AIE UK

Investigations into Steady-State and Stop-Start Emissions in a Wankel Rotary Engine with a Novel Rotor Cooling Arrangement 2021-24-0097

The present work investigates a means of controlling engine hydrocarbon startup and shutdown emissions in a Wankel engine which uses a novel rotor cooling method. Mechanically the engine employs a self-pressurizing air-cooled rotor system (SPARCS) configured to provide improved cooling versus a simple air-cooled rotor arrangement. The novelty of the SPARCS system is that it uses the fact that blowby past the sealing grid is inevitable in a Wankel engine as a means of increasing the density of the medium used for cooling the rotor. Unfortunately, the design also means that when the engine is shutdown, due to the overpressure within the engine core and the fact that fuel vapour and lubricating oil are to be found within it, unburned hydrocarbons can leak into the combustion chambers, and thence to the atmosphere via either or both of the intake and exhaust ports. As well as shutdown it also affects the startup process, where higher hydrocarbon emissions are caused due to the forced transfer of the unburned gases to the intake and exhaust ducts as the core depressurizes across the sealing grid when it is stationary. These emissions then sit in those volumes, possibly then escaping to the outside world; clearly this is also very important with respect to the SHED testing of any vehicle the engine might be fitted to.

The SPARCS concept is discussed with respect to how it functions versus a conventional wet sump arrangement (as employed by oil cooled rotor Wankel engines). Measurements are taken and steady-state emissions and fuel consumption results with and without pressurization of the core are presented; such a comparison has not been made before. In general, power output, brake specific fuel consumption, hydrocarbon emissions, and combustion efficiency are all better with a depressurized core, with only small improvements in cooling (defined by rotor air inlet temperature) being apparent when it is pressurized. A hypothesis for why this should be so is developed, the knowledge of which can help to guide further development.

The reasons for the engine on/off hydrocarbon issue are apparent. Using a solenoid valve as a means of venting the rotor core pressure directly to the engine intake just before shutdown is proposed as a means of alleviating this problem. This approach would feed the hydrocarbon-rich gases from the core through the combustion process and out through the catalytic converter just before the engine is switched off. In automotive applications this engine is to be used as a range extender and hence there is a great degree of control regarding all modes of its operation, including startup and shutdown, which is the approach investigated for mitigation here.

The results show that depressurizing the core in this manner results in a maximum reduction in total hydrocarbon emissions during warm shutdown and restart of 80% and 60%, respectively. However, it must be remembered that with the pressure relieved in the core, the cooling capability there is slightly reduced, and so the approach has to be calibrated correctly to achieve the best result for the whole system.

Further investigation into the optimum level of pressurization is recommended.