140 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. Shaft Rotor Exhaust pushed out Combustion pressure Spark plug ignites fuel mixture Exhaust port Intake port Air-fuel mixture Figure 6-31. Study the parts of a rotary automotive engine. The rotor spins inside the housing for smooth operation. There are no reciprocating pistons in this design. A B C D Fuel and air mixture Burning fuel mixture Exhaust or spent fuel Figure 6-32. The sequence of events in a rotary engine. A—Intake is starting between Points 1 and 3 compression is occurring between Points 1 and 2 power is being produced between Points 2 and 3 exhaust is finishing between Points 3 and 1. B—Intake continues between Points 1 and 3 compression continues between Points 1 and 2 power is finishing between Points 2 and 3. C—Intake continues between Points 1 and 3 spark occurs between Points 1 and 2 and power begins exhaust begins between Points 2 and 3. D—Intake is finished between Points 1 and 3 power is being produced between Points 1 and 2 exhaust is continuing between Points 2 and 3. Wankel (Rotary) Engine A Wankel engine, also termed a rotary engine, uses a triangular rotor instead of conventional pistons, Figure 6-31. The rotor turns or spins inside of a specially shaped housing. A rotary engine is one of the few alternative engine designs to be mass produced and installed in production vehicles. Figure 6-32 illustrates the basic operation of a rotary engine. While the rotor spins on its own axis, it also orbits around a main shaft. This eliminates the normal recipro- cating motion found in piston engines. One complete cycle (all four strokes) takes place every time a rotor face com- pletes one revolution. Since there are three rotor faces, three power strokes are produced per rotor revolution. A rotary engine is very powerful for its size. Also, because there is no up and down motion, engine operation is very smooth and vibration free. In the past, a complicated emission control system was needed to make the rotary engine pass emission standards. This has limited use of the traditional rotary engine. The newest rotary engine, called the Renesis design, has intake and exhaust ports in the engine endplates and to the side of the rotor housing. See Figure 6-33. The ports are no lon- ger in the periphery of the rotor housing. This helps reduce exhaust emissions because it eliminates the intake/exhaust port overlap of the older design, which contributed to higher emissions. Miller-Cycle Engine A Miller-cycle engine uses a modified four-stroke cycle. This engine is designed with a shorter compression stroke and a longer power stroke to increase efficiency. The intake valve remains open longer to delay compression. Because the intake valve remains open for a relatively long time, the air-fuel charge tends to travel back out the intake port. To compensate for this reverse flow, a supercharger is normally used to pressurize the intake manifold and block this flow, Figure 6-34. Theoretically, a Miller-cycle engine can produce more power and is more economical than a conventional four- stroke cycle engine of equal size. However, the engine is more complex to produce and maintain, partly due to the supercharger. Miller-cycle engines are currently used in limited applications in some passenger cars.