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Summary
  • Rotary engines usually appear in sports cars that need high performance.
  • Rotary engines have a four-step combustion cycle (intake, compression, combustion, and exhaust.)
  • The parts of the rotary engine are the rotor, housing, and output shaft.
  • The advantages of rotary engines are their powerful output in a small package, fewer moving parts, and smoother driving.
  • The disadvantages of rotary engines are poor fuel economy, high emission levels, and unsuitability for daily driving.

Aside from their appearance, what’s the difference between a high-performance sports car and a regular passenger vehicle? If you look under their hoods, you’ll see that they have different engines. Regular passenger vehicles typically have traditional piston engines, while powerful sports cars usually have rotary engines.

What Is a Rotary Engine?

Also known as a Wankel engine, a rotary engine is something you’ll typically only see in high-performance cars. While it was once considered the most efficient engine on the market, it was eventually replaced by piston engines.

A rotary engine is a type of internal combustion engine that powers a vehicle by going through the same combustion cycle as a piston engine. However, unlike piston engines, rotary engines have a dedicated space for each event in the combustion cycle.

How Does a Rotary Engine Work?

Just like piston engines, rotary engines need air and fuel to power a vehicle. When you turn on a rotary-powered vehicle, its engine goes through a four-step combustion cycle that transforms air and fuel into energy.

Intake

To set off the combustion cycle, the rotor in the engine passes by the intake port and moves inside the housing, causing the chamber to expand as it rotates. This draws the air-fuel mixture into the chamber, which is then carried over to the next stage of the cycle once the rotor end passes the intake port.

Compression

As the rotor continues spinning, the chamber once again decreases in size, compressing the air-fuel mixture in preparation for the next stage.

Combustion

In this stage, spark plugs ignite the compressed air-fuel mixture, which forces the rotor to expand, similar to the power stroke that occurs in a piston engine.

The exhaust gas continues to expand in the chamber until the rotor finally passes the exhaust port.

Exhaust

Finally, the burned gas exits the engine through the exhaust port, and the rotor continues to spin until it passes through the intake port where the combustion cycle begins once again.

Parts of a Rotary Engine

Of course, a rotary engine wouldn’t be able to go through a single combustion cycle without its unique parts, such as the following:

Rotor

At the heart of the rotary engine is a triangle-shaped rotor with concave sides that create a separate space inside the housing whenever the rotor spins.

As the rotor spins on the gears attached to a shaft in the center, each side is always in contact with the housing, creating three individual combustion pockets.

Housing

If you look at a rotary engine, you’ll notice it has an oval-shaped casing around the rotor and shaft. This is called the engine’s housing, which has spark plugs and fuel injectors inserted directly through its wall.

The housing’s oval shape maximizes the rotor’s movement so that every time it spins, its edges are constantly touching the housing’s inner wall.

Output Shaft

In the middle, you’ll notice an output shaft that spins in the center along with the rotor. When the compression and combustion generate energy, this shaft is responsible for transferring it to the drivetrain. This is what powers your wheels and moves your vehicle.

Advantages of Rotary Engines

Rotary engines obviously aren’t perfect, but they do come with their fair share of advantages. Consider some of these examples:

Small Yet Powerful

The rotary engine of the Mazda RX-7 only took up around one cubic foot, which is pretty small if you think about how much power it can produce. In fact, rotary engines are a popular choice for performance cars, especially because you can reach higher engine speeds with them compared to most piston engines.

Fewer Moving Parts

Have you ever noticed how many parts constantly move in a piston engine? Well, a rotary engine has a lot less parts, making it significantly lighter than a conventional piston engine.

Smooth Driving

Did you know that vehicles with rotary engines provide a smoother driving experience than those with piston engines? This is because rotary engines don’t have valves or pistons, greatly reducing engine vibration. This also means you can rev your rotary engine without worrying about the valvetrain losing control.

Disadvantages of Rotary Engines

Despite their advantages, rotary engines still aren’t as common as piston engines, and it’s all because of these drawbacks: 

Poor Fuel Economy

Rotary engines might sound more fuel-efficient, but it’s hardly the case in practice. Because they have long and narrow combustion chambers, rotary engines typically consume more fuel than piston engines.

Their apex seals also tend to wear faster, causing fuel to leak from inside the engine and leading to even more fuel consumption.

Rotary engines can be very difficult to own because it’s also common for their spark plugs to foul out, which would then require you to perform a special process to try and get it running again.

High Emission Levels

Another big red flag when it comes to rotary engines is their high emission levels. Rotary engines consume more fuel, so they naturally release more greenhouse gases. Unburned gas also tends to leak from their worn seals, increasing their hydrocarbon emissions even further.

Not Ideal for Everyday Driving

While rotary engines are great for performance vehicles, they aren’t all that suited for everyday driving. They have poor compression at lower revs, making them inefficient for driving around town.

Popular Cars With Rotary Engines

When it comes to popular cars with rotary engines, the Mazda RX-7 often tops the list. As one of Mazda’s fastest cars, the RX-7 is a popular sports car that offers a lighter and smoother driving experience than most piston-powered cars.

However, the Mazda RX-7 isn’t the only notable rotary-powered car out there. Consider some of these examples:

  1. Citroën M35
  2. Citroën GS Birotor
  3. NSU Ro 80
  4. Audi A1 e-tron concept
  5. Mazda Cosmo
  6. AMC Pacer
  7. Chevrolet Aerovette

Have you noticed what these cars all have in common aside from their rotary engines? They were all manufactured years ago, with the most recent one produced in 2010. Most of their manufacturers have stopped producing them too, so you might be out of luck if you want to buy one of these models.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Tony Harlin is a Master Gas and Diesel Diagnostic Technician with over 18 years of experience. He works full-time at a large independent automotive shop as a driveability and repair technician working on all types of vehicles with a focus on diesels. ASE certifications include A1-A9, L1 and L2, as well as X1.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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