Characterization of Pressure Waves in HCCI Combustion

The objective of this work was to understand the physics of combustion-generated pressure waves from Homogeneous Charge Compression Ignition combustion and the resulting audible noise that is produced. Experiments were performed with a single-cylinder engine operating in both SI and HCCI combustion modes, and comparisons were made between the pressure waves generation from the two types of combustion. Cylinder pressure oscillation amplitudes at the first circumferential mode frequency (5 to 6 kHz) generated in HCCI combustion are 5 to 10 times higher than those generated in SI knocking combustion without an undue increase in audible engine noise. Frequency analysis of the data showed that in knocking combustion a larger portion of the wave energy is contained within the higher order resonance modes. Cylinder block vibration measurements indicate that the cylinder liner significantly dissipates the wave energy below 8 kHz. A correlation for the pressure wave intensity was developed and used to successfully correlate all of the experimental data. The correlation was applied to data obtained from the literature from a HCCI engine operated under supercharged conditions, and was able to successfully explain the experimental observation that with higher boost levels the engine could withstand higher rates of pressure rise without an increase in audible engine noise.