What Do You Mean by Four Stroke?

What is a 4-stroke diesel engine?

As the name suggests, these engines work on reciprocating
actions of pistons. One stroke means one linear motion of the
piston in one direction. When the piston moves in the opposite
direction, that is counted as another stroke.

For a 4-stroke engine, the piston has to move up, then down,
then up again, and then down again to complete one cycle of the
engine. In that 4 stroke of the piston, the crankshaft will have
turned 2 revolutions.

The 4 strokes of the piston is to complete 4 stages of the
combustion cycle. As you might have known, in order for a fire
(or explosion) to occur to power the engine, we need a fuel, a
heat source, and oxygen coming together at the same time.

At the start of the cycle, air must be introduced into the
combustion space inside the cylinder. In the 4-stroke (also
called 4-cycle) engine, this is the air intake stroke. The
piston moving down will suck fresh air into the cylinder through
the air inlet valve(s) that is opened.

The next stroke is the compression stroke. With all the valves
at the cylinder head closed, the piston moves up again. The air
becomes compressed to a very high degree. So high that the
temperature of the air becomes sufficiently hot to ignite a
finely sprayed fuel mist on contact.

That is precisely what happens. Before the piston reaches the
top-dead center (the maximum height the piston can reach before
coming down again), the fuel is injected as a very fine mist
into combustion space at the piston top. In order for this fuel
to be injected into the highly compressed air inside the cylinder
the fuel itself must be at a higher pressure. This is achieved by
a fuel pump.

There is a reason for injecting the fuel slightly before the
piston reaches top dead center. The fuel takes some time to reach
the temperature that it can burn. So the timing of the injection
of the fuel is adjusted so that by the time the piston reaches
slightly beyond the top dead center, the fuel has acquired the
temperature high enough for ignition.

The fuel is supposed to burn so fast as to explode. So there is
no point in injecting it more than it can burn. If the fuel is
not able to catch fire, it will remain in the piston as unburnt

So the timing of the fuel injection will stop slightly after the
piston has reached top dead center.

The explosion inside the piston is the driving force for the
engine. It will create the momentum to turn the engine for the
next 2 cycles.

The next cycle is the exhaust stroke. In this stroke, all the
products of combustion is driven off from the piston top. The
exhaust valve(s) will have opened to allow the combustion gases
to escape. In an ideal engine, these include carbon dioxide gas,
water vapor and excess air.

However, in actual practice, the combustion may not be so perfect.
Carbon particles, unburnt fuel particles, lubricating oil
particles, sulphur dioxide, carbon monoxide, and ash may also be
discharged. As the piston will not be able to reach the the
cylinder head perfectly, not all the exhaust gas can be driven
off completely. A small quantity of these exhaust products will
remain inside the cylinder even as the next cycle starts.

Once the exhaust gases have been driven off, the next stroke
starts. This is the air intake stroke that we started off with.
The cycles repeat itself as the engine runs continuously.

Obviously the more the exhaust gas can be driven off, and the
more oxygen content the cylinder can contain, the more efficient
the combustion of the fuel can be achieved.

This can be achieved by supercharging and cooling the air intake.

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