BASIC TWO-STROKE OPERATION
Two stroke engines are a very simple and compact design which,
although not very fuel efficient, produce an excellent power-to-size ratio. That
is, the power of a given size two stroke engine will generally be higher than
that of the same size four stroke, although modern four strokes are getting
nearer. This is borne out by the fact that this year(2002) 990cc four strokes
are being allowed into the premier Grand Prix class, alongside 500cc two
strokes. The general concensus is that this will be the end of the two stroke
dominance but it does illustrate the excellent power/size ratio given by the
stroker.
There are three general two stroke designs:
PISTON PORT
REED VALVE
DISC OR ROTARY VALVE
But the same principles hold true for all designs, the main difference being
the way the inlet is controlled.
A two stroke engine makes power with every revolution, as opposed to
the four stroke which takes two revs for each power output. If the piston is at
top dead centre (TDC), i.e. the top of it's stroke, unburnt fuel has been
compressed and is ignited by the spark plug. The explosion of the gas fires the
piston back down the bore. The exhaust port, which is a whole in the wall of the
cylinder, is opened as the crown of the piston descends past it and the exhaust
gases travel into the exhaust pipe. At the same time, the movement of the piston
pressurises the crankcase causing the unburnt mixture drawn in the previous
cycle to be forced up through the transfer ports and into the cylinder. In
the case of reed valve engines, this pressure also closes the reed valve, thus
preventing mixture escaping back through the inlet.
The flow of the hot exhaust gases out through the port also lowers the
pressure in the cylinder, which once the descending piston has opened the
transfer ports, will draw the fuel mixture up into the cylinder. This unburnt
mixture also forces the last of the exhaust gases out and into the exhaust pipe.
However some of the fresh mixture is also lost into the exhaust. Two stroke
exhausts are designed so that they reflect the compression pressure wave back
which forces most of the fresh mixture back into the cylinder, although some is
lost, which is what makes two strokes relatively innefficient. The better the
design, the less unburnt fuel will be lost and more powerful the engine will be.
As the piston travels past bottom dead centre (BDC) and rises up the
cylinder it will close the transfer and exhaust ports and compress the unburnt
mixture that has been drawn in, while at the same time lowering the pressure in
the crankcase, thus drawing more fresh mixture in ready for the next cycle. Once
the piston reaches TDC the compressed mixture is ignited and the cycle starts
again.
