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Know your car: How the engine works

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Learning how an engine works just got easier. We're bringing it back to basics, to help you understand what goes on beneath your bonnet.

Gif of a four stroke engine cylinder cycle

Most petrol engines are made up of one or more cylinders, which work together to generate drive.

The best way to understand how an engine works is to focus in on a single cylinder, as it moves through the four-stroke cycle.

It's called a four-stroke cycle because each cylinder goes through 4 distinct stages.

Stage 1 - Intake stroke

The air intake valve opens and the piston drops, sucking air into the cylinder. At the same time, the fuel injector pumps fuel into the cylinder to be mixed with the air.

Stage 2 - Compression stroke

The intake valve closes and the piston is pushed back up, compressing the fuel-air mixture. This makes it more combustible.

Stage 3 - Power stroke

At the top of the compression stroke, the spark plug ignites the compressed fuel-air mixture, forcing the piston to the bottom of the cylinder. It's this stroke that generates power.

Stage 4 - Exhaust stroke

The exhaust valve is opened and the piston rises, pushing the spent fuel/air mixture out.

The cylinders in your engine will move through these stages continuously, with each cylinder at different stages of the cycle.

The four-cylinder setup

In the four-cylinder engine model below, you can see how several cylinders can work together, each at different stages of their four-stroke cycle.

How a four cylinder engine works gif

All of the pistons are connected to the crankshaft. The downward force of the combustion stroke rotates the crankshaft, which carries power out of the pistons and eventually to the wheels.

The timing belt runs from the crankshaft to the camshaft. This rotates in time, opening and closing the various valves at the correct stage of each piston sequence.

The camshaft, cams, fuel injectors and spark plugs need to be synchronised so that in each cylinder the valves open at the correct times, and each spark plug ignites the compressed fuel/air mixture at the right moment.

If the timing of any of these moving parts is off even slightly, the engine can lose power as it combusts less efficiently, or cease working altogether.

Engine configurations

There are lots of different engine configurations you might have heard of, and they’re generally named after the number of and layout of their cylinders:

Engine configuration diagrams

Hybrid cars

Hybrids use a combination of petrol and electric motors to produce power.

This means their petrol engines can often be smaller, with the electric motor picking up the deficit in a number of ways, including:

  • Diagram of regenerative braking
    Regenerative braking
  • Diagram of hybrid car drive assist
    Electric drive assist
  • Diagram of auto start shutoff
    Automatic start / shutoff
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Regenerative braking

This is a means of more efficently using energy that might otherwise be lost while braking.

An electric motor applies resistance to the drivetrain, slowing the car, and converts that kinetic energy to turn the electric motor.

This works as a generator, and sends power back to the battery. In conventional braking systems, this energy might be lost as wasted heat in the brakes.

Electric motor drive/assist

This is when an electric motor sends additional power to help the engine, usually during acceleration or going uphill. This means a smaller, more efficient engine can be used without assistance for regular driving.

Combustion engines are least efficient when driving or manoeuvring at very low speeds. In some hybrid cars, the electric motor alone powers all low-speed driving.

Automatic start/shutoff

This is when hybrid cars automatically shut off their engines when idle, and restart when you take your foot off the brake.

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