One of the possible methods for the improvement of thermal efficiency in SI Engine is fuel lean combustion. It contributes to the increase of the mixture specific heat ratio and the decrease of the net heat loss to the engine cylinder through low temperature combustion, in which both of them lead to the higher thermal efficiency. At first, this article introduces the latest study which attempts to increase the thermal efficiency of an SI engine through super-lean burn concept. After identifying the relevant combustion fundamentals required for establishing super lean-burn concept, fundamental studies which report the minimum ignition energy (MIE) transition and initial spherical flame propagations at different Lewis numbers are introduced. Then, some similarities between super lean-burn SI engine experiments and fundamental findings are discussed in terms of ignition enhancement at super lean conditions. Finally, possible correlations between the specific flame growth from small ignition kernel to flame propagations sensitive to the mixture Lewis numbers and elongated arc channel for the effective ignition of lean burn condition are discussed.
A low speed preignition (LSPI) is one of the critical issues in recent supercharged direct injection SI gasoline engines. LSPI is strongly demanded to be solved because knocking with high intensity which is called “super-knock” or “mega-knock” follows it, and sometimes provides severe engine damage. LSPI is occasional phenomena and is often occurs in a sequential manner intermittently including normal SI combustion cycles. Though some possible sources inducing LSPI have been found and reported, chemical and physical mechanisms of LSPI phenomena have not been cleared in detail, yet. This report reviews some studies and discussed about mechanisms of LSPI in supercharged DISI engines focusing attention on the effects of lubricant oil and its additives on LSPI and the LSPI induced by hot deposit fragments detached from the wall.
Spontaneous ignition of an isolated fuel droplet in hot lean premixed gas was numerically simulated in order to study the interaction between a fuel droplet and premixed gas. Ambient pressure was 2.5 MPa, and ambient temperature was 753 K, so that they were within the operation range of practical reciprocal engines. Fuel was n-heptane both for the droplet and the premixed gas. Initial droplet diameter and equivalence ratio of premixed ambient gas were varied. Initial droplet diameter was set near ignitable limit of hot flame in the given ambient conditions, that is, 45 μm and below. Premixed ambient gas was set so lean that its hot-flame ignition delay was far longer compared to that of a droplet, that is, equivalence ratio was 0.2 and below. Even such lean fuel in the ambient gas lowered ignitable limit of hot flame in terms of initial droplet diameter and decreased cool-flame and hot-flame ignition delays of the droplet. Besides, the heat release from the droplet induced the burning of entire premixed gas, and the hot-flame ignition delay of premixed gas was drastically shortened.
An increase in thermal efficiency of practical combustor is urgently demanded for the reduction of carbon dioxide emission. Numerical simulations, such as large eddy simulation (LES), are useful for rapid developments of next generation combustors, while the problems lie in that characteristics of turbulent combustion under high Karlovitz number conditions are not well understood and conventional turbulent combustion models are not validated in those conditions due to lack of data of fine experiments and direct numerical simulations. At the present, these challenges are resolved and turbulent combustion models are needed to be developed based on the turbulent combustion mechanism under high Karlovitz number conditions. This article reviews numerical simulation tools for LES and studies toward the understanding of local flame structure in turbulence. First, basic theories on flame structure of turbulent premixed combustion are introduced with a regime diagram of turbulent premixed combustion. Secondly, tools for LES and conventional turbulent combustion models are reviewed. Thirdly, numerical and empirical studies on local structure of turbulent premixed flame are reviewed with future requirements.
Recent advances in the research and development of ammonia combustion are introduced. They are mainly promoted by the Cross-ministerial Strategic Innovation Promotion Program (SIP), “Energy Carrier”. Ammonia is a potential chemical substance not only for a hydrogen energy carrier but also a carbon free fuel. Ammonia also has advantages in terms of storage and transportation because the thermal properties are almost the same as those of propane. The subjects of ammonia combustion research are on low combustion intensity, low radiation intensity, as well as high NOx emission. In this article, recently obtained fundamental ammonia flame characteristics, such as the features of NO formation, laminar burning velocity, Markstein length and reaction enhancement by hydrogen addition are summarized. Flame structures and burnt gas characteristics were experimentally and numerically evaluated. NO mole fraction decreased with an increase in equivalence ratio and it is caused by NHi (i = 2, 1, 0) generated from excess NH3 in the mixture. Laminar burning velocities and Markstein lengths were evaluated from spherically propagating flames up to 0.5 MPa. The maximum value of laminar burning velocity of ammonia/air is less than 7 cm/s and is about 1/5 of that of methane/air flame. 1D flame simulation with detailed reaction mechanisms were also performed and it was showed that the quantitatively predicted laminar burning velocities were inaccurate. The laminar burning velocity exponentially increases with the increase in the hydrogen ratio. On the other hand, the Markstein length varies non-monotonically with with an increase in the hydrogen ratio.
The flame inhibition effect by high-concentration water mist was investigated experimentally. Measurements of burning velocities for water-mist-loaded CH4/air premixed flames were conducted by the angle method for various equivalence ratios and the water-mist mass fractions. The characteristics of water mist, i.e., the particle size distribution, the mean droplet diameters, and the water mist mass fraction, were obtained basing on the Stokes' law. The vaporization Damkőler numbers were also evaluated in all conditions and were confirmed to be high enough leading to the complete vaporization in the preheat zone. The experimental results of burning velocities were compared to the numerical results and other experimental data. It was found that the higher water mist mass fractions were attained than those in the previous studies, and the experimental data of burning velocities became much smaller than those obtained by the numerical simulation.