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Investigation on Combining Partially Premixed Compression Ignition and Diffusion Combustion for Gasoline Compression Ignition—Part 2: Compression Ratio and Piston Bowl Geometry Effects
Abstract:
This work investigates the compression ratio (CR) and the piston bowl geometry effects in a modified version of the third generation of the gasoline direct injection compression ignition (Gen3 GDCI) engine using a research octane number (RON)92 E10 gasoline. The piston bowl geometry was redesigned for the partially premixed compression ignition-diffusion (PPCI-diffusion) combustion process.
The investigation was focused at 1500 rpm - 5 bar BMEP. Different combustion strategies were evaluated, including early PPCI, late PPCI, and PPCI-diffusion. Overall, increasing the CR from 14.3 to 16.3 led to improved fuel efficiency, notably reduced combustion losses, and enhanced combustion stability while maintaining low engine-out oxides of nitrogen (NOx) and smoke. The thermal environment at 16.3 CR was found to be more favorable for the RON92 E10 gasoline than 14.3 CR, thereby driving with less boost requirement and resulting in less parasitic losses. As a result of the enhanced thermal reactivity, hydrocarbon (HC) and carbon monoxide (CO) emissions were also markedly reduced.
Closed-cycle, full geometry, three-dimensional (3-D) combustion computational fluid dynamics (CFD) analysis showed that by introducing a more pronounced bowl pocket volume, the tailored piston bowl design was able to guide the fuel jets away from the spatially constrained cold regions and facilitated better in-cylinder air utilization. The benefit became more pronounced with load increase.
Finally, the performance for the PPCI-diffusion combustion strategy was mapped out over 1000-2000 rpm and 2-6 bar BMEP. Fuel injection strategy and air system operating boundary conditions were developed to achieve good fuel efficiency with low engine-out HC and CO emissions while keeping emissions of NOx at 1 g/kWh and smoke below 0.5 FSN.