Increasing the Load Range, Load-to-Boost Ratio, and Efficiency of Low-Temperature Gasoline Combustion (LTGC) Engines 2017-01-0731
Low-temperature gasoline combustion (LTGC) has the potential to provide gasoline-fueled engines with efficiencies at or above those of diesel engines and extremely low NOx and particulate emissions. Three key performance goals for LTGC are to obtain high loads, reduce the boost levels required for these loads, and achieve high thermal efficiencies (TEs). This paper reports the results of an experimental investigation into the use of partial fuel stratification, produced using early direct fuel injection (Early-DI PFS), and an increased compression ratio (CR) to achieve significant improvements in these performance characteristics. The experiments were conducted in a 0.98-liter single-cylinder research engine. Increasing the CR from 14:1 to 16:1 produced a nominal increase in the TE of about one TE percentage unit for both premixed and Early-DI PFS operation. Compared to fully premixed fueling, Early-DI PFS allowed significant CA50 advancement, resulting in higher TEs for both CRs, with peak gross-indicated TEs of 48.4% and 49.8% for CRs of 14:1 and 16:1, respectively. Early-DI PFS was also found to be very effective for increasing the high-load limit over a range of intake boost pressures for both CRs. For CR = 14:1, a maximum load of 19.4 bar IMEPg (gross indicated mean effective pressure) was achieved with an intake pressure (Pin) of 3.0 bar absolute using Early-DI PFS, compared to only 17.2 bar IMEPg for premixed fueling at this Pin. Perhaps more importantly, Pin = 3 bar is substantially less than the Pin = 3.45 bar required to reach this load for premixed fueling, which also required the use of a less reactive E20 gasoline. For Pin > 2.5 bar, the maximum load was lower with CR = 16:1; however, for 1.6 ≤ Pin ≤ 2.5 bar, higher loads were achieved with CR = 16:1.