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The 4-Stroke Engine

the most common internal combustion engine, as used in 99% of cars and motorbikes!

In "Suck; Squash; Bang; Blow!", I introduced and explained the Otto cycle, the principle on which the internal combustion engine works.

The 4-stroke reciprocating piston engine, illustrates the Otto-cycle very clearly, each of phase of the cycle completed in a separate stroke of the piston, in it's cylinder, as it 'reciprocates' up and down, attached to a crank-shaft. So, lets have a look at the thing, and see how it works

1 2 3 4
Suck Squash Bang Blow
Induction Compression Power Exhaust
Charge (fuel & air) drawn into the engine Charge put under pressure Charge ignited, and allowed to burn, releasing energy Burned charge expelled from the engine, ready for a fresh cycle to begin
First 'Stroke' of the Piston (coming down) Second 'Stroke' of the Piston (going back up) Third 'Stroke' of the Piston (coming down, again) forth 'Stroke' of the Piston (going up, again, to do it all over)

And explaining what we have in those pictures, a colour coded and labelled version. From the top, and going round clockwise:-

A) Spark Plug
See article "the Spark Ignition system", The spark plug sets fire to the charge in the cylinder at the beginning of the power stroke.
B) Cylinder Head
The cylinder head is the 'lid' on the engine, and holds the charge and everything in the cylinder. It usually also supports other bits and pieces of the engine like the camshafts and valves, and has holes in it to let the gasses in and out.
C) Camshaft (operating Exhaust Valve)
The camshaft is an 'eccentric' shaft; as it goes round, a lump on the shaft pushes open a valve against a spring. In this case, the exhaust valve.
D) Exhaust Valve
The valve generally looks like a penny on the end of a pencil. The head of the valve sits in a hole, called a 'seat' in the port. The shaft or stem of the valve sits in a tube called a guide and is acted on by the cam-shaft to push the head out of the hole and let gasses in or out of the engine. In this case, exhaust gasses. And is normally closed again by a spring sitting at the top.
E) Exhaust Port
A hole in the cylinder head controlled by the exhaust valve, to allow the exhaust gasses out of the engine
F) Combustion Chamber / Cylinder
The combustion chamber is usually the space in the cylinder head, above the cylinder when the piston is at the top of its stroke. The cylinder is the space beneath that, which the piston goes up and down in. They are both different regions of the big hole in the engine block, that's sealed by the cylinder head and the valves, where the 'action' happens!
G) Connecting Rod / Con-Rod
The con rod, is a rod, with a hinge at either end. It connects the piston to the crankshaft, and transmits the forces from the crankshaft to the piston or the piston to the crank, and in conjunction with that, allows the reciprocating, up and down motion of the piston to be converted to rotary motion on a shaft.
H) Crank-Case / engine 'block'
The 'block' is the casing of the engine, and supports all the other bits that go round, do work or move about. The main chamber, where the crank-shaft sits is called the crank-case, the chamber that the piston goes up and down in is called the barrel. Car engines generally have the crank-case and barrel cast in one piece of metal, which is then referred to as the 'block'. Motorbike engines often have the barrels cast separately from the crank cases. But designs vary, and some engines have the barrel cast integrally with the cylinder head. But, the important thing is that there is some structure, holding everything else together, and providing the enclosed space inside which combustion takes place, hence the 'internal' combustion engine.
J) Crank Shaft
The crank-shaft is either a stepped shaft or a wheel with a handle on it. As the 'main' shaft goes round, the step, handle or 'crank pin, goes around too, but in relation to the axis of the cylinder, it is going up and down, so in conjunction with the con-rod allows reciprocating motion to be converted to rotary motion, or the other way around. And in the engine, it does both. During the 'power' stroke, it takes force from the piston and turns it into rotary motion; but during the induction, compression and exhaust strokes, the crank shaft is pushing the piston up and down the cylinder.
K) Piston
The piston is a plug to seal the bottom of the cylinder; but its a clever plug that can move up and down, allowing the volume in the cylinder to change. Connected to the crankshaft via the con-rod, it converts the 'pressure' made by combustion and turns it into force to push the crank-shaft around. It usually has sealing or 'piston rings' around it to let it seal against the cylinder without being too tight a fit.
L) Inlet Valve
As the exhaust valve, the valve generally looks like a penny on the end of a pencil. The head of the valve sits in a hole, called a 'seat' in the port. The shaft or stem of the valve sits in a tube called a guide and is acted on by the cam-shaft to push the head out of the hole and let gasses in or out of the engine. In this case, the fresh charge in. And is normally closed again by a spring sitting at the top.
M) Inlet Port
A hole in the cylinder head, controlled by the inlet valve to allow the 'charge' into the engine.
N) Camshaft (operating Inlet Valve)
This engine has two cam-shafts, and would be known as a DOHC or Double Over-Head Camshaft engine. As the one operating the exhaust valve, it's an 'eccentric' shaft; as it goes round, a lump on the shaft pushes open a valve against a spring. In this case, the inlet valve.

And, the neat animation to the right there is showing it all 'in action'.

The important elements of the 4-stroke engine is it is an 'internal combustion' engine. The charge is burned inside it where it's energy is converted to motive power. Unlike an 'external combustion' engine, like a steam engine, where the fuel is burned in a hearth, which heats water in a boiler, to turn the energy into pressure that's then piped somewhere else to turn it into motive power.

It's a 'reciprocating piston' engine. That is it has a piston, that goes up and down in a cylinder, turning a crank-shaft.

But, the most important feature is that there are four 'strokes' of that piston in one 'cycle' of combustion, the Otto cycle.

If you watched carefully, you'll have noticed that the crank-shaft goes round half a revolution, as the piston falls on the induction stroke; goes around another half revolution as the piston comes up on the compression stroke, and likewise, does another half revolution when the piston's falling under power, and another half revolution when the piston is going up on the exhaust.

4 strokes of the piston

1 'cycle' of combustion

(2 revolutions of the crank-shaft)

I don't want to over complicate things or go into too much unnecessary detail. As shown the engine has a conventional crankshaft and con-rod arrangement, to turn the reciprocating motion into rotary motion, and it has two over head 'poppet' valves, operated by 'Double Over-Head Cam-Shafts'.

This is a pretty conventional, and typical arrangement of the 4-stroke engine. There are however numerous variations on it, by way of the type and arrangement of the valves, and even the crank-shaft.

One particular deviation I know of, uses a clever arrangement of rockers and rods so that the four strokes of the piston are actually different lengths, and the crank-shaft turns one revolution in four strokes. It is still a 4-stroke engine; but not a conventional one, and I mention it only because, it is important to stress that the 4-stroke description is referring to the number of strokes that the piston makes, in completing one combustion cycle.

However, in most 4-stroke engines, the crank is connected as shown in the illustration and goes round twice for each combustion cycle, and the crankshaft goes round twice.

This is the 'basics' of how the engine works; and doesn't explain anything about 'carburetion', or preparing the charge before its fed into the engine, the ignition system, that sets fire to the charge in the engine, or any of the detail of why the valves are arranged the way they are, or what the cam-shaft is doing to open them, or the 'tuning' of the engine to get best power or economy from it.

That 'hopefully' will be covered in other articles in this section, which I will add links to as I create them or get around to it!

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