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

The present work investigates a means of controllingengine hydrocarbon startup and shutdown emissionsin a Wankel engine which uses a novel rotor coolingmethod. Mechanically the engine employs a self-pressurizingair-cooled rotor system (SPARCS) configured to provideimproved cooling versus a simple air-cooled rotor arrangement.The novelty of the SPARCS system is that it uses the factthat blowby past the sealing grid is inevitable in a Wankelengine as a means of increasing the density of the mediumused for cooling the rotor. Unfortunately, the design alsomeans that when the engine is shutdown, due to the overpressurewithin the engine core and the fact that fuel vapour andlubricating oil are to be found within it, unburned hydrocarbonscan leak into the combustion chambers, and thence tothe atmosphere via either or both of the intake and exhaustports. As well as shutdown it also affects the startup process,where higher hydrocarbon emissions are caused due to theforced transfer of the unburned gases to the intake and exhaustducts as the core depressurizes across the sealing grid whenit is stationary. These emissions then sit in those volumes,possibly then escaping to the outside world; clearly this is alsovery important with respect to the SHED testing of any vehiclethe engine might be fitted to.The SPARCS concept is discussed with respect to how itfunctions versus a conventional wet sump arrangement (asemployed by oil cooled rotor Wankel engines). Measurementsare taken and steady-state emissions and fuel consumptionresults with and without pressurization of the core arepresented; such a comparison has not been made before. Ingeneral, power output, brake specific fuel consumption,hydrocarbon emissions, and combustion efficiency are allbetter with a depressurized core, with only small improvementsin cooling (defined by rotor air inlet temperature) beingapparent when it is pressurized. A hypothesis for why thisshould be so is developed, the knowledge of which can helpto guide further development.The reasons for the engine on/off hydrocarbon issue areapparent. Using a solenoid valve as a means of venting therotor core pressure directly to the engine intake just beforeshutdown is proposed as a means of alleviating this problem.This approach would feed the hydrocarbon-rich gases fromthe core through the combustion process and out throughthe catalytic converter just before the engine is switched off.In automotive applications this engine is to be used as arange extender and hence there is a great degree of controlregarding all modes of its operation, including startup andshutdown, which is the approach investigated formitigation here.The results show that depressurizing the core in thismanner results in a maximum reduction in total hydrocarbonemissions during warm shutdown and restart of 80% and 60%,respectively. However, it must be remembered that with thepressure relieved in the core, the cooling capability there isslightly reduced, and so the approach has to be calibratedcorrectly to achieve the best result for the whole system.Further investigation into the optimum level of pressurizationis recommended.