Influence of air and EGR dilutions on improving performance of a high compression ratio spark-ignition engine fueled with methanol at light load

• Air and cooled EGR has great dilution tolerance in the methanol engine.
• Cooled EGR shows more sensitive effect on the combustion process than air dilution.
• Air and cooled EGR dilutions had possibility in improvement of engine fuel consumption.
• Cooled EGR shows higher efficiency than air dilution under very low NOx level.
• Hot EGR have better combustion and relationship between output torque and NOx than cool EGR.

Methanol is a promising alternative fuel for the spark-ignition engines and it is applicable to dilution combustion for improving the thermal efficiency in the engine port load due to desirable fuel characteristics. However, under the pressures pursuing lower energy consumption rate and emission reduction of internal-combustion engines, how to achieve lower NOx emission while keeping the high thermal efficiency at the same time becomes a key challenge for dilution combustion. To get better “trade-off” relationship between specific fuel consumption and NOx emissions, the effect of three dilution substances, including air, cooled EGR and hot EGR on the engine combustion, performance and emissions was experimentally investigated at a port-injection spark ignition methanol engine which was modified from a diesel engine. The research results indicated that comparing with the air dilution, the cooled EGR has a more sensitive effect to the combustion characteristics. Under a same dilution level, the ignition delay and combustion duration were longer and cycle-by-cycle variations were increased for EGR dilution. Both air and cooled EGR dilutions have a possibility of improving the fuel consumption rate of the engine. Meanwhile, the cooled EGR dilution could decrease the NOx emission faster than the air dilution at the same dilution coefficients, and it also reduces the brake-specific emissions of HC and CO. If a higher NOx emission is tolerant, the air could be a superior dilution charge due to its higher torque output and lower application costs. However, the cooled EGR dilution can achieve a better balance relationship between torque output and NOx emission under the very low NOx level. Compared with cooled EGR, the hot EGR can improve the engine combustion performance, and it can achieve a better relationship between torque output and NOx emission under dilution coefficients of 1.45 and 1.65.