How Rotary Engines Work
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.
In this article, we'll learn how a rotary engine works. Let's start with the basic principles at work.
The Basics
Like a piston engine, the rotary
engine uses the pressure created when a combination of air and
fuel is burned. In a piston engine, that pressure is contained
in the cylinders and forces pistons to move back and forth.
The connecting rods and crankshaft convert the reciprocating
motion of the pistons into rotational motion that can be used
to power a car.
In a rotary engine, the pressure of combustion is contained in a chamber formed by part of the housing and sealed in by one face of the triangular rotor, which is what the engine uses instead of pistons.
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The rotor follows a path that looks like something you'd create with a Spirograph. This path keeps each of the three peaks of the rotor in contact with the housing, creating three separate volumes of gas. As the rotor moves around the chamber, each of the three volumes of gas alternately expands and contracts. It is this expansion and contraction that draws air and fuel into the engine, compresses it and makes useful power as the gases expand, and then expels the exhaust.
We'll be taking a look inside a rotary engine to check out the parts, but first let's take a look at a new model car with an all-new rotary engine.
Mazda RX-8
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Mazda has been a pioneer in developing production cars that use rotary engines. The RX-7, which went on sale in 1978, was probably the most successful rotary-engine-powered car. But it was preceded by a series of rotary-engine cars, trucks and even buses, starting with the 1967 Cosmo Sport. The last year the RX-7 was sold in the United States was 1995, but the rotary engine is set to make a comeback in the near future.
The Mazda RX-8 , a new car from Mazda, has a new, award winning rotary engine called the RENESIS. Named International Engine of the Year 2003, this naturally aspirated two-rotor engine will produce about 250 horsepower. For more information, visit Mazda's RX-8 Web site.
The Parts
A rotary engine has an
ignition
system and a fuel-delivery
system that are similar to the ones on piston engines. If
you've never seen the inside of a rotary engine, be prepared
for a surprise, because you won't recognize much.
Rotor
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.
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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.
Housing
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:
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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.
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.
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Now let's take a look at how these parts are assembled.
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.
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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.
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The center piece contains two intake ports, one for each rotor. It also separates the two rotors, so its outside surfaces are very smooth.
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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.
Next, we'll see how the engine actually makes power.
Producing Power
Rotary engines use the
four-stroke combustion cycle, which is the same cycle that
four-stroke piston engines use. But in a rotary engine, this
is accomplished in a completely different way.
The heart of a rotary engine is the rotor. This is roughly the equivalent of the pistons in a piston engine. The rotor is mounted on a large circular lobe on the output shaft. This lobe is offset from the centerline of the shaft and acts like the crank handle on a winch, giving the rotor the leverage it needs to turn the output shaft. As the rotor orbits inside the housing, it pushes the lobe around in tight circles, turning three times for every one revolution 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.
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.
The Four Strokes
Intake
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.
Compression
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.
Combustion
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.
Exhaust
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.
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.
Smoother
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).
Slower
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.
Challenges
There are some challenges in
designing a rotary engine:
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