that increased CR makes the engine more prone to knock due to increased combustion pressures and temperatures, but it may be less well understood why lean operation would exacerbate knock generation. For typical gasoline-range fuels, the laminar flame speed becomes very low (roughly only 20% compared to stoichiometric conditions) for an air-excess ratio (λ) of 2. Indirectly, this exacerbates the knock challenge in two ways; a) it may necessitate operation with a combustion phasing near Top Dead Center (TDC) to complete the combustion before expansion cooling occurs, b) it increases cycle-to-cycle variations, making it more challenging to operate near the knock limits. In addition, the high intake pressure required for lean operation (nearly a factor of two higher for λ = 2 compared to λ = 1) increases the oxygen concentration which promotes end-gas autoignition and knock generation.
Towards overcoming these challenges of lean combustion, this study aims to provide a better understanding of fuel autoignition under various conditions. First, to reveal the octane appetite under lean conditions, this experimental work utilized fuels of varying Research Octane Number (RON) and octane sensitivity (S). It was found that lean operation favored fuels that have high RON and high S since those were less knock limited. However, two compositionally different fuels with similarly high RON and S exhibited notable difference in knock limits under lean operation, indicating that RON and S may fail to accurately rank order fuels’ knock propensity. Second, the experiments show that under boosted conditions end-gas autoignition becomes sensitive to the level of trapped residual nitric oxide (NO), which in turn is very sensitive to variations of both actual λ and combustion phasing, among other factors. The results suggest that strong knock-suppression benefits could be realized if single-ppm NO mole fraction can be consistently maintained in the reactants. Finally, it is noted that maintaining knock-free operation is particularly important for lean operation because the lower peak combustion temperatures lower the speed of sound, which in turn shifts the frequency content of the in-cylinder knock to a lower frequency range. Lower knock frequencies can increase the transmission efficiency from the combustion chamber to the outer surfaces of the engine, potentially increasing engine noise levels if knock occurs.
Homogeneous lean or diluted combustion can significantly increase the efficiency of SI engines. These diluted engine charges suffer from poor ignitability and lead to unstable combustion. Actively fuelled pre-chamber ignition systems can overcome this problem with a high power ignition from a chemical reaction inside the pre-chamber. The active fuelling ensures good ignition conditions inside the pre-chamber even with high EGR rates or lean cylinder charges.
The active fuelling is done by volatile components of gasoline fuel using an in-house developed fuelling system. This system is adapted to either high-EGR or lean combustion as the requirements for the scavenging systems are much different in these two operation modes. This actively scavenged pre-chamber ignition system adds more parameters like quality, timing and quantity of the scavenging gas to the engine, so the correct calibration of these additional parameters is mandatory to optimize the combustion process. The pre-chamber is designed to fit a miniature pressure transducer to evaluate the combustion process inside the pre-chamber. Together with the other pressure transducers that are commonly mounted in research engines, a thermodynamic analysis of the combustion process is possible. Thus, a 1D-simulation model can be calibrated to evaluate unmeasured parameters like exhaust gas content and fuel-air-ratio inside the prechamber at ignition timing.
To understand the combustion processes with active pre-chamber ignition systems, optical investigations of the flame luminescence are performed inside an optical accessible engine through a piston crown window. To record single cycles with high temporal resolution, a high speed intensifier and a high speed camera are used to reach an imaging frequency of 36 kHz. The selected engine speed is 1200 rpm. These measurements show the progress of the engine combustion through the cylinder under real engine conditions including charge motion.
To gain deep insight into the ignition process of a pre-chamber, more detailed optical investigations are required. These measurements are performed inside an optically accessible combustion vessel with constant pressure. To analyse the jet ignition process, a special insert to this constantly scavenged chamber is developed to create a reactive environment but maintain the constant pressure during combustion. For these measurements, high speed schlieren images and the OH*chemiluminescence are recorded simultaneously. With the superposition of these two optical techniques, the combustion progress can be visualized in detail. It reveals that the reactive jet from the pre-chamber shows only a very weak combustion signal, the main cylinder combustion is started by a secondary ignition inside these reactive jets. This visualisation of the combustion process without the influence of charge motion or pressure gradients enables to develop and calibrate a combustion model.
The combination of optical measurements and the thermodynamic analysis can show the major interrelationships of the pre-chamber ignition system. To develop a combustion system for passenger cars, the optimisation of the entire system from the geometrical design of the pre-chamber to the active scavenging system is necessary. The paper contribution demonstrates the co-working of modern diagnostics and modelling in the development of advanced combustion systems.
High efficiency gasoline combustion engines with up to 50 % indicated efficiency are the target of current research work. To reach this goal, it’s essential to reduce all the combustion engine losses as much as possible. One major loss, with about 30 % of the total fuel energy, is the wall heat transfer loss. To reduce this loss, one solution is a temperature swing insulation, which is characterized by a low thermal conductivity and at the same time a low volumetric heat capacity. In order to analyze the thermal efficiency potential of different insulation materials, a thermal insulation model for one-dimensional engine process simulations has been developed and is presented in this paper. The insulation is discretized by a thermal node model and is coupled to an engine process simulation using the simulation tool GT-Power. The heat transfer and temperature swing behavior of the thermal insulation model is validated against 3D-CFD engine cycle simulations. A predictive detailed chemistry combustion model is used in 3D-CFD simulations to evaluate the influence of thermal insulations on the knocking tendency. Two different thermal insulation materials are investigated using the developed simulation model. One is yttria stabilized zirconia (YSZ) as a common thermal sprayed coating. The second material is produced by an electrolytic oxidation of the piston surface (anodizing). With thermal piston insulation, there is only a small increase in thermal efficiency in the range of 0.2 % to 0.6 % possible with both materials, whereby the potential increases at low speeds and medium loads. The reason can be found in the relative heat losses, which increases with a constant center of combustion and increasing load. An efficiency loss analysis shows that the exhaust losses increase by about two third of the reduced heat losses. Due to its lower volumetric heat capacity and similar heat conductivity, the anodized piston surface shows a thermal efficiency advantage compared to YSZ. The simulation results are finally validated by measurements conducted with a single-cylinder research engine for an YSZ coated piston surface as well as a hard anodized piston surface.
Particle emissions from engines have long been topic of research studies because of their detrimental impact on human health and environment. Light duty vehicles represent the main contributor of particles in the congested urban areas and in the next future thought there will be a drastic reduction of fossil fuels, an increasing of the electric vehicles and an improvement of battery technology, a good control of exhaust particle emissions must be yet guaranteed using accurate and effective measurements for their characterization.
This paper is an excursus of commercial and innovative techniques used for the evaluation of the particle emissions due to the engine exhaust. The effect of the main parameters on the soot formation and particle emissions were analyzed and described such as the influence of the injection strategies, the fuel type including commercial and not commercial fuels, and the lubricant oil. The particulate matter was characterized at the exhaust by means of the measure of number and size. Moreover, non-conventional diagnostics based on the optical detection of the natural flame chemiluminescence was applied to follow the combustion evolution and to evaluate the in-cylinder soot concentration. The correlation between the formation mechanisms of soot in the combustion chamber and the particle emissions at exhaust allows a comprehensive analysis.
Particular attention was payed to the particles smaller than 23 nm that will be object of the next European emission regulation. The difficulties correlated to their measurement caused by the large fraction of volatiles in the 10 - 23 nm range that can nucleate or condense on existing particles was highlighted. In this regard a methodological approach consisting in the sampling and conditioning of the particles at two different temperature settings was defined within the project Sureal-23 to evaluate the volatile organic fraction of these particles. The knowledge developed within the exhaust particle emissions allowed to develop an experimental layout and a methodology for the sampling and measurement of non-exhaust particles.
From the viewpoint of CO2 reduction, it is necessary to improve the thermal efficiency of internal combustion engines and to use carbon-neutral fuels. Recently, it has been reported that an oxygenated component blended fuel can expand the lean operating limit. From the results of the shortening of combustion duration in that fuel, one of the reasons for the expansion of that is presumed increase in turbulent burning velocity. Similarly, increase in turbulent burning velocity may be one of the reasons for an expansion of EGR limit. So, it is important to identify fuel composition that has the potential to expand lean limit or EGR limit by increasing turbulent burning velocity. In addition, given the current situation where various carbon-neutral fuel candidates are being studied, internal combustion engines must achieve high thermal efficiency by combustion that matches any fuels. It is also important to be able to immediately understand the requirements for achieving high thermal efficiency with any fuel. From the above, the purpose of this study is to clarify the effect of fuel composition on turbulent burning velocity under the conditions such as lean or EGR dilution and to establish a model considering that. Markstein number is a dimensionless number that indicates the change of burning velocity when the flame is stretched, and varies with fuel composition. In addition, it is considered an important indicator in combustion under EGR condition. Therefore, a modified model based on Peters’ model focusing on flame stretch and Markstein number was considered. In order to investigate the effect of fuel composition and to verify the modified model, experiments using Rapid Compression Machine (RCM) were performed to measure turbulent burning velocities of some fuels in EGR condition. In this time, from the viewpoint of clarifying the effect of the composition of possible carbon-neutral fuel candidates on combustion and verifying the influence of Markstein number, ethanol blended fuel and ETBE blended fuel were used. As a result of comparison between the turbulent burning velocity obtained from models and experiments, it was confirmed that the modified model reproduced the experimental results more accurately than the base model. Using the modified model, the effect of fuel composition on turbulent burning velocity was investigated. As a result, it was revealed that decrease in Lewis number by ethanol blend made turbulent burning velocity increase.
To further reduce the CO2 emissions of SI engines, maximum compression ratios must be realized in combination with high boost pressures. At high engine loads, knocking combustion then requires a retardation of the ignition timing at the cost of reduced efficiency and increased exhaust gas temperatures. Advanced computer-aided engineering tools enable prediction of the knock behavior and achievable peak efficiencies prior to actual thermodynamic testing, which significantly reduces development costs and times.
Current, well-calibrated 1D-models that include an estimate of the knock behavior can provide valuable insights into overall engine operation, even going so far as to optimize transient operation of hybrid powertrains for Real Driving Emissions. In contrast, 3D-CFD studies enable also the evaluation of the influence of geometric details in the early stages of development, as well as gaining physical understanding that can be applied to reduced, more cost-effective modeling approaches.
Reynolds-Averaged Navier Stokes (RANS) based turbulence models are able to represent a mean cycle since all turbulent structures are averaged. However, the mean cycle then typically does not exhibit significant knock intensities, so approaches must be found to nevertheless evaluate the knock tendency of the operating point. Depending on the combustion model, faster burning cycles can be represented by artificially enhancing the turbulent flame speed or by a virtual spark advance. These models provide both spatial location and knock onset timing information for a fast cycle.
On the other hand, Large-Eddy Simulation (LES) only models the turbulent structures smaller than the grid size. Therefore, LES is capable of also representing cyclic variations. The computational cost and the necessity of advanced combustion models due to insufficient spatial flame-resolution will be analyzed and discussed.
Finally, an outlook will be given on how the disadvantages of both RANS and LES can possibly be overcome to enable a cost-effective 3D-CFD evaluation of the knock tendency. Therefore, existing approaches based on Probability Density Functions (PDF) will be summarized and further developments based on the findings of the conducted research projects will be discussed.
The regulation of fuel economy of the transport sector has become strict to prevent global warming, and hence improvements of thermal efficiency of spark-ignition (SI) engines has an urgent necessity. Major solutions to increase thermal efficiency of SI engines are diluted combustion by exhaust gas recirculation (EGR) and an increase of compression ratio. On the other hand, a highly diluted mixture makes flame susceptible to stretch, which leads to deterioration of burning velocity and thermal efficiency. In addition, a high compression ratio oxidizes the air-fuel mixture before the arrival of propagating flame, and hence changes in chemical species and reactivity in the mixture due to the partial oxidation could affect dilution tolerance. In recent years, bio-fuel and synthetic fuel made from “green-hydrogen” have also received attention for the carbon neutralization of existing vehicles. These fuels have the capability to optimize their composition via their production path to increase octane number and octane sensitivity, the difference between research octane number (RON) and motor octane number (MON), to prevent knocking, whereas an increase in octane sensitivity suppresses the partial oxidation in a mixture, which could also influence dilution tolerance. Based on the background, effects of the partial oxidation and octane sensitivity on stretched flames under diluted conditions should be clarified for extension of dilution limit and thus improvement of thermal efficiency of SI engines with these renewable fuels. The paper presents a discussion of the effects of octane sensitivity on dilution tolerance for SI engines with high compression ratios. For this purpose, stretched flames with fuels that have various octane sensitivities are simulated using detailed chemical kinetics with an opposed-flow flame reactor model. The inlet mixtures of the opposed-flow flame reactor have different reaction progress variables to consider effects of the partial oxidation in unburned mixtures which are pronounced in high compression ratios. The mixtures are also diluted by complete combustion products to simulate EGR. Fuels with high and low octane sensitivities are examined with changing blend ratio of iso-octane, n-heptane, and ethanol. The simulation result shows that the fuel with the high octane sensitivity has a lower flame extinction limit under the diluted condition. For the partially oxidized condition, the extinction limits of both fuels extend and the values reach maximums at certain reaction progress variables. Furthermore, the high octane sensitivity fuel has the higher reaction progress variable at the maximum extinction limit than the low octane sensitivity fuel. This result suggests that the high octane sensitivity fuel requires a higher compression ratio to enhance dilution tolerance. For further understanding of the mechanism of the relation between the reaction progress variable at the maximum extinction limit and octane sensitivity, the results of sensitivity analysis and reaction path analysis are also discussed.
In spray combustion simulation, many breakup models have been developed and introduced into CFD simulation to predict the breakup process of spray droplets, which is very important for the mixture formation process. Theoretical models such as Taylor Analogy Breakup (TAB) Model are widely used as a droplet breakup model. To make TAB Model an experimental model, Improved TAB (ITAB) Model has been developed based on the observation results of the single droplet breakup process. In this study, Modified Improved TAB (MITAB) Model was developed as the new droplet breakup model. The model modifies calculation methods of droplet diameter and droplet velocity after breakup in ITAB Model based on observation results of the single droplet breakup process. WAVE-MITAB Model which combined with WAVE Model and MITAB Model was introduced into KIVA as the new spray breakup model, and CFD analysis was performed. In addition, the applicability of WAVE-MITAB Model to non-evaporating diesel spray was investigated by varying the calculation constant of WAVE Model and comparing with other spray breakup models.
Normal alkanes are important components of real-world and surrogate fuels, but discrepancies exist in their chemical kinetic mechanisms making combustion simulation during the engine design process less reliable. To provide ignition data for mechanism development and improve the fundamental understanding of the combustion process, reactivity and oxidation intermediates for two normal alkanes, n-heptane and n-dodecane, were measured in a modified CFR octane rating engine at an equivalence ratio of 0.25. The in-cylinder conditions of the motored engine covered a temperature range from 400K to over 1000K and a pressure range from atmospheric pressure to 22.9 bar with a compression ratio ranging from 4.0 to 15.7. Concentrations of detailed oxidation intermediate species in the exhaust gas were measured to provide simulation benchmarks. Existing chemical kinetic mechanisms were validated using a multizone model developed to simulate HCCI combustion in the motored engine. Prediction of CO concentration as an oxidation stage indicator was accurate in the negative temperature coefficient (NTC) regime, with less than 8% prediction error with the best-performing kinetic mechanisms. Prediction of the critical compression ratio (CCR, observed by sweeping the compression ratio until the onset of second-stage ignition) for n-heptane was improved from 7.9 to 7.0 by updating the H2/O2 sub mechanism, with the measured CCR being 7.1±0.08. Existing mechanisms showed incorrect predictions for n-dodecane reactivity. Although reaction pathways were similar, reaction rates differed between the mechanisms and greatly influenced the simulation results. All three evaluated mechanisms underestimated n-dodecane reactivity, with the best predicted CCR being 0.3 compression ratio too high. C12H24O production might need to be eliminated, and CH3CHO production enhanced to improve the n-dodecane mechanisms.
The unstretched laminar flame speed (LFS) plays a key role in engine models and predictions of flame propagation. It is also an essential parameter in the study of turbulent combustion and can be directly used in many turbulent combustion models. Therefore, it is important to predict the laminar flame speed accurately and efficiently. Two improved correlations for the unstretched laminar flame speed, namely improved power law and improved Arrhenius form correlations, are proposed for iso-octane/air mixtures in this study, using simulated results for typical operating conditions for spark-ignition engines: unburned temperatures of 300-950 K, pressures of 1-120 bar, and equivalence ratios of 0.6-1.5. The original data points used to develop the new correlations were obtained using the detailed combustion kinetics for iso-octane from Lawrence Livermore National Laboratory (LLNL). The three coefficients in the improved power law correlation were determined using a methodology different from previous approaches. The improved Arrhenius form correlation employs a function of unburned gas temperature to replace the flame temperature, making the expression briefer and making the coefficients easier to calculate. The improved Arrhenius method is able to predict the trends and the values of laminar flame speed with improved accuracy over a larger range of operating conditions. The improved power law method also works well but for a relatively narrow range of predictions. The improved Arrhenius method is recommended, considering its overall fitting error was only half of that using the improved power law correlation and it was closer to the experimental measurements. Even though φm, the equivalence ratio at which the laminar flame speed reaches its maximum, is not monotonic with pressure, this dependence is still included, since it produces least-rich best torque (LBT). The comparisons between the improved correlations in this study and the experimental measurements and the other correlations from various researchers are shown as well.
As part of the fight against global warming and to achieve greenhouse gas emission targets set by the different COP agreements, it is crucial to reduce the carbon footprint of ground transportation. Indeed, mobility needs are continuously growing with increase in population in urban areas. All these factors will lead to an upsurge in the energy demand for the mobility in the very next future. Consequently, the diversification of low carbon energy sources is urgently required.
Hydrogen can be used for mobility solution in its two energy conversion mechanisms: The Fuel Cell technology or the Internal Combustion Engine (ICE). The latter option, studied in the present work, offers the advantages of current fossil fuel engines – existing and proven technology, lifetime, controlled cost – with a very low carbon footprint.
The overall objective of the study is to define the specifications of a dedicated Hydrogen direct injection combustion system for ground transportation application with the best fuel efficiency and lower raw emissions, to minimize the aftertreatment needs.
A complete experimental and numerical study was carried out to get valuable information on various phenomena occurring throughout the engine cycle. The aim is to investigate and understand the influence of the hydrogen specificities on the different physical phenomena taking place in an ICE. The paper will present results obtained both on light-duty and heavy-duty engine configurations.
The very first step of the study consisted in performing experimental investigations. For this purpose, an all metal single cylinder engine originally designed for gasoline spark ignited combustion (tumble air motion, gasoline direct injection) was modified for hydrogen direct injection combustion. These experiments allowed to make several observations on the engine behavior and to list the requirements to operate a hydrogen fueled engine: compression ratio, tumble level, valve actuation strategies, dilution methods etc.
The gas-gas injection was experimentally studied in the High Pressure/High Temperature vessel available at IFPEN. Different optical diagnostics were implemented to observe the hydrogen jet evolution and provide qualitative and quantitative information on the jet behavior and the mixing evolution. Those measurements were used to calibrate the 3D CFD numerical approach and to adapt the methodology to get a realistic modeling of the hydrogen injection and of the mixture preparation.
Based on a 0D pre-study (boundary conditions) and using the injection modelling calibration introduced before, 3D CFD simulations have been then carried out with specific hydrogen kinetics properties.
Finally, this comprehensive study highlights the specificities of ICE running with hydrogen. It provides indications and guidelines for further developments and optimization of hydrogen combustion engines, including air fluid motion, compression ratio and general settings (piston shape, injector and spark plug positions).
Future transportation requires a variety of power sources, using sustainable and renewable energy sources. Further improvements in engine efficiency are crucial to meet the mandatory CO2 emissions regulations in the future. In this paper, ignition performances of spark based advanced ignition strategies were compared under both quiescent and flow conditions, including transient high current, and boosted glow phase discharge current. A constant volume combustion chamber emulating engine tumble flow was employed to investigate the impacts of spark discharge strategies and discharge control parameters on flame kernel initiation and combustion under lean burn condition. Ignition techniques were developed to generate transient high current (1700 A with 18 J ignition energy within 20 μs), generating an intensified thermal expansion. Efforts were also made to boost glow phase current up to 2 amps lasting 3 ms, which significantly enhance plasma stretching. The transient high current ignition strategy shows the best ignition capability under quiescent conditions because of the turbulence generated by the transient thermal expansion. However, the ignition capability is significantly mitigated under strong flow conditions, because of the much shorter discharge duration compared with traditional ignition strategy. The impacts of discharge current on flame kernel initiation were also investigated under same discharge duration. Under quiescent conditions, the change in discharge current amplitude has limited impacts on the flame propagation process. Under flow conditions, on the other hand, ignition volume is significantly enlarged because of the plasma channel stretching, and the enhancement of spark glow current is effective to enhance the ignition kernel development under high-speed flow. The experimental results indicate that an ignition strategy with moderate discharge current and sufficient discharge duration is most effective under flow conditions.
Anticipating the effect of implementing new fuel blends and combustion strategies is crucial in order to deal with the short-medium term green transition envisaged for passenger car fleets. In spark ignition engines, the use of alternative low-carbon fuel blends including biofuels, e-fuels and hydrogen, together with the stabilization of ultra-lean and highly diluted combustion conditions may accomplish both fuel economy and pollutant emission targets. In this framework, studying the very early ignition phase in terms of produced radicals is fundamental in order to optimize operating strategies. To this aim, optical analysis in dedicated engine configurations is a powerful device to extensively characterize those phenomena. The use of numerical tools further enhances investigative possibilities, as they provide insight into plasma-fluid interactions as well as related production of radicals and allow significant extensions of operative conditions along with the reduction of costs and efforts associated with experimental campaigns. This work is focused on the presentation of a numerical methodology supported by UV-visible emission spectroscopy on an optically accessible spark ignition direct injection engine. Focusing on the spark plug zone, the numerical tool traces the cyano radical (CN), which can be a key marker for ignition processes if considering its high sensitivity to the presence of CO2 and N2, namely the main components of charge diluent (e.g., when applying exhaust gas recirculation). The core of the methodology is a white-box zero-dimensional model for the simulation of non-equilibrium plasma chemical kinetics, accounting for the collisions with electrons and reaction schemes. The local mixture composition of the gaseous phase to be provided to the code is determined by means of three-dimensional CFD engine simulations performed with AVL-FIRE. Besides evaluating the effect of air-fuel ratio on CN, which is largely demonstrated in the literature (leaner mixtures feature lower CN production due to the smaller amount of carbon available for the CN-paths), the effect of the injection timing was investigated. Two different values of injection timings were tested for both stoichiometric and lean conditions at the same spark timing. The CN analysis showed that experimental data and numerical results are well correlated. The key role of the mixture local stratification on the CN production as a result of the injection timing was identified and discussed.
An Internal Combustion Engine (ICE) experiences higher injection quantity and extra friction loss during low operation temperatures. This feature provides further potential for improving the engine fuel efficiency with the additional control freedom in hybrid powertrain configurations. For energy management purposes, this paper presents a state-space engine thermal model that reflects the effects of low temperature. The system states include the engine coolant temperature, which reflects the excessive gas injection, and the lubricant temperature that fluences the extra friction loss. The thermal dynamical system's uncertain parameters are calibrated using the homotopy Levenberg–Marquardt algorithm for minimized residual RMS through the bench tests under pre-defined engine operation points. Their relationships with the operation point are regressed with linear functions. Exponential functions are used to describe the relationship between excessive injection/friction and thermal states. Validation with repeated tests shows that the established model can reasonably predict the engine's thermal behavior. Finally, we integrate the engine thermal model into a hybrid vehicle simulation model. Simulation results under a randomly generated driving circle reveal about 20% more fuel consumption considering the cold temperature influences.
Accurate and computational cost-effective modeling tools for the optimization of processes and devices of all kinds are needed in nearly all scientific fields. While experimental optimization entails high expenses in terms of cost and time virtual optimization may be a promising alternative. In this work, the suitability and accuracy of a 1D heterogeneous catalytic model is investigated. First, the influence of cell discretization and residence time on the convergence in a 1D catalyst model are investigated. Second, the catalyst model is investigated and validated with use of a stoichiometric steady state three-way catalyst experiment. With the help of these investigations the reaction mechanism is further developed and new reaction rates for two reactions are presented. The modeling results are compared to a 2D simulation approach in terms of computational time and catalyst conversion behavior. The presented model is capable to capture the experimental results with a drastically reduced computational time in comparison to the 2D simulation presented in literature.
Our diesel combustion calculation code using chemical calculation with a reduced elementary reaction model has enough accuracy for the practical use on engine developments. To aim for more calculation accuracy, a new idea was adopted to the fuel droplet breakup model. This idea is to apply a variable model constant to the breakup model. The model constant is varied according to the needle valve lifting of the injector. It works that the breakup right after the injection start is promoted by the variable model constant. And the heat release ratio estimation is remarkably improved. Some calculation results are shown in this paper, and the validity of the idea is discussed here showing some calculation results from an unstable in-injector flow and observations of spray characteristics with an experimental enlarged injector apparatus. In addition, an originally developed injector model is explained.
An axis-symmetrical diesel spray flame model coupled with momentum flux distribution measurement was developed as a tool for nozzle orifice development. Momentum flux distributions at several distances from the nozzle orifice exit are measured by a force sensor with a small-diameter aperture which traverses a cross-section, a set distance from the nozzle exit. A theoretical momentum flux profile is determined by fitting with the measured momentum flux profile at the cross-section. The spray velocity profile and equivalence ratio profiles are quantified from the theoretical momentum flux profile. By interpolating the profiles of the measured cross-sections, the axis-symmetrical diesel spray flame model can calculate the equivalence ratio and other physical values as a function of the distance from the nozzle exit (x) and the radius from the spray axis (r). The equivalence ratio distribution in a cross-section involving the spray axis is converted into soot formation and oxidation distribution by coupling with the improved soot φ -T map, which consists of contours of the soot particle diameter and OH mole fraction and shows the border between the soot formation and soot oxidation. The converted soot formation and oxidation regions in the axis-symmetric spray flame model exhibit a strong correlation with the measured soot (KL value) distributions, as determined by two-color pyrometry in a quasi-steady diesel spray flame in an optically accessible engine. The shear-stress, which is an important factor affecting soot formation/oxidation, is also quantified from the velocity distribution.
As various measures are being taken worldwide to restrain global warming, the internal combustion engine is required to realize a significant reduction in carbon dioxide emission. In addition, regulations of hazardous exhaust emissions in the global market are expected to become even more stringent near future. Regarding the diesel engine, it is well known that premixed charge compression ignition (PCI) combustion is effective in achieving both high efficiency and clean emissions. However, PCI combustion is associated with difficulty in controlling ignition timing and combustion noise, and this limits its operable range only to light load condition. In order to apply PCI combustion under higher load conditions, many researchers have been studying partially-premixed compression ignition (partially-premixed CI) combustion with use of multi-stage split fuel injection. Partially-premixed CI combustion can be expected to improve thermal efficiency by reducing combustion duration with a raised degree of constant volume of combustion, while suppressing exhaust emissions to a low level. In this type of combustion, PCI combustion occurs in the initial heat release process, and diffusive combustion becomes dominant in the latter part of combustion. If the spatial distribution of spray can be appropriately controlled to avoid interference between the burned region formed by early stage injections and the subsequent spray, it should be effective in forming leaner mixture in the latter combustion period. In this study, we investigated a new concept of partially-premixed CI combustion in which the spatial distribution of fuel spray is controlled with a dual zone combustion chamber composed of upper and lower zones. The effectiveness of the newly established combustion concept was validated by means of numerical analyses and engine experiments using a single cylinder research engine.
This study focuses on the technologies integration to aim about 60% indicated thermal efficiency (ITE) with a single-cylinder HD diesel engine, roughly equivalent to 55% brake thermal efficiency (BTE) with multiple-cylinder engines. A new thermodynamic cycle concept and a new heat insulation structure were discussed as the key integrated technologies. Otto cycle is the best thermodynamic cycle for ideal thermal efficiency. However, significant increase in heat release rate around top dead center results in higher cooling loss and sometimes in deterioration of late combustion under a higher compression ratio without any improvement in ITE. The newly proposed cycle is the hybrid of isobaric cycle and following steep pressure increase up to peak firing pressure (PFP) constraint from the timing when in-cylinder volume change rate becomes significant, which suppresses the average gas temperature. For further cooling loss reduction, the new in-cylinder wall insulating concept was investigated. The in-cylinder insulating effect was much significant by applying not only on the piston crown but also on the cooling side of piston. Although the test engine hasn’t been optimized yet, ITE was almost reached at about 60% by means of the multiple-injector concept with compression ratio of 23.5:1.
Compression-ignition engines usually have higher thermal efficiency than spark-ignited engines due to the design characteristics and operating mechanism. Under compression-ignition operation, fuel spray development and spray-piston interaction are essential topics to study to improve combustion performance fundamentally. This paper considers different combustion systems of piston designs and injector configurations to look at the combustion process, including spray development, premixed, diffusion, and late-cycle combustion. The experiment was performed using a single-cylinder research engine (SCE). The compression ratio (CR) was 17:1. Combustion profiles (heat release rate) processed from high-speed cylinder pressure measurement are compared from experiments with different combustion systems having varying levels of spray-wall interaction: wave (strong), open-bowl (low). Open-bowl system experiments were carried out under 12-hole and 16-hole injector configurations. The level of spray-wall interaction is stronger with the 12-hole system. Late-cycle combustion, a period where the combustion recession process begins, plays an important part in both combustion and emissions performance. It was found that late-cycle combustion occurred earlier under the open-bowl system than wave system. This was explained by the dependence of late-cycle combustion on spray-driven turbulence (more on the open-bowl system) or flame-piston interaction (more on wave system). There was ~6 degrees longer in combustion duration and ~6% point lower in gross thermal efficiency in open-bowl as compared to wave system. It was found that increasing injection pressure could improve the thermal efficiency of the open-bowl system more effectively than in the wave system. Potential improvements in combustion system design and operating strategies are considered to achieve higher thermal efficiency and minimize harmful emissions. Specifically, this study suggests that flame-wall interaction should occur near the peak of mixing-controlled combustion, especially before the start of late-cycle combustion. This provides an opportunity to combine the spray-driven turbulence that remains from the end of injection (EOI). The flame motion is guided by piston geometry and/or downstream flame-flame interaction. It was realized that an optimal combustion design should have a short late-cycle combustion duration (a period from MFB at the start of late-cycle combustion to MFB90) as defined by combustion aggressiveness analysis.
For further increase in thermal efficiency of a heavy-duty diesel engine, improvement of cooling loss reduction must be essential. Many attempts to reduce cooling loss in a cylinder have been carried out by applying various ceramic coatings or monolith to the piston and/or other in-cylinder surface aiming to increase temperature swing. However, their advantage was mostly very little or sometimes negative by their complex phenomena caused by the cyclic and unsynchronized variation of working gas and wall temperatures. This study investigated into cooling loss mechanism of a diesel engine in detail by two new insulation layers. A titanium thermal spraying piston intended to increase temperature swing by its thermal properties. The other structure was made of the aluminum layer with 3-5 μm thickness by vapor deposition process on the mirror-polished stainless-steel layer of the piston and the cylinder head. Aluminum was initially selected by its high natural reflectance for flame radiation to selectively absorb radiation at the soot deposit to achieve higher local temperature where spray flame impinged. Although further cooling loss reduction was achieved with the latter structure, the heat loss mechanism seems to be more complex than our hypothesis.
How to Improve the thermal efficiency of internal combustion engine is an eternal topic since it is directly related to energy consumption and pollution emissions. Among lots of technologies applied on heavy duty diesel engine, Miller cycle which increases the effective expansion ratio in relation to the compression is proved to be a good choice to balance engine performance with fuel economy and CO2/NOx/smoke emissions. However, limitation of peak firing pressure (PFP) will restrict the intake pressure or compression ratio of miller engines which in turn influence the thermal efficiency, and their in-depth trade-off relationship is not clear yet. Using the engine cycle simulation, this paper explores the most reasonable cylinder pressure waveform limited by PFP and found there exists an optimized pressure ratio values which slightly affected by other operating conditions for Sabathe-Miller cycle engine. Thereafter, effects of PFP limit on the performance of a large single cylinder engine were investigated considering the effect of engine load, miller degree, equivalence ratio as well as engine speed. It is found that under full load condition, the maximum brake thermal efficiency occurs when pressure ratio (PR) is about 1.2, and PFP constraint plays a dominant role on brake efficiency and could contribute more than 4.0% efficiency by increasing PFP constraint from 250bar to 350bar. Compared with full load condition, the brake thermal efficiency increases 4.6% and 6.3% respectively under fuel-rich (extremely high CR) and lean-burn (low heat loss) half load conditions. Meanwhile, miller degree was optimized for Sabathe-Miller cycle, and it plays a key role on reducing combustion temperature instead of increasing thermal efficiency compared with effect of PFP constraint.
Polyoxymethylene dimethyl ethers (PODEn) has the characteristics of high cetane number and high oxygen content. It is a promising alternative fuel for diesel engine. In order to explore the effect of natural gas substitution rate on the combustion and emission of PODEn pilot natural gas engine under low load, the combustion and emission parameters in the cylinder when the natural gas substitution rate is 40 ~ 90% are compared and studied by using the CONVERSE 3D simulation model. The results show that with the increased of natural gas substitution rate, the peak cylinder pressure decreased, the combustion phase advanced, the ignition delay period prolonged and the combustion duration increased. When the natural gas substitution rate is 40 ~ 60%, the heat release rate curve shows double peaks and the emissions of HC and SOOT are lower; When the natural gas substitution rate is more than 70%, with the increased of natural gas substitution rate, the incomplete combustion of natural gas in the cylinder increased, and CO and NOx emissions reduced. When the natural gas substitution rate is 90%, a low proportion of PODEn causes misfire. When the natural gas substitution rate is 80%, as the pilot fuel with SOI sweeping from 0 ° to - 25 ° ATDC, the peak in-cylinder pressure and heat release rate first increased and then decreased. When the injection timing is - 10 ° ATDC, the peak in-cylinder pressure and heat release rate are the highest. CO, NOx and SOOT emissions first increased and then decreased with the advanced of injection timing, while HC emissions first decreased and then increased with the advanced of injection timing. Early and late injection timing are not conducive to the combustion of natural gas.
Diesel jet controlled compression ignition (JCCI) with dual-direct injection was proposed to control the ignition timing and combustion phasing of premixed charge compression ignition (PCCI) actively and effectively. In diesel JCCI mode, the direct pre-injection gasoline prepares a flexible premixed charge, which is ignited by a small amount of jet-injection diesel near the compressed top dead center. In this research, the combustion and emission characteristics of diesel JCCI mode were optimized by using three-dimensional computational fluid dynamics (CFD) combined with genetic algorithm based on the test engine at 75% load. The simulation results show that under the current conditions, higher economy and lower emissions can be obtained simultaneously when the energy ratio is 50~59%, the initial in-cylinder temperature is 338~360 K and the pressure is 1.53~1.75 bar, meanwhile, the combustion phasing is in the range of 4~9 °CA ATDC. In addition, the combustion phasing has a positive proportional relationship with the diesel jet-injection timing, indicating that, in diesel JCCI mode with a reentrant type combustion chamber, the jet-injection timing plays a decisive role in the combustion process. Two typical cases are selected for deeper understanding. The Case A with both early pre- and jet-injection presents the single-stage high-temperature heat release process. By contract, the Case B with both late injections presents the two-stage high-temperature heat release process. Compared to Case A, the O2 at the corner of the combustion chamber at 90 °CA ATDC is more the that of Case A, and the THC and CO produced during the main combustion can be further oxidized. Therefore, the THC and CO emissions reduce by 2.17 and 15.09 g/kWh, respectively, the equivalent indicated specific fuel consumption (EISFC) reduces by 6.78 g/kWh compared with case A. Besides, the distribution of the spray particulars is leaner. So, the soot emission of Case B is about 0.0011 g/kWh lower than Case A. At the same time, the local temperature is lower, resulting in the NOx emission reduction by 0.06 g/kWh.
Homogeneous Charge Compression Ignition (HCCI) combustion with Pulsed Flame Jet (PFJ) and Exhaust Gas Recirculation (EGR) was studied using Rapid Compression Expansion Machine (RCEM). PFJ is a jet of burning gas issuing from a small cavity facing a main combustion chamber. Small amount of fuel-rich premixed gas is introduced to the cavity through the injector and ignited by a spark plug. The jet issuing from the cavity supplies heat and radicals volumetrically and initiates combustion in the main combustion chamber. Thus PFJ can advance the ignition timing of HCCI combustion. On the other hand, EGR retards the ignition timing and reduces pressure rise rate. In the previous study, the authors confirmed the possibility of reducing pressure rise rate, controlling the ignition timing and improving the thermal efficiency by using PFJ and EGR together. In the present study, OH radical injection and production behavior by PFJ was experimentally observed through chemiluminescence. Then, temperature and composition of the burned gas in the cavity of PFJ igniter were numerically evaluated by zero dimensional (0D) equilibrium calculation. OH and H are the main radicals. The combustion in the main chamber was also roughly estimated by 0D transient calculation. It was assumed that the burned gas issued from the cavity and the unburned gas in the main chamber are locally mixed at the front of the jet plume, and the autoignition of the mixture of PFJ burned gas and the unburned main gas was calculated. In the optical observation, the chemiluminescence of OH radical temporarily increased in the early stage of PFJ injection and then decreased. This was consistent with the tendency of the calculation, regardless of the PFJ/(PFJ+main) volume ratio. The optical observation also showed that the brightness of the OH radical chemiluminescence decreased with increasing EGR ratio. The calculations showed that the peak value of the mole fraction of OH radical decreased with increasing EGR ratio regardless of the PFJ/(PFJ+main) volume ratio. In order to estimate the effect of radicals supplied by PFJ on the initiation of HCCI combustion, calculations were also performed without the inclusion of radicals in the PFJ burned gas. The results imply that the heat supply effect of PFJ is dominant for the initiation of HCCI combustion in the main chamber, while the radical supply effect is not negligible in negative-temperature-coefficient (NTC) region.
As a zero-carbon fuel, ammonia is drawing more and more attention for application in marine power plants. High-pressure direct-injection of ammonia can be considered to be a promising way for marine engines. In this study, the spray and combustion characteristics of ammonia jets are studied under diesel-like conditions in a high-pressure high-temperature constant-volume combustion chamber (CVCC) with comparison to diesel jets. The spray tip penetration, cone angle, maximum liquid length and lift-off length are obtained by the high-speed Schlieren and Diffused back-illumination imaging techniques. The ammonia jet’s combustion intensity and heat release rate are evaluated by the capturing image and the pressure trace. The effects of the ambient temperature (800 ~ 1000 K), ambient density (15 ~ 21 kg/m3) and ambient oxygen concentration (0 ~ 21%) on the evaporating and burning ammonia jets are investigated. The results show that the maximum liquid length in the ammonia jets is negatively correlated with the ambient density and ambient temperature. Under the ambient temperatures of 900 and 1000 K temperatures with 21%vol. oxygen, the ammonia diffusion jet flame is successfully observed. The macroscopic structure of the ammonia jet is quite similar to that of diesel, however, the significantly transparent-like flame texture with lower combustion intensity is observed for the ammonia diffusion jet.
The transition to CO2-neutral sources of energy is a central issue of our time. In order to increase the amount of renewable energy sources in the electricity mix, effective and economical storage technologies are required. One technically and economically sensible option is the electricity storage by using hydrogen electrolysis and subsequent reconversion to the power grid using an H2 closed-cycle engine. During the engine operation, the aspirated air is substituted by an oxygen-enriched inert carrier gas. After the thermal conversion of H2 and O2, the resulting water is separated and the inert gas is recycled back to the combustion chamber. As a result of the substitution of the ambient air by an inert gas, NOx emissions are completely avoided. With this combustion concept, engine efficiencies up to 53 % are achievable.
The cam-roller pair is the core friction pair of the diesel engine fuel supply mechanism. Affected by complex transient conditions and interface micro morphology, it usually works in the mixed lubrication state of lubrication-contact coexistence. Under harsh working conditions, the interface roughness peak of cam-roller contacts, resulting in stress concentration and dry contact wear. In this paper, considering the influence of real surface roughness and transient working conditions, the 3D mixed lubrication characteristics of cam-roller pair are studied. Results show that the lubrication state at the fuel supply end angle is the worst in the working cycle, and the existence of surface roughness leads to drastic changes in the film state, when Rq exceeds 0.6um, the lubrication failure is easy to occur. The lubrication state of shaved surface is poor, and polished surface can significantly improve the lubrication performance of cam-roller pair. Under low-speed and heavy-load conditions, the film thickness is reduced clearly, which will may lead to excessive wear; By optimizing the structural parameters such as the eccentricity and base circle radius, the film lubrication contact state is significantly improved.
Officially from 2020, the upper limit of marine fuel sulfur has decreased to 0.5% in mass, leading to numerous bad effects on the engine performance. It is urgently necessary to ensure the stable operation of advanced-optimized engines with low-sulfur heavy fuel oil (HFO) in current shipping. Considering the various processes used to make low-sulfur marine fuel in the refineries, the compositions of low-sulfur HFO are diversified, which significantly affects the engine’s oil supply and lubrication systems, as well as the oil combustion performance. Hence, it is necessary to investigate the complex physicochemical processes of low-sulfur HFOs under high temperature and pressure conditions.
In this study, the spray experiments for low-sulfur HFO were firstly conducted, and the effect of low-sulfurization on the HFO spray macroscopic features is negligible. The ambient temperature shows little effect on the spray tip penetration and spreading cone angle, while the ambient density has a great impact. Under high temperature and pressure conditions, the tip penetration of HFO sprays behaves as a non-evaporating spray in the traditional sense, because the tip of spray is maintaining a certain opaque, and the opacity is sensitive to temperature. it is speculated that the substances leading to opacity are mainly formed by pyrolysis and polymerization of HFO in the environment of high temperature and pressure.
In order to confirm the pyrolysis and polymerization process of low-sulfur HFOs, the gas chromatography, mass spectrometry analysis and thermogravimetric analysis coupled with the spray experimental data were conducted. The results revealed that low-sulfur HFO sprays undergo a series of complex physicochemical process reactions in the inert gas, such as evaporation, pyrolysis and polymerization. A large number of aromatic hydrocarbons and macromolecular alkanes are consumed in high temperature environment, and the strong light absorbing substances similar to black carbon are produced. In the gaseous products, unsaturated olefins account for the main proportion instead of alkanes.
With the increasingly stringent requirements for carbon emissions, methanol fuel has attracted much attention due to its low carbon dioxide emissions. However, because methanol itself has the physical characteristics of low cetane number and large latent heat of vaporization, relatively large ignition energy is required. Therefore, it is difficult to achieve independent combustion of methanol in internal combustion engines. In this study, we propose a diesel methanol stratified injection technology based on the stratified water injection technology. The stratified injection technology is to inject two kinds of fuel into the injector in a stratified manner to make the spray out of the stratified state of the two sprays. In this study, we used CONVERGE software to model a marine medium-speed single-cylinder diesel engine in three dimensions. And through the test data to calibrate the model of the machine. Then the injector is modeled in the model, and the method of coincident point injector is used to simulate the stratified injection. By verifying the ignition delay, a combustion model that is suitable for both diesel combustion and methanol combustion is fitted. Then simulate the stratified injection of methanol and diesel to observe the combustion characteristics under the stratified injection mode to verify the feasibility of the scheme. After the verification is successful, we tried to formulate different diesel and methanol injection schemes with the number of layers unchanged. Observe the difference in cylinder pressure and heat release rate under different diesel and methanol ratios and finally choose one. The most suitable injection scheme for methanol and diesel stratified injection, which provides a theoretical basis for methanol stratified injection in the future.
Reducing harmful emissions and increasing thermal efficiency in internal combustion engines have been the foremost goals of modern engine research. Compression ignition (CI) engines offer advantages in terms of high-efficiency operation as the overall in-cylinder mixture is lean. High-pressure direct injection fueling systems, commonly employed in CI engines, allow for the precise control of the combustion through injection timing and heat release modulation with same-cycle multi-pulse injection. Diesel fuel has dominated the liquid energy sources for CI engines owing to its high reactivity and lubricity properties. The highly heterogeneous mixture criterion and the dependence on auto-ignition of CI engines allow for a wide range of fuels to be applicable for engine operation. Notably, ethers possess chemical properties similar to diesel such as high reactivity, albeit with improved volatility characteristics.
In this research, the mixture formation and burning behaviour of the high-pressure fuel spray have been optically examined under engine-like background conditions using a constant volume combustion chamber and high-speed camera. Specifically, dimethyl ether (DME) and ethanol were injected into a high-pressure and high-temperature reactive background media. Both fuels have matching chemical compositions (C2H6O), yet vastly different combustion characteristics. Diesel fuel was tested at matching conditions as a reference. A pneumatic pump was used for high pressure DME fueling in order to match the injection pressure to that of diesel systems. The coloured images were analyzed and quantified to characterize the behaviour of the reactive fuel spray.
The impingement of fuel sprays on the piston surface significantly affects mixture formation, combustion performance, and pollutant emissions in Direct-Injection Spark-Ignition (DISI) engines. To better understand the fuel adhesion behavior, the fuel adhesion characteristics of fuel spray impinging on the flat wall under cross-flow condition were investigated in this work. Refractive Index Matching (RIM) were employed to observe the propagation of fuel adhesion. Then the area, mass, and thickness of fuel adhesion were evaluated under various cross-flow velocities. Also, the propagation mechanism and lifetime prediction of fuel adhesion under the cross-flow conditions were revealed. The experimental results can provide necessary guidance on the operating conditions of airflow inside the engine.
In this study, formation and evaporation of fuel films on various surface roughness walls were investigated. Iso-octane was injected from multi-hole DISI injector. Injected spray was impinged on an aluminum wall having low or high roughness (Ra = 0.63 μm to 8.30 μm) and fuel film was formed on the wall. Wall surface temperature before fuel spray impingement was controlled by an electric heater (25°C to 130°C). Fuel film behavior with elapsed time under various wall conditions was captured by a high-speed camera. Fuel film spreading area change and evaporation lifetime of the fuel film were obtained from the captured top view image. As a result, it was found that the fuel film spreading area became narrower with an increase of the surface roughness. The evaporation lifetime of fuel film became shorter with an increase of the surface roughness. Therefore, it was considered that heat transfer from the wall to the fuel film increased owing to an increase of a substantial contact area between the wall surface and the fuel film.
Wall impingement and fuel film deposition in gasoline direct injection engines under cold start conditions are major concerns for emissions reduction. However, it is challenging to study the dynamics of film deposition under realistic conditions because of the difficulty of measuring the thicknesses of these microscale films. Low-coherence interferometry provides a quantitative optical film thickness measurement technique that can be applied to study this problem. This work presents the first high-speed spectral low-coherence interferometry measurements of impinging gasoline direct injection sprays. The feasibility and practical concerns associated with high-speed low-coherence interferometry systems are explored. Two approaches to spectral low-coherence interferometry: Michelson interferometry and Fizeau interferometry, were implemented and are compared. The results show that Fizeau interferometry is the better option for measurements of impinging sprays in closed spray vessels. The high-speed low-coherence interferometry system was applied in the Fizeau configuration to measure time-resolved film thickness of impinging sprays under engine-relevant conditions to demonstrate its capabilities.
Demands for high efficiency spark ignition engines have led to intake port and combustion chamber design achieving enhanced flow and turbulence. The present study reports a significantly improved version of the original endoscopic high-speed particle image velocimetry (eHS-PIV) developed and implemented in a high-tumble production engine to show flow structure details (Kim et al., 2020) and then their variations with engine operating conditions in several follow-up studies. The eHS-PIV has been improved for a finer pixel resolution while the PIV laser coverage is expanded to show the entire pent-roof region and upper half of the cylinder. This enables the time-resolved analysis of flow field and turbulence intensity distribution for the intake flow issued through the intake valves and its interaction with the piston. The new eHS-PIV also measures the in-cylinder flow/turbulence during the compression stroke. For the eHS-PIV measurement, a 532 nm Nd:YAG laser beam is supplied through a rigid endoscope wherein a series of rod lenses producing a 60-degree diverging planar laser sheet. Two laser endoscopes are installed on the spark plug hole vertically and the exhaust-side of the cylinder head horizontally. Normal to this plane, a camera endoscope is installed to capture the PIV signals. For PIV seeding, hollow-glass spherical particles are supplied at the upstream of the throttle body. The new HS-PIV diagnostic was applied to a selected multi-cylinder, naturally aspirated 1.6-litre engine operating at 1600 and 2000 revolutions per minute. The intake air flow rate and throttle position were controlled to simulate a fixed 60 Nm load condition. For each engine speed, a total of 100 motored cycles were recorded and the spatial filtering method with 7 mm cut-off length was used to extract turbulence intensity from each individual cycle image. The eHS-PIV was successfully demonstrated for the selected engine and operating conditions. It was measured that the high-tumble engine produces lateral intake flow and intense tumble flow that is skewed towards the exhaust side for both engine speeds with the higher engine speed resulting in higher intake flow magnitude and more lateral flow direction. During the piston compression, the upward flow motion is measured, which is also stronger at higher engine speed. The resulting tumble vortex formation is enhanced at higher engine speed and its interaction with the remaining upward flow vectors becomes more significant, leading to a more complex flow structure near top dead centre. This leads to not only higher flow magnitude but also increased turbulence intensity at higher engine speed. The turbulence is particularly higher at the interface between the tumble vortex and the remaining upward flow vectors.
Meeting stricter legal emission limits and the simultaneous introduction of new synthetic fuels are key challenges for current and future research in the field of engine combustion. A deep knowledge of spray behavior is mandatory to address these problems, as air‐fuel mixture and spray propagation in gasoline direct injection (GDI) are essential processes to achieve a highly efficient and clean combustion. Thus, a lot of effort is put into the identification of air‐fuel spray distributions. Most of them use lasers or X‐ray sources, which are accompanied by a high experimental complexity and further drawbacks.
In this work, the measurement technique application of high speed diffuse back illumination extinction imaging (DBIEI) is used to obtain quantitative information in the form of projected liquid volume fraction (PLV). The DBIEI setup is simplified to enable an easier and quicker application for different experimental environments, using a LED-Panel as light source, which fulfills diffuse back illumination (DBI) criteria. Measurements are done in a constant volume chamber, allowing easy optical access with up to 200 mm in diameter and enabling measurements at real world ambient engine conditions. An engine combustion network (ECN) Spray G injector is used. ECN ambient conditions G1 (3.5 kg/m³ ambient density at 300°C), G2 (0.5 kg/m³ ambient density at 60°C) and G3 (1.01 kg/m³ ambient density at 60°C) are chosen. Isooctane is used as fuel. The injector is mounted in a motorized rotational system, enabling measurements of the spray at defined and exact angles.
The DBIEI measurement technique suppresses the effect of beam steering at elevated ambient conditions, allowing the measurement of scattering based light attenuation by spray droplets. This requires a light source radiating uniformly over a certain angle‐range. Nevertheless, an inherent error in the quantification of liquid phase results from the detection of multiple scattered photons. The error is even more enhanced when using a non‐collimated light source. This leads to an underestimation of the optical depth (OD), which further results in a false calculation of the projected liquid volume. Therefore, beside very low‐density regions e.g. at spray boundaries, PLV results have to be assumed wrong. To enable the use of DBIEI in more dense spray regions, we present a simulation-based method correcting the ODs. Derived from this the corrected values indicate an underestimation of the OD of a factor greater than 2.
The corresponding PLV data at different viewing angles is then used to reconstruct three-dimensional data of the liquid volume fraction (LVF) with filtered back projection (FBP). Thereby we can obtain time and spatial resolved quantitative spray information using an easy experimental setup, with an approach to correct beam steering and multiple scattering, while the experimental effort is kept low by using LED light sources. This data can be used for comparison, calibration and evaluation of simulation data of transient sprays, leading to detailed knowledge of spray behavior and mixture formation in different conditions.
The fundamental concept of the LAS technique is to understand the fuel concentration by attenuation of both visible and ultraviolet light. The intensity of visible light is only attenuated by the scattering of droplets, while that of ultraviolet light is attenuated by the scattering of droplets and the absorption of vapors. The conventional LAS uses the ND: YAG pulse laser, CCD cameras and one shot for one spray, which takes time and effort. Moreover, temporal variation measurement of a single shot spray is not possible by the conventional LAS. To record the distribution of the whole vapor phase in an injection event and measure the liquid and vapor concentration inside the spray, a high-speed laser absorption scattering (HS-LAS) technique was developed applying continuous diode light source, high-speed video cameras, and image intensifier for UV light, which can provide the temporal variation of a single shot spray. In the experiment, a commercial seven-hole injector with a hole diameter of 0.123 mm allowing high injection pressure up to 100 MPa was used to avoid the potential inconsistencies with single-hole test injector. The diesel surrogate fuel which consists of 97.5 % n-tridecane and 2.5 % of volume-based 1-methylnaphthalene was used. The injection amount 5.0 mg/hole were selected to investigate the structure and mixture formation process of the spray. The findings of the experiments show that this imaging approach is a promising diagnostic technique for concurrently obtaining quantitative information on the quantity of vapor and droplets in a fuel spray. Furthermore, the turbulent/vortex fluid dynamics' temporal development/variation can be investigated.
This study focused on the factors for the decay of diesel-engine-vibration energy experimentally and numerically. We considered four factors of the vibration decay processes: internal damping of each engine part, friction between the contact surfaces of assembled parts, oil viscous friction on joint parts, and asperity friction on joint parts. First, we conducted hammering tests on each engine part to investigate the mode frequencies and modal damping ratios. A single part damps itself by internal damping. Second, we conducted hammering tests on the two parts assembled to investigate the influence of slip friction between the contact surfaces of two parts. We confirmed that if the mode shape of one part differs from that of the other part and its phase is in opposition to the other one, friction occurs on the contact surfaces and thus, the modal damping ratio of assembled parts becomes much bigger than that of each part. Third, we conducted engine firing experiments to investigate the effect of oil viscosity on the combustion-induced vibration. We confirmed that the decay rate with high-viscosity oil becomes smaller than that with low-viscosity oil at some frequencies. To detect the cause of this phenomenon, we performed simulation for different oil viscosities. The simulation results show that there is an asperity contact on the main bearing with low-viscosity oil while there is no asperity contact with high-viscosity oil and the asperity contact increases the decay rate in a wide frequency. Finally, we discussed the damping ratio for simulation. We found that containing the slip friction between the contact surfaces of two parts in Rayleigh damping can improve simulation compared with the Rayleigh damping ratio considering only internal damping.
In the maritime industry, marine lean-burn gas-engines have been expected to reduce emissions such as NOx, SOx and CO2. The methane slip, which is the unburned methane emitted from marine gas-engines has raised concern because of its impact on global warming. Because the number of the gas fuelled ships is increasing worldwide, the resulting increase in methane slip is now becoming a threat. It is therefore important to make a progress on the exhaust aftertreatment technologies for marine gas-engines. A methane oxidation catalyst supported the platinum group metals, especially palladium exhibits highest activity for CH4 oxidation in lower temperature, can be expected to be one of the most feasible countermeasures for the methane slip. Research and development on catalysts as aftertreatment has focused mainly on reducing emissions in natural gas vehicles. However, exhaust gas of the marine gas-engine is different from that of natural gas vehicles, such as lower temperature, and emission composition corresponding to widely use of engine load ranges. To apply the catalyst for the marine gas-engine, it is important that, in order to predict the catalyst performance considering exhaust gas components, the investigation on the effects of exhaust composition and exhaust temperature on the catalyst activity by comparing in actual exhaust gas of marine gas-engine and the simulated gas. Although the effects of specific exhaust composition, such as water on the catalyst activity are investigated in detail, the effects of comprehensive exhaust compositions are unclear. On the other hand, it also cannot be found the research on the performance of the catalyst under actual exhaust conditions coincident to a wide range of load conditions of marine gas-engines. Therefore, for the practical use of the catalyst, a comparative study on the catalyst performance evaluation in the actual engine exhaust gas and the simulated gas under same flow condition is necessary.
The actual exhaust gas test reveals that, despite lower exhaust temperature in low load condition, the catalyst exhibits extremely high performance nearly 100% CH4 conversion. In contrast, despite higher exhaust temperature in high load condition, the catalyst activity on CH4 conversion is reduced to 50%-60%. The simulated gas test shows that the catalyst activity is deactivated with increasing H2O and NO concentration and improved by CH4 and CO oxidation. Moreover, the results explain that high CH4 concentration in low load condition causes an increase of catalyst temperature and high catalyst activity by exothermic reaction, despite the low exhaust temperature and presence of H2O vapor. Because the measurement of NO adsorption on the catalyst at exhaust temperature exhibits that NO adsorption volume decreases with increasing temperature, NO adsorption can be one of the causes of catalyst deactivation especially at lower exhaust temperature.
Due to the rising concern about environmental effects and cost onboard, various scenarios including the use of clean fuels, renewable energies, and the optimization of power architectures have been proposed for green marine propulsions. As a midterm goal, the natural gas has been widely considered to achieve low CO2 and zero SOX pollutions. Meanwhile, renewable energies such as PV solar, wind have been introduced and coupled with energy storage systems to share the loads and provide assisted propulsion during the voyages. Considering the variable load and power features of natural gas fueled engines, the dual fuel engines have been used to drive the propeller directly, and the pure gas engines are mainly used to serve as generators.
The combination of gas engines and renewable energies has huge potential to reduce emissions over the whole life cycle in theory. However, the physical characteristics of new components and their control systems take uncertainty into the power systems in practice which requires more experimental investigations. Due to the scavenging efficiency and the time delay of turbochargers, as well as the low flame propagating velocity of natural gas, the transient load response of gas engines is weak, which may lead to an unsteady engine speed. In addition, the features of electronic components between renewable energy sources and power grids also cause the system property of low inertia, which may lead to fluctuations in the grid frequency. The electrification of the propulsion controls system and the diversification of power energies influences non-linearly the dynamic performance of the power systems.
As a result, improvements in the dynamic performance of power systems were conducted. From the point of view of gas engines, the optimization of structure and control methods of pre-chamber, valve timing, and turbochargers were explored to enhance the intake efficiency and accelerate the burning rate. On the side of the system level, operation condition control of ICE and power management including load distribution of batteries and supercapacitors were presented to accommodate the load response performance of gas engines. At the same time, the system-level real machine tests are of great importance and urgent to verify the credibility of simulations and to explore the mechanisms behind complex dynamic phenomena. Unfortunately, they are limited by the physical dimensions and costs of such a large-scale testbench.
This paper provides a review of the research on gas engine power electric systems onboard from literature. Aiming at the whole power system, many studies were conducted to predict the steady performance including the energy efficiency, reliability, electric quality as well as the power management strategies of gas-electric powertrains by simulations and small-scale physical platforms. However, the simulations and experiments on the load response and emissions during the transient operations of these power systems are fairly limited. Considering the differently on system tests and the lack of experimental results, scaling methods such as the similarity theory and Willans-Line model are developed to establish the relationship for different size systems. In addition, this paper summarized the theories and the state-of-the-art works of scaled model tests and simulation for marine gas engine power systems, and the boundary of the scaled model experiments for such power applications via the similarity theory was discussed.
Carbon dioxide emissions from heavy-duty vehicles can be reduced by converting, or retrofitting, conventional diesel engines into “dual fuel” mode operation using hydrogen and diesel fuel. Conversion can involve small and relatively cheap changes to design, thereby allowing a diesel engine to be fuelled by either conventionally, using diesel fuel (direct injection), or in conjunction with hydrogen (indirect injection – “fumigation”) in the intake manifold. We evaluate the conversion of a production, 6-cylinder 12.7L truck engine to run in dual fuel mode. Commercially viable conversions would permit, in the short term, the no-regrets ‘pump priming’ of a hydrogen pricing, production, storage and distribution infrastructure which, in the medium term, could also benefit the development of hydrogen-powered inshore marine vessels and trucks powered by, for example, fuel cells. The aim of this work was to establish the limitations and advantages of pursuing the conversion of an in-production heavy duty engine to dual fuel operation with the least changes to the engine.
The conversion used supervisory calibration, which consisted in maximizing the amount of hydrogen injection on top of diesel fuel, without any optimization of the other engine parameters. The strategy was, at a given operating point, to decrease the load seen by the diesel ECU while also increasing hydrogen injection until the torque at this operating point was recovered. As a result, the load ‘detected’ by the diesel ECU was lower when the engine was running on the dual fuel mode than when it was running on diesel fuel only. The displacement of diesel fuel by hydrogen directly resulted in a reduction in carbon dioxide emissions. Nevertheless, the variability of the hydrogen-air premixed mixture combustion was kept under control and, in particular, knock had to be avoided.
At low loads, up to 70% of the energy required to drive the truck could be provided by hydrogen. This percentage decreased quickly with increasing load and fell to about 25 − 35% at medium loads. Diesel fuel displacement increased with engine speed until a limit around 1800 rpm. In some areas, an increase in engine-out NOx emissions (increase from 257 ppm to 523 ppm at most) were noted when the engine ran on dual fuel, although in other areas, a reduction in engine-out NOx emissions was observed (decrease from 255 ppm to 139 ppm at most). These increases engine-out NOx emissions were converted by the SCR aftertreatment system which maintained a > 99% conversion efficiency when the SCR temperature was above 275°C, so the tailpipe emissions remained within current legal limits. Also, there were some areas with poor combustion efficiency, with a corresponding increase in exhaust hydrogen concentrations, which were due, at least in part, to the use of a combination of deliberately unoptimized parameters such as injection timings, boost pressure and EGR ratio because these were derived the original, diesel-only, calibration. Consequently, these parameters were set by the diesel ECU for that ‘detected’ lower load, rather than being optimised for dual fuel combustion.
The supervisory calibration was then road-tested. For an unladen truck, the test resulted in a 36.8% reduction in carbon dioxide emissions. Because the displacement of the diesel fuel by hydrogen was largest at low loads, the reduction in carbon dioxide emissions fell to 29.0% for a laden truck. This promising result was achieved for an unoptimized calibration. Future work will consist of optimizing the other engine parameters which may allow a higher diesel fuel displacement, resulting in higher reduction in carbon dioxide emissions, recovery of the loss in efficiency and mitigation of the increase in NOx emissions.
The purpose of this study is to visualize PREMIER combustion phenomenon in a micro-pilot diesel fuel ignited dual-fuel engine with differentiating the causes of knocking and PREMIER combustion. The auto-ignition characteristics in the end-gas region were investigated with combustion visualization and in-cylinder pressure analysis using a compression-expansion machine (CEM) fueled with a natural gas. The micro-pilot diesel fuel injection timing used to generate PREMIER combustion occasionally induced knocking and pressure oscillations, while normal and PREMIER combustion did not cause the pressure to oscillate. The auto-ignition in the end-gas region of both knocking and PREMIER combustion was found to cause a second rise in pressure before the propagation premixed flame reached the cylinder wall. As the auto-ignition in PREMIER combustion was later than in knocking, the area and mass of the end-gas region were smaller. The KI factors, which represented the intensity of the pressure oscillations, were higher in the case of knocking than in the case of PREMIER combustion. As the knock intensity increases in CEM, the dominant frequencies of 10 kHz and 14.5 kHz are generated in addition to the dominant frequency of 6.4 kHz. The auto-ignited flame area spread faster during knocking than during PREMIER combustion. Knocking caused a sudden pressure difference and imbalance between the flame propagation and end-gas regions, followed by pressure oscillations.
To deal with stringent exhaust emission regulations, an exhaust gas recirculation (EGR) system is widely used in internal combustion engines. However, appropriate control of EGR is difficult because of the gas transport delay and unavailability of the EGR rate. In this paper, a model-based control approach for engine airpath systems is proposed. A physical model of engine airpath considering the transport delay in EGR is constructed. An adaptive observer is introduced to estimate the EGR rate by using available quantities. Feedback error learning control is applied to the airpath system to deal with a characteristic change of the controlled plant. We validate the proposed method by numerical simulations.
This paper closely scrutinizes the application of machine learning techniques for the intelligent monitoring of ship propulsion engines. The information acquired online from the sensors is subjected to statistical processing with a factor analysis method. The resulting principal factor loadings characterize the contribution of every measured and analyzed parameter to the variance of the principal factor. Owing to the fact that the incipient failures cause deviation of different groups of propulsion engine parameters, the variation of principal factor loadings forms the anomaly pattern unique for every failure. A set of unique patterns is highly suitable for applying clustering and classification algorithms. Factor loadings sets, responsible for every failure, are used to train data-driven anomaly models based on Support Vector Machine (SVM) and Self-Organized Map (SOM). Once trained, these models are able to discriminate newly-acquired data samples as belonging to either a normal state or one of the trained fault patterns. In addition, the factor analysis method is used to derive a performance index recognizing engine state deviation from the normal condition. A combination of the performance index with the classification analysis provides a robust framework for the early detection and identification of incipient engine faults at an early stage.