The conversion of
into power in an
starts when petrol is mixed with air in a device called a
, to form a highly combustible mixture.
The mixture is drawn into the
, compressed to about an eighth or ninth of its original
, and then ignited by a sparkplug.
Rapid expansion of the burning
, drives the piston down the cylinder.
The downward thrust is changed by the
to rotary movement of the
in much the same way as a cyclist pressing his foot on the pedal turns the chain wheel.
of the piston is known as the
in a four-stroke
it occurs only once in every four strokes of the piston's up-and-down movement.
The cycle starts with the
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
The upward movement of the piston which follows is the
. 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
usually in the
or in the top of the piston.
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
, and is called the
The camshaft continues to rotate, the exhaust valve closes and the inlet valve opens and the four-stroke cycle starts again.
The firing order
The sequence in which the sparkplugs ignite the mixture in each of the engine cylinders is known as the
This is controlled by the
, which directs the flow of
to each plug at the correct time during the engine's four-stroke cycle. The camshaft is designed to open and shut the valves in the required sequence.
The spark occurs just before the piston reaches top dead centre (
) on the
The cylinders of an in-line engine are usually numbered from front to rear, starting with No. 1 cylinder.
If the plugs were fired in numerical order from one end to the other, the successive power impulses from the pistons would cause the engine to run very unevenly and vibrate excessively.
In a four-cylinder engine, vibration is reduced with a firing order 1, 3, 4, 2 or 1, 2, 4, 3.
Whenever the high-tension leads are removed from the sparkplugs they must always be reconnected in the correct sequence, to maintain the proper firing order.
If in any doubt, label the leads with their cylinder numbers on pieces of sticky tape.
of the rotating
also helps to smooth out the cyclic variations and minimises vibration of the engine.
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Most people have some idea of what an engine's power is, but are hazy about
exactly what the torque figure represents. In fact, many cars that feel
powerful are showing the effects of strong torque rather than high power