Experimental Research On Performance Of CNG-Diesel Dual Fuel Engine

Natural gas,as a clean energy source with rich reserves and low price,can be used in the internal combustion engine through dual fuel engine technology.The dual fuel engine can greatly reduce NO_x and soot emissions while ensuring the same power performance as the original engine.Due to its good economic performance and emission performance,dual fuel engines technology are increasingly valued by scholars from all over the world.In order to find the optimal operating parameters of the dual-fuel engine under various operating conditions,so that at each operating point,the maximum replacement rate of the dual-fuel engine can be increased as much as possible without sacrificing greater thermal efficiency.In this paper,an experimental study was conducted on a CNG-diesel dual fuel engine which is independently modified by a 135 single-cylinder diesel engine.Firstly,the performance of the dual-fuel engine was tested under various loads of the maximum torque rotation speed of1200r/min,rated rotation speed of 1500r/min and equal to rated torque rotation speed of1050r/min.It was found that under the same load,the thermal efficiency of each rotation speed from high to low was 1500r/min>1200r/min>1050r/min,and the maximum replacement rate was ranked from high to low:1200r/min>1050r/min>1500r/min,considering the thermal efficiency and the maximum replacement rate,the performance of the maximum torque speed of 1200r/min is better,so the rotation speed of 1200r/min was selected as the control group in the experiment.On the basis of the control group,in order to looking for technical solutions to further improve the performance of the dual fuel engine,experiments were conducted on the pilot diesel injection pressure,pilot diesel injection timing,compression ratio,the number of injector holes and the angle of the fuel beam using the control variable method to explore the impact of these factors on the economics and emission performance of dual fuel engines.The experimental results show that compared with other speeds,the dual fuel engine has a higher substitution rate at a maximum torque speed of 1200r/min,and the maximum substitution rate can reach 88.7%at 100%load.At this time,the NO_x and soot emission levels are the lowest,and compared with the original engine the thermal efficiency is only reduced by 0.4%under dual fuel mode.Though increasing the pilot diesel injection pressure can effectively increase the maximum natural gas replacement rate and thermal efficiency,and the NO_x content basically remain unchanged,while HC and CO emissions are significantly reduced.Increasing injection advance angle of the pilot diesel,the thermal efficiency gradually increases under each load,and the maximum natural gas replacement rate basically remain unchanged.However,at medium and high loads,the injection advance angle is too large,it will are prone to knock,and as the load increases,the pilot diesel injection advance angle when knocking occurs become smaller.As the pilot diesel injection advancement angle increases,the NO_x content gradually increases,and the HC and CO content both decrease.Therefore,selecting a larger pilot diesel injection advance angle for medium and low loads can effectively improve thermal efficiency while maintaining a high replacement rate.However,at high loads,try to reduce the pilot diesel injection advance angle aimed to avoid knocking.As the compression ratio increases,the natural gas replacement rate and thermal efficiency are improved,and the natural gas replacement rate and thermal efficiency can be increased by 0.9%and 2.65%respectively.However,under high torque and high load conditions,excessive compression ratio will cause knocking phenomenon.In the dual fuel mode,compared with the 7-hole nozzle injector,it is more suitable for the use of8-hole nozzle injectors,which can expand the fuel injection range,increase the ignition area,and improve the power performance of the the dual fuel engine.Therefore,in this experiment the optimal injection strategy obtained is to select the pilot fuel injection pressure of 150MPa;at 50%load,the injection timing is chosen as-15°CA ATDC injection timing,and at 75%and 100%load,-13°CA and-11°CA ATDC injection timing is chosen respectively;higher compression ratio 17 can be used at low load,and compression ratio should be appropriately reduced at medium and high load,using 16.5compression ratio;the injector should be chosen as the 8-hole 150°oil beam angle nozzle injector;this experimental scheme can ensure high thermal efficiency at different operating points,and avoid the occurrence of knocking phenomena,which can effectively improve the performance of dual fuel engines,but at the same time may cause a slight increase in NO_x emissions.