The Pros & Cons of Rotary Engines

I like rotary engines. This post will explain what I think is good and bad about the rotary engine. However before all that I should explain how the rotary engine works and how it’s different from a piston engine.

How It Works

The rotary engine or Wankel engine as Mazda call it works on the same four stroke cycle as a piston engine (intake, compression, ignition, combustion) it’s the way it goes about it that makes it different.

Pistons

Internal combustion engine pistons of partial cross-sectional view
Internal combustion engine pistons of partial cross-sectional By Mj-bird CC BY-SA 3.0, via Wikimedia Commons

A piston engine works by rotating a crank shaft with an offset allowing the pistons to move in time with each other. While this is happening the crankshaft is turning a belt turning a camshaft which opens and closes valves, allowing air into the cylinder mixing with fuel allowing combustion to occur when the spark plug fires at what’s called top dead centre.

4-Stroke-Engine-with-airflows
4-Stroke-Engine.gif: by UtzOnBike (3D-model & animation: Autodesk InventorCC-BY-SA-3.0], via Wikimedia Commonss

Rotary

Wankel-1
A Wankel engine in Deutsches Museum in Munich, Germany by Softeis at German Wikipedia CC-BY-SA-3.0, from Wikimedia Commons

A rotary engine does the same thing in a different way.

The rotor is triangle shaped to allow it to hold the air fuel mixture and move it around the housing. As it passes the first part of the housing it allows air and fuel into the housing through valves and a fuel injector. After spinning some more to the rotary equivalent of top dead centre where spark plugs ignite the air fuel mixture spinning the rotor, turning the central crankshaft, turning the transmission, turning the differential and finally turning the wheels. You might think that is the end but it isn’t, as the rotor spins a little more past an exhaust valve and as the burnt air fuel mixture passes it, it opens and releases the mixture through the exhaust.

Wankel Cycle anim en
*Animation of Wankel engine, with English annotationBy User:Y_tambe CC-BY-SA-3.0], via Wikimedia Commons

Pros

There are many pros of the rotary engine.

Firstly how smoothly it runs. The rotary engine runs so smoothly because it only has two moving parts. The way it combusts the air fuel mixture allows it to rev up more easily than piston engines.

Secondly the sound. This is a controversial topic as some think the rotary’s signature popping and banging is just annoying but most Japanese car fanboys love bouncing the engine off the redline and the way it crackles and pops. I think it sounds cool.

Thirdly the power delivery. The fact that there are only two moving parts, only one of them delivers power to the flywheel so there is less energy loss through extra parts so it delivers power a lot more easily and smoothly.

Cons

As we all know when there is good there is bad and it is the same with the rotary. Firstly the way it drinks oil. Because the edge of the rotor needs lubrication against the housing, as the fuel is injected into the combustion cycle some oil gets injected onto the corner of the rotor, during the combustion stroke it gets burnt off, so every cycle is destroying oil.

Secondly the unreliability. Rotary engines are famous for blowing but most people don’t know why. The reason is that the edges of the rotor can split and cause incomplete combustion and misfires causing a lot of bad stuff.

Summary

Overall the rotary is a great idea to simplify the engine and with some tweaks it can have a very high power output. However I would not recommend using it in a car you use day to day but as a fun project it is a good choice.

Comming next - Cars my dad should be considering for his midlife crisis.

Mazda-rx7-3rd-generation01
Mazda RX7 3rd generation By Taisyo CC-BY-SA-3.0 or CC BY-SA 2.1 jp], from Wikimedia Commons

KLC