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 NO_x 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 NO_x 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 NO_x 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).

Landscape of advanced compression ignition combustion strategies aimed at achieving low temperature combustion with gasoline and diesel fuel. Strategies are positioned according to the level of fuel stratification at the start of combustion relative to conventional diesel combustion, which is highly stratified. Source: Dempsey et al. Click to enlarge.

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.

CFD simulation results for conventional diesel combustion (CDC) strategy at 6.5° ATDC. (a) Cylinder pressure, heat release rate, and injection velocity versus crank angle. (b) In-cylinder mixture conditions (local equivalence ratio vs local temperature). Each datum is an individual CFD cell. (c) In-cylinder image of local equivalence ratio. (d) In-cylinder image of local temperature. Cut planes are coincident with the injection axis. Dempsey et al. Click to enlarge.		CFD simulation results for the partial fuel stratification (PFS) strategy with PRF68 fuel at 5.6° ATDC. (a) Cylinder pressure, heat release rate, and injection velocity versus crank angle. (b) In-cylinder mixture conditions (local equivalence ratio vs local temperature). Each datum is an individual CFD cell. (c) In-cylinder image of local equivalence ratio. (d) In-cylinder image of local temperature. Cut planes are coincident with the injection axis. Dempsey et al. Click to enlarge.

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 NO_x 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.

Resources

• 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 NO_x and soot emissions: Effects of in-cylinder fuel stratification” International Journal of Engine Research doi: 10.1177/1468087415621805