How the heart of the car works.

The engine is the heart of your car. It is a complex machine built to convert heat from burning gas into the force that turns the road wheels.

3y ago

The engine is the heart of your car. It is a complex machine built to convert heat from burning gas into the force that turns the road wheels.

The chain of reactions which achieve that objective is set in motion by a spark, which ignites a mixture of petrol vapour and compressed air inside a momentarily sealed cylinder and causes it to burn rapidly. That is why the machine is called an internal combustion engine. As the mixture burns it expands, providing power to drive the car.

To withstand its heavy workload, the engine must be a robust structure. It consists of two basic parts: the lower, heavier section is the cylinder block, a casing for the engine's main moving parts; the detachable upper cover is the cylinder head.

The cylinder head contains valve-controlled passages through which the air and fuel mixture enters the cylinders, and others through which the gases produced by their combustion are expelled.

The block houses the crankshaft, which converts the reciprocating motion of the pistons into rotary motion at the crankshaft. Often the block also houses the camshaft, which operates mechanisms that open and close the valves in the cylinder head. Sometimes the camshaft is in the head or mounted above it.

The simplest and most common type of engine comprises four vertical cylinders close together in a row. This is known as an in-line engine. Cars with capacities exceeding 2,000cc often have six cylinders in line.

The more compact V-engine is fitted in some cars, especially vehicles with eight or 12 cylinders, and also some with six cylinders. Here the cylinders are arranged opposite each other at an angle of up to 90 degrees.

Some engines have horizontally opposed cylinders. They are an extension of the V-engine, the angle having been widened to 180 degrees. The advantages lie in saving height and also in certain aspects of balance.

The cylinders in which the pistons operate are cast into the block, as are mountings for ancillary equipment such as a filter for the oil which lubricates the engine, and a pump for the fuel. An oil reservoir, called the sump, is bolted underneath the crankcase.

Both block and head are usually made of cast iron. But sometimes aluminium is chosen for the head, because it is lighter and dissipates heat more efficiently.

The conversion of fuel energy into power in an engine starts when petrol is mixed with air in a device called a carburettor, to form a highly combustible mixture.

Gas flow



On the induction stroke the piston is descending, the inlet valve is fully open and the exhaust valve closed.

Gas compression

As the piston rises on its compression stroke the exhaust valve is still closed and the inlet valve is closing.

Spark ignition

The power stroke drives the piston downwards as the ignited gases expand. Both the inlet and exhaust valves are closed.

The hot gasses in the cylinder escape through the open exhaust valve as the piston rises again for the exhaust stroke.

The mixture is drawn into the cylinders through valves, compressed to about an eighth or ninth of its original volume by a piston, and then ignited by a sparkplug.

Rapid expansion of the burning gas, the combustion, drives the piston down the cylinder.

The downward thrust is changed by the connecting rod to rotary movement of the crankshaft in much the same way as a cyclist pressing his foot on the pedal turns the chain wheel.

The downward stroke of the piston is known as the power stroke in a four-stroke cycle it occurs only once in every four strokes of the piston's up-and-down movement.

The cycle starts with the induction stroke. With the exhaust valve closed, a downward movement of the piston sucks fuel mixture from the carburettor into the cylinder. The mixture enters through the inlet valve, which has been opened by the camshaft turning.

The upward movement of the piston which follows is the compression stroke. The exhaust valve remains closed and the inlet valve also closes, so the mixture in the cylinder is compressed by the rising piston into a small space known as the combustion chamber usually in the cylinder head or in the top of the piston.

A spark from the sparkplug ignites the mixture and causes it to expand rapidly, driving the piston down in the power stroke.

As the piston rises once more, the inlet valve remains closed but the exhaust valve opens. This movement allows the waste products of the burned mixture to escape through the exhaust system, and is called the exhaust stroke.

The camshaft continues to rotate, the exhaust valve closes and the inlet valve opens and the four-stroke cycle starts again.

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