A rotary engine is an internal combustion engine, like the
in your car, but it works in a completely different way than
the conventional piston engine.
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
The rotary engine (originally conceived and developed by
Dr. Felix Wankel) is sometimes called a Wankel engine,
or Wankel rotary engine.
In this edition of HowStuffWorks,
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.
The rotor and housing of a rotary engine from
a Mazda RX-7: These parts replace the pistons,
cylinders, valves, connecting rods and camshafts found
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 alternatively 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.
In the next section, we'll take a look inside a rotary
engine and check out the parts.
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 2003.
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.
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
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 epitrochoid in
shape -- check out this
Java demonstration of how the shape is derived). 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:
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.
The output shaft (Note the eccentric
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
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
One of the two end pieces of a two-rotor
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
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
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
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
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 stokes of
the engine looking at one face of the rotor.
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
When the peak of the rotor passes the intake port, that
chamber is sealed off and compression begins.
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 shape of 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
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 stokes. 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
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
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:
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
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