Research On Scavenging Process And Turbocharging Matching Of An Opposed-piston Two-stroke Diesel Engine

With the advantages such as high efficiency, high power density and excellent balance, opposed-piston two-stroke diesel engine(OP2S) has attracted wide attention at home and abroad. Based on the scavenging theory, the gas exchange process and turbocharger matching of the OP2 S folded crank-train engine was investigated with a combined method of numerical simulations and experiments.In order to investigate the characteristics of the gas exchange process in opposed-piston two-stroke(OP2S) diesel engines, a specific tracer gas test method was developed and experiments were carried out to analyze the gas exchange performance under different working conditions. The results show that: â—‹1the difference between intake pressure and exhaust pressure has a significant impact on the scavenging efficiency, and an increase of pressure difference results in a higher scavenging efficiency; â—‹2the trapped gas mass is determined by the exhaust pressure. Therefore more gas mass is trapped inside the cylinder if the exhaust pressure increases with the same delivery ratio; â—‹3the delivery ratio ranges from 50% to 140% is chosen to achieve an optimization between steady running and minimum pump loss.With the demand of the supercharger matching of the OP2 S diesel engine, an equivalent flow coefficient of 0.292 was calculated by the results obtained by both the gas consumption experiment of the OP2 S prototype engine and the gas consumption theory. Besides, the profile of gas consumption characteristic under different ratios of intake pressure and exhaust pressure was obtained for further supercharger matching of the engine.In this paper, a thermodynamic model of the OP2 S diesel engine with a scavenging pump was established based on equivalent model and assumptions. The simulation model of gas-exchange process and in-cylinder working process had been verified by the experiment results. To achieve a maximum IMEP with an excellent fuel economy, the optimal exhaust back pressure and delivery ratio were determined in different working conditions. The following results were obtained: the IMEP(Indicated Mean Effective Pressure) reach its maximum value(1.234MPa) when the exhaust pressure is 0.14 MPa at a speed of 1200r/min(delivery ratio is 120%). And the methods and procedures were obtained which was applicable to OP2 S diesel engine supercharging matching.A compound supercharging system including turbocharger and scavenging pump was adopted based on the OP2 S exhaust energy analysis. Then a comparison between different compound supercharging systems was made with the goal of optimization between the fuel economy and feasibility and subsequently an adjustable speed compound supercharger which composed of a centrifugal supercharger driven by a motor as a high-pressure stage and a VGT turbocharger as low-pressure stage was employed. Lastly, the working condition of the supercharger can be flexible adjusted according to the demand of the change of the gas-exchange quality.Three types of boost mode were mentioned in this paper: supercharging mode, compound supercharging mode and turbocharging mode. The design process of boundary switch was achieved as follows: 1) After analyzing the ranges of delivery ratio and nozzle ring diameter, the switch boundary of turbocharging mode was determined. 2) The remaining load interval was simulated using the supercharging model and compound supercharging mode relatively with an optimization goal of excellent fuel economy, and the switching boundary model was determined.In this paper some achievement are made in the simulation and experiment research of the gas-exchange process and supercharging process of OP2 S diesel engine, which are of great use in the design of OP2 S engine to achieve high boost pressure without too much harm to the gas-exchange quality.