Air System Proposal and Testing for a Downsized Two-Stroke Diesel Engine

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 of the work presented in this paper was to test engine air loop system devices selected based on 1D-simulations and to verify their characteristics used for 1D-simulations. 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. Due to the high boost pressure ratios above five and low mass flows, all boosting devices got at their limits or out of their working range even in the two stage configuration. The simulation part of the work has been presented on SAE World Congress 2012 held in Detroit (USA) (SAE paper no: 2012-01-0831), but the main results will be also presented in this paper to highlight the main issues met during the whole research work. 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. After the selection of suitable boosting devices, its testing followed. There had been two types of supercharger tested, Roots type and mechanically driven centrifugal compressor with Continuously Variable Transmission (CVT). Concerning turbocharger, there has been tested device with waste-gate turbine at open loop test rig at different temperatures at turbine inlet. The measured data has been then compared with “paper characteristics” delivered by producers of tested boosting devices for simulation use. The greatest differences in measured and “paper data” had been detected at T/C turbine efficiency.