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

Firstly we must clarify a point which often causes confusion.  Heat and temperature are not the same thing but are however closely related.  It is important not to confuse the two!

In a petrol engine we use use petrol and air mixed as a gas for fuel.  Early Physicists such as Robert Boyle and later Lord Kelvin and Avogadro researched the relationships between temperature, pressure and volume.  This research led to some very important discoveries which we now call The Gas Laws.  The gas laws told us that if we change the temperature, pressure or volume of a gas we change at least one of the others.  For example, if we decrease the volume of a gas without letting any escape the temperature and pressure will increase.      

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 (HEAT ENERGY FROM THE BURNING GAS TO KINETIC/MOVEMENT ENERGY), 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.  We call this an engine cycle, we must first introduce our fuel to the system, we must then burn it to cause an increase in volume.  Then in order to achieve this again we must decrease the volume and refuel the system.

TO FIND OUT HOW THIS IS DONE PRESS- HOW IT WORKS