Technical Papers
Exhaust Emissions from Marine Diesel Engines Using Residual Fuel
Kazuyuki Maeda**, Yutaro Wakuri***, Koji Takasaki****, Shin-ichi Morishita**
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 disadvantages 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 non-evaporated 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, and it is found necessary to comprehensively examine these two factors.
1. Introduction
Recently the air pollution caused by the exhaust emissions from the marine diesel engines has become a more serious problem, and the condition of the harmful composition to be emitted from ships on the sea has been examined1), 2), and studies3) 4) on the method to reduce NOx emission have been made. The residual fuel containing much residues in the refining process of the crude oil is used in most of marine diesel engines whose cylinder diameter is 200 mm or over. Because the residual fuel is inferior5) to the distilled oil in evaporation, ignition and combustion, there is a possibility of emitting more harmful compositions in the combustion.
The main engines of the ships are generally operated around the service load while the ships are on the ocean, but are often operated at the low load condition1) in such areas as ports and narrow channels where ships are concentrated. In the combustion at the low load condition, the ignition delay is likely to be long, and it seems that the premix combustion is increased and NOx emission is promoted.
Further, when the fuel spray reaches the piston crown during the ignition delay, the combustion is extremely deteriorated5), causing generation of the exhaust smoke, and it is also expected that CO2 emission is increased through deterioration of the fuel consumption.
In this study, to clarify the effect of the properties of the residual fuel on the NOx emission, exhaust smoke, and CO2, the experiment was carried out using the small high-speed marine diesel engine in which the difference in property of the fuel appears sensitively on the operational performance. In addition, the emission characteristics when the residual fuel is used for medium and large-sized marine diesel engines were also considered.
* Translated from Journal of MESJ Vol. 32, No. 10 (Manuscript Received May 21, 1997) Lectured Oct. 16, 1997)
** National Fisheries University (Shimonoseki City)
*** Fukuoka University (Fukuoka City)
*** Kyusyu University (Fukuoka City)
2. Experimental Apparatus and Procedures
The experimental apparatus is shown in Fig. 1. Table 1 indicates the principal particulars of the 4-cycle, small and high-speed marine diesel engine which was used in the experiment. In Fig. 1 a sampling probe made of stainless steel spirally provided with total 6 suction holes to sample the exhaust gas is fitted in the vicinity of the outlet of the turbo-charger. The exhaust gas was guided to the pre-treatment device through the heating pipe, and the gas was dried, the concentration of NOx, O2, CO and CO2 were measured by the exhaust gas analyzer.
In the experiment, Bunker Fuel Oil (hereinafter, referred to as BFO) and Gas Oil on the marked (hereinafter, referred to as GO) were used. Table 2 indicates the properties of these test fuels and Fig. 2 shows the distribution of the distillation temperature.
