Initial Investigations into the Benefits and Challenges of Eliminating Port Overlap in Wankel Rotary Engines

Abstract

<div class="section abstract"><div class="htmlview paragraph">The Wankel rotary engine historically found limited success in automotive applications due in part to poor combustion efficiency and challenges around emissions. This is despite its significant advantages in terms of power density, compactness, vibrationless operation, and reduced parts count in relation to the 4-stroke reciprocating engine, which is now-dominant in the automotive market. A large part of the reason for the poor fuel economy and high hydrocarbon emissions of the Wankel engine is that there is a very significant amount of overlap when the ports are opened and/or closed by the rotor apices (so-called peripheral ports). This paper investigates the benefits of zero overlap from a production engine with this characteristic and the effect of configuring a peripherally-ported Wankel engine in such a manner. As discussed in the paper, arranging this condition for peripherally-ported engines unfortunately reduces the trapped compression and/or expansion ratios significantly, such that when naturally-aspirated operation is simulated, a large reduction in performance ensues.</div><div class="htmlview paragraph">In order to demonstrate the potential of zero port overlap in Wankel engines with respect to emissions, a 2007 model year Mazda RX-8 was rebuilt, run-in, degreened, and tested on a chassis dynamometer. As standard, the engine in this vehicle is configured with no port overlap through the adoption of side intake and exhaust ports. This testing was performed in order to see subjectively how successful such an approach could be in controlling emissions. The vehicle easily met Euro 5 limits for all criteria emissions and was even better in terms of hydrocarbon emissions versus Euro 6 on the WLTP cycle, giving the lie to the belief that a Wankel engine can no longer meet current automotive emissions targets.</div><div class="htmlview paragraph">The analytical work reported here studies the result of eliminating overlap on the performance of a peripherally-ported single-rotor Wankel engine using a 1-D model. This was implemented and correlated to the in-production Advanced Innovative Engineering (UK) Ltd 225CS engine used in the UK government-funded ADAPT project. The initial port study focused on advancing and retarding the exhaust and intake port respectively to achieve zero port overlap and then sweeping their zero-overlap positions together around the trochoid housing. The best location for the ports was then identified; this was essentially an “Otto” timing set, with broadly equal compression and expansion ratios. Notwithstanding this, potential performance was found to be severely curtailed, as was to be expected given the marked reduction in trapped compression and/or expansion ratios necessary due to peripheral porting.</div><div class="htmlview paragraph">Countermeasures to this reduction are discussed. Those that will be studied later in the project will be reported in a later publication.</div></div>

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

“…The subject of geometry and tradeoffs in the design of the Wankel engine are dealt with in numerous texts, including [12,13]. However, it should be noted that because of this and the requirement to provide cooling air to the rotor it is harder to arrange side ports, which are useful in reducing overlap, the importance of which has been covered by the authors in other publications [4,5].…”

“…Chiefly this distinction is driven by the layout of the intake ports, although the exhaust port configuration also plays an important part, since with full peripheral porting overlap between the ports can be completely eliminated. The importance of minimizing this has been discussed by Ohkubo et al [6] and Turner et al [4,5]. Unfortunately, it is not easy to combine side ports which ACRs, as will be explained next.…”

“…In [4,5] more than satisfactory fuel consumption was shown to be possible in Wankel engines provided an appropriate technology level is adopted and that their operating regime can be suitably constrained. Particularly in REx applications the latter is made possible by the fact that control of the REx is independent of vehicle operation by the driver.…”

“…The equivalence of best fuel consumption in the full operating map was shown to be the case versus similartechnology reciprocating 4-stroke engines in [5] so in actuality, one may expect no disadvantage for the Wankel. Similarly, in [4] it was shown that there is no reason why, with the right technology, Wankel 10/19/2016 engines cannot easily meet emissions limits, and so be a very viable engine type for REx use.…”

Lean-burn characteristics of a turbocharged opposed rotary piston engine fuelled with hydrogen at low engine speed conditions

“…Although two-stroke engines can provide high power density, they have low brake thermal efficiency and high exhaust emissions due to low compression ratio and large overlaps of intake and exhaust valves [8]. Wankel engines have long and narrow combustion chambers and large port overlaps, which contribute to high exhaust emissions because of incomplete combustion [9][10][11][12]. ORP engines, as a new type of internal combustion (IC) engines have high power density and are free of connecting rod mechanisms [13][14][15][16].…”