Rotary vs. Piston Engines
Advantages & Disadvantages
The Wankel Rotary (In Comparison to Piston Engines)
- Rotary engines are usually simpler than other engines due to the fewer amount of moving parts. A two rotor engine has 3 moving parts (2 rotors and the eccentric shaft) where as the simplest 4 cylinder engine has at least 40 moving parts, including pistons, connecting rods, camshaft, valves, valve springs, rocker arms, timing belt, timing gears, crankshaft and so on.
- Not only does fewer moving parts make the engine simpler to produce, it is also considerably lighter.
- Also due to less moving parts, there is less energy and frictional losses due to the transfer of energy and motion from part to part. This makes the engine more efficient and able to produce more power and at higher rpms than a piston engine of comparable size.
- Fewer moving parts also provides the engine with better reliability causing many aircraaft companies to choose rotary over piston engines.
- Rotary engines are smoother. Since the rotary engines spin constantly in one direction, they are smoother than piston engines that must convert reciprocating motion into rotational motion and violently change direction as the pistons do.
- Reliability is also increased due to the rotors spinning at 1/3 the speed of the eccentric shaft. This means the moving parts in a rotary engine move slower than those of a piston engine.
- More/smoother power delivery due to rotors having 3 faces and the varying rotational rates of the rotors and eccentric shaft. This causes the rotary engine to have a 50 percent longer duration of combustion than a piston engine. A rotary engine delivers power via combustion for 3/4 of each revolution of the eccentric shaft, where as in a piston engine combustion only occurs for a 1/4 of each revolution of the crankshaft. This results in a higher horsepower to liter (size) ratio for the rotary engine over a piston engine. Thus, an equivalent piston engine to a rotary engine is 1.5 to 2 times the size of the rotary engine.
- One of the most important factors is that Wankel rotary engines that operate within their normal design parameters are almost immune to catastrophic failure. A rotary engine that loses compression, cooling or oil pressure will lose a large amount of power, but the engine is still able to produce power and operate. Piston engines under similar circumstances are prone to seizing or breaking parts that will result in major damage of internal engine parts resulting in an instant loss of power and operation.
- Can withstand being ran for longer periods of time at maximum power than piston engines.
- Very difficult to make a rotary engine pass US emission regulations.
- In a rotary engine, the trailing side of the combustion chamber develops a squeeze stream which pushes back the flamefront. This allows there to be more carbon monoxide and unburnt hydrocarbons in the exhaust stream than produced by a piston engine. This makes it more difficult to pass emissions testing.
- Due to the limited number of these engines produced, the manufacturing costs can be higher.
- Rotary engines typically consume more fuel than a piston engine because the thermodynamic efficiency of the engine is reduced by the long combustion-chamber shape and low compression ratio (because they're often turbocharged).
- Rotary engines are extremely susceptible to damage (possibly damaging an apex seal) from pre-ignition, aka knocking, resulting in dramatically losing compression. This is the reason I had to rebuild the engine in my Rx-7.
- The apex seals are often less efficient at sealing the chambers than piston rings, which can be a factor in decreasing its efficiency and longevity. This sealing is less critical than in piston engines due to the leakage being between different chambers rather than into the crankcase and oil sump as in piston engines.
- Due to deficiencies and damages of the apex seals that provide the engine's sealing and compression, the average life span for a rotary engine is normally less than that of a piston engine. This is normally result of poor maintainance and frequent abuse from racing the engine. Often these engines are turbocharged which can also produce additional stress to lower their lifespan.
- In comparison to the piston engine, the rotary engine produces less torque at low rpms due to its smaller displacement and low compression. Although its overall horsepower will still be greater than that of a comparable piston engine. This lack in torque will mostly be noticeable in "daily driving" but not during racing conditions.
- Most rotaries burn a small amount of oil by design because it is metered into the combustion chamber in order to preserve the apex seals. Thus, owners must add small amounts of oil which slightly increase the running costs.
- As previously mentioned, the true engine displacement remains questioned as a result of the varying rotational rates of the rotors and eccentric shaft.
- Less technical support for repair and maintainance are available, as well as fewer parts available for purchase, leading to an increase in the cost of repairs due to the lower production rate in comparison to piston engines.
When weighing all of these factors into consideration, it can be seen that a rotary engine is best suited in either high performance sports cars or race car applications that desire the simplicity, light weigt & compact design, and high rpm, high power output the engine provides while not caring about the difficulty in passing emission regulations, increased fuel and oil consumption, decrease in low rpm torque, and decrease in longevity of the engine.
Page 1: Intro & Combustion Engine Basics
Page 2: The Physics Behind Rotary Engines
Page 3: The Physics Behind Piston Engines
Page 5: Nick's Rotary Powered Mazda Rx-7
Page 6: Works Cited
Information Provided By Evan Reed (4/19/09)
Edited By Nick Perza (4/19/09)