In World War II, airplanes began to run into a unique problem. At the time, all planes had engines like modern truck engines, only much bigger. These engines powered large propellers. The problem was that aircraft speed only increase as a function of engine and propeller speed. In a fast dive, the tips of the propellers of a P-51 Mustang or Spitfire would approach the speed of sound. If the blade tips went supersonic, they”d tear the plane apart.
Even if the propellers didn’t tear the plane apart, the engine might. The back-and-forth motion of the pistons in an engine has a safe upper limit, effectively limiting RPM. Pilots eventually found the solution to these problems in the jet engine. Jet engines don’t have reciprocating parts – everything moves in one smooth, continuous rotation. Furthermore, the blades inside a jet are much smaller in diameter than the tips of a large propeller, and thus can turn much faster before the tips approach the sound barrier.
Car manufacturers don’t have to deal with propellers, but do benefit from some of the same things that benefit planes. For example, since cars develop their peak horsepower at high RPM, cars designed to go very fast benefit from eliminating reciprocating motion at high speeds, just like those WWII planes. Because cars don’t fly, there is not quite as much emphasis on reducing weight, but the power to weight ratio is still important.
Turbine engines were one way to create high amount of power for low weight. Jet engines have many other advantages as well, including fewer moving parts, continuously moving parts (everything just spins), no need for frequent oil changes (since the oil doesn’t get contaminated with fuel), , and less vibration. Gas turbines also have fewer total parts, and don’t have valves, tie rods, pistons, or cams. Since a gas turbine is naturally air-cooled by the huge amount of air constantly flowing through it, there’s no need for a radiator. Gas turbines don’t stall like traditional engines, and can use just about any fuel.
Finally, jet engines increase in speed as torque load is reduced, which exactly matches the need for automobile applications. So, for turbine engines, this means nearly an elimination of need for a multi-speed transmission.
With all these advantages, it would seem that jet cars should be ubiquitous. Why are there absolutely none in production?
Well, gas turbines are used in the military. The incredibly successful M1 Abrams tank is powered by a 1,500 HP gas turbine. And in 1963 Chrysler created a gas turbine car that had many of the advantages outlined above.
But the primary disadvantage of both the tank and the car is fuel consumption. A jet engine idles at about 20,000 RPM, and runs at peak efficiency when operated at continuously high speed. Stopping and starting, or idling for any length of time, something that land vehicles do a lot, uses a huge amount of fuel compared to reciprocating engines.
Gas turbines also have a different acceleration curve than piston engines. While a piston engine will accelerate quickly off the line, a gas turbine must be revved to high RPM before developing peak torque. So a jet powered car wouldn’t be great at accelerating away from a traffic light, but would be the ideal vehicle for passing on a two-lane highway.
A third type of engine, the Wankel or rotary engine, somewhat fills the gap between piston engines and gas turbines.
Like a reciprocating engine, the Wankel engine has enclosed chambers that contain compression, ignition, and exhaust phases. Wankel engines run on gasoline or diesel fuel, just like reciprocating engines. The essential principles of combustion are basically the same as a reciprocating engine. However, the shape and motion are more similar to a gas turbine. Although the Wankel engine does not have a turbine, it has a single triangular ôpiston that rotates continuously just like the turbine in a jet engine.
Also, similarly to a turbine engine, the lubrication in a Wankel engine is completely separated from the fuel system. A reciprocating engine has blow-by, which is fuel and exhaust that may leak past the piston heads into the oil system. A jet has no equivalent of blow-by, as air continuously flows through the entire engine. A Wankel engine has blow-by, but leakage is between adjacent chambers undergoing subsequent stages in combustion, not between the combustion chamber and oil system. This translates only to a slight reduction in power and efficiency, and an increase in unburned hydrocarbons, but not a fouling of the oil system.
Another advantage of the rotary engine is that the power stroke is around 270 degrees of rotation, while the power stroke on a reciprocating engine is closer to 180 degrees. This, combined with the fewer necessary parts, means that Wankel engines are smaller and lighter than piston engines for an equivalent power output. High output Wankel engines can weigh as little as one pound per horsepower output.
Because of their continuously rotating operation, Wankel engines share with jet turbines the tendency to be very smoothly running, with very little vibration compared to the back-and-forth motion of piston engines. They also share the ability to run on a variety of fuels such as gasoline, hydrogen, or kerosene.
Unlike jet turbines, Wankel engines operate at similar rotation speeds as reciprocating engines. Instead of idling at 20,000 RPM, a rotary engine idles at about 1000 RPM, or a little under, which is typical for piston engines. This is because the Wankel has the same combustion process as a reciprocating engine, albeit in a different shape.
This fusion between the power to weight advantages of a jet engine and the relative efficiency of a reciprocating engine means that the Wankel engine has enjoyed a relatively significant stint as a standard production engine.
Mazda has been making Wankel engines since the 1960s, and enjoyed a certain cult following for a while. Although enthusiasts enjoyed the benefits laid out above, production of the last Mazda rotary engine was in 2011, in the RX-8.
The reason for the phase out was partially because in a world where gas is often upwards of $3 or $4 a gallon, any engine that is inherently less fuel efficient is avoided, regardless of the potential improvement in power. Another significant obstacle is tighter restrictions in emissions testing. Rotary engines, with their tendency toward exhausting a higher percentage of unburned fuel, have a much harder time meeting emissions requirements.
Although the rotary engine has been completely phased out of Mazda production, it hasn’t been abandoned on paper. Research continues, and perhaps some breakthrough will allow for production of the rotary engine to resume once again.