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Know your car , Maintenance and tyres · Actualizado el 2026-10-18

How a car engine works, in plain language

You do not need to be a mechanic to drive well, but a basic picture of what happens under the bonnet makes advice about servicing, warning lights and used cars much easier to follow. This guide explains the common piston engine in everyday terms.

The basic idea

A petrol or diesel engine turns the energy in fuel into motion. It does this by burning a small amount of fuel mixed with air inside a closed space, thousands of times a minute. The burning gas expands and pushes a piston. The pistons turn a crankshaft, the crankshaft passes the turning force to the gearbox, and the gearbox sends it to the wheels.

The four steps

Most car engines work in a repeating four-stroke cycle in each cylinder.

1. Intake: the piston moves down and draws in air (and, in many petrol engines, fuel).
2. Compression: the piston moves up and squeezes the mixture into a small space.
3. Power: the mixture burns, expands and pushes the piston down. This is the step that actually produces force.
4. Exhaust: the piston moves up again and pushes the burnt gases out.

With several cylinders firing in turn, the engine runs smoothly. That is why you hear numbers like three, four or six cylinders when engines are described.

Petrol and diesel: the main difference

In a petrol engine a spark plug ignites the mixture. In a diesel engine the air is compressed so strongly that it becomes hot enough to light the fuel when it is sprayed in, so no spark is needed. Diesels are usually built stronger and tend to be efficient on long trips, while petrol engines are often lighter and quieter. For a buying angle, see diesel or petrol.

The parts that support the engine

• Valves and camshaft: open and close to let air in and exhaust out at exactly the right time. A belt or chain links them to the crankshaft; see timing belt vs chain.
• Oil system: a film of oil keeps moving metal parts from rubbing directly on each other, carries away heat and holds dirt until the filter catches it. More in engine oil explained.
• Cooling system: coolant circulates through the engine and the radiator to keep the temperature in a safe range. Details in coolant and fluids.
• Fuel system: a pump and injectors deliver the right amount of fuel at the right moment.
• Air intake and filter: clean air is needed for burning. A clogged filter makes the engine work harder.
• Exhaust system: carries gases away, quietens the noise and cleans the emissions with a catalytic converter and, on some cars, a particulate filter.
• Electronics: the engine computer reads sensors and adjusts fuel and timing continuously.

Why engines wear

Most engine wear happens in particular situations: starting cold, when oil has not yet reached everything; overheating; dirty or old oil; and long periods of very short trips, when the engine never gets properly warm and moisture builds up. Regular oil changes, a healthy cooling system and a warm-up period of gentle driving go a long way toward a long life.

Signs something is wrong

• Knocking, ticking or rattling that is new or grows louder.
• Blue smoke from the exhaust, which can point to burning oil; white smoke that lingers can mean coolant is getting in; black smoke often means too much fuel.
• Oil or coolant level dropping between services.
• Rough idle, shaking or hesitation when you accelerate.
• A temperature gauge that climbs, or warning lights; see dashboard warning lights.

These signs do not always mean an expensive repair, but they are reasons for a workshop visit rather than for waiting.

Engine size and power

Engine size is the total volume of its cylinders, usually given in litres or cubic centimetres. A bigger engine can make more power, but modern small engines with a turbo can match older, larger ones. Power says how quickly the engine can do work, while torque is the pulling force you feel when accelerating. See turbo explained for how a small engine can feel big.

When buying a used car

The engine is the part where a poor history is most costly. Ask for service records, listen to the car starting from cold, look for leaks and check the oil and coolant levels. Our inspection guide and the buying checklist walk you through it.

What to remember

• An engine burns fuel and air to push pistons, which turn the wheels through the gearbox.
• Oil, coolant and clean air keep it healthy.
• Cold starts, overheating and neglected oil are the main causes of wear.
• New noises, smoke or falling fluid levels deserve a workshop visit.

Beginner Diesel Engine Oil Petrol Service history