Low-temperature gasoline combustion (LTGC) engines can provide high efficiencies and extremely low NOx and particulate emissions, but controlling the combustion timing remains a challenge. This paper explores the potential of Partial Fuel Stratification (PFS) to provide fast control of CA50 in an LTGC engine. Two different compression ratios are used (CR=16:1 and 14:1) that provide high efficiencies and are compatible with mixed-mode SI-LTGC engines. The fuel used is a research grade E10 gasoline (RON 92, MON 85) representative of a regular-grade market gasoline found in the United States. The fuel was supplied with a gasoline-type direct injector (GDI) mounted centrally in the cylinder. To create the PFS, the GDI injector was pulsed twice each engine cycle. First, an injection early in the intake stroke delivered the majority of the fuel (70 - 80%), establishing the minimum equivalence ratio in the charge. Then, a second injection supplied the remainder of the fuel (20 - 30%) at a variable timing during the compression stroke, from 200° to 330°CA (0°CA = TDC-intake, 360°CA = TDC-compression) to provide controlled stratification. For both CRs, second DI timing sweeps were performed for a range of intake pressures from highly boosted to naturally aspirated conditions, allowing the CA50 control authority at each condition to be determined. By varying the late-DI timing, CA50 could be adjusted as much a 12°CA, from near the misfire limit (overly retarded CA50 with COV-IMEPg > 3%) to well beyond the acceptable knock/ringing limit (overly advanced CA50 with RI > 5 MW/m2). For different conditions, the amount of DI timing retard and CA50 advancement was limited by either engine knock, combustion instabilities, or high NOx emissions (NOx > 0.27 g/kWh). For most conditions, approximately 6-8°CA of CA50 control was possible with good stability and acceptable NOx emissions.
Low-temperature gasoline combustion (LTGC) engines can deliver high efficiencies, with ultra-low emissions of nitrogen oxides (NOx) and particulate matter (PM). However, controlling the combustion timing and maintaining robust operation remains a challenge for LTGC engines. One promising technique to overcoming these challenges is spark assist (SA). In this work, well-controlled, fully premixed experiments are performed in a single-cylinder LTGC research engine at 1200 rpm using a cylinder head modified to accommodate a spark plug. Compression ratios (CR) of 16:1 and 14:1 were used during the experiments. Two different fuels were also tested, with properties representative of premium- and regular-grade market gasolines. SA was found to work well for both CRs and fuels. The equivalence ratio limits and the effect of intake-pressure boost on the ability of SA to compensate for a reduced Tin were studied. For the conditions studied, =0.42 was found to be most effective for SA. At lower equivalence ratios the flame propagation was too weak, whereas =0.45 was closer to the CI knock/stability limit, which resulted in a smaller range of CA50 control and Tin compensation. At =0.42, SA worked well from Pin = 1.0 to 1.6 bar, but the range of effective Tin compensation dropped progressively with boost from 21 °C at Pin = 1.0 bar to the equivalent of 12 °C at Pin = 1.6 bar. The amount of control authority using SA was demonstrated by varying the spark timing, advancing CA50 to the onset of strong knocking and then retarding CA50 to near misfire. SA provided good control, however the CA50 control range decreased from 7.2° CA at Pin = 1.0 bar to 4.2° CA at Pin = 1.6 bar. For all intake pressures at these well-mixed conditions, NOx emissions for SA were less than for compression ignition only, and all were below the US-2010 Heavy Duty limit.