Experimental and numerical investigations to extend the operating load range of a premixed charge compression ignition engine through injector and fuel modifications

Premixed charge compression ignition (PCCI) presents an intriguing alternative to conventional diesel combustion (CDC), facilitating exceptionally low oxides of nitrogen (NO x ) and soot emissions by reducing local in-cylinder combustion temperatures and enhancing fuel-air mixing. PCCI combustion mode has been extensively investigated through high exhaust gas recirculation (EGR) levels and early or late injection timings to extend the ignition delay. One of the significant challenges in PCCI mode is controlling combustion initiation and a narrow engine operating load range due to early ignition and knocking combustion with high-reactivity diesel. The present work addresses this shortcoming by modifying injector orientation and utilizing a blend of high-reactivity diesel and low-reactivity gasoline. Two fuel blends are investigated, viz. 10% (D90G10) and 20% (D80G20) of gasoline mixed with diesel on a volume basis. The mechanical fuel injection system in the test engine is replaced with a common-rail direct injection (CRDi) system, and the compression ratio is reduced from 17.5 to 15 to achieve PCCI combustion. An initial parametric investigation revealed that early direct injection and high fuel injection pressure restricted the load range to 30% of the rated load in PCCI mode using diesel fuel. However, with diesel and EGR, the load range can extend to 60% of the rated load, resulting in high emissions of unburned hydrocarbons (HC) and carbon monoxide (CO). The engine cylinder head is modified to accommodate a vertical injector orientation, enabling operation with diesel-gasoline blends to address these shortcomings. The experiments were conducted at a constant rated speed with varying load conditions, and injection timing was optimized to achieve the maximum operable load range while maintaining stable combustion, maximizing brake thermal efficiency, and keeping NO x emissions below 100 ppm, at each load in PCCI mode to assess the effects of injector orientation and the diesel-gasoline blend. The results show that using a diesel-gasoline blend significantly improved the engine’s operable load range and reduced unburned emissions in PCCI. The longer ignition delay associated with the gasoline blend promotes enhanced premixed combustion, resulting in lower local equivalence ratios and lower in-cylinder combustion temperatures. This enables a substantial extension of the engine load range up to 73.4% of the rated load. Concurrently, NO x emissions are reduced to below 100 ppm, and soot emissions decrease by 69.4%. Improved combustion phasing and in-cylinder thermodynamic conditions at higher engine loads result in an 8.8% enhancement in brake thermal efficiency when using a modified injector operated with a diesel-gasoline blend, compared to the unmodified injector operated with diesel (reference case). Additionally, HC and CO emissions are reduced by 52.2% and 84.3%, respectively, at 60% of the rated engine load.