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To complete the firing cycle the piston must move along the cylinder four times. These movements called strokes.

The suction (drawing in, intake) stroke.

In this stroke the piston is moving down the cylinder. The inlet valve is open. A mixture of air and petrol is drawn into the cylinder above the piston.

The compression (pressing together) stroke.

In this stroke the piston is moving up the cylinder. Both valves are closed. The piston moves up as far as it can. It pushes the mixture of air and petrol in front of it. It compresses it into the space above the cylinder.

The explosion (firing, power) stroke.

In this stroke, the spark fires the mixture and it explodes. The energy released by the burning mixture forces the piston down the cylinder.

The exhaust (emptying out) stroke.

In this stroke the piston is moving up the cylinder. The exhaust valve is open. Smoke and gases remain after the explosion. The piston pushes them in front of it. They are pushed through the exhaust valve opening. They leave the cylinder through the pipe. Now the cycle begins again.



The carburetor has two main parts. There is a container (the chamber) which controls the flow of petrol. The pump forces the petrol through a pipe into the chamber. If the chamber is full, the float rises. This causes the needle valve to close. Petrol cannot enter. As the chamber empties, the float sinks. The valve opens. More petrol enters the chamber. There is also a choke tube. This tube is open at the top so that air can be sucked in. Pipes lead from the bottom of the tube to the inlet valves of the cylinders. The air and petrol mixture is sucked into the cylinder. This flow of mixture is controlled by a throttle valve. This is a round piece of metal like a large coin. It is moved by the accelerator pedal.

A small pipe leads from the chamber to the choke tube. The petrol flows through this. There are very small holes (jets) at the end of the pipe. As the induction stroke sucks air down the tube, it also sucks petrol through these jets. The petrol is broken up into thousands of very small drops to form a vapor. It mixes with the air. The mixture is sucked into the cylinders. When the accelerator pedal is pushed down, the throttle is opened. More air is sucked into the carburetor. More petrol and air mixture goes to the cylinders. The explosions in the cylinders are stronger. The car goes more quickly. When the pedal is let out, the throttle closes. There is less mixture. The explosions are weaker. The car goes more slowly.

The carburetor. Gasoline in its raw liquid form will not burn satisfactory to operate an internal-combustion engine. It must first be broken up into small drops, or atomized. After being atomized, the gasoline is vaporized and mixed with air in proper proportions for combustion. While these proportions vary slightly for different makes of engines, one part of gasoline to fifteen parts of air by mass is the average mixture used. The carburetor, therefore, is a device which automatically vaporizes and mixes the gasoline and air in the proper proportions necessary for starting, idling, acceleration, and power at various speeds.



The piston goes up and down but the car wheels go round. So something must change the up-and-down movement. The connecting rod is connected to the piston. The connecting rod can swing from side to side. The big end is connected to a shaft (the crankshaft). The crankshaft can move round inside the big end. As the piston goes up and down, the connecting rod causes the crankshaft to go round.

When you ride a bicycle, your legs go up and down. The pedals and the chain wheel go round. The movement of the connecting rod is like your leg movement. So the connecting rod causes the crankshaft to go round. This circular movement goes through the gears to the car wheels




As the crankshaft turns, it turns another shaft. This is the camshaft. There is a wheel with teeth at one end of the crankshaft. This wheel is connected by a chain to a bigger wheel. This bigger wheel is at the end of the camshaft. So when the crankshaft turns, the camshaft turns too. The camshaft has cams for each cylinder. They open and close the inlet and exhaust valves on each cylinder.

The cams turn with the shaft. As the high part of the cam comes up, it pushes up a rod. This rod pushes up one of a lever (the rocker arm) the other end of the lever pushes down the valve and opens it.

On each valve there is a strong spring. As the lower part of the cam comes round, the spring forces the rocker arm up. This closes the valve. The turning of the shaft is set in a certain way. It makes the inlet and exhaust valves open and shut at the right time.

There is a gear wheel in the middle of the chamber of the camshaft. It does two things. It turns the distributor. It works the oil pump.

The camshaft turns the distributor. The distributor distributes electricity to the sparking plug. The sparking plug fires the petrol and air mixture in the cylinder, at the right time in the firing cycle. So the camshaft controls the firing cycle.


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