by Jim Kerr

Say “rotary” and most knowledgeable drivers would instantly think of Mazda. True,
Mazda has been the long time proponent of this compact engine design but they have
not been the only ones to work on rotary engine development or build production
vehicles. It started many years before, with the collaboration of Dr. Felix Wankel
and the NSU Corporation.

Felix Wankel conceived the idea of a rotary engine in 1924 and received his first of
many patents in 1929. It took until 1951 for NSU to become interested in the rotary
concept and the first truly functional Wankel rotary engine ran in February 1957.
Later that year, a prototype was able to run for two hours and produce 21
horsepower.

While NSU built rotary-powered cars such as the Wankel Spider )1964 to 1967) and the Ro80 (1968 to 1977) before being assimilated by Volkswagen, the NSU name was not well known in North America.

Many other manufacturers have spent millions of dollars on rotary powerplant development for everything from chainsaws to motorcycles. Companies such as John Deere, Rolls Royce and Curtis-Wright have worked with the design. Some have built experimental cars such as the Mercedes C111 Gullwing coupe and the Corvette XP-882. There are a few manufacturers that actually took it to production.

Arctic Cat, Johnson, Evinrude and Polaris were snowmobile manufacturers that used the rotary engine in production machines. Arctic Cat even offered a rotary-powered lawn mower. Motorcycles such as the Suzuki RE-5 and the Hercules Wankel 2000 made the market, while other motorcycle manufacturers such as Yamaha and Norton had experimental models. Rotary engines have even made their way into light aircraft.

Even with all this interest in rotary engines over the years, Mazda is the one name that has become best associated with the compact design. The first production rotary-powered Mazda model was the Cosmo Sport in 1967. It wasn’t until 1968 though that we saw Mazda’s rotary in North America with the introduction of the R100 coupe. Design improvements over the years have now lead to the engine used in today’s RX-8 sports car.

While following the history of rotary development can seem complex, the principle of
rotary engine operation is simple. Start with a triangular-shaped rotor. Place it in
an hourglass-shaped cylinder and put the crankshaft into the centre of the rotor.
There are four strokes of operation to produce power, just like conventional four
stroke engines.

As the rotor is rotating inside the cylinder, the space between one side of the
rotor and the cylinder increases. Air and fuel are pulled into this space through
ports or passages in the cylinder wall. As the rotor is rotating further, the space
between the rotor and cylinder starts to decrease. This is the compression stroke. A
spark plug fires, causing the gases to burn and expand. This forces the rotor to
rotate further around in the cylinder. The rotor is connected to the crankshaft by
internal gears so this motion turns the crankshaft, producing the power stroke.
Finally, the rotor rotates further in the cylinder until exhaust ports are exposed
and the burnt gases flow out.

Because the rotor has three sides, each side acts as a one-cylinder engine. By
stacking two rotors and two cylinders beside each other, the equivalent of a
6-cylinder engine can be built. Some manufacturers have built 3 and 4 rotor engines
for more power but 2 rotor designs seem to be the right combination for space and
power in automobiles. The 2-rotor engine is very compact, often smaller than the
transmission it is bolted to.

Because all the motion in a rotary engine is circular rather than reciprocating, the
engine runs very smmooth. In practice they can b operated faster and faster and will
keep producing more power until the point where they disintegrate. Carburetted Mazda
rotary vehicles had a warning buzzer when engine rpm’s were too high. Fuel injected
rotary models simply cut the fuel before engine damage can occur.

Problems with rotary engines include large combustion chamber areas that makes it
difficult to burn all the fuel completely, so fuel economy is lower and exhaust
emissions are high. Another problem is wear on the sliding seals that are necessary
to seal the rotor to the cylinder sides as the rotor turns. Improvements in
materials have cured most seal problems while fuel injection, cooling system
modifications and exhaust system after treatments have taken care of emissions.

RENESIS, Mazda’s next generation rotary engine, was introduced in the RX-8 and named International Engine of the Year in June 2003. Using ports in the side plates of the cylinder, the intake and exhaust timing can be better tuned than previous engines that had the ports in the cylinder wall. This provides about 30% more intake area than previous Mazda rotaries, making the RX-8 a spirited performer.

The rotary engine is also well suited to hydrogen fuel, because the intake,
compression, power and exhaust strokes are separated into different areas of the
cylinder. Perhaps it will gain more widespread use, as hydrogen becomes more
available.

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