Investigation on injection strategy affecting the mixture formation and combustion of a heavy-duty spark-ignition methanol engine

Methanol is thought as a carbon neutral fuel and one of the most promising alternative fuels for spark-ignition (SI) engine. On the basic combustion theory of SI engines, mixture formation plays an important role in the SI engine performance. However, there is a lack of investigation on the mixture formation process of heavy-duty (HD) port fuel injection (PFI) SI methanol engines. Therefore, experiments and simulations were conducted on a HD PFI SI engine fueled with pure methanol. The effects of the injection timing (alpha inj) on the mixture for-mation process, combustion and emission characteristics were investigated. Methanol engine performance can be predicted by the mixture homogeneity which is deeply affected by injection timing. When alpha inj was 240 degrees CA, 360 degrees CA and 480 degrees CA, there were three typical mixture formation modes, namely "spray", "premix and spray" and "premix" mode, respectively. The mixture homogeneity increased first and then decreased while the in-cylinder temperature at the spark timing increased monotonically as alpha inj varied from 240 degrees CA to 480 degrees CA at the interval of 120 degrees CA. The "premix and spray" mode was the optimal mixture formation strategy from the perspective of promoting mixture homogeneity and decreasing the in-cylinder mixture temperature. When alpha inj was from 120 degrees CA to 300 degrees CA, the leaner mixture around the spark plug and lower mixture prolonged or retarded CA0-10, CA10-90 and CA50. The above combustion characteristics contributed to a lower engine brake thermal efficiency (BTE) at alpha inj of 120 degrees CA to 300 degrees CA. As alpha inj varied from 0 degrees CA to 720 degrees CA, HC emissions increased first and then decreased while CO emissions did the opposite. Both HC and CO emissions at alpha inj of 360 degrees CA were at low levels. The more HC emission was caused by the more regions with the richer mixture in the crevice formed by the piston and liner. The CO emission was determined by the volume of regions with the richer mixture in the combustion chamber and in-cylinder mixture temperature. The "premix and spray" mode should be adopted to improve the BTE and reduce both HC and CO emissions.