Lets first take a quick look at what we have learnt so far.

Volume, temperature and pressure are all related, if we change one we must also change another.

If a heat engine is to work continuously it must return to its original state.  It MUST OPERATE IN A CYCLE!

The petrol engine uses a special cycle called the OTTO cycle. 

SUCK - SQUASH - BANG - BLOW

1. Intake Stroke (SUCK): The inlet valve opens as the piston moves down.  By increasing the volume of the cylinder the pressure and temperature are kept the same because the open valve allows fuel and air to be sucked in.  The inlet valve then closes as the piston reaches the bottom of the cylinder.     2. Compression stroke (SQUASH): With both valves closed the piston moves upwards reducing the volume of the cylinder.  With nowhere to go the fuel and air in the cylinder are squashed, this increases the temperature of the gas to a point where a spark can set the fuel on fire.

3. Power stroke (BANG): You will see the spark plug flash.  The small electrical spark sets fire to the fuel which burns very quickly.  Burning the fuel causes the temperature and pressure to increase.  This increase in pressure forces the piston down which increases the volume to compensate.     4. Exhaust stroke (BLOW): Finally when the piston has reached the bottom of the cylinder and all the gas has been burned we want to get rid of the left over which is trapped in the cylinder.  So the exhaust valve opens as the piston moves upwards forcing out the waste gas.

JUST A QUICK SUMMARY

• The piston moves downwards sucking in fuel through the the open valve. 
• The valve then closes so the fuel can be squashed to increase its temperature and pressure.
• A spark sets fire to the fuel, the increase in temperature increases the pressure.  The increase in pressure causes an increase in volume by pushing the piston downwards.
• The exhaust valve opens so as the piston moves upwards the waste gas can be forced out.

THE CYCLE THEN STARTS AGAIN!

	VALVE: There are two valves, the one on the left opens as the piston is moving down on the inlet stroke to draw fuel and air into, the cylinder.  The valve on the right opens after the fuel has been burned, so as the piston moves up the burnt (waste) gas is pushed out ready to start again.
	PISTON: The piston moves up and down within the cylinder, it first moves down to suck in the fuel and air, it then moves up to compress the gas.  The fuel is then burned increasing its temperature and pressure which increases the volume by forcing the piston down.  Finally the piston moves up to force out the waste gas.