Research On Low Temperature Cold Start Assist Control Of Low Compression Ratio Diesel Engine

Diesel engine is developing towards higher power density.In order to realize high power density,low compression ratio and high turbocharging are usually adopted.This can increase the IMEP with a acceptable mechanical load and heat load.But the reduction of compression ratio will bring the difficult problem of cold start.The measures to assist the cold start of the diesel engine are to optimize the injection parameters and additional auxiliary measures.In this paper,through the comparison of various auxiliary cold starting measures and combining with the actual project,the flame preheating is selected as the assist cold start scheme.A preheating system experimental bench is design and build.The experimental study of flame parameter characteristics was carried out.A mathematical model for the air preheating of the preheating system and the internal temperature rise of a flame preheating plug was established.And the simulation model of the flame preheating system was built.The simulation of the preheating system under different boundary conditions was carried out.One dimension simulation model of low compression ratio diesel engine cold start was built.The simulation of cold start performance under different boundary conditions was carried out and the influence of the injection parameters and preheating assist on the cold start performance of the diesel engine was studied.The parameter optimization of cold start based on orthogonal test and cold start test with a preheating system were carried out on a low compression ratio diesel engine.(1)At low temperature conditions,fuel atomization and combustion is not complete,when the temperature decreases by 5℃,post-preheating temperature reduces by 2%.The high speed wind will lead to a increasing loss of the heat of the preheating plug,and flame is blown away from the flame preheating plug at high seed wind.At 7m/s,flame preheating plug has a good performance of fire and combustion.The amount of oil and the increase of post-heating temperature does not increase linearly.At a certain wind speed and ambient temperature,the amount of fuel preheating plug can burn is limited.When the amount of fuel exceed to 14ml/min,visible unburned droplet can be observed.(2)The intermittent heating strategy can save about 25% of the electricity compared with the original strategy,while ensuring the combustion stability and increasing the service life of the preheating plug.At low temperature environment,the oil supply rate should be increased to make up for the negative effect of combustion deterioration caused by the decrease of ambient temperature.(3)The reduction of compression ratio and the environment temperature,and the elevation of the altitude will all cause the delay of the first fire cycle and the decrease of the rate of speed rise.There is an optimal value of the main fuel injection mass,the main injection advance angle and the pre-injection mass,with which of the diesel engine has a minimum ignition time.Preheating can greatly improve the cold start performance of the diesel engine,and the optimum oil mass of cold start is reduced by 14% with preheating.The best mass of preheating oil is 8ml/min～11ml/min.(4)In the range of test parameters,the order of primary and secondary influences on starting time is main fuel mass > pre-injection advance angle > pre-injection mass > main injection advance angle.With the application of preheating system,low compression ratio diesel engine can start successfully at low environment temperature in a relatively short time(3.5s).