The effect of exhaust gas recirculation (EGR) on combustion characteristics and thermal efficiency has been investigated in homogeneous charge compression ignition (HCCI) engines using diesel engines with single- and multi-cylinder having compression ratios of 17.8 and 18.2 respectively. These HCCI engines were operated by natural gas premixed with a small amount of dimethyl ether (DME) as an ignition source. In the present study, the effect of EGR was focused on improvement in the brake thermal efficiency. It is noticed that the brake thermal efficiency higher than 4% was achieved by the high EGR ratio of 0.5 compared with the case without EGR, and almost equal improvements due to EGR are obtained in both single- and multi-cylinder engines. It is found that an increase in brake thermal efficiency due to EGR is based on the improvement in combustion efficiency mainly resulting from a significant reduction in the unburned hydrocarbon emission.

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This research examined the effect of the residual gas condition on the HCCI combustion process, about which there have been many unclear points heretofore. Specifically, the influence of the residual gas condition on low-temperature reactions, ignition and the combustion state following ignition was investigated. A spectroscopic measurement technique was used to investigate the effect on low-temperature reactions. By analyzing the emission and absorption spectra, the formation behavior and wavelength bands of the chemical species produced during low-temperature reactions were made clear. In other words, the use of emission and absorption spectroscopic measurements provides a method for ascertaining the activity level of low-temperature reactions.

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HCCI engines have been widely investigated over the past few years because of their high efficiency potential with low NO_x emission, but they have some problems which prevent their current commercialization. Beside problems with the control of ignition timing and low specific power output, they show increased HC and CO emissions. Although these emissions could be aftertreated with oxidation catalyst, a better solution would be to decrease the amount of emissions from the cylinder. By lowering emissions of HC and CO from the engine cylinder the combustion efficiency will be increased and by that the overall efficiency of the engine will also be increased. Some previous studies have shown that significant sources of HC and CO emissions are crevices and boundary layers near cylinder walls. In order to further investigate their influence on these emissions and possible solutions for lowering them, a computational analysis of the influence of crevice volume and cylinder wall temperature will be done. For this analysis, a, newly developed HCCI multi zone computational model will be used. The model incorporates a crevice, boundary layers, and centre zones, and uses detailed chemical kinetics in order to predict the heat released from combustion, and also to calculate the actual amount of HC and CO compounds. The model also incorporates mass transfer between zones in order to maintain the boundary layer thickness, and also the heat transfer between zones and between boundary layer zones and cylinder walls. A high pressure cycle multi zone model is integrated into the cycle simulation code which calculates processes during gas exchange. In this manner, the initial conditions at the start of a high pressure cycle are calculated, and not predicted. In order to have temperature inhomogeneity at the start of the high pressure cycle, a model of temperature distribution between zones has also been developed.

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Combustion processes with fully premixed cylinder load combined with self-ignition, e.g. HCCI, provide high combustion efficiency and low emissions of Nitrogen Oxides (NO_x) and particulate matter at the same time. However, a high effort in the control of combustion start has to be made and afterwards no further control of the ongoing combustion is possible since it is only reaction controlled. In addition to that these combustion systems are always limited in load. Partially premixed systems offer the possibility to control both start and progress of combustion because of the presence of slower diffusion processes. Nevertheless a significant reduction of the raw emissions is possible due to an increase of the share of premixed combustion. Since the number of diesel operated passenger cars is still rising it is certainly of economical interest wether such a combustion process can be realized with an ordinary DI-Diesel engine which is operated with conventional diesel or diesel-like fuels. Here, a fuel variation is of great interest, because the fuel properties can be seen as one of the key-factors for a successful realization. In this study, the influence of several fuels (different chemical composition and cetane numbers) and injection strategies on the functioning chain of combustion is analyzed in a transparent single-cylinder CR diesel engine by means of optical measurement techniques. These optical diagnostics (laser-based and non-laser-based) are used simultaneously to study the fuel distribution, ignition and combustion inside the test engine. The investigation shall demonstrate to what extent a partially premixed mixture can be reached with different fuels and how this affects the spatial distribution of the ignition locations as well as the combustion progress.

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In this study, investigation was conducted on prediction of ignition timing and combustion process in a gasoline homogeneous charge compression ignition (HCCI) engine by means of one-dimensional engine cycle simulation and zero-dimensional chemical kinetics calculation. In-cylinder gas thermal states, air-fuel ratio and EGR ratio at intake valve closing time were obtained by means of cycle simulation and applied to initial conditions for chemical kinetics calculation. A detailed kinetic model (101 species and 592 reactions) obtained from the elementary reaction schemes for iso-octane and n-heptane was modified for zero-dimensional calculation with regular gasoline of octane number 91, and validated by engine experiments. The combustion characteristic timescale model by Kong et al. was applied to zero-dimensional chemical kinetics calculation for considering the influence both of the chemical reaction and turbulent mixing. As a result, ignition timing and combustion processes were predicted reasonably at various supercharging pressure and engine speeds.

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The fuel specific chemical mechanism responsible for compression ignition is investigated by measuring transient species existing during the two-stage ignition in a motored single cylinder engine. N-heptane was used as the fuel in this study. Crank angle resolved profiles of n-heptane, formaldehyde and hydrogen peroxide were obtained by means of mass-spectrometric detection of gas sampled from the engine cylinder through a pulse actuated valve. Partial fuel consumption and intermediates formation taking place at the timing of the first stage (low temperature) ignition were observed. The percent fuel consumption and product yields relative to initial fuel amount are weak function of equivalence ratio. These trends were well predicted by simulations of homogeneous chemical reaction using the detailed kinetics model for primary reference fuel combustion. Compared with the previously studied case of dimethyl ether fuel, the n-heptane fuel consumption is considerably larger, whereas the partial heat release of the first stage is close. Exhaust gas analysis under suppression of high temperature ignition stage was conducted to detect species undetectable or less sensitive by the mass-spectrometry. A Fourier transform infrared spectrometer was used for this purpose. Formic acid, methanol and ethylene were detected in this method, where amounts of the last two species were in disagreement with the numerical prediction. Based on these results, it is considered that the first stage heat release is governed by reactivity of the transient species, which are grouped into olefins, aldehydes and ring-oxides, in the low-temperature chain reaction mechanism.

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The present research focuses on the application of Light Cycle Oil (LCO) in a middle speed diesel engine under PCCI (Premixed Charge Compression Ignition) combustion conditions. LCO is a middle distillate from the fluid catalytic cracking (FCC) process during petroleum refining. The industries gaining interest in the use of LCO in diesel engines comes mainly due to its low sulphur content (<0.2%) in order to conform to the tight future emission restrictions for marine vessels. The use of LCO in marine diesel engines, however, is known to be problematic due to the bad combustion properties of LCO. This study shows the application and suitability of LCO by means of PCCI combustion in comparison with Marine Diesel Oil (MDO), usually used in ship engines. Experiments were carried out on a medium size, single cylinder, two stroke diesel engine (NDT) with 190mm in bore and 350mm in stroke. The visualization of the PCCI combustion was realized using an optical laser setup for shadowgraph imaging. The investigation of the shadowgraph pictures indicate that no soot was formed during PCCI combustion. Further the NO_x emissions of the two-stage combustion, composed of PCCI- and diffusive combustion were reduced of nearly 10% compared to ordinary diesel combustion. The thermal efficiency was increased of nearly 1.5% to 34.6% by using LCO compared to MDO due to shifting of the ignition to TDC. Furthermore intake air cooling from 65℃ to 34℃ results in an additional increase of thermal efficiency to 34.9% at constant level of NO_x emission.

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A variable valve timing (VVT) mechanism has been applied in a high-speed direct injection (HSDI) diesel engine. The effective compression ratio (ε_<eff>) was lowered by means of late intake valve closing (LIVC), while keeping the expansion ratio constant. Premixed charge compression ignition (PCCI) combustion, adopting the Miller-cycle, was experimentally realized and numerically analyzed. Significant improvements of NO_x and soot emissions were achieved for a wide range of engine speeds and loads, frequently used in a transient mode test. The operating range of the Miller-PCCI combustion has been expanded up to an IMEP of 1.30 MPa.

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A new multizone premixed-diffusion combustion model was developed, assessed and applied to the analysis of the burning process and emission formation in two different DI diesel engines, one working with a conventional combustion system and the other with a Premixed Charge Compression Ignition (PCCI) one. The combustion chamber was split into a liquid fuel zone, an unburned gas zone, a rich mixture of fuel vapor and unburned gas, with the related premixed burned-gas zone, and several diffusive burned-gas zones. All of these are treated as homogeneous. Basically, according to a combustion mechanism close to the one of Dec, a fuel rich zone is generated first, giving rise to a premixed flame surrounding the fuel-vapor/air mixture at the liquid jet tip. This forms a plume, which entrains the oxygen required to oxidate the combustion products at its periphery, and thus completes its oxidation in a nearly stoichiometric diffusion flame, consequent to an unburned gas induced mass dilution. The computed thermo-dynamic and thermo-chemical properties in the burned gas zones allowed the post-processing analysis of nitric oxide (NO), particulate matter (PM) and carbon monoxide (CO) formation. The model calibration was made by comparing the experimentally determined engine combustion efficiency to the combustion efficiency that was calculated by applying the energy conservation equation to the whole cylinder charge. The model was tested and assessed for two distinct commercial-type 16V, DI Common Rail (CR) diesel engines. For the conventional combustion engine, the model was applied to the heat release and emission formation analysis in a NO - PM trade-off mode, by changing the EGR mass rate. In particular, in order to estimate its effectiveness and robustness, the model was calibrated on the test condition with the highest EGR level and the calibration parameters were kept constant when lower EGR rate conditions were investigated. In addition, the model was used to analyze the combustion process at full load conditions, for different engine speeds. For the other engine, an investigation in PCCI combustion mode was carried out. The transition from conventional to PCCI mode was made by strongly increasing the EGR rate. With reference to NO emissions, the model outcomes showed an excellent agreement with experimental data for all test conditions, and good results were also obtained for the prediction of CO and PM emission levels. It was ascertained that higher local A/F ratios were required in PCCI combustion mode than in the conventional one.

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Aiming to further improve Diesel engine fuel efficiency and exhaust emissions, we conducted evaluation of a newly devised Twin Jet Injection (TJI) Diesel fuel system, where two jets were formed separately and then interacted into the nozzle exit hole. l ransmission and Schlieren photography and phase-Doppler anemometry were used to quantity transient spray and evaporation dynamics. The nozzle produces a nearly homogeneous fuel spray with wider cone patterns up to 68℃ and finer droplet sizes less than 22 microns. The concentration of fuel droplets into the spray is two orders of magnitude lower vs. conventional Diesel highly stratified jets. The TJI spray pattern results in higher mixing and rapid evaporation that minimizes the soot and NO_x emissions inside the engine cylinder. Ideally, to achieve better control of compression ignition (CI) combustion at high speed and load the fuel system needs an injector that provides homogeneous fuel distribution. An extended Diesel homogeneous-charge CI (HCCI) also requires totally vaporized fuel prior to the combustion reactions. A homogeneous fuel-air mixture is one in which the composition and the thermodynamic conditions are uniform throughout the reaction phase. The CI- and HCCI-combustion efficiency is mainly controlled by injection, i.e. its timing and rate-shape. The TJI fuel injection technology allows converting highly-stratified injection spray into a nearly homogeneous injection.

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The effect of injection rate modulation, i.e., periodical fluctuation of injection rate, on the spatial dispersion of fuel droplets and inner structure of fuel spray were investigated by using an electronically controllable fuel injection system. The major experimental parameters were the modulation frequency and the modulation amplitude. It was confirmed that the shape of spray with injection rate modulation becomes wider at some frequencies, i.e., 5 kHz - 6 kHz, 7 kHz and 13 kHz in our experiments. It was also confirmed there are two mechanisms of wider spray, i.e., the effect of larger maximum injection rate and the effect of high frequency modulation itself. From the temporal movement of fuel droplets obtained by high-speed photographs, the reason of this phenomenon was discussed.

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In diesel combustion, spray evaporation and mixture formation during ignition delay period play an important role in ignition, combustion and emission production. Therefore, this study focused on diesel fuel evaporation process during ignition delay period. Two types of shadowgraph photography method were employed in this experiment: single and double nano-spark shadowgraph photography methods. Single nano-spark shadowgraph photography enables us to capture relatively clear still image of evaporating spray including fuel droplets, liquid phase and vapor phase. On the other hand, double nano-spark shadowgraph photography can record the double exposure images of droplets and liquid phase that can measure droplets velocity and their flying direction. Droplets velocity and flying direction have an influence on droplets evaporation and, furthermore, mixture formation processes. Therefore, the velocity vector distribution of the droplets gives suggestive information on mixture formation during ignition delay period. Results show that spray evaporation begins immediately after start of fuel injection under the condition of high ambient temperature, in particular, at the middle of the spray. Spray atomization is fast promoted at this region, leading to ignition. The ambient temperature and injection pressure affect the droplet size and the number of droplets. Furthermore, droplets behavior, in particular flying direction at spray boundary, is strongly affected by the shear stress caused by the ambient temperature and injection pressure.

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The group-hole nozzle has a series of group of holes and each group consists of two small holes with a small spatial interval and a small angle. It will be expected that minimized holes enhances fuel atomization and evaporation, in addition, small spatial interval between two holes maintains the spray tip penetration. Studies of quantitative spray/ mixture properties injected by the group-hole nozzles under several angle between two holes were conducted via the laser absorption scattering (LAS) technique. Since the spray from the group-hole nozzle is not axisymmetric, the original LAS technique based on the assumption of the axisymmetric spray structure is not applicable. The original LAS technique was extended to obtain the accumulative mass and mass distributions of vapor/ liquid phase per unit projection area of the group-hole and the conventional hole nozzle sprays. The in-cylinder ignition and combustion processes of the group-hole nozzle and the conventional multi-hole nozzle were investigated by using the optical research engine. In the case of the impinging spray, the group-hole nozzle enhances the fuel evaporation as well as the spray tip penetration. Form the in-cylinder ignition and combustion process visualization, it was found that the group-hole nozzle produces lower mean flame luminosity than that of the standard-hole nozzle. Much air can be expected to be entrained into the spray, and a suitable air-fuel mixture for low soot combustion was formed by the group-hole nozzle.

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Ignition characteristic of wall impingement two diesel sprays which were interacted with each other were investigated experimentally. Moreover, the measured results were compared with those obtained in a single spray case. Appearance positions of OH and C_2 radical luminosities, and luminous flame kernel were stereoscopically observed using a two-way fiber optical system. The impingement wall was placed in a pressure chamber. Parallel two sprays were simultaneously injected with two nozzles. A separate distance between two sprays was 35mm. Impingement distance from nozzle tip to the wall was fixed at 30mm. These sprays were interacted with each other after wall impingement. As the result, the delay for luminous flame appearance in parallel impingement sprays became shorter than that in the single impingement spray when the ambient temperature was relatively low (under 700K). In the case of parallel impingement sprays, appearance positions of OH and C_2 radical luminosities and luminous flame kernel appeared in a spray bulge formed between two sprays.

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This work is to simulate the droplet interactions and collision in spray-to-spray systems except for the jet-to-jet of liquid fuel and jet dispersion induced by mutual impingement. In the spray field, the realistic collision event results in various possible collision regimes, for instance bouncing, coalescence, reflexive separation, and stretching separation. Especially in the dense spray systems like inter-spray impingement systems, atomization characteristics is outcome of competition between these collision event and the droplet breakup. Therefore, in order to simulate the dense spray systems with accuracy, it is necessary to consider as many collision outcomes as possible. Hence, the aim of the current study is to evaluate the predictive model for binary droplet collisions. To get the goal of the study, the atomization characteristics of the inter-spray systems was simulated by using the KIVA code which adopts a collision model that considers the possible collision outcomes.

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The aim of this study is to investigate the effect of the fuel temperature on the spray behavior and atomization characteristics. As a test fuels, it was used biodiesel fuel derived from the soybean oil and the ultra low sulfur diesel (ULSD). In order to analyze the spray characteristics such as the spray tip penetration, spray cone angle and injection delay, the spray images were obtained from the visualization system. The droplet sizes (Sauter mean diameter, SMD) of test fuels were analyzed to investigate the atomization characteristics by using the droplet analyzer system It was revealed that the injection quantity increases a little according to the increase of the fuel temperature of because the decrease of internal friction and dynamic viscosity by increasing of fuel temperature. When the fuel temperature increases, the spray tip penetration of diesel fuel has similar values, however, that of biodiesel fuel decreases due to the fuel evaporation and reduction of spray momentum. As the fuel temperature increases, the ratio of detected droplet for the smaller size droplets decreases, and the ratio of the larger size droplets increases, remarkably. These results lead to the increase of the droplet size at the high fuel temperature.

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Unlike conventional diesel engines, which have a negative ignition dwell, many strategies for low-emissions diesel combustion operate with a positive ignition dwell mode, where the ignition delay exceeds the injection duration. Although nitrogen oxides and particulate matter emissions can be reduced by operating with a positive ignition dwell, unburned hydrocarbon and carbon monoxide emissions typically increase. Sources of these emissions can stem from characteristics of the fuel spray after the end of injection, which may differ significantly from the main injection period where most spray models have been developed. To provide fundamental details of spray mixing during the end-of-injection transient, we have studied liquid-phase spray penetration and evaporation using simultaneous high-speed shadowgraph and Mie-scatter imaging for a single-hole, common-rail injector. Experiments were conducted over a wide range of ambient temperature and density in a constant-volume vessel. The experiments show that during the injection-rate ramp-down, the liquid penetration decreases (recedes towards the injector) from the quasi-steady-state distance for most diesel conditions. A transient jet entrainment model, coupled with the assumption of mixing-limited spray vaporization and direct measurement of the vaporized jet spreading angle, shows that this behavior is caused by a slower fuel delivery interacting with an increased rate of ambient entrainment during the injection-rate ramp-down. This increased mixing travels downstream as an "entrainment wave", permitting complete vaporization at distances closer to the injector than the quasi-steady liquid length. The position of the entrainment wave relative to the quasi-steady liquid length determines how far, and how quickly, the liquid recedes towards the injector. The tendency of recession increases with increasing ambient temperature and density because the transit time of the entrainment wave to the liquid length is shorter than the injection-rate ramp-down transient. Alternatively, the liquid-length recession is zero for conditions with low ambient temperature or density because the entrainment wave does not reach the quasi-steady liquid length until after the end of the injection-rate ramp-down.

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The optimization of a modern direct injection diesel engine in terms of fuel consumption, power and pollutant emission requires an accurate study of the dynamic of the injection system for delivering, cycle to cycle, the right fuel quantity on the whole range of operative conditions. This paper reports the results of an experimental and numerical investigation on the stability of delivered amount of fuel in pilot injections supplied by a common rail system for heavy duty engines. The study aims to evaluate the performances in terms of fuel mass stability and cycle to cycle dispersion for short duration of energizing currents. Tests have been taken using an 8 holes, 0.21 mm diameter, 154° spray angle, having a flow rate of 1100 cm^3/30 sec@10 MPa. Two different Programmable Electronic Control Units (a commercial and a home-made type), able to set different multinjection strategies, have been used to evaluate the performances of the system. Experiments have been focused on the analysis of the instantaneous flow rate and the fuel spray morphology both under non evaporative conditions and evaporative ones with the presence of a swirl motion. The fuel injection rate profiles for the implemented strategies have been measured by an AVL Injection Rate Gauge System working on the Bosch tube principle with time resolution up to 7.6 μs. Under non evaporative conditions, the spray evolution has been analyzed injecting the fuel in a quiescent high-pressure optically-accessible cylindrical vessel at different gas densities. Under evaporative conditions, the experiments have been carried out on a crank-case scavenged single-cylinder 2-stroke direct injection Diesel engine equipped with an optically accessible swirled combustion chamber. The engine is suitable to stabilize, during the compression stroke, a well structured swirl flow typical of real heavy duty engines. Images of the spray evolution, in single-shot mode, have been captured by a high resolution CCD camera, synchronized with the fuel emerging from the nozzle at different instant from the start of injection. They have been off-line processed by professional software to estimate the spray tip penetration and cone angle. A numerical investigation has been carried out by using the 3-D Star-CD code with the k-s turbulence model, Huh-Gosman atomization model and Reitz-Diwaker secondary break-up model adopted to predict the fuel spray evolution within the prototype single cylinder 2-stroke Diesel. The grid of the engine has been made using STAR-CD tools and the experimental results have been used to give the initial conditions as well for the comparison of the prediction of the air velocity field and spray penetration within the combustion chamber. The initial pressure and temperature conditions have been imposed according to experimental data whereas the flow field distribution within the combustion chamber has been tested by comparison with the PIV results.

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The present paper reports an experimental investigation of the effect of nozzle geometry of port fuel injector PFI on exit flow velocity, the stability of primary spray behavior and spray droplet size. Several types of PFI with were tested. The investigation of exit flow velocity and stability (wavelength) of spray structure was performed using an ultra high-speed video camera (max. camera speed 1Mfps) with a long-distance microscope (whole image). The visualized experiments were carried out in a closed chamber at the atmospheric pressure. With back-lighting, the time-series images of the spray behavior were obtained. Using ultra high-speed camera with long- distance microscope and Barlow lens, droplet diameter could be visualized with high temporal resolution (magnified image). Experimental results of average exit velocity compared with NOZZLE FLOW model where experimental results of stability and droplet diameter compared with KH-RT breakup model. Experimental investigation showed that exit velocity, droplet diameter and stability (wavelength) influence by nozzle geometry.

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The spray-wall interaction is a key factor for optimizing the in-cylinder mixture formation and combustion of a direct injection (D.I.) gasoline engine. In this study, the laser absorption scattering (LAS) technique for non-axisymmetric sprays was newly developed based on the LAS for axisymmetric sprays. The LAS technique for non-axisymmetric sprays makes it possible to quantitatively measure the vapor and liquid distributions and their total mass in the spray. The influence of wall impingement on the evaporation process and the tip penetration of the spray injected by a hole-type injector were investigated. The experimental results were compared with the simulation results using three-dimensional computational fluid dynamics (CFD). The input variables of the spray sub-model parameters and injection conditions were optimized by the genetic algorithm, where the spray tip penetration and the mass of vapor phase obtained by the experiment was selected as objective functions. As for numerical analysis of the mixture formation process in the spray, the accuracy has been improved.

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The spray from a multi- hole injector applied to direct injection spark ignition (DISI) engine was investigated. The spray has been injected into a constant volume chamber and has been visualized with a high speed video camera with a long distance microscope and quantified in terms of droplet velocity and diameter with HiDense PDA system. HiDense PDA system permits accurate measurements in spray with extremely high particle concentrations, and it is the only PDA system available that provide high quality measurements in the core region of the spray cone. The spray close to the nozzle has been investigated. Also the spray at the center axis far from the nozzle has been investigated at different injection pressure. PDA data have been processed by using a time-dividing method which divides the spray information into four distinct periods (F, C, R and T). As a result it was found that large particles 〜 (85 pm) which are injected at high speed 〜 (90 m/s) in the earliest stage lose their velocity rapidly due to breakup into smaller droplets and are overtaken by smaller but slower particles which are emitted during a later stage. The results also show that within the measured range the effect of injection pressure on droplet size was rather small.

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A novel experimental system has been developed in order to provide full access to the transient two-phase flow present inside a real-size transparent six-hole diesel injector nozzle. The purpose of this experiment is to investigate the formation and development of cavitation inside the sac volume and nozzle holes during an injection event as a function of geometric and operating parameters. A conventional common rail fuel injection system has been employed but whose metallic injector tip is replaced by a transparent section of identical geometric characteristics to those of the production nozzle. A CCD and an advanced high speed camera have been employed to record images of the in-nozzle flow in all holes during an injection event lasting about 4ms at a 250bar rail pressure within a VCO and a MiniSac nozzle. The obtained images of the cavitation patterns are compared with observations obtained previously at City University London using enlarged nozzle models of similar geometry as well as real-size nozzles but with optical access to only one of the holes. In this paper first the experimental setup is briefly described, which allows high quality images using the CCD camera as a function of the needle lift and operating conditions to be obtained, followed by an investigation of the transient flow using a high speed camera which is able to record 10000 frames per second at a 512X512 pixels resolution. Combination of the images from the two cameras with their complementary spatial and temporal resolutions, has provided very good understanding of the formation and development of both the geometric and string cavitation in the sac volume and the holes of production diesel injector nozzles.

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Characterisation of the wall film development in port fuel injection (PFI) motorcycle and car engines is very important in achieving higher performance, lower emissions and better fuel consumption. As part of an on-going research programme, an in-house system was developed based on the Laser Induced Fluorescence (LIF) technique to allow the accurate measurement of the fuel film thickness in a simulated intake port, complemented by visualisation of the film development using high speed video imaging. Two fuels have been investigated: iso-octane doped with 5% (v/v) 2,3-hexanedione and conventional gasoline. Parametric studies have been carried out by investigating the effect of injection duration/quantity, air velocity and port backpressure. Representative data on the characteristics of the fuel film for the two examined fuels are presented, followed by discussion of the results and conclusions.

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Ignition timing and control, as well as predictability of engine emissions, are critical factors in advanced-engine system design. The use of detailed chemical kinetics is key to simulating ignition performance and to predicting emissions. This paper describes a collaborative and systematic effort that is underway to enable computationally efficient use of accurate kinetics in engine simulation. The collaboration is focused on building a database of chemical information and on developing a complementary set of software tools that provide efficient engine-simulation capabilities. The goal is predictive simulations that capture the detailed behavior of complex fuels, such as gasoline and diesel, under homogeneous charge compression ignition (HCCI) and related operating conditions. Since directly accounting for all of the hundreds of constituent molecules in a fuel during simulation of real-fuel combustion is intractable, we employ a "model-fuel" or surrogate-fuel approach instead. Mixtures of model-fuel molecules can be determined to adequately represent important real-fuel properties and engine-combustion characteristics. In this work, model fuel compositions are determined by matching a mixture behavior to that of the real fuel, focusing on distillation-curve characteristics, net-heating value, hydrogen-to-carbon ratio, and octane/cetane numbers. This surrogate-definition process requires detailed chemical-kinetics mechanisms for a variety of model-fuel compounds. To build such a database of model-fuel component mechanisms, we have used a combination of automatic mechanism-generation and manual mechanism-development approaches. These methods adhere to a systematic set of class-based rules in determining elementary reaction rates, as well as thermodynamic and transport properties of species. In addition, a comprehensive validation study of the mechanisms, using a wide variety of both fundamental and engine experiments, has allowed refinement of these rules and improvement of both the mechanisms' predictions and their consistency across components. Even though model fuels have a small number of components, their detailed mechanisms contain large numbers of species (>1000) and reactions (>10000). Systematic mechanism reduction is therefore required for many engineering applications. To this end, we have also developed a package of automated mechanism-reduction techniques. In addition, we have advanced the solution algorithms used in the kinetics simulations and developed a multi-zone engine model that provides good predictions of ignition behavior and emissions. We report on selected results of this systematic approach to using detailed kinetics in engineering simulation, as well as the challenges encountered.

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Autoignition characteristics are an important parameter in designing diesel or PCCI engines. Especially diesel spray flames are lifted from the nozzle and initial flame is formed by an autoignition phenomena. The lifted nature of diesel spray flames influences soot formation since air will be entrained into the spray core by the entrainment of air between the region of nozzle and lifted flame base. The objective of the present study is to identify the autoignition characteristics by adopting a coflow jet as a model problem. The propane jets have been injected into high temperature air and autoignition and lifted flame characteristics have been investigated experimentally. The result showed that the fuel jet in the high temperature coflow air with the initial temperature larger than 860 K was autoignited by itself without having any external ignition source. The liftoff height of the autoignited lifted flame increased nonlinearly with the fuel jet velocity, in much the similar behavior as to non-autoignited lifted flames at relatively low temperature. However, the data reduction demonstrated that the liftoff height of the autoignited flame exhibited marked difference as compared to non-autoignited cases, where the liftoff height was controlled by the balance of the propagation speed of lifted edge flame and local flow velocity. At a certain excessive jet velocity, the autoignited lifted flame had a repetitive behavior of extinction and reignition at a critical liftoff height. An analysis for this critical autoignition showed that the ignition delay time for the adiabatic case should be modified by considering heat loss to ambience. A liftoff height correlation for autoignited lifted flames has also been determined.

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By using "Contribution Matrices", the authors had clarified that a major reaction path during thermal ignition preparation phases of DME and n-heptane was a set of reactions named "H_2O_2 reaction loop", and suggested that it may be a universal rule that the heat accumulated by this loop plays a dominant role in preparing the thermal ignition. In the present study, this suggestion was confirmed by using a higher octane number fuel, iso-octane, and by changing the initial temperature. It was confirmed that in all cases, an ignition delay, a period of the thermal ignition preparation phase, depends on the H_2O_2 concentration at the beginning of this phase. Reaction path analyses on higher initial temperature conditions skipping reactions in LTO (cool flame) and NTC ranges result in lower H_2O_2 concentrations and lower heat release during thermal ignition preparation phases. Low and high octane number fuels show similar ignition characteristics when the initial temperature is higher than NTC range temperatures. When a low octane number fuel reacts at a lower initial temperature than NTC range temperatures, the fuel is given an "H_2O_2 bonus by LTO".

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In this study, the authors tried to understand the knocking phenomenon in a spark ignition engine fueled by LPG-DME mixture. The authors' original 3-D CFD code "GTT" linked to the CHEMKIN-II subroutines, called "GTT-CHEM", was used for not only understanding this knocking phenomenon but also estimating the knock intensity, which is usually measured by the completely different way on the test bed, from the CFD results. Then, a several different shapes of piston cavity were examined by both CFD and actual engine tests. As to the 3-D chemical kinetics calculation technique, for largely saving the computational time with little deterioration of accuracy, the authors constructed a reduced elementary reaction scheme for LPG-DME mixture by combining and reducing the GRI-Mech's scheme for LPG (mostly propane) and the Curran's scheme for DME. This reaction scheme was employed in the GTT-CHEM code, into which a modified version of the Kong's turbulent combustion model based on elementary reactions was incorporated. The GTT-CHEM code with the original reduced elementary reaction scheme was able to reproduce the knocking phenomenon reasonably well in the LPG-DME mixture fueled spark ignition engine. Finally, after actual engine experiments about different piston cavities, it has been found that a knocking intensity estimation index derived from HCHO behavior in CFD results is a good prophetic index to expand the knocking limit.

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A flame kernel initiation of methane-air combustible mixtures in the spark ignition process has been numerically investigated using a two-dimensional theoretical model including a detailed description of gas phase chemical kinetics, shock capturing scheme and transport parameters. The two-dimensional cylindrical coordinate system has been employed and assumes axial symmetry, and numerical domain is bounded by a solid wall. A chemical reaction scheme for a methane-air mixture consists of 34 species and 164 reactions. The thermodynamics and transport properties have been evaluated by a theoretical model in detail. In order to capture the blast wave behavior, a TVD scheme is employed in all equations. Fourth-order Runge-Kutta scheme is used to integrate a set of conservation equations in time. In the early stage, the behavior of the hot gas is dominated by a flow which is induced by the blast wave with an application of high ignition energy. Although a high temperature gas, which spurts out from the electrode gap, quenches, the gas at the electrode gap is self-sustained with an application of low ignition energy. After a certain period of time, the flame kernel gradually grows out of the electrode gap. The induction time of the flame kernel initiation increases with decrease in ignition energy. In the process of the flame kernel development, the local equivalence ratio at the electrode gap is larger than that in the outer region with an annlication of low ignition energy Diffusion of methane into the hot as region is recognized.

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Spherically propagating laminar and turbulent flames at elevated pressures in a large volume bomb were studied using iso-octane / air mixtures. The properties of the iso-octane flame at elevated pressures, especially the burning velocity and the effects of the flame stretch acting upon it are quite important to the modeling of combustion in internal combustion engines. Turbulent burning velocity correlations with the turbulence Karlovitz and Markstein numbers were proposed in this study. Experiments were carried out in the wide range of the equivalence ratio from 0.8 to 1.4 at two turbulence intensities of 0.80 and 1.59m/s varying the initial pressure from 0.10 to 0.50MPa. The Markstein number decreased for increasing pressures and for large equivalence ratios. The ratios of turbulent to unstretched laminar burning velocities at a constant Peclet number increased with increasing turbulence Karlovitz number and decreasing Markstein number at a constant pressure. However, the burning velocity ratios did not increase with increasing pressure although the Markstein number reduced with pressure. Turbulence spectrum affecting flames at higher pressures might be limited due to the smaller flame radii under the condition of a constant Peclet number.

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The premixed flames of DME-hydrogen-air premixed mixture were studied by the outwardly expanding flame and schlieren photography. The influences from equivalence ratio, hydrogen addition and initial pressure on the flame speed and combustion characteristics are analyzed. The results show that the flame speed, the laminar burning velocity and the mass burning rate increase with the increase of hydrogen addition. Increasing the ratio of hydrogen to DME will shorten the combustion duration. In the case of small hydrogen addition, the Markstein length decreases with the increase of equivalence ratio, and this reveals that lean mixture has better flame front stability than of rich mixture. In the case of large hydrogen addition, the Markstein length increases with the increase of equivalence ratio, and this indicates that rich mixture has better flame front stability than that of lean mixture. The initial pressure has larger influence on the combustion pressure than that from hydrogen addition. The flame development duration decreases with the increase of hydrogen addition and/or with the increase of equivalence ratio. Addition of hydrogen into DME can improve the ignitability of the mixtures and fasten the combustion.

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Abnormal laminar combustion of LPG-hydrogen-air mixture, including flame instability and uncompleted combustion phenomena, was experimentally studied with high speed photography in a constant volume chamber for different excess air coefficient (0.6〜1.4), initial pressure (0.09〜0.124MPa) and hydrogen fraction (0〜60%) at room temperature. The experiments and analysis showed that, with the increase of excess air coefficient, the Markstein length increases and therefore the stability of laminar flame increases, but the flame propagation speed is lowered, then buoyancy plays an important role in the combustion process and flame propagation. When LPG-hydrogen-air mixture is either too lean or too rich, the flame propagates very slow and the profile of flame front is no longer spherical and so-called "flame brush" phenomenon occurs; moreover, the flame doesn't spread all over the chamber, but is limited to only part of the chamber volume, leads to uncompleted combustion. Based on the above analysis, the lean flammability limit (lower flammability limit) and rich flammability limit (upper flammability limit) of LPG-hydrogen-air mixture, in volume fraction of fuels and excess air ratio separately, were plotted in a 3D coordinate system for different hydrogen fraction and initial pressure. The plots indicated that the flammability limit is remarkably expanded with the increase of hydrogen fraction.

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A newly developed small-sized IES (inductive energy storage) circuit with semiconductor switch at turn-off action was successfully applied to an ignition system. This IEC circuit can generate repetitive nanosecond pulse discharges. In this paper, the ignition system using repetitive nanosecond pulse discharges was investigated as an alternative to conventional spark ignition systems. The experiments were conducted using spherically expanding flame configuration for C_3H_8-air mixtures under various conditions. In conclusions, the ignition system using repetitive nanosecond pulse discharges was found to improve inflammability of lean combustible mixtures, such as extended flammability limits, shorted ignition delay time and extended dilution limits, compared with conventional spark ignition systems.

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The combustion characteristics of eight cylinders, four valves per cylinder racing engine have been studied by using flame chemiluminescence sensors. The time-series chemiluminescence of OH^*, CH^*, and C^*_2 were measured by optical filters and photomultipliers. The flame emissivities and a pressure signal were obtained simultaneously. And the local flame characteristics of each cylinder were calculated. Then, we analyzed. Cycle-to-cycle and cylinder-to-cylinder variations of local air fuel ratio and flame propagation speed were investigated with changing engine operating conditions, and the differences of these flame characteristics between cylinders were also discussed.

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In order to reduce emissions of particulate matter and nitrogen oxides by diesel engines, it is very important to understand the physics and chemistry of spray combustion in a real engine. However, most research into the characteristics of diesel spray combustion has used a constant volume combustion chamber or an optical access engine. This paper describes the combustion characteristics obtained using a micro-Cassegrain sensor and discusses how the sensor can be used to directly sense the flame characteristics for future control of diesel combustion. We connected a chemiluminescence sensor to a production diesel engine for various loads, engine speeds, and injection modes. The flame chemiluminescence of OH^*, CH^*, and C^*_2 band and narrow band spectra intensity at 650 and 850 nm were measured using a new micro-Cassegrain sensor. The cylinder pressure and injection timing were also acquired simultaneously. We investigated the relationship between the radical intensities and flame characteristics and found that the intensities of the radical spectrum in a cycle changed with the load. We also demonstrated differences in the radical intensity ratios.

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A recently-developed, three-dimensional measurement approach for high-speed combustion visualisation has been applied to two different engine concepts. Three photomultiplier-based camera devices are linked with each other to simultaneously measure the ultraviolet combustion radiation in the engines from different viewing angles. The resulting two-dimensional images provide a basis for the subsequent reconstruction of 3D information on combustion phenomena. In this paper, we focus on the character of auto-ignition processes as they take place in Controlled Auto-ignition (CAI) engines as well as - unintentionally - in turbocharged spark-ignition engines with high compression ratios.

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The reduction of total hydrocarbon (THC) emissions during cold start conditions is very important in order to achieve a tight control over exhaust emissions. The key factors to reduce these emissions include the spray characteristics, mixture formation, ignition, and injection control. In order to understand cold start combustion in a cylinder, the air-to-fuel ratio (A/F) near the spark plug was measured using a micro Cassegrain system (MCS). The local A/F and flame propagation speed were detected under the cold start conditions. This optical system consisted of a micro Cassegrain sensor (MC sensor), a spectroscopic unit with optical filters, photo-multipliers, and associated software. The A/F was determined from the intensity ratio of the radicals measured using their flame chemiluminescence. A practical engine and an M14 spark plug MC sensor were used for the measurements. The A/F and the THC concentration at the exhaust pipe were also measured by a conventional oxygen sensor and a flame ionization detector (FID). The A/F near the spark plug was compared to the A/F and THC concentration at the exhaust pipe. The A/F cyclic variation, which can barely be measured by an O_2 sensor, could be measured by the MCS just after starting the engine. The A/F near the spark plug was around 10 immediately after starting the engine, which is considered rich. It became leaner and converged after 50 cycles. The initial flame propagation became slower as the A/F near the spark plug became richer because the ignition delay and initial flame propagation were slower. The THC concentration was higher when the A/F was rich just after starting the engine. The MCS could evaluate the mixture formation of different injectors under cold start conditions and is therefore useful for achieving a reduction in the THC emissions.

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The mixture concentration in the vicinity of the spark plug at the time of spark ignition is a critical parameter for the performance of SI engines. Infrared absorption is often used to measure this parameter in firing cycles. It was already made clear that the mixture concentration near the spark plug is a primary factor explaining combustion stability in direct injection engines, although the effect of the local mixture concentration has not been confirmed for a nearly homogeneous mixture. In this research, the mixture concentration in the vicinity of the spark plug was quantitatively measured by using infrared absorption with an optical fiber sensor built into the spark plug of a port injection engine. Measurements were made for two cases, by changing the injected fuel amount from a rich to a lean air-fuel ratio between the cycles and by keeping the fuel amount constant between the cycles at three different overall air-fuel ratios. It was experimentally confirmed that the mixture concentration near the spark plug has little impact on combustion stability with a homogenous mixture distribution. It has also been confirmed that fiber-sensor-based infrared absorption can be used for cycle-by-cycle analysis of the local mixture concentration even for a homogeneous mixture distribution.

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A series of experiment of the in-cylinder flow measurement has been carried out by using a laser Doppler anemometer (LDA). The purpose of the study is for making standard database of the flow structure of in-cylinder flow. The database can be used for the verification of the numerical simulation. The velocity components of vertical- and swirl-directions are measured individually. The test engine made by Fuji heavy industrial has transparent sapphire cylinder for optical access. The test condition is 600 rpm with motoring condition. The mean velocity maps are indicated by using ensemble averaged data with crank angle window of 1 degree. The rms velocity is also calculated from the data. The flow structure by each crank angle can be clarified. In addition, the effect of tumble generation valve (TGV) on the flow structure is also evaluated.

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It is reported that the oxygenated fuels play significant roles in reducing soot emission. However, the mechanisms of soot formation and oxidation in oxygenated fuels and conventional diesel fuel so far have not been well understood. Laser induced incandescence (LII) is particularly suited to measure the instantaneous spatial distribution of the soot volume concentration, which can offer much needed detailed information of soot distribution for better understanding of soot formation and oxidation. In this paper, a two-color laser induced incandescence (2C-LII) technique was implemented for measuring absolute soot volume fraction in a laminar diesel fuel diffusion flame. Based on LII signal of the same point in the flame recorded at two wavelengths, the temperature of the laser-heated soot particles was first derived and later the soot concentration of the point was acquired by calibration. The 2D soot concentration distribution in the flame was obtained by mapping. The 2D concentrations of polycyclic aromatic hydrocarbons (PAHs) which are main precursor of soot were obtained by laser induced fluorescence (LIF). Measurements were performed in a laminar diesel and biodiesel/air diffusion flame by 2C-LII and LIF. It was found that with the biodiesel blended in the diesel fuel, the maximum soot and PAHs concentration decrease as well as their distribution area.

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This paper reports an investigation of the mechanisms of fuel jet interaction with an inclined plate in direct Diesel injection. The analysis is based on the application of optical diagnostics for the study of mixture, auto-ignition and soot formation. The interaction of a jet issuing from a single hole Common Rail Diesel injector with a flat inclined plate is observed in a high pressure, high temperature cell that reproduces the conditions which are typically encountered in the combustion chamber of a Diesel engine during injection. An analysis of the fuel air mixture by Laser Induced Exciplex Fluorescence (LIEF) shows that the presence of the wall significantly affects the mixture structure. Jet impingement on the inclined wall leads to confinement of the jet near the wall in the upstream region. This results in the development of a coherent structure at the jet tip which leads to improved mixing in the jet periphery. Furthermore, the mechanisms of air entrainment in the tail of the jet and near to the injector nozzle are significantly modified to such an extent that the fuel concentration in the upstream zone is very different depending on the configuration tested. A subsequent analysis of the combustion process reveals that the auto-ignition delay increases in the case of the wall impinging jet and that this effect is amplified with the relative angle between the jet and the wall. A combined analysis of the mixture structure by LIEF and of auto-ignition by Laser Induced Fluorescence excited at 355nm (355 LIF) shows that this effect is due to confinement of the jet near to the wall which inhibits auto-ignition and thus results in an increased ignition delay. An analysis of the formation of PAH and soot by LIF 355 and Laser Extinction Method (LEM) respectively reveals that the wall impinging jet has two contradictory effects on soot formation. Improved mixing related to the development of the coherent structure at the jet tip leads to a soot reduction for low incident jet-wall angles. However, this effect is countered by poorer mixing as a result of confinement in the near wall region, leading to an increase in soot concentration when the incident angle is increased.

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In order to examine the mechanism on which diesel soot emission is reduced by using Fischer-Tropsch Diesel (FTD) fuel compared to the case of conventional diesel fuel (JIS#2) under EGR conditions, the soot formation processes in a diesel spray flame of two different fuels (FTD and JIS#2) under different ambient oxygen concentrations (21 to 10%) were investigated via Excitation-Emission Matrix (EEM) analysis of polycyclic aromatic hydrocarbons (PAHs) and high-speed laser shadowgraphy of soot particles. The experiments were conducted using an optically accessible constant volume combustion vessel under a diesel-like condition (Ta= 940K and Pa= 2.5MPa). In the FTD-fuelled diesel spray flame, the timing and region for the first appearance of PAH LIF and soot particles in the flame were delayed and shifted downstream compared to the case of JIS#2. In the JIS#2 case, the LIF appeared first in the shorter wavelength region (350 to 400nm) and then shifted to longer wavelength region (above 400nm), while in the FTD-fuel case, the LIF was observed not in the shorter wavelength but only in the longer wavelength region. The production of soot in the flame was increased by lowering the ambient oxygen concentration from 21 to 15% in either fuel cases, while the timing and region for the first appearance of soot and PAHs in the flame were delayed and shifted downstream. By lowering the oxygen concentration further down to 10%, the timing and region for the first appearance of PAHs and soot were further delayed and shifted downstream and the production of soot was decreased.

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In-cylinder imaging of unburned hydrocarbon (UHC) distributions was performed via planar laser-induced fluorescence (PLIF) in a light-duty diesel engine employing a partially-premixed compression ignition (PPCI) combustion scheme. Measurements were acquired in the bowl and clearance volume at a light load baseline condition with optimized injection timing. With the fueling rate held constant, the injection timing was both advanced and retarded to help clarify the source of high level UHC emissions. The UHC composition was further investigated by spectral analysis of the fluorescence signal. Three major UHC sources were identified: nozzle dribble from the injector tip, low-temperature mixture of incomplete reactants within the cylinder core surrounding the injector and crevice region fuel accumulation along the cylinder sidewalls. Injection timing was found to influence UHC composition and concentration in each of the identified regions.

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We have developed a two-dimensional Raman technique which provides the hydrogen mole fraction and the hydrogen temperature by directly probing the hydrogen and the nitrogen molecular number densities and the temperature sensitive population distribution of the hydrogen molecules in different rotational and vibrational molecular energy levels. In addition to the benefit that no tracers are needed, this measurement technique profits from a simple experimental setup and a straight forward data evaluation procedure with a prior non-extensive calibration. To develop this 2-D Raman technique for simultaneous mole fraction and temperature analysis inside a field of 20 × 23 mm with a local resolution of 200 μm and a temporal resolution of a few nanoseconds, hydrogen was injected with a temperature of 473 K into pressurized nitrogen with a temperature of 297 K at 1.1 MPa absolute pressure. Different measurement strategies acquiring different combinations of Raman rotational signals and Raman vibrational signals were elaborated to find the most suitable combinations for different conditions.

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New computational procedures are proposed for experimentally evaluating air-fuel ratio, mass fractions of exhaust emissions as well as EGR, oxygen mass fraction and thermal capacity of the inducted charge in IC engines running with diesel oil, gasoline or any alternative liquid or gaseous fuel, such as LPG or CNG Starting from the chemical reaction of fuel with air, from gaseous and smoke level measurements in the raw gases, the procedures calculate the volume fractions of oxygen in the combustion air and of compounds in the exhaust gases, including those that are not usually measured, such as water, nitrogen and hydrogen. The methods also take the effects of various fuel and combustion air compositions into account, as well as to the presence of water vapour, CO_2, Ar and He in the combustion air. The algorithms are applied to three different automotive engines under wide ranges of steady-state operating conditions: two turbocharged diesel engines with high-pressure cooled EGR, and a SI naturally aspirated bi-fuel engine running on either gasoline or CNG. The computed air-fuel-ratios are compared to those obtained from directly measured air and fuel mass flow rates as well as from more conventional UEGO sensor data. The mass emissions are then worked out in terms of both brake specific mass emissions and emission indexes of each pollutant species, and the results are compared to those obtained by applying SAE and ISO recommended practices. Finally, the sensitivity of results to the main engine working parameters, the influence of environmental conditions (in particular the effect of air humidity on NO_x formation) and the experimental uncertainties are determined.

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A laser 2-focus velocimeter (L2F) has been applied for measurements of velocity and size of droplets in the core region of diesel spray. The L2F had a micro-scale probe which consisted of two foci. The focal diameter was about 3 μm, and the distance between two foci was 17 μm. The data sampling rate of the L2F system has been increased to 15 MHz. Investigated was the fuel spray injected intermittently into the atmosphere from an injector nozzle with the orifice diameter of 0.113 mm. The injection pressure was set at 40 MPa by using a common rail system. Measurement position was located at 15 mm from the nozzle. The number density of droplet was estimated with an interval of time between droplet appearances. It was confirmed that the velocity of droplet showed the highest at the spray center and decreased toward the off-axis direction. The spatial distributions of velocity and size of droplet were axisymmetric during injection. The mean number density in the spray core region was found to be about an order of 30,000 1/mm^3. It was understood that the mass flow rate was highest at the spray center and decreased towards the off-axis region.

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Diesel combustion is a complex process, including autoignition, premixed combustion and diffusion combustion. A novel TI (Time-scale Interaction) combustion model has been developed for simulating diesel combustion phenomena with high accuracy from premixed combustion to diffusion combustion. This model takes into account the characteristic time scales of chemical reactions and turbulence eddy break-up so as to match actual combustion modes. Performance tests were conducted with a single-cylinder direct-injection diesel engine that was used in the experimental validation of the TI combustion model. Comparisons of the calculated and measured pressure histories and heat release rates under various engine operating conditions showed good agreement. The diesel combustion mechanism was analyzed under several fuel injection timings and engine loads using the TI combustion model.

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