Where Has The Wankel Engine Gone? And How Does It Work?
The Wankel engine is, without doubt, not commonplace in the automotive world. We want to publish an article about Wankel-powered bikes, but we thought it is best that we start with an article about the engine, since it is a rarity. How many cars and bikes use that format, apart from the now-defunct Mazda RX-7 and RX-8, Suzuki RE5 and Nortons? The answer is none as of 2019, although Mazda said in 2018 that they will have a Wankel-powered hybrid car in 2020.
Developed by German Engineer Felix Wankel, he received his first patent for the engine in 1929. But he only developed the concept in 1951 before completing a working prototype in 1957.
The engine was seen as more advantageous compared to the usual piston engine, as all its parts rotate in one direction, resulting in simplicity, smoothness, compactness, higher revolutions and ultimately higher power-to-weight and power-to-capacity ratio.
Schematic of the Wankel : 1 : Intake 2 : Exhaust 3 : Stator housing 4 : Chambers 5 : Pinion 6 : Rotor 7 : Crown gear 8 : Eccentric shaft 9 : Spark plug. Source: Fred the Oysteri
Piston engines, on the other hand, have more moving parts that follow their own planes of motion, but are tied together (read: forced) to convert their different motions to circular motion at the crankshaft. For example, the piston reciprocates (moves up and down) instead of going round and round. The connecting rod also reciprocates but since its big end is connected to the crankpin, its motion resembles that of a pendulum that swings back and forth, and up and down (when viewed from either end of the crankshaft). Consequently, these reciprocating parts need to change direction quickly at every 180 degrees of crankshaft rotation, resulting in high stresses, vibration and power losses.
Schematics of a piston engine
In comparison, a Wankel engine places a fattened (for want of a better word) rotor or rotors that spin in only one direction in a housing. The housing is oblong in shape, allowing for the four Otto cycles—i.e. intake, compression, power and exhaust—to occur simultaneously. That also means the engine doesn’t require camshafts, poppet valves, valves springs and everything that goes with them. So, it’s just a matter of continuing with a circular motion from the point of combustion, through the gearbox, out the propeller shaft to driveshafts and finally the wheels.
The output shaft is geared directly to the inside of the rotor. As the shaft turns three times faster than the rotor, the rotor produces one power pulse per revolution, as opposed to a four-stroke piston engine which produces one power pulse per two crankshaft revolutions. This means that if the engine is turning at 3,000 RPM, there are 3,000 power pulses in a Wankel engine, whereas there are only 1,500 power pulses in a four-stroke engine.
This was why the Mazda RX-8’s Renesis engine produced 238 hp from just 1.3 litres without the help of a turbocharger. Certain variants of the RX-7 (FD) had the twin-turbo 13B-REW Wankel engine that made 276 hp, also from only 1.3 litres.
It was due to this “unconventional” power output from such small displacement that vehicle importation/taxing authorities and racing regulatory bodies in some countries consider the Wankel as “punching above its weight”. To them, this engine is equivalent to 1.5 to 2 times its displacement. Some racing series even banned Wankel-engine cars altogether.
Mazda RX-7 Type R Bathurst R. Only 500 were made.
But there are drawbacks.
The engine has a reputation for astronomical fuel consumption. But unlike some reputations that were born out of scorn, the Wankel certainly had problems with fuel efficiency and emissions. And heat.
Although it may seem that gasoline burns quickly, it doesn’t burn quickly enough in internal combustion engines, especially at high RPMs. If this is already a problem for piston engines, it’s magnified in a rotary engine.
This is because the rotor sweeps the mixture along the chamber, rather than being contained in a single space. As a result, the combustion chamber becomes longer and longer, and the fuel mixture ends up not fully burnt. Also, there are small gaps between the face of the rotor and housing where the combustion flame could not reach. Hence, this unburnt fuel mixture finds its way into the exhaust outlet and into the atmosphere.
Damaged Wankel rotor and housing.
Not being able to burn all the fuel lowers the engine’s thermal efficiency (as in how much of the fuel is converted into heat).
Other problems include consuming the engine oil being injected into the rotor space to lubricate the rotor. Some of that oil gets burnt while some ends up in the exhaust pipe. The burning of engine oil creates ash and carbon deposits in the engine.
Also, the apex seals (the seals that sit at the tips of the rotor) are the culprit. The seals “cordon” off each part of the rotor so that the fresh charge, expanding gasses and exhaust gasses don’t mix. Centrifugal forces push the seals against the wall of the housing, but gas pressure at low engine speeds can lift the seals, causing all the gasses to mix.
Finally, there’s the problem of heat. As only one part of the housing is used for combustion, that part is super-hot while the rest are cool. For comparison, the combustion chamber of a piston engine is also used to induct and contain fresh charge which cools the area. Therefore, sealing off different areas of the rotor housing becomes a challenge. To overcome this, heat pipes are fitted to draw the heat from the hottest part to the cooler parts. Doing so brought operating temperature down from 231°C to 129°C, while temperature difference between hottest and coolest regions came down from 159°C to just 18°C. It also makes the engine more complex and heavier.
The future of the Wankel.
Every engine design is a set of compromises but how much of that compromise leads to usability or otherwise determines which format lives or dies. In the end, it’s the piston engine that won as the choice spark ignition and compression ignition powerplants.
Mazda did not upgrade the RX-8’s engine for Euro 5 regulations in 2012, leaving no current automobile with the Wankel engine. However, they have future plans to employ the engine as range extenders in hybrid vehicles, as shown in the XEV Concept. It works by running at a constant RPM and charges the vehicle’s batteries, rather than being the primary source of power. The Wankel is most efficient when running at a constant RPM rather than when accelerating or decelerating.
Mazda XEV Concept. Image credit: thedrive.com
Other car manufacturers are also considering the same application, but nothing in the motorcycle segment.
The engine is used widely in other applications, however, including in go-karts, aircraft auxiliary power units and personal watercraft.
The US Army is also considering using a small Wankel auxiliary power unit for the M1 Abrams battle tank. As the beast uses a gas turbine (modified jet engine) for propulsion, the auxiliary power unit can still power other things such as the electronics when the main engine is off or disabled.