The dictionary defines an engine as a machine with moving parts that converts power into motion. That being so, when we consider how a car engine works we can ignore many of the extra parts (the water pump, the alternator, the starter motor and so on) that many people would also consider to be a part of the engine.
They are there in the sense that they help give the car’s engine life, but they are not directly involved in the business of making power.
How does a car engine make power?
A sequence of highly controlled explosions pushes down pistons (they look like upturned mugs) that are attached to metal rods, called connecting rods. These rods are attached to a much larger and extremely strong piece of metal at the bottom of the engine that lies at right angles to them. It’s called the crankshaft.
The up and down motion of the pistons and rods is converted into a rotary motion by the spinning crankshaft. All sorts of things are connected to the crankshaft, including the gearbox and transmission.
What causes the explosions?
In a petrol-powered engine they’re caused by spark plugs (one per piston, but occasionally two). When an electrical charge is passed through them, they generate a spark that ignites a mixture of petrol and air.
This all takes place in the combustion chamber, a small space between the top of the piston and the cylinder. The cylinder is what the piston moves up and down in. Engines are often known by the number of cylinders they have. A four-cylinder engine, with the cylinders arranged in a line, is the most common.
The hot gases produced by the spark plug igniting the fuel-air mixture rapidly expand inside the combustion chamber, pushing the piston down the cylinder.
In a diesel-powered engine there are no spark plugs. Instead, the explosion is caused by the piston compressing the air in the combustion chamber to such a degree that it gets very hot. At that point, diesel is squirted into it and spontaneously ignites, causing an explosion which, again, forces the piston down.
How do the air and fuel get into the combustion chamber?
Set into the top of the combustion chamber (in effect, the top of the cylinder) are, depending on the type of engine, two or four valves. They’re slim discs of machined metal with a stem extending from them. When they’re in the closed position, they sit flush against the top of the combustion chamber.
In an engine with four valves per cylinder (an increasingly common arrangement), two of them are inlet valves that allow air into the combustion chamber, while the other two are exhaust valves, because they allow the hot gases to escape. The fuel is introduced by an injector set into the top of the combustion chamber that squirts it in just as the air enters.
What operates the valves?
The valve stems poke out of the cylinder to make contact with something called the camshaft. If the engine has four valves per cylinder (as most modern engines do) it will have two camshafts. They lie next to each other across the top of the cylinders.
Each camshaft operates two valves per cylinder. Along the length of the camshaft, and arranged at different points on its circumference, are small rounded protrusions called lobes. It’s the lobes that the valve stems are in contact with.
The camshafts have a small pulley at one end around which a toothed rubber belt or a chain runs. The other end of the belt or chain runs around a pulley on the end of the crankshaft at the bottom of the engine. It has a cover over it, so you can’t see it and a tensioner that keeps it taut.
As the pistons move up and down and turn the crankshaft, so the belt, or chain, also turns. This motion turns the camshafts, causing the lobes along their length to momentarily push on the valve stems, opening the inlet and exhaust valves in the combustion chamber in a pre-determined sequence.
The valves don’t all open at once but are staggered because of how the lobes are located. As each lobe turns away from the stem, a spring raises the open valve back into its closed position.
What are the pistons doing while all this is going on?
We’ve left the pistons until last but in fact they’re the heart of the whole process. A car engine works on what is called the four-stroke combustion cycle. It was a principle invented by a German engineer called Nikolaus Otto in 1876, which is why it’s also called the Otto cycle.
The cycle begins with the piston at the top of its travel. In an engine with four valves per cylinder, the camshaft lobes push the inlet valves open.
As the piston starts to move back down the cylinder, it draws air through the open inlet orifices into the space between it and the top of the cylinder.
This is the intake stroke. At the same time, a small amount of fuel is squirted into the space by the injector.
Once the piston has reached its lowest point it rises back up, compressing the air and fuel mixture in the combustion chamber as it does so.
Just as it reaches the very top of its travel (called top dead centre) and point of maximum compression, the spark plug generates a spark, detonating the fuel mixture.
This forces the piston down once more. When it reaches the bottom of its stroke again, the exhaust valves open, allowing the hot gases to escape, and the whole cycle begins again.
What’s the crankshaft doing?
It’s crucial to the smooth running of the engine. It is designed in such a way that the pistons, which are attached to it by connecting rods, don’t all go up and down at the same time, but are staggered.
This way, each piston is at a different point in the four-stroke cycle, an arrangement that keeps all the parts of the engine turning smoothly. The crankshaft is also very strong because if it wasn’t, the pistons might break away and burst out of the engine.