Decreasing the compression ratio improves thermal efficiency in a premixed charge compression ignition engine for the range of investigated octane numbers due to improvement in the degree of constant volume combustion and decreased cooling loss as the ignition timing approached TDC. The indicated thermal efficiency with different octane numbers is almost unchanged if the compression ratio is optimized. The thermal efficiency benefits of lower compression ratios decreased with higher homogeneity achieved by advancing the injection timing at due to a corresponding deterioration in combustion efficiency.
