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4 crankcase-supercharged 4-stroke spark-ignition engine was made and tested in both the motoring and firing modes. It was found that a 20 to 30 % torque rise compared with the naturally-aspirated baseline engine was obtained in the low and middle engine speed ranges without any sacrifice in brake specific fuel consumption. Additionally, the throttle response of the modified engine was better than that of the naturally-aspirated baseline engine, and there was no deterioration in part-load brake specific fuel consumption.

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.

On 4-stroke cycle engines, the authors reported that under certain conditions the cylinder pressure, measured during the suction stroke with a low-pressure pickup, dropped below the critical pressure. Noting this considerable negative cylinder pressure during the suction stroke, the authors devised a Jet Air Suction Port (JASP), which was opened and closed by piston movement near bottom dead center. As a result, it was possible not only to burn lean mixtures steadily, but also to decrease the fuel consumption 10 to 30 % at idle by the jet air flow generated automatically from the JASP. Reductions in THC, CO and NO at idle were also noted.

In order to improve fuel consumption, the effects of cylinder cutoff on engine performance were investigated on a 4-stroke cycle in-line 6-cy linder gasoline engine. It was found that the cylinder cutoff obtained by deactivating both the inlet and exhaust valves was very effective for reducing the fuel consumption at part-load and idle, as both the pumping power for the inlet and the power for activating the inlet and exhaust valves were noticeable decreased.

As a mechanical supercharging system for 4-stroke cycle engines, crankcase-supercharging seems attractive because of its low production cost and simple construction. Therefore, a crankcase-supercharged 4-stroke cycle spark-ignition engine was developed and tested in both motoring and firing modes. It was found that the motoring volumetric efficiency could be increased over the entire engine speed range, but the increase was more noticeable at lower engine speeds. In the firing mode it was proved that a favorable throttle response and a 20 to 30% torque rise could be obtained by crankcase-supercharging without sacrificing the brake specific fuel consumption of the naturally aspirated baseline condition.

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.

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.