Simulation On Combustion Performance Of Diesel/LNG Dual Fuel Engine Under Overload Conditions

With the advantage of high torque,high thermal efficiency and good economic performance,the internal combustion engine is widely used in the fields to save human labor.Diesel engine as the heart of the ship is more important.At present,serious environmental pollution and shortage of fossil energy promote the shipping industry to find alternative sources of energy.The natural gas caught the attention of people because of the characteristics of cleaning and efficient,which promotes diesel/natural gas dual fuel engine development.Currently the research on the dual fuel engine is mainly the working characteristic of the rated condition,but few people make the research of engine working characteristic on overload condition.Sometimes due to the reason of bad sea conditions the ship will run in overload conditions,if we can not guarantee the engine power,the ship will be in danger.The passage is mainly conduct an investigation into the combustion performance of dual fuel engine under the overload condition.With AVL FIRE software to establish the 3D simulation model of engine and carry out simulation study on the engine combustion process with the different diesel replacement rate,diesel injection moment,inlet gas temperature and air amount.The results showed that:(1)With the increment of replacement rate,the temperature and pressure in cylinder is dropping,and the time of peak becomes more later.When the replacement rate increased from 30 % to 80 %,the highest pressure in the cylinder decreased by about nine percent,the highest pressure decreased by about 23 %,the peak time lag about 5°CA.The average NO mass fraction in cylinder reduces as the replacement rate increasing.Comprehensive comparison,in order to ensure the performance to improve fuel economy,choosing diesel replacement rate 50% is more appropriate with the overload condition.(2)With fuel injection spray time lagging later,the temperature in cylinder peak decreases,but the peak time lags later;the peak pressure in cylinder is reduced.Compared with the diesel fuel injection time 20 ° CABTDC,the 5 ° CABTDC highest pressure dropped by about 11 percent,the peak lag about 5 ° CA.In order to improve engine combustion performance under the overload condition,the diesel injection time can be slightly early 0-5°CA.(3)With the higher initial intake gas temperature,when combustion in cylinder the average temperature is higher,the average pressure in cylinder is reduced,at the end ofdiesel spray in cylinder the flame speed is faster.When the inlet gas temperature increase from 300 K to 360 K,the maximum cylinder pressure dropped by 6.2 percent,and finally NO average mass fraction in cylinder increased by 40 percent.Therefore under overload conditions in order to improve the engine emissions performance,we can lower inlet gas temperature 20 K.(4)As excess air coefficients increasing,inlet air volume increases,the initial pressure increases,the average temperature in cylinder in combustion process will reduce,the average pressure in cylinder increase.While the excess air coefficient is beneficial to the fuel combustion and lower the NO content,but the natural gas quality fraction is lower,the flame speed becoming precarious and SOOT content increase.So under overload conditions,we can increase the excess air coefficients to 2.2 in order to improve engine combustion and emissions performance.