Introduction: How to Build a Wankel Engine (and How It Works)

This is for my AP Physics C class.
Wankel Rotary engines are quite ingenious. They arose as an attempt to challenge piston-based engines, and proved that sufficient power can be created without the reciprocating motion of pistons. They rely on very few moving parts to produce a power output and manage to utilize these few parts in a very clever multitasking manner. Let's become familiar with the components of a Wankel Rotary engine before we can build one.

Step 1: The Rotor Housing

The rotor housing is the home to all the action in a Wankel engine. It remains stationary during the engine's operation and serves the purpose of containing the combustion cycle. The wall of the housing allows for the proper conditions for the combustion cycle (to be outlined in a later step) to occur by enabling the proper flow of air in and out of it. The housing has a port in its lower left corner for exhaust gases to leave, and a series of coolant jackets around its perimeter to control the temperature of operation through conductive heat transfer.

*Images contain hoverable notes that assist the written explanation

Step 2: The Rotor

The rotor is the first moving part in the engine. Its size is smaller than the housing it sits in, but it has a very important property that enables it to fill the housing's full volume when moving. The rotor only fits in a single position within the housing based on how it is angled (two permutations are displayed), and the only way to move it around the housing requires both rotation and translation. Although the rotor does not move in a circular manner, its center executes a circle when the rotor completes a full cycle around the housing. The engine exploits this property and translates the motion of the rotor to a direct circular motion.

The rotor also contains geared teeth which mesh with the engine's backplate to guide its motion.

Step 3: The Eccentric Shaft

As the second (of two) moving parts, the eccentric shaft is what's used to translate the rotor's off-center motion into a uniform circular output. The shaft's widest part fits into the round opening of the rotor, and its narrower parts can be hooked up to another rotor's eccentric shaft or the drivetrain of a vehicle.

The eccentric shaft relies on the principle of torque for its movement and circular output. The pivot point is the center of the output shaft. The axis of rotation spans from the center of the output shaft to the center of the larger eccentric part. The force applied comes from the combustion of the engine and the expanding gases (to be discussed) at a right angle to the axis of rotation. The torque output is modeled by τ=rFsinθ where θ=90.

Step 4: The Frontplate

The frontplate covers the front side of the engine. It has a hole for the output shaft and should also have two ports for air intake. An additional model that contains these ports is attached.

Step 5: The Backplate

For the purposes of this demonstration, the backplate is reduced to the part closest to its stationary synchronizing gear, which meshes with the rotor's gearing. In actuality, the plate looks much like the frontplate, and contains ports for air intake.

Step 6: Assembly!

Even though this is a simplified model of the Wankel engine, this design can be scaled up to real life. Having so few parts and understanding their interrelation makes for a speedy assembly.

1. Begin with laying the housing flat. In real life, this is the part when gaskets would be applied.

2. Align the holes of the backplate to the corresponding holes of the housing and connect the two with bolts.

3. Turn the assembly over, so that the backplate lays flat on the surface.

4. Insert the rotor and make sure its gear teeth mesh with the teeth of the stationary gear on the backplate.

5. Insert the eccentric shaft so that its output portion goes through the backplate and its wide part fits within the rotor's opening.

6. Overlay the frontplate and secure it with bolts.

Step 7: The Engine in Action

The engine undergoes four steps to its combustion cycle: air intake, air compression, ignition, and exhaust. The rotor splits the housing into three sections which host each part of the cycle.

The cycle begins in chamber 1. As the rotor spins about the stationary gear of the backplate, the volume of chamber 1 expands. This causes lower air pressure within the chamber, and external air rushes in through the intake pots. Fuel and oil are injected at the same time. As the rotor continues spinning, the volume of the chamber decreases, and the pressure increases per the perfect gas law, PV=nRT. In the provided picture, all the air from chamber 1 has been compressed into the small volume of chamber 2 and at that point, ignition occurs. The stored chemical potential energy of the air-fuel mixture is released through combustion and the expanding gas exerts a force on the rotor which causes it to continue its motion. This same force is transferred through the rotor to the eccentric shaft and the resulting torque spins the output shaft. The cycle is concluded when the expanded gas is forced out the exhaust port. The cycle repeats in the same order, with each side of the rotor executing portion simultaneously.

A point to be noted is that for every full rotation of the rotor, the eccentric shaft spins three times. This allows for higher revolutions per minute from a smaller number of combustions.

A website called Animated Engines serves as a nice supplement to the provided gif of the engine's motion:

Step 8: History

Like much of today's technology, the initial rotary engine designs are not what is used now. Felix Wankel's initial concept of the rotary engine had both the rotor and its housing turning (shown in gif). This allowed for higher revolutions to be achieved but also introduced complications due to the high number of moving parts.

The design most common in the present is came in the engine's next iteration. The housing was held stationary, and only the rotor and its eccentric shaft were allowed to move, which brought the general design to its present simplicity.

Although it was simple, it had a few downsides- the engine struggled to produce torque at lower revolutions, and had a chronic problem of oil consumption and seal wear due to the lack of adequate materials. When Mazda became involved in the engine's development, it introduced designs with multiple rotors that produced better results in torque and reduced oil consumption, but nevertheless fell short of the performance of the commonplace piston-based engines.

Mazda gave up their efforts in the rotary engines in 2012, with the last rotary-powered RX-8 being produced. The engines had problems with their thermal efficiency and gas consumption.

The Wankel rotary is a very clever concept, but has shown to be ineffective in practice.

Step 9: Improvements

To address the issue of low torque output, I would suggest increasing the diameter of the eccentric shaft, as well as the opening in the rotor. Doing so would enable a longer axis of rotation, as well as a greater contact area across which the force of combustion can be transferred, thereby improving the engine's torque capabilities both in low and high RPMs.

Step 10: Acknowledgements

The model I 3D printed for this demonstration came from user mming1106 on Thingiverse:

I familiarized myself with the general history of the Wankel engine from this article by Dave Pratte on Speed Academy :

Finally, Engineering Explained's videos were great help in visualizing what happens in the engine: