Public Servant Lifestyle

A car engine can look like a big confusing jumble of metal, tubes and wires to the uninitiated. You might want to know what's going on in there simply out of curiosity. After all, you probably ride in a car every day.

Or maybe you are tired of going to a mechanic who tells you things that are totally meaningless and then charges £250. Or perhaps you are buying a new car, and you hear funny words like '3.0 litre V-6' and 'dual overhead cams.'' What does all of that mean?

To understand how a reciprocating internal combustion engine works, it is helpful to have a good mental image of internal combustion. A good example is an old cannon. You will have seen movies where soldiers load a cannon with gunpowder and a cannon ball and light it. It's hard to imagine that having anything to do with motoring, but that is an internal combustion engine.

Reciprocating Internal Combustion Engine

The principle behind any reciprocating internal combustion engine is: If you put a tiny amount of high-energy fuel (like petrol) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas.

And if you can create a mechanism that allows you to set off explosions like these hundreds of times per minute, and if you can then harness that energy, what you have is the core of a car engine.

Almost all cars use what is called a four-stroke combustion cycle to convert petrol into motion. The four-stroke approach is also known as the Otto cycle, in honour of Nikolaus Otto, who invented it in 1867.

The four strokes are:

The intake stroke

The compression stroke

The combustion stroke

The exhaust stroke

A device called a piston is connected to a crankshaft by a connecting rod. As the crankshaft revolves, it has the effect of 'resetting the cannon.'

The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder full of air and petrol. This is the intake stroke. Only the tiniest drop of petrol needs to be mixed into the air for this to work. Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful.

When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the petrol. The petrol charge in the cylinder explodes, driving the piston down.
Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe.
Now the engine is ready for the next cycle, so it intakes another charge of air and gas.

Rotational Motion

Notice that the motion that comes out of an internal combustion engine is rotational. In an engine the linear motion is converted into rotational motion by the crankshaft. The rotational motion is handy because it turns (rotates) the car's wheels.

Two other things that are good to note:

There are different kinds of internal combustion engines. The gas turbine engine is another form of internal combustion engine, but has the disadvantage of extremely high manufacturing costs

There is such a thing as an external combustion engine. A steam engine in old-fashioned trains and steamboats is the best example of an external combustion engine. The fuel (coal, wood, oil, whatever) in a steam engine burns outside the engine to create steam, and the steam creates motion inside the engine

It turns out internal combustion is a lot more efficient (takes less fuel per mile) than external combustion, plus an internal combustion engine is a lot smaller than an equivalent external combustion engine

Efficient (compared to an external combustion engine)

Inexpensive (compared to a gas turbine)

Easy to refuel (compared to an electric car)

These advantages beat any other existing technology for moving a car around. Here's a quick description of each one, along with terms to help you understand what all the car ads are talking about.

Cylinder - The core of the engine is the cylinder. The piston moves up and down inside the cylinder. The engine described here has one cylinder. That is typical of most lawnmowers, but most cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine the cylinders are usually arranged in one of three ways: inline, V or flat (also known as horizontally opposed or boxer).

Different configurations have different smoothness, manufacturing-costs and shape characteristics that make them more suitable in some vehicles.

Spark plug - The spark plug supplies the spark that ignites the air/fuel mixture so combustion can occur. The spark must happen at just the right moment for things to work properly.

Valves - The intake and exhaust valves open at the proper time to let in air and fuel and to let out exhaust. Note that both valves are closed during compression and combustion so that the combustion chamber is sealed.

Piston - A piston is a cylindrical piece of metal that moves up and down inside the cylinder.

Piston rings - Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder.

The rings serve two purposes:

They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion

They keep oil in the sump from leaking into the combustion area, where it would be burned and lost

Most cars that 'burn oil' and have to have a litre added every 1,000 miles are burning it because the engine is old and the rings no longer seal things properly.

Combustion chamber - The combustion chamber is the area where compression and combustion take place. As the piston moves up and down, the size of the combustion chamber changes. It has some maximum volume as well as a minimum volume. The difference between the maximum and minimum is called the displacement and is measured in litres or CCs (Cubic Centimetres, where 1,000 cubic centimetre equals a litre).

So if you have a four-cylinder engine and each cylinder displaces half a litre, then the entire engine is a '2.0 litre engine.' If each cylinder displaces half a litre and there are six cylinders arranged in a V configuration, you have a '3.0 litre V-6.'

Generally, the displacement tells you something about how much power an engine has. A cylinder that displaces half a litre can hold twice as much fuel/air mixture as a cylinder that displaces a quarter of a litre, and therefore you would expect about twice as much power from the larger cylinder (if all else is equal).

So a 2.0 litre engine is roughly half as powerful as a 4.0 litre engine. You can get more displacement either by increasing the number of cylinders or by making the combustion chambers of all the cylinders bigger (or both).

Connecting rod - The connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves and the crankshaft rotates.

Crankshaft - The crankshaft turns the piston's up and down motion into circular motion just like a crank on a jack-in-the-box does.

Sump - The sump surrounds the crankshaft. It contains some amount of oil, which collects in the bottom of the sump.

More information available in Maintenance, DIY