Experimental And Numerical Study On The Gas Exchange Process Of A Two-stroke Poppet Valve Diesel Engine

Energy-saving and emission reduction are important driving forces to promote the development of internal combustion engine technology.The two-/four-stroke switching technology that juxtaposes two-stroke and four-stroke engines have the potential to further reduce fuel consumption because of the powerful characteristics of two-stroke engines and the good fuel economy of four-stroke engines.However,the gas exchange quality of the two-stroke/four-stroke switching engine in the two-stroke operation directly affects the engine’s power,economy,and emission performance.Therefore,the study of the engine’s gas exchange process in the two-stroke operation is the focus and difficulty of the research,and it is also the breakthrough of the engine’s performance optimization.This article first analyzed the experimental test principle of the in-cylinder sampling method and used CO₂ as the monitoring gas.By analyzing the mass distribution of CO₂ before and after the gas exchange process and in the tailpipe,the calculation formulas for scavenging efficiency and short-circuit rate were deduced in detail.The trapping efficiency was able to be derived from the short-circuit rate and the global fuel-air ratio.Then,a two-stroke poppet valve single-cylinder diesel engine with two-stroke/four-stroke switching was carried out to study the CO₂ sampling in the cylinder to study the engine’s gas exchange characteristics at different speeds and intake and exhaust pressure differences.Finally,a GT-POWER simulation model was built based on the existing test data,and the influence of the valve phase on the engine’s gas exchange characteristics was analyzed.In the experiment of CO₂ sampling in-cylinder for a two-stroke poppet valve single-cylinder diesel engine,the in-cylinder sampling CO₂ test bench was built based on the analysis of the engine structure and gas exchange characteristics,and the measurement results at different cylinder sampling probe depths were compared.The results showed that when the sampling probe was at an appropriate sampling depth,the in-cylinder CO₂ sampling method was suitable for studying the gas exchange characteristics of a two-stroke poppet valve single-cylinder diesel engine.Also,the object measured by the in-cylinder sampling method was the change in the concentration of CO₂ in the cylinder and would not be affected by combustion factors.Therefore,this method had better test accuracy.Then,according to the test bench built,the change of the engine’s gas exchange characteristics at different speeds and intake mass flow was obtained.The results showed that with the increase of the delivery ratio,the trapping efficiency gradually decreased,and the scavenging efficiency first increased and then gradually leveled off.The two were between the ideal scavenging models.And when the delivery ratio was less than 1.0,the scavenging efficiency was closer to the perfect displacement model,and the scavenging efficiency was taken as the optimization target,and the appropriate range of the delivery ratio was 0.67～1.3.Based on GT-POWER,the research on the valve phase of the two-stroke poppet valve single-cylinder diesel engine was carried out.The results showed that a single postponement of IVO or advancement of EVO was able to effectively reduce the in-cylinder pressure at the opening of the intake valve,thereby reducing backflow to the intake pipe.Properly retarded IVO or advanced EVO could enable the engine to obtain greater torque output and better fuel economy.The exhaust blow-down period should be used as the main period of exhaust gas in the cylinder.By optimizing the gas exchange in this phase,it was able to a bring better scavenging effect for the engine.Comprehensively,properly advanced EVO was able to make the exhaust gas mass flow larger than retarded IVO,and properly retarded IVO can bring better gas exchange effect and fuel economy than advanced EVO.The DOE test with torque as the optimization target showed that when the IVO was 140°CA and the EVO was 115°CA,the engine had the greatest power and fuel economy.