Interactive control of combustion stability and operating limits in a biogas-fueled spark ignition engine with high compression ratio

The use of high compression ratios on spark ignition engines enables the increase of thermal efficiency, but also contributes to the reduction of high load limit because of the higher auto-ignition tendency in the end-gas. Gaseous fuels provide a good option to expand the high load limits because of their high octane ratings, mostly in small engines. Biogas is a renewable fuel, mainly composed by CH4 and CO2 that exhibits high auto-ignition temperature and slow laminar flame speed. When biogas is burned in spark ignition engines partial and total misfire at low loads counteract the benefits achieved at high loads in which knocking combustion is reduced, hence the design of an effective control of the operating range based on the real-time observation of combustion instabilities is desirable. This paper presents an interactive control of the safe operating range through the modification of the spark time, equivalent ratio and throttle valve opening, taking as feedback the combustion instabilities, which are calculated from the in-cylinder pressure evolution for a biogas-fueled high compression ratio spark ignition engine. The interactive control was tested on a modified diesel engine converted to spark-ignition, original compression ratio of 15.5:1 and fueled with biogas. Control was able to keep a safe operating range with a maximum throttle valve opening of 39%, equivalence ratios within 0.6 and 1, and spark advances in the range of 329–358 crank angle degree. The coefficient of variation of IMEP was lower than 8%, whereas the maximum average knocking intensity was close to 2.5.