Recently the air pollution caused by marine diesel has become a more serious problem. For such engines heavy fuel, e. g. bunker fuel oil, is usually used. High residual portion and long ignition delay are the main disadvantage of heavy fuel. Their effects are especially severe at low load, which is the running condition of ships near the seashore where the pollution is a more serious problem.
In this study the emission from a relatively high speed marine diesel engine using heavy fuel is examined. Because the effects caused by heavy fuel can be observed more clearly on a high speed diesel engine, such one was used for the experiments instead of a low speed engine.
The experiments show clearly that heavy fuel causes higher smoke density than gas oil. The higher smoke density is caused by direct impingement of the unevaporated residual portion onto the piston.
On the other hand the effect of heavy fuel on NOx emission can not be explained simply, because it is influenced by two counteracting effects : the higher pre-mixed combustion rate, which increases NOx formation, and the later ignition timing, which decreases it.