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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.

In order to improve the part-load and idling characteristics of 2-stroke cycle diesel engines, the effect of throttling on engine performance was investigated on a separately scavenged engine with a Roots blower and on a crankcase-scavenged engine. For the first engine, it was found that decreasing the delivery ratio through bypassing a part of scavenging air into the inlet side, that is, by recirculation of scavenging air, made it possible not only to decrease the fuel consumption but also to raise the scavenging temperature which improved combustion. Also, a vehicle test using a bus powered by the same type of engine showed improved mileage through throttling. For the crankcase-scavenged engine, it was found that a slight increase in delivery ratio was very effective for improving the performance at low speed with heavy load, and throttling was also effective in improving not only the part-load but also the idling operation.

Two hybrid Combustion engines, wherein a premixed gasoline homogeneous charge was ignited by a small amount of injected diesel fuel under high compression ratio, were made and tested by modifying crankcase scavenged 2-stroke cycle diesel engines, one uniflow scavenged with an open chamber and the other loop scavenged with a prechamber. 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 factors affecting the performance of these hybrid combustion engines. The effect of inlet throttling on the brake specific fuel consumption (BSFC) in the light load region was also examined.

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.