Rotary Engines

Rotary engines, also known as Wankel engines is a type of internal combustion engine in which the method of operation is different compared to a conventional four stroke engine. Rotary engine adopts eccentric rotary design to complete the combustion process.

In a four stroke engine, all four jobs- intake, compression, combustion and exhaust takes place in the same volume space of the cylinder. Whereas, in a rotary engine all jobs take place at different parts of the engine housing.

History:

Wankel Rotary engine was patented by a German  engineer, Felix Wankel in 1929. NSU Motorenwerke completed a working prototype of the design in 1957.

Design:

Wankel rotary engine employs a eccentric triangular rotor instead of a cylindrical piston in order to compress and burn the air-fuel mixture. The housing is oval or epitrochoid ( as shown in fig 1, a roulette traced by a point 'p' attached to a circle of radius 'a' rolling outside the circle of a radius 'r' ) in shape. The use of a triangular rotor and the epitrochoid design of the housing creates three different volumes of gas inside the housing.

Fig 1: Epitrochoid explanation

There is an intake port on the upper left portion of the housing, and an exhaust port at the lower left portion of the housing. One or two slot(s) are provided at the mid right portion of the housing to house the spark plugs.

The entire housing is enclosed in coolant jackets. An eccentric shaft (output shaft) runs through the centre of the housing. It has round lobes mounted eccentrically around the output shaft, therefore making the lobes slightly offset from the axis of the output shaft.

A triangular shaped rotor is fixed over the rotor journal provided in the eccentric shaft. Internal gears are provided in the rotor that mesh with the central output shaft or the eccentric shaft.

Each face of the rotor is provided with a cavity or pocket to help in the combustion process. The cavity increases the displacement of the engine and also allows some space to the air-fuel charge.

The apex of each face acts as a seal for the three volumes of gas to the outside of the chamber.

Assembly:

The rotary engine is made of several layers:

• The outermost layer is a layer of coolant jacket. Coolant liquid flows through the passages provided in the jacket.


• The next layer from outside is the oval or epitrochoid housing layer which also has a exhaust port. This layer is very smooth so that it can help the rotor to rotate with minimum friction.


• The next layer is the centre piece which contains the inlet port.


• The centre piece has a circular port in the centre which houses the output shaft with circular lobes. The rotor rotates around the circular lobe and it also has internal gear that is meshed with a smaller gear fixed to the housing.

Working:

As the rotor rotates around the chamber, all three volumes of gas expands and compresses alternately. This alternate expansion and compression of the volumes creates a suction force for the air-fuel mixture, then compresses the fresh charge, then ignites the compressed charge with the help of spark plugs and eventually drives out the exhaust gases.


The offset lobe on the output shaft will rotate three times for every one revolution of the rotor. When the rotor rotates, the three chambers created keep varying in sizes.

• Intake :  Intake starts when one of the apex of the rotor crosses the inlet port. The volume of the chamber expands, thereby drawing in  more air-fuel mixture. The intake stroke is over when the consecutive apex crosses the inlet port and no more air-fuel charge enters the chamber.

• Compression: As the rotor continues its rotation in the housing, the volume of the chamber keeps getting reduced and the air-fuel mixture is compressed.

• Combustion: As soon as the face of the rotor makes it around the spark plugs, combustion process starts. Normally two spark plugs are employed for complete combustion. This produces enormous power that pushes the rotor in the direction that increases the volume of the chamber filled with exhaust gases unless the peak of the rotor crosses the exhaust port.

• Exhaust: Once the apex of the rotor crosses the exhaust port, the combusted gases are free to move out through the exhaust port to the tail pipe. The continuous rotation of the rotor shrinks the chamber volume and this forces the exhaust gases to move out of the chamber. Later the apex reaches the inlet port and the entire cycle starts again.

One salient feature about the rotary engine is that all three faces can act as a combustion chamber and thus three combustion or power stroke per revolution of the rotor. But the output shaft rotates thrice for every one revolution of the rotor. Therefore, one combustion stroke per revolution of the output shaft.


Advantages:

• A four stroke piston engine has a minimum of 40 moving parts including camshaft, crankshaft, valves, rockers, timing gears, etc. Whereas, rotary engines have only three main moving parts (i.e) two rotors and a output shaft.


• The entire operation is smoother. Rotary engines are balanced internally with counterweights to cut down the vibration.


• The power delivery is smooth. For every one revolution of rotor, three combustion strokes are produced.


• Rotary engines are reliable because of the slower moving parts such as the rotor which has one-third of the speed of the output shaft.

Disadvantages:

• Doesn't meet the emission norms set by the U.S government.


• Expensive in production compared to piston engines.


• Higher fuel consumption.


• Lower compression ratio.