3.3　Characteristics of CO₂ Emission

Fig. 11 shows the relationship between the fuel injection timing and the fuel consumption. The experiment was carried out under the condition of the constant load (75%) on the marine characteristics, and it can be considered that the CO₂ emission is proportional to the fuel consumption. In this figure, the fuel consumption of BFO is higher than that of GO by about 5% at any injection timing (the CO₂ emission is more). This is believed to be affected by the delay in combustion of the residues adhered to the piston crown, and deterioration of the cycle efficiency caused by the delay in the ignition timing, i.e., the delay in starting the heat release.

Fig. 12 shows the relationship between the ignition timing and the fuel consumption. In comparison at the same ignition timing, the difference in the fuel consumption by difference in the fuel properties is reduced by half compared at the same fuel consumption. It can be concluded that CO₂ emission can be suppressed by selecting the optimum injection timing for the fuel properties. It is also understood that it is necessary to improve the incomplete combustion of the residues and the slow combustion speed to obtain the same fuel consumption (CO₂ emission) of BFO as that of GO.

　

4. Examination

In the previous chapter, it is clarified by the experiment using the small high-speed diesel engine how the properties of the residual fuel which contains large amount of residues and is long in the ignition delay affect the emission characteristics of NOx, smoke, and CO₂. In this chapter, examination is made by applying the results to the medium and large-sized marine diesel engines in which the residual fuel is used.

Fig. 13 shows the relationship⁵⁾ between the temperature of the compressed air and the ignition delay by the difference in the kind of fuel oil. Marine Diesel Oil (hereinafter, referred to as MDO) in the figure is A-oil having the distillation temperature distribution close to that of GO. Sample Fuel is the residual fuel long in ignition delay, in which the light distillation content is prepared in the residues of approximately same quantity as that of BFO (in which the intermediate distillation content superior in ignition is intentionally removed).

The ignition delay is correlated to the temperature of the compressed air at the ignition, and it is well known that the ignition delay is extremely long when the temperature of the compressed air is low, which is clearly indicated in this figure. The ignition delay becomes longer in the order of MDO, BFO, and sample Fuel, irrespective of the temperature of the compressed air, and when the temperature of the compressed air is low, the difference over 1 ms can be observed between MDO and Sample Fuel.

　

Fig. 11　Relationship between fuel injection timing and specific fuel consumption

Fig. 12　Relationship between ignition timing and specific fuel consumption

Fig. 13　Difference in ignition delay by fuel property

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