A Heat Engine is a machine that turns heat energy into work.

If we increase the temperature of a gas two things can happen:-  The pressure or volume can increase.  If the volume (its size) is not allowed to change the pressure must increase.  Alternatively, If we don't allow the pressure to change its volume must increase.  Pressure is a force that we encounter everyday, if you squeeze a tennis ball you put pressure on it which forces it to change its shape.

Similarly if we squash a gas, making its volume smaller, we increase its temperature and pressure.  IT WORKS BOTH WAYS!

Its all a bit confusing at the moment, but remember that pressure volume and temperature are all related, that is, if we change one we change at least one of the others depending on how we do it.

A heat engine uses these LAWS OF PHYSICS by burning a contained gas to increase its temperature and pressure.  The increase in pressure causes an increase in volume in a chosen direction.  This increase in volume can be used to move an object, but once the gas has finished expanding it can move no more so we must then start again.  A result of this is a heat engine can only work in a cycle.  If we want it to work continuously we must the system to the state it was in initially.