Initial Investigations into the Benefits and Challenges of Eliminating Port Overlap
in Wankel Rotary Engines 2020-01-0280
The Wankel rotary engine has historically found limited success in part due to poor combustion efficiency and challenges around hydrocarbon emissions. This is despite its significant advantages in terms of power density, compactness, vibrationless operation, and reduced parts count in relation to the now-dominant 4-stroke reciprocating engine. A large part of the reason for the poor fuel economy and high hydrocarbon emissions of the Wankel engine is the very significant amount of overlap that the use of ports opened and closed by the rotor edges creates. This paper investigates the feasibility operating a Wankel internal combustion engine with zero port overlap in order to mitigate this effect. As discussed in the paper, arranging this condition unfortunately significantly reduces the compression and/or expansion ratio of the engine, so compounding (using turbomachinery) is applied here in order to recover any expansion energy sacrificed in configuring the engine to have no overlap.
In order to conduct this investigation a baseline 1-D model was implemented and correlated to the in-production Advanced Innovative Engineering (UK) Ltd 225CS peripherally-ported single-rotor Wankel engine. The initial port study focused on advancing and retarding the exhaust and inlet port respectively to achieve zero port overlap and then sweeping their zero-overlap positions together around the epitrochoidal housing. The best location for the ports was then identified considering the geometric reduction in either the compression or expansion. At this point, turbocharging was investigated in order to recover the lost work. This helped to understand the amount of lost energy through wastegating.
The turbocompound model was then built, with the excess turbine work being assumed to be returned to the output electrically. In this specification, boost pressure was increased in order to see the effect of full compounding on the full engine system efficiency. Finally, brief investigations were made in terms of turbine efficiency, increased geometric CR (extremely difficult to do physically but investigated here to gauge the benefits of attempting to undertake this), and the effect of heat loss in the transfer duct from engine exhaust to turbine. The latter was undertaken because although pressure losses are expected to be minimal in this part of the engine (due to the absence of any valve mechanism between rotor and turbine), the high temperatures make the concept of applying ceramics in this area potentially very attractive.
The final adopted concept results in a blending of the Otto and Brayton cycles, which in itself is more akin the Humphrey cycle since it features constant volume combustion followed by full expansion down to exhaust back pressure. Areas for investigation to increase the efficiency of the full concept are also discussed.
James Turner, Matthew Turner, Giovanni Vorraro, Toby Thomas