Advanced Compression‐Ignition Combustion for High Efficiency and Ultra‐LowNO_Xand Soot

Abstract

Low temperature combustion of gasoline or gasoline‐like fuels has significant advantages over conventional piston‐engine combustion processes. This combustion process, which is based on the compression‐ignition (CI) of a premixed or partially premixed dilute charge, is capable of achieving thermal efficiencies at or above those of diesel engines, engine‐out NO X and soot emissions well below mandated standards, and loads as high as those typical of turbo‐charged diesel engines. However, additional research and development are required to make this process commercially viable. There are many variations of this combustion process, involving charge mixtures that range from well premixed (homogeneous charge compression ignition, HCCI) to those that are mildly or even highly stratified. This chapter provides a comprehensive overview of low temperature gasoline combustion (LTGC), beginning with the fundamentals, which are discussed primarily in terms of HCCI, the simplest form of LTGC. An emphasis is placed on explaining the physical and chemical processes responsible for key operating parameters such as load limits, fuel effects, and intake‐pressure boosting. This approach is also applied to discussions of the use of controlled charge stratification to extend the operating limits and improve efficiency. Building on this understanding, methods for the practical implementation of LTGC are reviewed, along with two more‐recently introduced techniques that show promise: one using two fuels with different autoignition reactivities and the other using diesel‐type direct injection of gasoline‐like fuels.

On the effect of injection timing on the ignition of lean PRF/air/EGR mixtures under direct dual fuel stratification conditions

“…In previous studies [21,71] , it was found that as the degree of thermal inhomogeneity of the fuel/air charge increases at the on- set of the main combustion, the overall combustion is more likely to occur by the deflagration mode of combustion, which in turn reduces the peak HRR by slowing down the overall combustion process and prolonging the combustion duration. In addition, it was also found that the negatively-correlated T − φ distribution in the mixture has an adverse effect on the HCCI combustion because the evaporation cooling effect of direct fuel injection offsets that of mixture inhomogeneity by narrowing the differences in the ignition delays of local fuel/air mixtures with different reactivity, consequently leading to a shorter combustion duration and an excessive HRR [23,33,38,[71][72][73] . Recently, Bansal et al [38] demonstrated that the HCCI combustion with compression heating is more apt to occur by the spontaneous ignition mode than that under constant volume because the compression heating tends to reduce temperature gradients before the main combustion.…”

A perspective on the range of gasoline compression ignition combustion strategies for high engine efficiency and low NOx and soot emissions: Effects of in-cylinder fuel stratification

“…Subsequent direct injections occur during the compression stroke to create slight levels of fuel stratification in an attempt to create sequential autoignition events while maintaining ultra-low NOx and soot emissions. [15][16][17][18][19][20][21][22][23][24][25][26][27][28] MFS increases the stratification level by reducing to the amount of premixed fuel and typically features all direct injections during the compression stroke. In addition, MFS strategies typically feature a fuel injection event near top dead center (TDC) of the compression stroke to trigger combustion.…”

Prediction of ignition modes of NTC-fuel/air mixtures with temperature and concentration fluctuations

“…Advanced low-temperature-combustion (LTC) strategies of downsized and boosted engines are capable of offering ultra-low emissions and higher efficiencies [2][3][4]. In these engines, combustion is chemically driven by the autoignition process with no direct means to control the ignition timing and combustion rate.…”