Two stroke cycle Engines

1 - compression stroke , which start by closing the inlet and exhaust ports .

2 - power or expansion stroke, which start by firing and the inlet and exhaust are still closed .

The two-stroke cycle , starting at top , or inner,
dead centre firing stroke , can said to compromise
first a combined power and Exhaust stroke as
the piston moves down , and then induction and
compression as it moves up again . However,so
of the overlap of these functions at BDC , this is
perhaps a slight over-simplification.

Doubling the number of power strokes per
revolution might be thought to offer potential for
a power output double that of the four - stroke
engine , but it does not . Indeed, the outputs of
two - stroke engines range from only about 10 to
40% above those of equivalent four-stroke units.

This situation arises partly because the Pumping losses in the two-stroke engine are generally higher, but mainly because , for reasons to be explained later , it is not possible to develop such high Mean Effective pressure as with the two-stroke cycle.

The Principle Disadvantage :

Which is that its Fuel consumption is high because over most,if not all, of its speed range some of the incoming charge inevitably is lost through the Exhaust ports during the overlap period.

Although both efficiency and specific power output can be improved by measures such as Fuel injection after the exhaust ports are closed,incorporating Poppet type exhaust valves into the head , scavenging the exhaust gases more effectively by Supercharging or even incorporating extra cylinders for scavenging by providing extra air,all involve increasing the complexity of the engine , which reduces its attractiveness relative to a four-stroke unit.

The Principle advantages :

Mechanical simplicity , low cost, greater mechanical silence,smooth torque owing to the shortness of the intervals between combustion impulses, and consequently the small flywheel and therefore light weight) were valid, they would still have to be set against the apparently inescapable disadvantages.

These are greater noise due to the sudden uncovering of the ports by the pistons, high specific fuel consumption, excessive hydrocarbon content of the exhaust gas, and some more, including difficulty of starting and irregular firing at idling and light load with some types of two-stroke engine.