Rotary Vs. Piston Engine – Pros and Cons

Most vehicle enthusiasts believe that the traditional piston engines are the only type of engine used in automobiles. 

Although this assumption has lingered for a long time, it is important to note that another type of engine, known as the rotary engine, was also popular at the time.

Rotary engines were designed differently from regular piston engines. However, they both have their uniqueness. This article is geared toward revealing the major differences between these two, and the rotary vs. piston engine argumentative article will walk you through the needed information. 

What Is A Rotary Engine?

A rotary engine is a unique type of engine built with a radial layout, which operates with an odd number of cylinders. The engine is designed with a uniquely shaped long combustion chamber and two spark plugs. Virtually all parts of the engine are designed to rotate; hence, the name – is the rotary engine. 

The engine’s combustion process is ignited by the spark plugs, which trigger the air-fuel blend to generate enough pressure to push the rotor further. The rotor turns due to the expansion of the exploding gas in order to generate power.

The engine is also referred to as the Wankel engine, which was coined from the inventor’s name – Felix Heinrich Wankel. The German mechanical engineer invented the rotary engine in 1954 as an alternative internal combustion engine to the regular reciprocating engine.

Wankel and others who modeled his work introduced a different mode of operation to power the internal combustion system of the rotary engine. However, the invention had a couple of pros and cons.

rotary engine cars

Pros Of Rotary Engine

If you’re contemplating the difference between Mazda rotary engine vs. piston engine, some of the benefits of the rotary engine include the following:

Quieter and smoother operation

Unlike conventional engines with pistons moving up and down, the Wankel rotary engine’s operation involves its components circling in one direction to generate power. This simple operation makes the entire system quieter and smoother.

Low maintenance cost

Due to the minimal moving parts in the rotary engine, the tendency for quick wear and tear is minimal. This simply means lesser maintenance costs when compared to that of its counterparts.

Long term reliability

Since the components in a rotary engine engage in a circular movement in one direction to power a vehicle, the operation is relatively slower. This minimizes the strain on the components, unlike the speed required for a piston engine to power a car.

Due to the lesser strain on a rotary engine, the components tend to wear slowly, thereby resulting in long-term reliability than their counterparts.

Lightweight

The Wankel rotary engine is relatively smaller and lighter than its counterparts, irrespective of the extent of power output. This is mainly due to the fewer components within the engine. It is arguably one of the best engines with an advantageous power-to-weight ratio.

Cons

Poor gas mileage and emission

Although rotary engines deliver a great power output, the compression ratio and the fuel that makes their way to the exhaust cause the engine to deliver poor emissions as the engine sucks more fuel. This is a major limitation due to the recent strict emissions regulations.

Oil consumption

Besides the fuel consumption and emission issues, the Wankel engine also burns oil. The oil consumption is meant to lubricate the engine in order to minimize faults or damage. However, the oil consumption rate is a disadvantage when compared to their counterparts.

Frequent maintenance

The excessive oil consumption in a rotary engine is mostly accompanied by oil leaks, which require frequent checks and maintenance to keep the engine in good condition. This can be very stressful.

Fewer technical experts

Due to the fact that rotary engines are not so common in modern vehicles, the probability of finding an auto mechanic to fix a faulty Wankel engine is slim. The cost can be another factor, even when you find an expert that can do the job due to their scarcity.

What is a piston engine?

A piston engine is an internal combustion engine that operates with one or more reciprocating pistons to convert high pressure and temperature to rotational motion. A classic example is a 4-stroke cycle internal combustion piston engine. 

Some of the components of the 4-stroke engine include the piston, crankshaft, intake camshaft, exhaust camshaft, spark plugs, connecting rode, water jacket, and valves (inlet and outlet valves).

A vehicle with a four-stroke cycle piston engine generates power through four different stages. The process begins with the intake cycle, where the intake valve of a cylinder opens as the piston goes down into the cylinder. This process draws air and gas into the combustion chamber.

After that, the intake valve will close and compress the air-fuel mixture in the combustion chamber. As the various pistons push up, the air-fuel mixture is compressed, and the combo is ignited by the spark plug, which later pushes the pistons down.

Once the piston gets to the cylinder’s bottom, the exhaust valve opens to release the remaining fuel and air in the combustion chamber. While most vehicle enthusiasts believe the piston engine type is better, it also comes with some benefits and drawbacks.

Piston Engine

Pros of Piston Engine

  • Low Nitrogen oxide emissions.
  • Mechanical simplicity.
  • Less manufacturing cost.
  • Low turbine operating temperature.
  • Flexibility and reliability.
  • Easy to start the piston. 
  • Highly suitable for waste heat recovery.
  • The high degree of maneuverability.
  • Offers the HCCI combustion process.
  • Internally balanced.

Cons

  • Poor part-load efficiency.
  • High combustion rate.
  • Requires reduction gearing

Rotary Vs. Piston Engine Differences

The major difference between rotary engines vs. piston engines is the mode of operation. The following are some of the basic distinctions between the two engines.

rotary vs piston engine

The direction of moving components

While the rotary engine engages its components in a circling movement in one direction to generate power, piston engines operate with a set of pistons moving up and down to convert high pressure and temperature into rotating motion.

Number of parts

The rotary engine consists of fewer parts compared to its piston counterpart. Basically, a rotary engine has about three major parts, while a piston engine consists of several parts that enable it to generate power.

Rotary vs. Piston Engine Similarities

Although there are some differences between the rotary engine and the piston engine, the underlying similarities are pretty obvious from their mode of operation. Some of the similarities include the following.

Internal combustion

First, both rotary and piston engines are internal combustion engines, even though their mode of operation differs. They engage in the conversion of chemical energy to thermal energy. Then, thermal energy is further converted to mechanical energy. Further, this is transformed into kinetic energy to enable a vehicle to move.

Four-stroke cycle

Rotary engines and piston engines are subjected to the four-stroke cycle, which comprises of the intake, compression, power, and exhaust strokes. These stages are important to both engines.

Requires air, fuel, and spark

Since both the rotary and piston engines are internal combustion engines, they both need fuel, air, and spark to generate power.

FAQs

Q: Is a rotary engine better than a piston engine?

A rotary engine is relatively less risky and so less adverse than a piston engine. This is because its design helps to minimize the occurrence of damage due to a sudden breakdown. Unlike the rotary engine, a piston can seize up in a piston motor during operation and cause serious havoc.

Then again, a rotary engine may lose power and continue to generate power to sustain a vehicle until it eventually declines completely. Undoubtedly, rotary engines are safer than piston engines, especially when a problem surfaces within the components.

Q: Why are rotary engines better?

Rotary engines are better than regular engines because they consist of fewer moving parts, which helps to improve performance and reliability. In addition, the fewer moving parts also mean less maintenance is required to keep the engine running smoothly.

The engines are also lighter, quieter, and relatively more durable than their counterparts. The summary of the comparison between rotary engines and conventional engines is that they are more cost-effective due to less maintenance.

Q: Are rotary engines more efficient?

If you’re analyzing rotary engine vs. piston engine efficiency, rotary engines are more efficient than their counterparts because they engage minimal working parts in the combustion process. Unlike piston engines, they are more durable and reliable.

Rotary engines do not have valves, rocker arms, camshaft, flywheel, or timing belts. This simply means relatively lesser weight, minimized probability of unexpected malfunctions, and less costly repair.

Q: Why don’t we use rotary engines?

The low thermal efficiency challenge due to the long combustion chamber and the passage of unburnt fuel making its way through to the exhaust, resulting in poor emissions, is a major reason why the rotary engine is no longer used.

The engine also has a problem with the rotor sealing due to the inconsistent temperature in the combustion chamber. This is mainly because the combustion happens in a portion of the rotary engine.

Q: How many miles can a rotary engine last?

A rotary engine can last around 80,000 – 100,000 miles or more. However, the engine’s ability to last long depends on the car owner’s driving style and maintenance culture. In other words, a bad driving style and poor maintenance culture will reduce the extent that the engine can go.

Cars like the Mazda RX-7 and RX-8 operated with rotary engines. These cars could attain 100,000 miles or more with standard maintenance on the part of the car owner.

Q: Do rotary engines burn oil?

Yes, rotary engines burn oil. As a matter of fact, some vehicle enthusiasts believe that rotary engine burns oil as a result of faults. This is not necessarily the case. A rotary engine is built to use oil squirters which collect little metered quantities of oil to mix up with the fuel in order to lubricate the seals as the engine runs.

Although rotary engines have a couple of advantages due to their fewer moving parts, there are also a few shortcomings, like poor oil consumption rate and gas mileage.

Q: Why do rotary engines rev so high?

Rotary engines rev so high because they do not have reciprocating mass like conventional engines. They are built to operate with rotational mass. Rotary engines operate with minimal vibrations; hence, their ability to rev high, delivering about 7,000 – 8,000 RPM.

However, Mazda manufactured the RX-8 with its redline at 9,000 RPM. The car was designed to make 232 horsepower at 8,500 RPM. Nevertheless, those two RPM levels are pretty high for a production car engine.

Q: What kills a rotary engine?

Low thermal efficiency is a major problem that kills the rotary engine. This is due to the uniquely designed long combustion chamber, which further results in unburnt fuel accessing the exhaust.

The high oil consumption, poor fuel economy, and emission issues are some major challenges of the rotary engine. Irrespective of its ability to manage a breakdown by not seizing like a piston engine, the rotary engine still has its limitations.

Q: Can rotary engines explode?

Yes! One of the frightening truths about rotary engines is that they can explode due to detonation, especially when the engine runs lean. This is mostly the case for turbo rotaries. Even though it is advantageous to supercharge a rotary, it is important to take note of the downsides of doing so.

But in the case of piston engines, excessive heat due to insufficient lubrication is the major cause of friction and cracks that cause the engine to explode.

Q: Do rotary engines use more fuel?

Of course, rotary engines consume more fuel because of the extent of power they generate as the engine’s rev gets higher. The fuel mileage issue is a challenge associated with rotary engines.

Besides the fuel consumption issue, the excessive burning of oil is another challenge with rotary engines to consider.

Q: Do airplanes use rotary engines?

Yes! Rotary engines powered about 80% of World War 1 aircraft. At a time, they were arguably the most common aircraft power plants, especially around the early years of the first World War.

Some of the aircraft that used rotary engines include the following; Nieuport, Vickers, Morane-Saulnier, Sopwith, Bristol, Thomas-Morse, Caudron, etc. As a matter of fact, about 50 percent of the top 10 aces’ airplanes had rotary engines.

Q: Does any car still use a rotary engine?

Rotary engine cars have become uncommon as very few manufacturers produce their vehicles with rotaries. Although most people thought Mazda stopped producing the engine in 2012 after the last major appearance in the RX-8, the manufacturer claims they still produce rotary engines.

Piston engines have taken over and are still being used in most modern vehicles. The number of pistons in different vehicles varies from 4, 6, or 8, as the case may be.

Final Words

Although piston and rotary engines are both four-stroke cycle internal combustion engines, they are not the same. The major differences between the rotary vs. piston engine have been unveiled above in this article. Also, the similarities have been captured to strike a balance.

The major drawbacks of rotary engines reduced their availability in most modern vehicles in the automotive industry. However, Mazda and a few other manufacturers still utilize the Wankel engine in some of their models.

If you’re asking, “which is better, rotary or piston engine?” Kindly consider the analysis of rotary engine vs. piston engine pros and cons above before choosing between the two.

Osuagwu Solomon

Osuagwu Solomon is a seasoned automotive technician for the past 9 years, and a technical writer. He loves writing about auto professional repair guides, DIY repair guides, and buyer’s guide. After spending six years in the automotive workshop, he decided to impact his knowledge to people aside his domain, and he has achieved this by centering his Automotive writing skills on REPAIRS.

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