How an engine makes power

An engine turns fuel into movement. Inside it, metal plugs called pistons slide up and down in tubes called cylinders. Each time a little fuel and air burn above a piston, the pressure shoves it down.

Each piston is joined by a connecting rod to a crankshaft, a shaft with offset arms, like the pedals on a bike. As the pistons push down in turn, the crankshaft spins. That spin goes through the gearbox to the wheels.

Most car engines have three, four or six cylinders. When a car is sold as a “2.0-litre”, that number is the space all the pistons sweep through between the top and bottom of their travel. A bigger engine can take in more air and fuel each turn, which is one way to make more power.

Every cylinder repeats the same four steps, called strokes. A stroke is one trip of the piston, either up or down. Mechanics remember them as suck, squeeze, bang, blow.

1. Intake (suck). The intake valve opens and the piston moves down, drawing in air and fuel.
2. Compression (squeeze). Both valves close and the piston moves up, squeezing the mixture into a small space at the top of the cylinder.
3. Power (bang). Near the top, the spark plug fires. The mixture burns in a fraction of a second, the gas inside gets hotter than 2,000°C (3,632°F), and its pressure can pass 40 bar (580 psi). That pressure drives the piston down. This is the only stroke that makes power.
4. Exhaust (blow). The exhaust valve opens and the piston moves back up, pushing the burnt gas out into the exhaust.

Then it starts again with intake. Each stroke is half a turn of the crankshaft, so one full cycle takes two turns. At 3,000 rpm, each cylinder fires 25 times a second.

With four cylinders taking turns, one of them is on its power stroke every half turn, which keeps the crankshaft spinning smoothly. A heavy wheel on the end of the crankshaft, the flywheel, carries it through the gaps between pushes.

The valves open and close at exactly the right moments because a camshaft pushes them. A timing belt or chain drives the camshaft from the crankshaft at half its speed, so each valve opens once every two turns.

Petrol engines light the mixture with a spark. Diesel engines have no spark plugs: they squeeze air so hard it gets hot enough to light the fuel as it is sprayed in.

When an engine loses power or runs rough, it usually comes down to a cylinder not doing its full share.

• Misfire. A cylinder fails to burn its mixture, often because of a worn spark plug, a failing ignition coil or a blocked injector. The engine shakes, feels down on power and the engine warning light comes on. If that light flashes, ease off and get it checked soon: unburnt fuel reaching the exhaust can overheat and ruin the catalytic converter.
• Knock. Part of the mixture explodes on its own instead of burning smoothly, making a metallic rattle under load, sometimes called pinking. Modern engines hear it with a knock sensor and fire the spark later to stop it, which costs power. Fuel with too low an octane rating is a common cause.
• Lost compression. Worn piston rings or a failed head gasket let the mixture leak while it is squeezed. Blue smoke means the engine is burning oil past the rings. White smoke that smells sweet, plus coolant going missing, points to the head gasket.

An engine’s power depends on how much air and fuel it can burn each second. Adding fuel is easy; getting more air in is the hard part. So nearly every power mod does one of three things: gets more air in, burns it better, or lets the exhaust gas out more easily.

• Turbochargers and superchargers force extra air into the cylinders, so the engine can burn more fuel each stroke.
• Remaps change the engine computer’s settings: when the spark fires, how much fuel goes in and, on turbo engines, how much boost the turbo makes.
• Intakes and exhausts help the engine breathe. On a standard engine the gains are usually small.

There is always a trade-off. More power means more pressure and heat in every cylinder, which puts more strain on the pistons, rods and head gasket and makes knock more likely. That is why good tuners leave a safety margin rather than chase the biggest number.