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Beginner's Guide: What Is a Rotary Engine (and How Does It Work)?

What is a rotary engine (and how does it work)?

We have previously explained how four-stroke piston engines work, but that is not the only way to turn fuel and air into power. Perhaps the most unusual engine design to ever find its way into a production car, the Wankel Rotary Engine is elegant in its simplicity; it has about 75% less moving parts than a similar power piston engine. There have been literally dozen of different rotary engine designs that eliminated reciprocating pistons (see this Wikipedia list) but only Felix Wankel's design has been successfully used in motor vehicles.

Before you get your head around the concept of the rotary engine, let's review the basic workings of the normal piston-combustion engine. The piston is connected to a crankshaft via a connecting rod, and as it moves up and down it makes the shaft rotate. Opposite the crankshaft is a combustion chamber with valves that let in fuel and air, and let out exhaust gasses. The piston moves up and compresses this explosive air/fuel mixture, which is then ignited by the spark plug as the piston approaches maximum compression. This causes an explosion which forces the piston downwards. This causes rotational motion at the crank, which then goes into the gearbox and ultimately turns the wheels.

Now a rotary engine also uses the same principles of combustion - but in a completely different way.

Instead of having cylinders it has rotor housings, and within the rotor housings are the roughly triangular rotors. These rotors have rounded, bow-shaped flanks, and they fit into the oval-like housings (the correct name is epitrochoid, but think of it as an oval with pinched-in sides). The rotors are geared to an eccentric shaft (which is the equivalent of the crankshaft) which allows the rotor 'tips' to remain in contact with the walls of the housing at all times as it spins. The shape of the rotor and housing creates voids which expand and contract depending on where the rotor is during its cycle, and each of these voids handles one aspect of the combustion cycle.

There are no valves in a rotary engine, which is one of the reasons why they can often be spun to 10,000 rpm or more. The portion of the chamber with the intake port is large, sucking fuel and air into it as the rotor exposes the port. As the rotor turns it compresses the mixture, expands the chamber again as it ignites, then squeezes the exhaust gasses out of the exhaust port.

Wankel Rotary Engine

The way the rotary engine is designed, for every one revolution of the triangular rotor three combustion events take place. Because the output shaft is geared to spin three times for each revolution of the rotor, there is one combustion stroke for each revolution of the output shaft and the power is very smooth.

The rotary engine carries out the same process as a combustion engine but with a fraction of the components. There are no valves, no timing gears, no con-rods, no pistons, no crankshaft and only three main moving parts compared to the myriad of parts involved in making a 'normal' engine work. 

Intake

As the tip of the rotor passes the intake port - a hole in the housing through which metered amounts of fuel and air pass - the movement of the rotor 'sucks' the fuel and air mixture into the engine. As the next tip of the rotor passes the intake port the fuel and air mixture is 'trapped' and swept along to the next portion of the housing…

Compression

As the rotor continues its motion around the housing the volume of the chamber reduces, compressing the fuel/air mix. As rotor moves the mixture to the part of the chamber with minimum volume, it approaches the spark plug, or plugs and we're on to the next step…

Combustion

With the fuel/air mix squeezed into the tiny area where the oval housing is pinched, the spark plugs (usually two per rotor) do their thing and combust the mix. As the explosion expands, it forces the rotor to rotate, and creates the 'power' which drives the engine...

Exhaust

The expanded combustion gassed are swept along and then forced out of the exhaust port - basically another hole in the housing - as the rotor again squeezes the volume down on the opposite side of the oval housing. As the tip sweeps past the exhaust port, it uncovers the intake and the whole process starts again.

Rotary vs Piston

PROS
• The nature of the engine means that a much smaller displacement can produce considerably more power than a comparably sized piston engine - a Mazda RX-8 is technically a 1.3 liter, yet produced around 230 hp.

• The engines are physically much smaller, lighter, and have less moving parts to go wrong.

• Due to the nature of the engine they are internally balanced - the rotors act like spinning counterweights, phased to cancel each other out. This means there are less vibrations, so the engine is smoother and will spin to higher revolutions (10,000 rpm is by no means unheard of) without damage.

CONS
• Rotary engines are less fuel efficient than piston engine equivalents as they are less thermally efficient.

• Emissions are poor due to overlap between intake and exhaust events, and none meet current regulations.

• Rotor tips, also known as apex seals, are under a huge amount of stress, and are prone to failure - this was a huge issue on older Wankels, and has yet to be completely addressed in modern variants.

• Oil consumption is high due to the need to keep the rotors and seals lubricated internally.

• Due to the small amount of eccentricity in the shaft, in comparison to the stroke of a crankshaft, rotary engines have a fraction of the torque of conventional motor at low rpm.

Mazda has been the biggest producer of rotary engines, and the only maker to use them since the late 1970s. General Motors was developing their own more than 40 year ago, but smog laws and the first oil embargo in 1973 caused them to give it up before it was finalized for production. NSU and Citroen in Europe did sell cars in small numbers, and Hercules, Norton and Suzuki produced motorcycles, but no one made anywhere near as many as Mazda. The Mazda Cosmo first appeared with rotary power in 1965, followed by the R100, R130, RX-2, RX-3, RX-7, Luce, Rotary Pickup Truck, RX-7, and finally the RX-8 which was made until 2012.

There has been some research recently into producing small rotary engines to power the generator part of a hybrid, due to their compact size and smoothness. It is felt that running at a constant speed to generate power the Wankel engine may finally be able to overcome its fuel efficiency and emission issues.