Engine operation just above and below the knocking threshold, using a blend of biogas and natural gas

This research involves a knocking effect analysis of the operation and performance of a spark ignition engine with high compression ratio using a blend of 50% biogas with 50% natural gas. A diesel engine was converted to spark ignition mode. During testing, the output power, equivalence ratio, fuel blend composition and engine speed were kept constant while the spark timing was modified. Three knock intensities were evaluated using three different spark timing to evaluate the combustion parameters and engine performance. Eighteen tests were developed for repeatability analysis. A knocking analysis compare the engine performance with negligible and low knocking intensities. The knocking effect was analyzed with respect to instantaneous pressure in the combustion chamber, heat release rate, crank angle location for mass burned, combustion duration, coefficient of variation of indicated mean effective pressure, knock peak pressure, specific fuel consumption, thermal efficiency, and emissions. The thermal efficiency reached was 28.76% with an output power of 7.5 kW. High values of efficiency result from high CR and high output power, close to the knocking threshold. A knock peak pressure in the range between 0.3 and 0.5 bar averaged over 200 cycles is established as the knocking threshold.