Research On Knock Characteristic Of Natural Gas Engine

In order to meet the shortage of traditional oil resources and increasingly stringent emission regulations,natural gas as a substitute fuel has better performance of economy and emission,making natural gas engines widely used in passenger cars,heavy trucks and other fields.However,knocking behavior has become one of the most important factors that restricting the improvement of the power and economy characteristics of spark-ignition natural gas engine.Therefore,it is beneficial to deeply understand the knock trend and the reason of knock by investigating the influence of different engine state parameters and the components in natural gas on the knock of natural gas engines,which plays a significant role in the exploration of knock control strategies.In the project,based on the spark-ignition natural gas engine,the reduced kinetic mechanism of natural gas that can be applied to predict knock was constructed and two 3D simulation models were developed including the model for inlet flow simulation and the model for in-cylinder combustion simulation that adopts G-equation coupling the reduced kinetic mechanism of natural gas.The prediction for ignition delay using the reduced kinetic mechanism and the description for normal and knocking combustion by the 3D combustion simulation model were verified,implying that the model is reliable.The established 3D combustion simulation model was used to study the influence of equivalence ratio,inlet temperature and the ethane volume fraction of the fuel on the entire combustion process including knocking combustion stage and knock characteristics.During the research,the total energy which burned in cylinder was kept constant.Knocking is mainly affected by equivalence ratio through two ways.First,the equivalence ratio increases,the flame propagation speed in the cylinder increases,heat release of combustion is faster,the first heat release peak value is higher,as a consequence,the temperature of unburned mixture before knocking is higher,which increases the low-temperature oxidation rate of methane and shortens the ignition delay=.Second,the higher methane concentration in the mixture due to the greater equivalent ratio results in the shorter ignition delay.Therefore,when the equivalence ratio is 0.85,CH2O,which is the intermediate product during methane oxidation at low temperature,generates at the fastest rate,the knock occurs at the earliest time;additionally,the second peak of heat release caused by knocking combustion advances,and a severe pressure oscillation occurs in the cylinder;in this case,the knock intensity is greater than the other three equivalent ratio conditions.When the equivalence ratio is 0.7,there is no obvious heat release peak in the later combustion period,and the pressure oscillation in the cylinder is weak;consequently,it can be regarded as no knocking.Hence,reducing the equivalence ratio can effectively suppress the occurrence of knocking.In this paper,the temperature of in-cylinder mixture at the 10?A after the closing of intake valve is taken as the intake air temperature for comparative study.The results show that the inlet air temperature has no obvious effect on the flame propagation velocity due to the difference of the inlet air pressure.The influence of the inlet air temperature on the engine knocking characteristics is mainly achieved by changing the ignition delay of the air-fuel mixture.As the intake temperature increases,the rate of low-temperature oxidation of methane increases,the point of knock onset is advanced,the intensity of knock is greater and the heat release rate of knocking combustion is faster,resulting in an increase in the overall pressure and temperature in the cylinder.The influence of the volume fraction of ethane on the combustion process and the knocking characteristics of the engine is similar to the intake air temperature,which affects the fuel ignition delay time.However,ethane promotes the low temperature oxidation process of methane mainly through the accelerated generation of active free radicals.When ethane is added to methane,the low temperature oxidation reaction of ethane is preferentially performed because of the high activity.Therefore,the consumption of ethane is faster and the mixture has a shorter ignition delay,resulting in stronger knock tendency.knocking is more likely to occur.