Abstract
Gasoline compression ignition (GCI) is a promising powertrain solution to simultaneously address the increasingly stringent regulation of oxides of nitrogen (NOx) and a new focus on greenhouse gases. GCI combustion benefits from extended mixing times due to the low reactivity of gasoline, but only when held beneath the threshold of the high temperature combustion regime. The geometric compression ratio (GCR) of an engine is often chosen to balance the desire for low NOx emissions while maintaining high efficiency. This work explores the relationship between GCR, variable valve actuation (VVA) and emissions when using GCI combustion strategies. The test article was a Cummins ISX15 heavy-duty diesel engine with an unmodified production air and fuel system. The test fuel was an ethanol-free gasoline with a market-representative research octane number (RON) of 91.4–93.2. In the experimental investigation at 1375 rpm/10 bar BMEP, three engine GCRs were studied, including 15.7, 17.3, and 18.9.
Across the three GCRs, GCI exhibited a two-stage combustion process enabled through a split injection strategy. When keeping both NOx and CA50 constant, varying GCR from 15.7 to 18.9 showed only a moderate impact on engine brake thermal efficiency (BTE), while its influence on smoke was pronounced. At a lower GCR, a larger fraction of fuel could be introduced during the first injection event due to lower charge reactivity, thereby promoting partially-premixed combustion and reducing smoke. Although increasing GCR increased gross indicated thermal efficiency (ITEg), it was also found to cause higher energy losses in friction and pumping. In contrast, GCI performance showed stronger sensitivity towards EGR rate variation, suggesting that air-handling system development is critical for enabling efficient and clean low NOx GCI combustion.
To better utilize gasoline’s lower reactivity, an analysis-led variable valve actuation investigation was performed at 15.7 GCR and 1375 rpm/10 bar BMEP. The analysis was focused on using an early intake valve closing (EIVC) approach by carrying out closed-cycle, 3-D CFD combustion simulations coupled with 1-D engine cycle analysis. EIVC was shown to be an effective means to lengthen ignition delay and promote partially-premixed combustion by lowering the engine effective compression ratio (ECR). By combining EIVC with a tailored fuel injection strategy and properly developed thermal boundary conditions, simulation predicted a 2.3% improvement in ISFC and 47% soot reduction over the baseline IVC case while keeping NOx below the baseline level.
