Enhancing the Efficiency of a Rotary Thermal Propulsion System through Advanced Simulation Techniques

Abstract

Electrification of transport is essential if we are to achieve the drastic reductions in CO2 emissions required to mitigate the effects of climate change and uncontrolled global temperature rise. But this transition will take time. Until the development of battery energy storage technology allows the necessary capacity and charging speed, transitional technologies such as range-extended electric vehicles will have an important role in the short and medium term. The Wankel rotary engine, with its superior power-to-weight ratio, simplicity, compactness, perfect balance, and lower cost compared to reciprocating piston engines, should be a great candidate for the prime mover in a range-extended electric vehicle. However, until very recently, the Wankel engine has not been present in mainstream automotive applications, having historically struggled with poor combustion efficiency, high fuel consumption and unburnt hydrocarbon emissions. The purpose of this research is therefore to develop and apply advanced computational methods, such as large eddy simulation, to study the in-chamber flow in a Wankel engine in order to better understand how such limitations may be overcome. The subject Wankel engine is the peripherally ported 225CS rotary engine, designed and manufactured by Advanced Innovative Engineering of Lichfield, UK. The corresponding computational model successfully predicts the instantaneous chamber pressure within 4% of the experimental test result. In terms of simulated flow structures, it is found that eddies generated during the intake process play an important role in turbulence generation and promoting flame propagation. The main limitations on Wankel engine efficiency are flame speed, the elongated combustion chamber shape and pinch point, and restricted compression ratio. While the simulation study shows that optimizing ignition timing improves thermal efficiency to an extent, the presence of the pinch point, which is inherent to Wankel engine combustion chambers, will always inhibit flame propagation unless more radical design changes are implemented. To address this and other weaknesses, three efficiency enhancement concepts are designed and simulated. The first is jet ignition. Implementing jet ignition on a Wankel engine will demand certain design changes to the jet ignition systems designed for reciprocating engines, however several designs show promise. The second concept concerns a two-stage Wankel engine, inspired by a Rolls-Royce prototype. It solves the issues of low compression ratio and port overlap, as well as the combustion chamber pinch point problem, all at once, albeit with a weight, cost, and complexity penalty. Since Wankel engines usually operate with high exhaust temperatures as a result of under expansion, the third concept is a novel hybrid steam-gas turboexpander, which is simulated and optimized to be used in an Inverted Brayton Cycle-based heat recovery system. In these ways, the research contained in this thesis enables further optimization and re-design opportunities that should allow the Wankel engine to become highly competitive as an electric vehicle range extender.

Date of Award	4 Dec 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Aaron Costall (Supervisor), Sam Akehurst (Supervisor) & Chris Brace (Supervisor)