Combustion and NOx Emission Characteristics in a DI Methanol Engine Using Supercharging with EGR

In this study, we investigated the combustion technology for the direct injection (DI) methanol engine for a heavy-duty vehicle that makes use of the fuel characteristics of methanol and achieves smokeless burning with high efficiency and low NOx emissions under the heavy load condition. A 3.3-liter 4-cylinder spark-assisted DI methanol engine was tested to investigate the combustion and NOx emission characteristics under the full load condition with supercharging and/or EGR.

We believe that supercharging suppressed the stratified charge combustion, but accelerated the premixed combustion to increase the indicated mean effective pressure. Moreover, supercharging was helpful in carrying out EGR under the full load condition without deteriorating the thermal efficiency. Furthermore, heavy EGR during supercharging reduced the NOx emissions dramatically while maintaining the high thermal efficiency and controlling the unburned hydrocarbons emissions. We believe that this was because the heavy EGR considerably suppressed the initial combustion to extend the mixing time for the fuel-air charge, and accelerated premixed combustion. The test results also showed that the high thermal efficiency was maintained because the accelerated premixed combustion prevented the shortage of oxygen during the combustion period even under the full load condition which involved high injection quantity.

Based on the above results, we were able to resolve the trade-off relationship between the brake specific NOx emissions and the brake thermal efficiency by combining supercharging and heavy EGR. We also verified that the brake specific NOx emissions were reduced to the low level of 1.0 g/kWh or less at the brake thermal efficiency of 38% under the full load condition.