Characterizing the Multiscale Knock Energy of the in-Cylinder Pressure of Compound Combustion Engines Fueled with Dimethyl Ether

A two-cylinder, direct injection, and four-stroke naturally aspirated diesel engine is reformed to the compound combustion engine fueled with dimethyl ether. The wavelet energy spectra of the in-cylinder pressure with normal combustion and knocking combustion in a compound combustion engine are experimentally studied. The effects of the port fuel quantity, engine load, and engine speed on them are analyzed. The multiscale knock energy characteristics of the in-cylinder pressure are investigated based on wavelet energy and the Shannon entropy-energy ratio. The result shows that the in-cylinder pressure oscillation tends to be violent with the increase of port fuel quantity, the peak in-cylinder pressure increases, and its crank angle phase advances. With the increase in port fuel quantity, the wavelet energy of high-frequency detail signals d1, d2, and d3 obtained from wavelet decomposition all increases and the Shannon entropy-energy ratio decreases. The high-frequency detail signal d1 is more sensitive than the other detail signals. The frequency band of 5-10 kHz is the knock characteristic frequency band. The energy of the detail signal d1 increases significantly during knocking combustion, and the oscillation range enlarges and moves forward. The wavelet energy of detail signal d1 is the largest, and the Shannon entropy-energy ratio is the smallest at different brake mean effective pressures and different engine speeds. The effect of brake mean effective pressure and engine speed on the values is not obvious, and the port fuel quantity is the main factor.