Numerical Simulation Study On Combustion Characteristics Of Diesel-nature Gas Dual-fuel Engine Under High Load

Under the double pressure of increasingly stringent emission regulations and energy shortage,the theory and technology of energy saving and emission reduction of internal combustion engines have been paid much attention in the field.Adopting low-carbon alternative fuel and optimizing combustion strategy is one of the effective ways to realize energy saving and emission reduction in internal combustion engine.Natural gas is a promising clean alternative fuel,this paper replaces some diesel with natural gas,using the three-dimensional fluid simulation software CONVERGE to simulate the working process of diesel fuel-natural gas dual fuel engine.The effects of different injection strategies and their coupled EGR rates on the combustion and emission characteristics of diesel-natural gas dual-fuel RCCI engines were compared and analyzed.Optimize the combustion strategy to solve the problems of engine rough work and poor combustion and emission under the heavy load condition.Under the secondary fuel injection strategy,the combustion characteristics of pre-injection time,main injection time and pre-injection specific gravity change of refined diesel fuel on the engine in-cylinder pressure,temperature and heat release rate and so on were analyzed.The results show that the combustion can be optimized and the engine performance can be improved when the fuel injection is properly advanced under high load,but the premature injection timing of fuel will lead to the deterioration of in-cylinder combustion and decrease of combustion efficiency.When the main injection time is fixed,the peak values of characteristic parameters such as cylinder temperature,cylinder pressure and heat release rate are obtained when SOI1=-105°CA ATDC,and IMEP made a breakthrough rise,the engine has the best power and economy at the moment.At the fixed pre-injection time,the engine has higher indicated thermal efficiency and combustion efficiency when SOI2=-25°CA ATDC,the engine performance is better at this time.Compared with the single injection strategy,the second injection has a higher potential in improving engine performance such as power performance,economy and emission performance.When the proportion of pre-injection oil is between 0.5～0.7,the fuel is in the best stratified state,the cylinder temperature and oxygen concentration are moderate,the combustion duration is longer,the engine combustion efficiency is higher,and the lower emission of HC and CO is realized at the same time.Finally,the optimal injection time is combined with different EGR rate conditions to calculate and analyze the in-cylinder combustion process.The study shows that a large EGR rate under heavy load will lead to the deterioration of in-cylinder combustion and the increase of emissions.The EGR rate of about 45% can make the engine have high power performance,economy and emission,reduce the tendency of rough work under heavy load,and stabilize the combustion process.This combustion strategy can improve engine performance to a certain extent,but it can also cause excessive carbon emission.