Rotary engines, also called Wankel engines after the inventor, are combustion engines that perform the same four tasks as a standard four-stroke engine. However, the design of the engine is completely different from conventional automobile engines. Rotary engines were installed in the Mazda RX-7 line of car, and the company plans to put a variation of that rotary engine in its upcoming RX-8. Because of the design of the engine, there are many advantages to using it over standard four-stroke engines.

No Pistons?

The biggest difference between a rotary engine and a four-stroke engine is that the rotary engine has no pistons. Instead, the engine derives its power from things called "rotors¹." These rotors have three convex sides and a hole in the center. Imagine the part of a Ouija set that you rest your hands on. This rotor is set in a roughly elliptical housing, which the three points of the rotor slide against to create a tight seal. The rotor's central hole has teeth in it, which are used to rotate a central shaft. The shaft has an elongated lobe to increase torque as the rotor turns it.

Confused Yet?

Because the design of a rotary engine is so much different from a four-stroke engine, it can be quite a daunting task to understand its workings. The rotor essentially does each job² at the same time. The shape of the housing and the rotor make three "chambers," each between the housing and one wall of the rotor. Let's take a look at one of these chambers.

Because of the path the rotor takes, the chambers vary in size along the route. Just before intake, the chamber is just at its smallest size. Then, one point of the rotor passes the intake port³ and the chamber begins to fill with fuel. The chamber expands in size, and then the second point covers the intake port, making a sealed chamber. As this chamber moves towards the spark plugs it begins to get smaller, compressing the fuel mixture.

The two spark plugs are situated on the opposite side of the housing from the intake and exhaust ports. When the chamber passes these spark plugs it is at its smallest size. The spark plugs ignite, burning the mixture and forcing the chamber to expand. By expanding the chamber rotates the rotor, turning the shaft which provides power. At its maximum size, the point of the rotor passes the exhaust port, which expels the combustion products. The second point passes the port, closing it off and readying that chamber for the next intake cycle.

Three Times The Fun

The rotor makes three different "chambers" with the wall of the housing. So, in one revolution of the rotor there are three combustions - 75% of the cycle produces power. Compare that to a standard four-stroke engine, where one piston produces power one-half of every other stroke, or 25%. The central shaft also turns three times per revolution of the rotor, so there is one revolution per combustion stroke.

Advantages

Rotary engines are, in many ways, much more reliable than four-stroke engines. A rotary engine with two rotors has only three main moving parts: two rotors and the shaft. Compare to over forty moving parts in a four-stroke engine. Fewer moving parts means fewer things that can go wrong.

In addition, because of the circular motion of the rotary engine, there is less violent vibration. A four-stroke engine's pistons are constantly changing direction, creating a lot of vibration. The rotary engine also produces combustion through more of its rotation, so it is smoother than a four-stroke engine. Spinning counterweights also cancel out any adverse vibration.

Furthermore, the rotor moves only one-third as fast as the output shaft, which increases reliability. Slower movement translates into less violent clashing between parts, which reduces stress.

Disadvantages

The biggest disadvantage facing rotary engines is meeting emissions regulations. It is typically more difficult for a rotary engine to meet US emissions guidelines. However, it is not an impossible feat.

Manufacturing costs are also usually higher than a four-stroke engine. Rotary engines, due to relative lack of demand, are not produced in as large a quantity as four-strokes. Therefore, the unit price is normally higher.

Fuel efficiency is also less than a four-stroke engine. Due to less compression and a long combustion chamber, fuel is not burned as efficiently.