The potential for using low temperature combustion (LTC) in compression ignition engines (i.e., diesel) to reduce NOx and PM while maintaining high efficiency has attracted a great deal of research interest over the past several years. While achieving LTC with diesel fuel over a wide operating range has been shown to be difficult for several reasons, gasoline, with its high volatility and low chemical reactivity, offers a more attractive fuel option for LTC.
Accordingly, in the industry’s quest for lower fuel consumption and emissions, a number of schemes for achieving LTC gasoline compression ignition have emerged. Now, a team of researchers from Oak Ridge National Laboratory’s National Transportation Research Center (NTRC) has published a comprehensive, open-access review of a variety of fuel injection strategies being investigated for LTCGCI. The paper is available for download from the International Journal of Engine Research.
Simply speaking, the goal of LTC is to achieve sufficient premixing between the fuel and air in a globally dilute environment, such that combustion takes place at lower temperatures… Providing time for fuel/air mixing avoids high soot formation rates. Dilution, with either air or exhaust gas recirculation (EGR), reduces the peak combustion temperatures to avoid high NOx formation rates. However, if the fuel and air are overly mixed, resulting in equivalence ratios that are too low, the local combustion temperature becomes too low to support complete fuel oxidation. Although LTC can yield high thermal efficiency with simultaneously low NOx and soot emissions, there are challenges associated with combustion efficiency (i.e., unburned hydrocarbon (UHC) and CO emissions), narrow operable load range, high pressure rise rates, and combustion timing controllability, all of which in-cylinder fuel stratification aim to address while maintaining low NOx and soot emissions.—Dempsey et al.
To illustrate the impact of fuel stratification on gasoline compression ignition, the authors selected three representative operating strategies: partial (PFS), moderate (MFS), and heavy fuel (HFS) stratification. The team used CFD modeling of the in-cylinder processes during the closed valve portion of the cycle to illustrate the differences between the various levels of fuel stratification, making comparisons with conventional diesel combustion (CDC).
The authors stressed that because the different operating strategies will be sensitive to combustion chamber and injector design, their results are not “all-encompassing” and should not be used to create overly wide generalizations. However, the results do highlight some overarching opportunities and challenges, while also emphasizing that there is a continuum between all of the defined stratification bins.
Among their overall findings:
All of the operating strategies yielded high gross indicated efficiency (GIE) (~46%) due to the high compression ratio and globally lean air/fuel ratio.
There is a trade-off between fuel stratification and combustion efficiency, with the more stratified conditions yielding higher combustion efficiency. The lower combustion efficiencies observed for the more premixed concepts stem from overly lean regions which result in overly low combustion temperatures (i.e. less than ~1400 K / 1127 ˚C).
Despite the lower combustion efficiency, the more premixed strategies, such as MFS and PFS, still yielded high indicated efficiency due to reduced heat transfer losses and more favorable thermodynamic properties of the working fluid, both stemming from the lower combustion temperatures.
The more stratified cases yielded higher NOx emissions than the more premixed operating strategies due to locally high combustion temperatures.
All of the LTGCI strategies investigated appear to form very little soot due to the fact that all injection events have ceased prior to the start of combustion.
Combustion noise displayed a non-monotonic trend with fuel stratification—a finding that must be exploited to minimize engine noise.
Each mode of LTGCI operation has unique opportunities and challenges that can be exploited in different regions of the engine operating map.—Dempsey et al.
Adam B Dempsey, Scott J Curran and Robert M Wagner (2016) “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” International Journal of Engine Research doi: 10.1177/1468087415621805