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Most of the information and photos on this page are courtesy of HowStuffWorks.com | MeASpec.com | RotaryEngineIllustrated.com
A rotary engine is an internal combustion engine, like the engine in your car, but it works in a completely different way than the conventional piston engine. In a piston engine, the same volume of space (the cylinder) alternately does four different jobs -- intake, compression, combustion and exhaust. A rotary engine does these same four jobs, but each one happens in its own part of the housing. It's kind of like having a dedicated cylinder for each of the four jobs, with the piston moving continually from one to the next.

The rotary engine (originally conceived and developed by Dr. Felix Wankel) is sometimes called a Wankel engine, or Wankel rotary engine.

Animated view of a turbocharged Model

The following © information can be found at:

The Basics
A Rotary Wankel Engine are greatly different from piston engines, but it retains the familiar intake, compression, power, and exhaust cycle. Instead of a piston, cylinder, and mechanical valves, a triangular rotor revolves around the eccentric shaft. Three apexes, or tips, of this rotor remain in constant. The only other moving part is the crankshaft. Here is a very accurate set of visual animations Rotary Engine Illustrated.

The Parts
The Rotary has 40 % fewer parts and roughly 1/3 less the bulk and weight of a comparable reciprocating engine. In addition to the simplicity of design, there is little or no vibration.

13B Twin Turbo Disassembled
Typical Single Overhead Cam 4 Cylinder Engine

There are less or no problems with heat dissipation, hot spots, or detonation, all are properties of piston engines. Rotaries are liquid-cooled, capable of running at unusually high speeds for long periods of time. The motor exhibits a high power-to-weight ratio and an exceptionally good torque curve at all engine speeds, and tremendous horsepower for its size.


The rotor has three convex faces, each of which acts like a piston. Each face of the rotor has a pocket in it, which increases the displacement of the engine, allowing more space for air/fuel mixture.

At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber. There are also metal rings on each side of the rotor that seal to the sides of the combustion chamber.

The rotor has a set of internal gear teeth cut into the center of one side. These teeth mate with a gear that is fixed to the housing. This gear mating determines the path and direction the rotor takes through the housing.

Photo courtesy of RotaryEngineIllustrated.com ©
The housing is roughly oval in shape (it's actually an epitrochoid). The shape of the combustion chamber is designed so that the three tips of the rotor will always stay in contact with the wall of the chamber, forming three sealed volumes of gas.

Each part of the housing is dedicated to one part of the combustion process. The four sections are: Intake, Compression, Combustion & Exhaust.

The intake and exhaust ports are located in the housing. There are no valves in these ports. The exhaust port connects directly to the exhaust, and the intake port connects directly to the throttle.
Photo courtesy of RotaryEngineIllustrated.com ©

Output Shaft
The output shaft has round lobes mounted eccentrically, meaning that they are offset from the centerline of the shaft. Each rotor fits over one of these lobes. The lobe acts sort of like the crankshaft in a piston engine. As the rotor follows its path around the housing, it pushes on the lobes. Since the lobes are mounted eccentric to the output shaft, the force that the rotor applies to the lobes creates torque in the shaft, causing it to spin. (Note the eccentric lobes.)

Photo courtesy of RotaryEngineIllustrated.com ©
Animation courtesy of howstuffworks.com ©

How It's Put Together
A rotary engine is assembled in layers. The two-rotor engine we took apart has five main layers that are held together by a ring of long bolts. Coolant flows through passageways surrounding all of the pieces.

The two end layers contain the seals and bearings for the output shaft. They also seal in the two sections of housing that contain the rotors. The inside surfaces of these pieces are very smooth, which helps the seals on the rotor do their job. An intake port is located on each of these end pieces.

Photo courtesy of RotaryEngineIllustrated.com ©
The next layer in from the outside is the oval-shaped rotor housing, which contains the exhaust ports. This is the part of the housing that contains the rotor.
The part of the rotor housing that holds the rotors
(Note the exhaust port location.)
Photo courtesy of howstuffworks.com ©

The center piece contains two intake ports, one for each rotor. It also separates the two rotors, so its outside surfaces are very smooth.
The center piece contains another intake port for each rotor.

In the center of each rotor is a large internal gear that rides around a smaller gear that is fixed to the housing of the engine. This is what determines the orbit of the rotor. The rotor also rides on the large circular lobe on the output shaft.

Photo courtesy of RotaryEngineIllustrated.com ©

Producing Power
Three chambers are formed by the sides of the rotor and the wall of the housing. The shape, size, and position of these chambers are constantly altered by the rotor's clockwise rotation and the faster rotation of the eccentric shaft. The faster the rotor opens the intake port and the fuel and air enter as in the conventional engine.
The rotor continues, closing the intake port by passing beyond it; then compression begins, followed by ignition, combustion, and expansion for the power stroke until the apex seal at the tip of the triangle opens the exhaust port.
The exhaust cycle then takes place, again with no speed-restricting valve mechanism. The engine is unique in that the power impulse is spread over approximately 270 degrees of crankshaft rotation, as compared to 180 degrees for the conventional engine.

-Main moving parts are: 2 rotors and the eccentric shaft.
-These rotate continuously in a single direction.
-They do not jerk backwards and forwards.
-Housing where the rotor tip trace an epitroichoid curve.

Piston engines restrict their engine speed with the valve mechanism. Valve mechanisms are complex because of the number of parts involved in it. Valves need to be recalibrated after a certain time so that the car will run at its optimum efficiency.

While the rotary is a valve free mechanism and the intake port is regulated by the rotor itself and it does not changes after time, one has to physically redesign the intake and the out take port or the rotor itself. Rotaries are so smooth they are almost electric in feel.

The piston engine ( reciprocating ) does not travel in one continuous and smooth rotation like the rotary and this cause shaking and rattling. In the piston engine there is power lost when the piston moves around ( there are points where the piston has zero momentum and speed, that is at the very top and the very bottom of the cycle )

This concept can be easily illustrated by the rotary saw and the hand saw. Even though the rotary is a smooth rotating there are some slight disturbances that Mazda countered with another rotor on the other side that is positioned 180° away from it. There have been three and four rotor designs from Mazda and even a six rotor design from an independent company.

As the rotor moves through the housing, the three chambers created by the rotor change size. This size change produces a pumping action. Let's go through each of the four strokes of the engine looking at one face of the rotor. If you watch carefully, you'll see the offset lobe on the output shaft spinning three times for every complete revolution of the rotor.

Animation courtesy of howstuffworks.com ©
The Four Strokes

The intake phase of the cycle starts when the tip of the rotor passes the intake port. At the moment when the intake port is exposed to the chamber, the volume of that chamber is close to its minimum. As the rotor moves past the intake port, the volume of the chamber expands, drawing air/fuel mixture into the chamber. When the peak of the rotor passes the intake port, that chamber is sealed off and compression begins. If you watch carefully, you'll see the offset lobe on the output shaft spinning three times for every complete revolution of the rotor.

As the rotor continues its motion around the housing, the volume of the chamber gets smaller and the air/fuel mixture gets compressed. By the time the face of the rotor has made it around to the spark plugs, the volume of the chamber is again close to its minimum. This is when combustion starts.

Most rotary engines have two spark plugs. The combustion chamber is long, so the flame would spread too slowly if there were only one plug. When the spark plugs ignite the air/fuel mixture, pressure quickly builds, forcing the rotor to move. The pressure of combustion forces the rotor to move in the direction that makes the chamber grow in volume. The combustion gases continue to expand, moving the rotor and creating power, until the peak of the rotor passes the exhaust port.

Once the peak of the rotor passes the exhaust port, the high-pressure combustion gases are free to flow out the exhaust. As the rotor continues to move, the chamber starts to contract, forcing the remaining exhaust out of the port. By the time the volume of the chamber is nearing its minimum, the peak of the rotor passes the intake port and the whole cycle starts again.

The neat thing about the rotary engine is that each of the three faces of the rotor is always working on one part of the cycle -- in one complete revolution of the rotor, there will be three combustion strokes. But remember, the output shaft spins three times for every complete revolution of the rotor, which means that there is one combustion stroke for each revolution of the output shaft.

An epitrochoid is a curve traced by a point P on one circle as it rolls around another circle. The equation for an epitrochoid depends on the radii r and R of the two circles and on the distance h of the point P from the center of the circle.

The rotor of the Wankel engine is an equilateral triangle with curved sides and the bore is a curve known as an epitrochoid. The bore must be carefully designed to allow smooth rotation of the triangular rotor.

e=eccentricity. The amount of offset between the
eccentric shaft centerline and the rotor centerline

R=radius. Generating radius is the distance between
the centerline of the rotor and the apex

b=width. The trochoid chamber width

Vh=working chamber volume or single camber capacity
( displacement ) Vh=3 x ( square root of 3 ) x R x e x b

Epitrochoid ( Eq.A )

x = e cos 3å + R cos å
y = e sin 3å + R sin å

Where e is the eccentricity and R is the rotor center-to-tip distance. For given values of e and R, Equations give the x and y coordinates defining the housing shape when å is varied from 0° to 360°. The rotor shape may be thought of as an equilateral triangle, as shown in. Because the rotor moves inside the housing in such a way that its three apexes are in constant contact with the housing periphery, the positions of the tips are also given by equations of the form of Eq.1 & 2:

Rotor ( Eq.B )

x = e cos 3å + R cos ( å + 2n¶ / 3)
y = e sin 3å + R sin ( å + 2n¶ )

Where n = 0, 1, or 2, the three values identifying the positions of the three rotor tips, each separated by 120°. Because R represents the rotor center-to-tip distance, the motion of the center of the rotor can be obtained from Eq.1 & 2 by setting R = 0. The equations and Figure indicate that the path of the rotor center is a circle of radius e. Note that Equations A and B can be nondimensionalized by dividing through by R. This yields a single geometric parameter governing the equations, e/R, known as the eccentricity ratio. It will be seen that this parameter is critical to successful performance of the rotary engine. The power from the engine is delivered to an external load by a cylindrical shaft.

The shaft axis coincides with the axis of the housing. A second circular cylinder, the eccentric, is rigidly attached to the shaft and is offset from the shaft axis by a distance, e, the eccentricity. The rotor slides on the eccentric. Note that the axes of the rotor and the eccentric coincide. Gas forces exerted on the rotor are transmitted to the eccentric to provide the driving torque to the engine shaft and to the external load. The line labeled e rotates with the shaft and eccentric through an angle 3å, while the line labeled R is fixed to the rotor and turns with it through an angle å about the moving eccentric center. Thus the entire engine motion is related to the motion of these two lines. Clearly, the rotor (and thus line R) rotates at one-third of the speed of the shaft, and there are three shaft rotations for each rotor revolution.

Key Differences
There are several defining characteristics that differentiate a rotary engine from a typical piston engine.
Fewer Moving Parts
The rotary engine has far fewer moving parts than a comparable four-stroke piston engine. A two-rotor rotary engine has three main moving parts: the two rotors and the output shaft. Even the simplest four-cylinder piston engine has at least 40 moving parts, including pistons, connecting rods, camshaft, valves, valve springs, rockers, timing belt, timing gears and crankshaft.

This minimization of moving parts can translate into better reliability from a rotary engine. This is why some aircraft manufacturers (including the maker of Skycar) prefer rotary engines to piston engines.

All the parts in a rotary engine spin continuously in one direction, rather than violently changing directions like the pistons in a conventional engine do. Rotary engines are internally balanced with spinning counterweights that are phased to cancel out any vibrations.

The power delivery in a rotary engine is also smoother. Because each combustion event lasts through 90 degrees of the rotor's rotation, and the output shaft spins three revolutions for each revolution of the rotor, each combustion event lasts through 270 degrees of the output shaft's rotation. This means that a single-rotor engine delivers power for three-quarters of each revolution of the output shaft. Compare this to a single-cylinder piston engine, in which combustion occurs during 180 degrees out of every two revolutions, or only a quarter of each revolution of the crankshaft (the output shaft of a piston engine).

Since the rotors spin at one-third the speed of the output shaft, the main moving parts of the engine move slower than the parts in a piston engine. This also helps with reliability.

There are some challenges in designing a rotary engine:
Typically, it is more difficult (but not impossible) to make a rotary engine meet U.S. emissions regulations.

The manufacturing costs can be higher, mostly because the number of these engines produced is not as high as the number of piston engines.

They 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.

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