The use of fuel chemistry and property variations to evaluate the robustness of variable compression ratio as a control method for gasoline HCCI

On a gasoline engine platform, homogeneous charge compression ignition (HCCI) holds the promise of improved fuel economy and greatly reduced engine-out NOx emissions, without an increase in particulate matter emissions. In this investigation, a variable compression ratio (CR) engine equipped with a throttle and intake air heating was used to test the robustness of these control parameters to accommodate a series of fuels blended from reference gasoline, straight run refinery naphtha, and ethanol. Higher compression ratios allowed for operation with higher octane fuels, but operation could not be achieved with the reference gasoline, even at the highest compression ratio. Compression ratio and intake heat could be used separately or together to modulate combustion. A lambda of 2 provided optimum fuel efficiency, even though some throttling was necessary to achieve this condition. Ethanol did not appear to assist combustion, although only two ethanol-containing fuels were evaluated. The increased pumping work from throttling was minimal compared to the efficiency increases that were the result of lower unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. Low temperature heat release was present for all the fuels, but could be suppressed with a higher intake air temperature. Results will be used to design future fuels and combustion studies with this research platform.