Comparing Knock between the CFR Engine and a Single Cylinder Research Engine

The confluence of increasing fuel economy requirements and increased use of ethanol as a gasoline blend component has led to various studies into the efficiency and performance benefits of higher octane numbers and high ethanol content fuels in modern engines. As part of a comprehensive study of the autoignition of different fuels in both the CFR octane rating engine and a modern, direct injection, turbocharged spark ignited engine, a series of fuel blends were prepared with varying composition, octane numbers and ethanol blend levels. The paper reports on the second part of this study where cylinder pressures were recorded for fuels under knocking conditions in both a single cylinder research engine (SCRE), utilizing a GM LHU head and piston, as well as the CFR engines used for octane ratings. In the SCRE, spark timing and air-fuel ratios were adjusted to achieve a consistent level of knock based on peak-to-peak values of the filtered cylinder pressures, over a range of engine speeds and manifold air pressures. The CFR engines were operated at standard RON and MON test conditions, achieving standard knock intensities measured by the ASTM prescribed instrumentation. In addition, stoichiometric mixtures, incipient knock and knock-free conditions were measured. Results revealed different manifestations of autoignition and knock in the two different engines and different responses of the different fuels tested.