Simulation Study of a Turbocharged Two-Stroke Single Cylinder 425cc SI Engine

An afterburner-assisted turbocharged single-cylinder 425 cc two-stroke SI-engine is described in this simulation study. This engine is intended as a Backup Range Extender (REX) application for heavy-duty battery electric vehicles (BEV) when external electric charging is unavailable. The 425 cc engine is an upscaled version of a 125 cc port-injected engine [26] which demonstrated that the selected technology could provide a specific power level of 400 kW/L and the desired 150 kW in a heavy duty BEV application. The 425 cc single cylinder two-stroke engine is an existing engine as one half of a 850 cc snowmobile engine. This simulation study includes upscaling of the swept volume, impact on engine speed and gas exchange properties. In the same way as for the 125cc engine [26], the exhaust gases reaches the turbine through a tuned exhaust pipe and an afterburner or oxidation catalyst. The intent with the afterburner is to convert some of the air and hydrocarbons (HC) to heat to provide turbine power at a lower turbine pressure ratio. The turbocharger of the upscaled 425 cc engine was also linked to an electrical machine which was able to either absorb or extract power from the turbocharger shaft. This technique is sometimes referred to as super compounding. Downstream the turbine another oxidation catalyst was installed and utilized as a HC clean up catalyst.

It is demonstrated that the upscaled engine obtains optimum gas exchange conditions at a lower engine speed because of the port area relationship to the swept volume. The crank case compression ratio (CCR) increases due to the upscaling which has an impact on the optimum pressure difference between the air and exhaust side to optimize the gas exchange process. The upscaled engine also operates at higher air mass rates which allows the use of larger turbocharger compressors and turbines able to operate with higher efficiencies. Higher turbocharger efficiencies also contribute to lower pressure ratios on the turbine side for any given compressor pressure ratio which improves scavenging properties. The clean-up catalyst makes it possible to oxidate all the remaining HC in the exhaust gases in certain engine speed ranges.