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Four-stroke cycle

Home Motoring About my car Car components Four-stroke cycle

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The petrol engine in most modern cars is called an internal combustion engine and it operates on a four-stroke cycle: induction, compression, power and exhaust.

• The air/fuel mixture is ignited in combustion chambers in the engine cylinders.
• Engines can have one cylinder but most have multiple cylinders: two, three, four, five, six, eight, ten and sometimes twelve cylinders are common in vehicles.
• When the key is turned to the start position, the starter motor rotates the engine and the pistons begin to move up and down the cylinders.

Induction

• The piston moves down the cylinder.
• The inlet valve is open and the air/fuel mixture is drawn into the cylinder through the inlet port.
• The air is drawn through the air filter and is mixed with the fuel as it passes through the carburettor or in the case of a fuel-injected vehicle the mixing takes place in the inlet manifold.
• When the piston reaches the bottom of the cylinder (the end of its stroke) the inlet valve closes.

Compression

• The piston now begins its journey back up the cylinder.
• Both the inlet and exhaust valves are closed.
• As the piston moves upwards the air/fuel mixture is compressed.
• The upward movement of the piston causes the volume of mixture to rapidly decrease and the temperature of the compressed mixture increases.
• The mixture is now highly volatile.
• A spark from the ignition system, provided by the spark plug, ignites the air/fuel mixture as the piston reaches the top of the cylinder (top dead centre).
• The mixture burns rapidly.

Power

• The mixture expands as it burns and exerts pressure on the top of the piston.
• The piston is forced down to the bottom of the cylinder.

Exhaust

• When the piston reaches the bottom the exhaust valve opens.
• The piston now moves upwards again, expelling the spent gases out past the exhaust valve, into the exhaust manifold and out through the exhaust.
• When the piston reaches the top of the cylinder, the exhaust valve closes and the cycle begins again.
• All this happens very quickly in multiple cylinders and provides the power to drive your car.