Impact Of Intake Dilution On Combustion And Emissions Of Dual Fuel Engine At Low Load

The lack of global energy resources and environmental degradation is an important topic of concern in the world in recent years,the development of efficient clean energy is the general trend.Natural gas is widely used because of its abundant resources,clean combustion and low price,and it has become an alternative fuel for traditional gasoline and diesel fuel.Diesel/natural gas dual fuel engine is one kind of natural gas engine,using natural gas as the main fuel instead of diesel,and microscale of diesel being used as pilot fuel.Because of the diesel/natural gas dual fuel engine is modified on the basis of the diesel engine,so there is the problem of low thermal efficiency,high CO and THC emissions when dual fuel engine is running at low load.The problem of low thermal efficiency and high THC emissions at low loads is due to the low flame propagation speed resulted from the very low concentration of the mixed gas.In order to improve the situation of low thermal efficiency and high THC emissions of diesel/natural gas dual fuel engine at low load,it is necessary to improve the concentration of the mixed gas in the cylinder.The addition of EGR(Exhaust gas recirculation)can reduce the oxygen concentration in the cylinder to improve the concentration of mixture,namely EGR dilution.There is other direct way to improve the concentration of the mixture in the cylinder is installing a throttle valve in the engine.By reducing the TOD(opening degree of throttle valve)reduce the amount of air entering into the cylinder,thus increase the concentration of the mixture in the cylinder,and namely air dilution.And also we can combine EGR dilution with air dilution,namely EGR combined with air double dilution.But diesel/natural gas dual fuel engine needs to burn high dilution mixture to keep NOx emissions at a lower level,so there is a "trade-off" relationship between thermal efficiency and NOx emissions.This paper carried out some bench tests on a diesel/natural gas dual fuel engine to improve the situation of low thermal efficiency and high THC emissions of diesel/natural gas dual fuel engine at low loads and kept NOx emissions at a lower level at the same time.Three kinds of dilution strategies:air dilution,EGR dilution and EGR combined with air double dilution were comparatively studied in this research.During the experiment,three working conditions were selected as A25(1335 r/min,25%load),B25(1656 r/min,25%load)and C25(1978 r/min,25%load).Besides,there were three energy substitution rate of natural gas selected as 85%,90%and 95%.In order to analyze the data,the A25 and C25 working conditions were analyzed when natural gas energy substitution rate is 90%.The main conclusions are as follows:All of the three dilution strategies can improve the situation of dual fuel engine at low loads,increasing the thermal efficiency and decreasing THC emissions at the same time.Comparison of three kinds of intake dilution methods to improve the effective thermal efficiency of dual fuel engine:EGR combined with air double dilution>air dilution>EGR dilution;Comparison of three kinds of intake dilution methods to decrease THC emissions of dual fuel engine:EGR combined with air double dilution>air dilution>EGR dilution;While Comparison of three kinds of intake dilution methods to decrease NOx emissions of dual fuel engine:EGR combined with air double dilution>EGR dilution>air dilution.Considering the effect of the three kinds of intake dilution on thermal efficiency and emissions of dual fuel engine at low load,EGR combined with air double dilution is the best.At A25 working condition,when TOD=45%,EGR rate=9.5%,BTE=35.87%,NOx specific emissions is 4.2 g·kW-1·h-1.Compared with the condition without EGR,without throttle valve,the brake thermal efficiency increase 2.79%(from 33.08%increase to 35.87%),CO emission decrease 26%(from 798.4 ppm decrease to 590.7 ppm),THC emissions decrease 28.4%(from 6102.4 ppm decrease to 4370.6 ppm);At C25 working condition,when TOD=37.4%,EGR ratee=11.4%,BTE=34.28%,NOx specific emissions is 6.5 g-kW-1·h-1.Compared with the condition without EGR,without throttle valve,the brake thermal efficiency increase 7%(from 33.08%increase to 35.87%),CO emission decrease 44.3%(from 1280.3 ppm decrease to 712.6 ppm),and THC emissions decrease 28.4%(from 5758.7 ppm decrease to 3506.5 ppm).