Air System Conception for a Downsized Two-Stroke Diesel Engine 2012-01-0831

This paper introduces a research work on the air loop system for a downsized two-stroke two-cylinder diesel engine conducted in framework of the European project dealing with the POWERtrain for Future Light-duty vehicles - POWERFUL. The main objective was to determine requirements on the air management including the engine intake and exhaust system, boosting devices and the EGR system and to select the best possible technical solution. With respect to the power target of 45 kW and scavenging demands of the two-cylinder two-stroke engine with a displacement of 0.73 l, a two-stage boosting architecture was required. Further, to allow engine scavenging at any operation, supercharger had to be integrated in the air loop. Various air loop system layouts and concepts were assessed based on the 1-D steady state simulation at full and part load with respect to the fuel consumption. Among the investigated boosting devices were the positive displacement and centrifugal superchargers driven from the crankshaft and placed upstream or downstream of the turbocharger with either the waste gate or variable turbine. Additionally, the electrically driven compressor as well as the electrically assisted turbocharger was assessed, too. Moreover, middle pressure and low pressure EGR loops were also compared. Due to the high boost pressure ratios above 5 and low mass flows, all boosting devices got at their limits or out of their working range even in the two-stage configuration. The best compromise regarding the feasibility, power target and fuel consumption was the configuration with the positive displacement supercharger placed downstream of the waste gate turbocharger. However, the biggest drawback of this solution was the necessity of sufficient air cooling between the stages due to the limitation of the maximum temperature at the outlet of the supercharger at 150°C. On the other hand, the boosting system with the supercharger upstream of the turbocharger required small compressor wheel to avoid surge and was ruled out due to the turbocharger procurement feasibility. Moreover, difficulties with EGR at the part load appeared at that configuration. If procurable for the whole engine mass flow range, the variable turbine geometry would provide better fulfillment of the power target and scavenging requirements than the waste gate turbine. The simulation showed no particular benefit of the variable drive of the supercharger in comparison with the dual drive gearbox due to the higher mechanical friction. Finally, the electric devices were put aside because of the unfeasible procurement and high demand on electric power of approx. 5 kW to achieve the power target.