A Six-Stroke DI Diesel Engine Under Dual Fuel Operation 1999-01-1500
A six-stroke DI diesel engine proposed by the authors had second compression and combustion processes which were added on a conventional four-stroke diesel engine. This engine had the first and second power strokes before the exhaust stroke. Numerical predictions and experiments previously carried out had shown that this six-stroke diesel engine could reduce NO exhaust emission. Further, the ignition delay of the second combustion process could be shortened by a high temperature effect in the second compression stroke. This advantage of short ignition delay could be utilized for an ignition improvement of a fuel with low cetane number. In the engine system reported here, a conventional diesel fuel was supplied as the fuel of first combustion process, and in the second combustion process, methanol was supplied. Injection timings and injection flow rates of the first combustion fuel (diesel fuel) and the second combustion fuel (methanol) were varied independently, and the allocation ratio between diesel fuel and methanol were changed from 10% to 70%. As a result, remarkable reductions of NO and soot emissions appeared. Especially, the almost zero emission of soot was achieved even if a small amount of methanol (less than 10% of the total fuel) was supplied in the second combustion process. The engine performance and heat release rate of the diesel fuel six-stroke engine and the dual fuel six-stroke engine were investigated to make clear the combustion characteristics of the second combustion process. Furthermore, a four-stroke DI diesel engine with the same displacement was evaluated for comparison with this six-stroke engine. The indicated specific fuel consumption (I.S.F.C.) of the six-stroke diesel engine was slightly lower than that of the four-stroke engine. Then it could be concluded that the diesel fuel-methanol dual fuel DI engine has a high performance and low exhaust emission potentials.