One of the problems with conventional car
designs is that the
move in a straight line up and down in their
, to produce what is
Yet the road wheels require a different kind of motion -
convert reciprocating motion into rotary motion, the pistons are linked to the
so that, as the pistons go up and down, they cause the crankshaft to
rotate. The rotary motion of the crankshaft can then be transmitted to the road
wheels to drive them round.
A car engine would be a lot simpler if the pistons could rotate instead of
moving up and down because the rotary motion thus produced could then be
transmitted directly to the road wheels (though gearing would still be
A further advantage of such a
would be that the pistons would
always be travelling in the same direction - a circle. None of the engine's
power would be wasted by stopping the pistons at the end of their
accelerating them again in the opposite direction, as happens in a
Despite the appeal of the idea, only one type of rotary engine has ever been
used successfully in cars. This is the Wankel engine, developed by Felix
He began researching into rotary
in 1938. After World War Two he
teamed up with NSU (a German car manufacturer later to become part of VW Audi)
to turn his compressors into a practicable
internal combustion engine
By 1957, Wankel had built an experimental rotary engine which was running on
a test bed, and in 1964 this engine was offered to the public in the NSU Wankel
Spyder. This small, rear-engined sports car had a 498cc Wankel engine, yet it
could develop 50bhp and had a top speed of 95mph (152km per hour).
The Spyder never really caught on with the public, and the car that really
brought the Wankel engine fame was the NSU R080, which was acclaimed Car of the
Year in 1968. This has a twin-rotor engine of 995c and could reach 110mph
(176km per hour).
Inside the Wankel
The heart of the Wankel engine is a three-sided piston called the rotor
revolving inside the
. On each side of the housing is an
The sides of the rotor are curved into three lobes and the rotor housing is
shaped roughly into a fat figure of eight so that, as the rotor rotates, the
gap between each side of the rotor and the housing becomes alternately larger
and smaller. This constantly changing gap is the key to the
/air mixture is timed to enter the housing at a point when the
between the housing wall and one of the lobes of the rotor is
increasing. As this volume increases it creates a
, drawing in the
fuel/air mixture through ports in the housing and the endplate.
As the rotor moves round, this volume starts to shrink, compressing the
fuel/air mixture. This mixture then passes over the
, set into the
wall of the housing. The
plug fires to ignite the mixture, causing it to
expand and drive the rotor on round its
. At this point the volume between
the rotor and the housing increases to allow this expansion of the gases.
Finally, the volume decreases again, forcing the waste gases out through the
The rotor thus goes through the same four-stroke cycle as a reciprocating
, power and exhaust - but each of the three
lobes of the rotor is going through this process continuously, so there are
for each revolution of the rotor.
Running through the centre of the rotor is an
, to which the
rotor is linked by a system of
similar to that in an automatic
gearbox (see Systems 44 and 45). The gearing allows the rotor to follow an
orbit so that the three rotor tips are continually touching the
As the rotor rotates, it drives this shaft around. The shaft carries this
rotary motion to the
and so to the road wheels.
The design of the Wankel engine means that it has no
- the fuel/air
mixture simply enters and leaves the chamber through ports in the rotor housing
and the endplate. Therefore it also has no rockers,
This means that the Wankel has about half the number of parts of a
reciprocating engine. It is also lighter and more compact. However, it still
needs many of the same ancillaries as other engines -
and so on. Once the
engine is installed with all these, it loses much of the advantage of its own
compactness and lighter weight.
Nevertheless, the Wankel engine in the Ro80 was widely praised for its
smooth running and lack of vibration. This was partly due to the engine's
having two rotors set in-line with each other but in separate housings. Each
rotated about the same output shaft, but their timing was set 180° out so that
produced by one rotor would be cancelled out by the same
forces of the other rotor, and so that they would jointly produce a more even
Once the basic design of the Wankel had been established, problems soon
became apparent. One was seal wear. The rotors are sealed on all sides to
ensure that gases do not seep past the tips from the high-compression parts of
the housing to the low-compression parts. These seals were prone to wear and
breakdown, causing the engine to lose compression and hence power.
On a reciprocating engine, this sealing is done partly by the valves and
partly by the
, but the seals on the Wankel engine posed particular
The seals were least effective at low engine speeds, where they need to be
fitted with springs to keep them pressed against the side of the housing.
But at high engine speeds a combination of
force the seals much harder against the housing. The resulting
meant a loss of power and considerable wear on the seals, which soon
Early Wankels had seals made of
, but later designs had special
cast-iron seals, which proved more durable. To provide extra protection the
inside of the housing and the endplates were given a hardwearing coating.
The second major problem is wear of the eight-shaped running surface caused
by 'chattering' of the seals. This results in corrugations on the running
surface and shortens the life of the engine.
The other problem with the Wankel engine is the shape of the
. In a typical reciprocating engine, the chamber is roughly
hemispherical, which helps to ensure that the fuel/air mixture burns evenly and
progressively. In a Wankel engine, the combustion chamber is inevitably long
and flat, a shape that makes optimum combustion much more difficult.
A partial solution to the combustion chamber problem was to
plugs positioned a short distance apart. Mazda - whose RX-7 is now the only
Wankel-engine car on sale in the UK today (see below) - took this principle a
stage further by fitting two plugs, with one plug firing a fraction of a second
later than the other one. This arrangement requires two separate
systems with two
Lack of success
Despite the Wankel's power and smooth performance it has so far failed to
catch on among the vast majority of car manufacturers.
The main reason is its high fuel consumption caused by the tendency of the
fuel/air mixture to burn unevenly. Uneven combustion in the Wankel engine also
creates another problem - high
levels of part-burnt
In the years since the R080 brought the theoretical advantages of the Wankel
engine to prominence, there have been various oil crises and continuing
pressure from governments and the public for lower exhaust emission levels and
better fuel consumption.
Neither of these demands favour the Wankel engine and, furthermore, it has
meant that most car manufacturers have had to devote a lot of time and money to
improving the efficiency of their existing engines.
Stop wasting time on YouTube and get serious!
The Ultimate Car Mechanics video course
Learn everything about modern cars from our new video series.
Almost all car engines work on the four-stroke cycle, so called because it
takes four strokes of the piston induction, compression, ignition and exhaust -
to produce one firing of the fuel/air mixture. This means that the crankshaft
rotates twice to complete each cycle.