Studies on Improvement of Combustion with an Accumulator-type Fuel Infection System Coupled with Pilot Injection

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Studies have been carried out to examine both the effects of high pressure fuel injection and pilot injection on the improvement of Combustion, that means reductions of BSFC and exhaust smoke density without increase in NO emission, in a Diesel Engine. The effects were clarified not only by operation tests with the test engine equipped with a newly developed accumulator-type fuel injection system, but also from analyses of high sped photographs of fuel spray and flames taken in another test engine with a visualized combustion chamber. As the results of experiment following Conclusions have been obtained.(1) It is effective on the improvement of combustion characteristics to use the injection nozzle with smaller hole diameter corresponding to the high injection pressure, because of the increase in the excess air ratio of fuel spray.(2) The pilot/pre injection followed by high pressure injection is effective on reductions of BSFC and exhaust smoke density without increase of NO emission.(3) Residual fuel spray is more difficult to be dispersed by air swirl and needs high swirl intensity for the optimum combustion than marine diesel oil.(4) Combustion of impingement of fuel spray on the piston wall and air swirl is an effective means to improve the combustion of residual fuel spray in the diffusion combustion period by talking off the gaseous part and small droplets from the liquid core and supplying the spray with fresh oil.(5) Increase of combustion air temperature in the combustion chamber improves the combustion of residual fuel spray by accelerating the evaporation of distillate components.(6) Attention should be paid to the fact that the heat release rate of residual fuels goes down extremely with the increase of ignition delay at low compression air temperature.