Understanding how engines work - basics

How do engines turn petrol into power?


Petrol engines work by turning chemical energy into heat energy, this heat energy into a gas pressure and this pressure into kinetic (movement) energy. Chemical energy is the potential energy that can be released from a chemical reaction. Engines burn petrol, burning is a chemical reaction with oxygen and another chemical that gives off heat, the oxygen in this case comes from the air. The heat causes air to expand and raise the pressure rapidly within a hollow cylinder. Inside the cylinder is a piston which is pushed down inside the cylinder by the pressure. The piston turns a crankshaft, so the engine turns the power from burning petrol into a turning force to turn the wheels. So far, the above description also loosely describes a steam engine. There are obvious differences between a petrol and a steam engine, so let’s look at how a petrol engine works: 



Diagram 1:








Diagram 1 shows how the vast majority of petrol engines work and describes the ‘four stroke cycle’. Most engines have more than one cylinder, the picture only shows one individual cylinder but at the four distinct stages of operation. On engines with more than one cylinder, they all work in the same way but are at different stages of operation at any one time. Each stage of operation sees the piston move (stroke) one way inside the cylinder - down on the power stroke, up on the exhaust stroke, down on the intake stroke and up on the compression stroke. 


Power stroke: At the start of the power stroke both the inlet and exhaust valves are closed and the piston is at the top. A mixture of compressed air and vaporised fuel in the area of cylinder above the piston are ignited by a high voltage electrical spark between the electrodes of the spark plug. The fuel explodes, (an explosion describes a very fast burning which violently expands) and pushes the piston down, turning the crankshaft.


Exhaust stroke: When the piston reaches the bottom of the power stroke the exhaust valve opens and, since the crankshaft is still turning, the piston is pushed back up inside the cylinder and this pumps the burned fuel out through the exhaust valve and into the exhaust pipe.


Inlet stroke: When the piston reaches the top of the exhaust stroke the exhaust valve closes and the inlet valve opens. Since the crankshaft is still turning, the piston is pulled back down inside the cylinder and this sucks a new mixture of air and fuel into the cylinder through the inlet valve (and intake manifold).


Compression stroke: When the piston reaches the bottom of the inlet stroke, the inlet valve is closed so now both valves are closed. Since the crankshaft is still turning the piston moves up inside the cylinder. As the piston moves up inside the cylinder the air and fuel become compressed and the whole process is repeated as we return again to the power stroke.