(3-05) Effect of EGR and Preheating on Natural Gas Combustion Assisted with Gas-Oil in a Diesel Engine((AF-2)Alternative Fuels 2-Gas Engines)

Abstract

In order to reduce NOx and smoke simultaneously and also to improve markedly the trade-off between smoke and NOx without deteriorating fuel consumption, natural gas was charged homogeneously into the intake air and was burned igniting by a small amount of gas oil injection in a four cylinder naturally-aspirated DI diesel engine. Combustion tests were carried out by changing the ratio of the amount of natural gas and the amount of gas oil first, secondarily the intake preheating temperature, and thirdly the EGR rate respectively. Effects of the respective parameter on the ignition and the burning rate of natural gas, exhaust emissions and specific fuel consumption were investigated. It is found that significant improvement of Smoke-NOx trade-off can be obtained without deteriorating fuel consumption by the suitable combination between the natural gas charge rate, the EGR rate and the intake preheating temperature for each engine load condition as shown in Fig. 1. Concluding remarks are as follows ; (1) A high burning rate of natural gas results in shortening the combustion duration, then, leads to lower fuel consumption. (2) Too high burning rate of natural gas results in increase of NOx, on the other hand, too low burning rate of natural gas results in increases of fuel consumption and THC. (3) Increases in fuel consumption and THC due to incomplete combustion of natural gas at the low load are improved drastically by raising the intake charge temperature up to 120℃, which increases the burning rate of natural gas. (4) Increase in NOx due to high burning rate of natural gas at the high load is improved by lowering the intake charge temperature below 60℃, which suppresses the burning rate of natural gas. (5) High EGR rate shows suppression effect on the burning rate at the high load, however, the burning rate is hardly affected by EGR at the low load. (6) NOx is reduced effectively by EGR without deteriorating fuel consumption, which is due to the water content brought by EGR as well as the decrease in oxygen concentration of the intake charge.