How Does a Car Engine Work?
FixMyCar explains how combustion engines work in cars.
Book a car repair nowEllie Dyer-Brown
Monday April 29 2024
8 min read
Most of us aren’t engineers, so trying to understand how a car engine works can be daunting - especially because there’s often lots of jargon involved. If you want to learn more about the internal combustion process without swallowing a car-related dictionary, this guide is for you. It covers all the key information in simple terms.
How does an internal combustion engine work?
Combustion engines are based on the idea that if you take a small amount of high-energy-density fuel (like petrol) in an enclosed space and ignite it, it will release a massive amount of energy in the form of expanding gas. You can use this gas for many purposes, including powering a car.
To do this, you must make a cycle that sets off hundreds of these explosions every minute and find a way to harvest the energy.
Engine parts
It’s easier to understand how an engine works when you know a little more about the parts it contains. Here are the main ones you need to be aware of. We’re focusing on petrol engines here - if you want to know how diesel engines differ, we’ll cover that later in this guide.
Engine block
The engine block - also known as the cylinder block - holds all the other parts of the engine together. It is usually made of cast iron or aluminium and divided into three sections:
Engine block
Cylinder head
Crankcase
Cylinders
Fuel is combusted in cylinders to power your car. Each cylinder contains a piston propelled by the energy of the combustion process, as well as inlet and exhaust valves.
Lawnmowers typically have one cylinder, whereas cars have multiple cylinders - commonly four, six or eight. The more a car has, the more power it can produce.
Find out why your car might be losing power.
The cylinders are organised in one of three ways: flat (also known as horizontally opposed or boxer), inline or V. Different configurations have different advantages, but most cars designed for everyday use have an inline four configuration (pictured below).
Spark plugs
Spark plugs are found in petrol engines. They supply the spark needed to ignite the air-fuel mixture. You can find out more about how they work in this guide.
Valves
The exhaust and intake valves must open at precisely the correct time to let air and fuel in and out. All valves are closed during compression to ensure the chamber is sealed for combustion.
Pistons
Pistons are cylindrical metal parts that move up and down inside the cylinders.
Piston rings
Piston rings create a sliding seal between the cylinder's inner edge and the piston's outer edge. Their purpose is to:
Prevent oil from the sump from leaking into the combustion area.
Prevent the air/fuel mixture from leaking into the sump during combustion and compression.
Connecting rod
This rod connects the piston to the crankshaft. It can rotate at each end to account for the movement of the other parts.
Crankshaft
The crankshaft changes the up-and-down motion of the pistons into a circular motion, just like the crank handle on a jack in the box. The engine cylinders are aligned along the crankshaft so that as one piston goes down, another goes up. This configuration balances the forces and maintains momentum.
The crankshaft is also connected to a flywheel at one end that keeps it rotating even if there is a slight decrease in power. When the engine is going, this momentum pulls some pistons down to draw fuel in while others go up to compress fuel that is already in the system.
At the other end of the crankshaft is a cog connected via a belt to the other engine parts - this is used to start the engine. Before starter motors were invented, you had to crank the crankshaft manually to get it going.
Sump
The crankshaft is surrounded by a sump, which contains oil.
Camshaft
The camshaft is a metal rod with lobes positioned along it connected to the crankshaft by the timing belt or chain. Its job is to open and close the inlet and exhaust valves at precisely the right time.
Timing belt or chain
Cars have a timing belt (also known as a cambelt) or a timing chain. Both parts perform the same job, controlling the camshaft's rotation to ensure the engine’s valves open and close at the right time.
The four-stroke engine cycle
Most cars use a four-stroke engine cycle, also called the Otto cycle, after Nikolaus Otto invented it in 1867. The four strokes are:
Intake - the air-fuel mixture is drawn in.
Compression - the air-fuel mixture is compressed.
Power - combustion forces the piston down.
Exhaust - the piston pushes out exhaust gas.
In many modern cars, fuel is injected straight into the cylinders near the top of the compression stroke, while other engines pre-mix the air and fuel during the intake stroke. In both cases, the spark plugs ignite the air and fuel mixture just before the piston reaches the top of its travel (top dead centre).
After this, hot burning gases expand and force the piston down during the combustion stroke, which moves the car wheels. As the stroke reaches the bottom dead centre, exhaust valves open so that combustion gases can leave the engine as the pistons rise again. When the exhaust gas is gone, the exhaust valves shut at the top dead centre, and the process begins again.
In cars with multiple cylinders, the cylinder cycles are offset and evenly spaced to prevent combustion strokes from happening at the same time, keeping the engine as balanced as possible.
Petrol vs diesel engines
Diesel and petrol engines are very similar, but they have a few differences. In petrol engines, fuel is mixed with air first, and the mixture is compressed by pistons and ignited by spark plugs. But in diesel engines, the air is compressed before it’s mixed with the fuel, and since the air gets hotter when it’s compressed, it ignites the fuel.
Petrol cars use spark plugs to ignite the air-fuel mixture, whereas diesel cars use glow plugs (pictured below) to help them get going in cold conditions. Instead of producing a spark, glow plugs heat up at one end to aid the combustion process and produce less emissions.
Learn more about the differences between spark plugs and glow plugs.
How can you produce more engine power?
There are many ways to make car engines more powerful.
Increase the compression ratio
Higher compression ratios produce more power to an extent. However, the more the air-fuel mixture is compressed, the more likely it is to burst into flames spontaneously. You can use higher-octane fuel to reduce the risk of this happening. That’s why high-performance sports cars often need higher-octane petrol: they use a higher compression ratio than a regular car.
Fit more air/fuel into the cylinders
Cramming more air and fuel into the same size cylinder allows it to produce more power - this is what turbochargers and superchargers are for. They pressurise the incoming air to fit more into the cylinder, leaving space for more fuel.
This guide explains how a turbocharger works.
Increase displacement
Engine displacement measures the total volume of an engine’s cylinders. The more displacement, the more power you have - you can achieve this by adding more cylinders or making them bigger.
Cool incoming air
When air is compressed, its temperature increases, which is bad news because the hotter the air, the less it expands during combustion. That’s why many turbocharged and supercharged cars have a radiator that the compressed air passes through to cool it down, known as an intercooler.
Allow air to enter more easily
Air resistance makes it harder for the piston to move down in the intake stroke. You can reduce the impact of this resistance by adding another exhaust valve to each cylinder.
Allow exhaust to exit more easily
Similarly, if air resistance makes it more difficult for exhaust gases to leave the system, the engine’s power is reduced. You can counter this effect by adding another exhaust valve to each cylinder.
Make everything lighter
Lightweight parts increase engine performance because they require less energy to move.
Looking for affordable car repairs and maintenance? Whatever you need, FixMyCar can help you find the right garage at the right price.
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