Almost all car
work on the four-stroke
, so called because it
and exhaust -
to produce one firing of the
/air mixture. This means that the
rotates twice to complete each cycle.
Some smaller engines, however, notably those fitted to some mopeds or
motorcycles, operate on a two-stroke cycle - the piston is on a
every time it moves down the
so the crankshaft turns only once during
each cycle. A few cars have used this engine too, such as the Wartburg Knight
and some early Saabs.
The earliest two-strokes were of the uniflow type. With this design, the
fuel/air mixture is forced into the cylinder by a rotary blower (
driven by the engine. There is no inlet
: instead, there is an elongated
hole, called a port, in the side of the cylinder near the bottom of the
piston's stroke. The port is opened or closed as the piston passes up and down
the cylinder. The exhaust gases usually pass out through a conventional
The cycle starts with a down-stroke in which burning fuel pushes the piston
down. When the piston uncovers the inlet port at the bottom of its stroke, fuel
and air is pushed in above it. On the upstroke the exhaust
is forced out
and fuel is compressed, ready to be fired. To allow this to happen, the exhaust
valve opens just before the descending piston uncovers the inlet port, so there
to the incoming
The two stroke cycle
Most modern two-stroke engines work slightly differently. Instead of having
a blower to
the fuel/ air mixture into the cylinders, they use what is
This type of engine needs no conventional valves. The inlet ports lead into
the bottom of the cylinder which is open to the crankcase: higher up the
cylinder on the opposite side are another set of ports leading to the
. A transfer port leads back up to the cylinder from the crankcase,
entering at a slightly higher level than the inlet port, but a little lower
than the exhaust port.
During the upstroke the piston uncovers the inlet port and allows the
fuel/air mixture to rush into the crankcase, underneath the piston. Sometimes
there is a cut-out in the side of the piston through which the mixture can pass
to reach the crankcase.
When the piston reaches the top of the cylinder, the compressed fuel/air
mixture is fired by a
, forcing the piston down on the power
As the piston descends, it compresses the fuel/air mixture in the crankcase,
and it also uncovers the exhaust poit closely followed by the transfer port.
The exhaust gases start to escape as the exhaust port is uncovered, and are
further scavenged (forced out) by the fuel/air mixture coming in from the
transfer port under slight
from the crankcase.
To help scavenge the exhaust gases out of the cylinder, the top of the
piston is often shaped to deflect the incoming mixture upwards. The mixture
then doubles back when it strikes the
, flows down the exhaust
port side and pushes the exhaust gases out.
The momentum of the gases from the transfer ports, which will have been open
since near the bottom of the downstroke, continues to expel the exhaust
products until the exhaust ports are closed. This system of expelling exhaust
gases is known as loop
The design of the exhaust is more critical in a two-stroke engine than
it is in a four-stroke engine. The burnt exhaust gases are not positively
forced out by the upward-travelling piston, so it is essential that the
offers the minimum amount of resistance to the gases'
With most two-strokes, the inward rushing inlet charge helps to sweep
the residual exhaust gases out of the cylinder. The problem is that some of
the inlet charge — unburnt fuel — can be lost to the atmosphere because
both the inlet and exhaust ports are open together for some time. However,
the design of the exhaust pipe and silencer can be exploited to minimize
When an exhaust charge leaves the cylinder, it sends a pulse —a shock
wave — down the exhaust pipe, which is reflected back from the end of the
pipe. By paying careful attention to the design of the exhaust, engineers
can arrange a system that can use the returning exhaust pulse to push the
inlet charge, which is trying to follow the exhaust gases down the exhaust
pipe, back into the cylinder.
In most engines the crankcase and
contain the oil to lubricate the
engine's moving parts. But with a crankcase compression two-stroke, the
crankcase cannot do this because it is needed for initial compression of the
fuel and air.
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