The
Two Stroke
FROM
A HUMBLE BEGINNING TO A SIMPLE END.
In
the mid 1880's the world was introduced to the first two stroke
engine that worked on the principle of compressing an air/fuel
mixture in the operating cylinder. From that point it was
only a short ten years or so for the concept to develop close
to the highly efficient engine as we know it today, with ports
for fuel induction, transfer and the removal of burnt gasses.
For the next 100 years the only advancement has been to improve
the efficiency of introducing fuel and removing the exhaust
gasses. There have been attempts to completely separate those
two functions, but they have only been successful in achieving
one thing, to destroy the one big advantage of the two stroke
engine, It's simplicity.
THE
BASIC ENGINE
The basic two stroke operates by compressing the air/fuel
mixture both in the crankcase and the cylinder, and moves
the mixture around by making use of the vacuum and pressure
created by the piston movement. The term "Two Stroke"
means the engine performs all functions in two strokes of
the piston, (down and up) and the spark plug fires each time
the piston is approaching the top of it's travel or top dead
centre (TDC).
BASIC
OPERATION
To take you through a logical sequence of events we will assume
the spark plug has ignited the compressed air/fuel mixture
in the upper cylinder which causes the hot gasses to expand
rapidly, driving the piston downwards.
This act in itself will cause the bottom of the piston to
compress a new amount of mixture which was introduced into
the crankcase when the piston was in it's most uppermost position.
As the piston approaches TDC it exposes the induction port
allowing the new mixture to be drawn into the crankcase vacuum.
With the piston now travelling in a downward direction in
an almost simultaneous operation the introduction of a new
mixture into the combustion chamber via the transfer ports
will aid in the expulsion of exhaust gasses via the exhaust
ports.
The new mixture is also an aid to cooling the engine and is
very important to high revving engines. As the piston moves
in an upward direction again, it first closes the transfer
port, followed by the exhaust port, and then compresses the
new mixture for the next ignition phase.
ENGINE
SPEED OR RPM
The frequency of the complete cycle is dependent on the efficiency
of the inlet, transfer and exhaust ports to put fuel mixture
in, and get rid of the hot exhaust gasses. The more efficient
engines are capable of greater revolutions per minute or RPM,
with some recent engines limited to their ability to stay
together and expend the tremendous heat generated in the engine
reaching some 19,000 RPM plus.
The workload on the piston to compress the fuel mixture as
well as fill and clear the cylinder has the effect of reducing
the efficiency of the engine, and therefore reduces the power
at the crankshaft.
IMPROVING
THE BASIC ENGINE
Improvements to the basic two stroke that have had the most
impact to increase maximum revs, and power are, Reed and Rotary
valve induction systems. These two systems go a long way towards
increasing the efficiency of the engine, and provide a more
manageable and definable power band than the piston port engine
described previously.
Other improvements to the basic out of the box engine are,
blueprinting, or bringing the engine up to manufacturers specifications,
and head modifications to alter the size and shape of the
combustion chamber and squish band.
REED
VALVE
The reed valve consists of thin flexible blades that respond
to the crankcase pressure to let the fuel mixture flow into
the crankcase. That is, for a large proportion of the upward
travel of the piston, the vacuum in the crankcase allows the
reed to bend inwards allowing fuel to flow.
Once the piston approaches the end of it's upward travel the
natural spring in the reed blade allows it to close the port
preventing blow-back and is aided by pressure in the crankcase
created by the piston travelling downwards after ignition.
The reed valve engine is capable of very high revs, but also
provides more low and medium rev power. Despite this power
advantage karters still tend to use them most in the high
rev range (around 18,000-19,000 revs) where some fuel starvation
and therefore overheating problems exist.
Recent technological advances in reed block design has meant
more reliability in the high rev range, and this has increased
the popularity of the reed engine.
ROTARY
VALVE
The rotary valve engine is not as popular in Australia but
is the best of the three designs with more power over a wider
band or rev range far outweighing the disadvantage of mechanical
design that has made them unpopular.
Because the basis of the rotary induction system is a rotary
disc with a fuel inlet opening attached to the crankshaft,
the disc and carby have to be mounted to the side of the engine,
increasing the width and introducing mounting problems for
karts.
The introduction of the compulsory noise induction silencer
which mounts on the side of the rotary valve engine has also
meant reduced popularity for these engines. The rotary valve
engine has none of the high rev fuel starvation problems of
the reed valve motor and the fuel induction window can be
easily adjusted.
BLUEPRINTING
When your engine was mass produced by the manufacturer, the
various parts were made to an ideal size, give or take a bit.
Unfortunately with all the parts giving or taking a bit you
end up with an engine that is not all that ideal. On the other
hand, if you have an engine made up from all ideal parts,
you have a real beauty and don't let it out of your sight.
Blueprinting is just a method of adjusting or machining parts
to bring them back to the specified tolerance without giving
or taking. This is a highly specialised job, and should be
done by experts in conjunction with the latest rule book.
HEAD
MODIFICATIONS
The method of modifying the two stroke head varies between
engine builders, but the reason is mostly the same. To get
the head to the minimum combustion chamber volume permitted
by the rule book.
The size of the squish band gap and the shape of the combustion
chamber is what varies in achieving that goal. The squish
is a part of the head face on the outside of the combustion
chamber and is the same shape as the piston top.
The reason for the squish is to force the fuel mixture towards
the spark plug as the two faces come together, and the combustion
chamber shape is designed to create turbulence to thoroughly
mix the fuel and any remaining gasses before detonation.
The squish and combustion chamber shape are also important
to engine cooling, so care must be taken with modifications.
WARNING
Any modifications on restricted class engines should be done
by a reputable engine builder. A mistake could make your engine
illegal.
Look for results when choosing someone to blueprint, rebuild
or modify your engine. The engine builder with the best results
will have the best prepared engines.
