Visualization Study On Effect Of Multiple Injections On Combustion Process Of Direct Injection Gasoline Engine

Direct injection petrol engines have a great potential to reduce fuel consumption and emissions and are becoming a mainstream trend in the development of petrol engines.In order to ensure that direct injection petrol engines fulfil their greater potential for energy saving and emission reduction,more precise analytical tools are needed to elucidate this.Visualization technology is an important tool in engine research and development.Besides the traditional bench test and numerical simulation,it can obtain information in a more direct way,especially for the specific state of spray,mixing and combustion,which can not be obtained by traditional bench test or can only be obtained indirectly.In this thesis,in order to explore the influence of injection strategy on the combustion process of in-cylinder direct injection gasoline engine in secondary injection mode,a visual optical test platform was built,and the software of the optical test platform was developed by d SPACE rapid prototyping control system.Combining MATLAB/Simulink with RTI,an optical engine control system is built.Specifically,it includes the synchronization of crankshaft signal and camshaft signal,the establishment of in-cylinder direct injection and ignition control system,the establishment of electronic throttle control system,the establishment of intake pressure and intake temperature control module and the establishment of high-speed camera trigger module.The effect of the injection strategy on the combustion process of a direct-injection petrol engine in multiple injection mode was investigated on the above-mentioned inhouse built optical test platform.The effect of the secondary injection strategy on the combustion process of the in-cylinder direct injection petrol engine was investigated by varying the secondary injection timing and the injection ratio.Combined with the combustion images and test data,the effect of the injection strategy on the combustion process of the direct injection petrol engine was investigated under multiple injection mode.The results of the study show that.1)When the fixed excess air coefficient is the equivalent ratio and the fixed injection ratio is 5:5,the second injection at 125°CA BTDC has the best stratification effect and the highest combustion quality,at which time a stratified mixture with a good concentration gradient can be formed near the spark plug.The earlier the time of the second injection,the more homogeneous the mixture becomes.The stratified mixture formed at this point has a better performance in terms of improving smoke emissions,but not in terms of power gain.When the second injection is later than 125°CA BTDC,the stratified mixture in the cylinder is less effective and is more likely to be overly rich or too lean,resulting in poor combustion quality and reduced combustion efficiency.2)Fixed excess air coefficient for the equivalent ratio,the second injection at 125 °CA BTDC,the injection ratio of 5:5 in-cylinder combustion effect is the best.When the injection ratio is 8:2,because the first injection ratio increases,the fuel spray in the cylinder has more than enough time to mix and the second injection moment in the middle of the compression stroke,at this time,because the second injection ratio is too small,the maximum spray penetration distance will be reduced,so it will not produce a large number of fuel wet wall phenomenon,the cylinder mixture as a whole tends to be homogeneous mixture.When the injection ratio is 2:8,as the second injection ratio increases,a denser stratified mixture is formed near the spark plug,which is prone to incomplete combustion resulting in increased carbon smoke.In addition,as the first injection ratio decreases,during the second injection,the fuel spray hits the top of the piston to form a wet wall oil film and diffuse combustion occurs,which further leads to an increase in carbon soot particles.3)At a fixed excess air coefficient of 1.3 and a fixed injection ratio of 5:5,the best stratification of the in-cylinder mixture and the best combustion effect with lower soot emissions is achieved with the second injection at 75°CA BTDC.Under lean combustion conditions,the second injection at 50°CA BTDC does not allow sufficient time for the fuel and air to mix before ignition due to the late timing of the injection,resulting in a partial over-concentration of the mixture.When the second injection is carried out before 75°CA BTDC,the mixture in the cylinder is stratified but the effect is not very obvious and tends to be homogeneous as a whole.The combustion quality is higher and therefore the carbon smoke emissions are lower.The difference is that the optimum secondary injection time is 125° CA BTDC for the equivalent ratio and 75°CA BTDC for the diluted combustion condition.For equivalent ratio,relatively late injection can improve engine dynamics and thermal efficiency of combustion.4)At a fixed excess air coefficient of 1.3 and a fixed secondary injection moment of 75°CA BTDC,the in-cylinder combustion effect is best when the injection ratio is also 5:5 and the combustion efficiency is higher.When the injection ratio is 8:2,a large amount of fuel enters the cylinder during the first injection,and the fuel and air have sufficient time to mix in the engine cylinder.When the injection ratio is 2:8,as the second injection is late in the compression stroke,most of the mixture accumulates in the vicinity of the spark plug,which makes incomplete combustion very easy and generates a lot of carbon smoke,and the afterburning phenomenon is more serious and the combustion lasts longer.Compared to other injection ratios,the combustion quality at this ratio is the worst,and the power and economy of the engine are not good.