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In order to reduce diesel idle knock, the effects of EGR on the idling characteristics were investigated on three 4-stroke cycle engines having three different combustion chambers (precombustion, swirl and direct injection). It was found that EGR was effective not only for reducing idle knock, but also for decreasing fuel consumption, smoke density and exhaust emissions at idling operation regardless of the type of combustion camber. The same test results were obtained on a crankcase-scavenged 2-stroke cycle diesel engine (direct injection type) by means of inlet throttling.

Two hybrid Combustion engines, wherein a premised gasoline homogeneous charge was ignited by a small amount of injected diesel fuel under high compression ratio, were made and tested by modifying open chamber and prechamber 4-stroke cycle diesel engines. It was found that the premixed gasoline was effective not only for decreasing the fuel consumption but also for reducing the smoke density both in the heavy and over-load regions. The mixing ratio of the gasoline and diesel fuels, the injection timing of the diesel fuel and the compression ratio were found to be the main parameters affecting the performance of these hybrid combustion engines. The effect of introducing a small amount N2 gas for suppressing the diesel knock in the heavy load region also was examined.

In order to reduce the knock of spark-ignition engines in the heavy load region, the effects of EGR on engine performance were investigated on two engines, one with a turbocharger and the other naturally aspirated with a high compression ratio. It was found that EGR was effective not only for reducing the knock, but also for improving the brake specific fuel consumption compared with the ignition timing retarding system.