A sealed container, usually plastic and shaped to fit around the rear axle and the exhaust, holding about 40–70 litres. The filler neck runs up to the fuel flap; inside, baffles stop the fuel from sloshing and a float sender reports the level to the dashboard gauge. The cap seals the tank so that fuel vapour cannot escape.
A small electric motor driving a pump, submerged in the tank so that the fuel around it keeps it cool. It pushes fuel along the line at a steady pressure — a few bar for port injection, with a second, mechanical high-pressure pump added for direct injection and diesel. You can hear it hum for a second when you switch the ignition on, as it builds pressure.
A cartridge of fine paper or mesh that catches rust, dirt and water before they can clog the injectors, whose openings are finer than a hair. On many petrol cars it sits inside the tank and is never serviced; on a diesel it is a regular service item that also separates water, which is drained off.
Fuel rail and pressure regulator
A metal pipe that feeds every injector from one shared line, like a manifold for fuel. The pressure in it is held steady — by a regulator, or by the engine computer controlling the pump — so that each injector sprays the same amount in the same time. The injectors it feeds are described in the Engine section.
Petrol evaporates, and the vapour in the tank must not be released into the air. A canister of activated charcoal soaks it up; when the engine is running, a purge valve draws the stored vapour into the intake to be burned. A faulty purge valve or a loose fuel cap are classic reasons for a check-engine light.
Exhaust gas recirculation: a valve that sends a small part of the exhaust back into the intake. These gases no longer burn, so they dilute the mixture and lower the peak temperature of combustion, which cuts the formation of nitrogen oxides. Diesels use it most, often with a small cooler on the way, and the valve can get clogged with soot over time.
Cast iron or stainless-steel pipes that gather the gases from each cylinder into one. The lengths matter: a well-shaped manifold helps the cylinders empty cleanly. It glows hot, so it is bolted close to the engine, which also helps warm the catalyst quickly. On a turbo engine it leads straight into the turbine.
A sensor in the exhaust that measures how much oxygen is left after the burn, which tells the computer whether the mixture was too rich or too lean. The computer then trims the fuel by tiny amounts, continuously. One sensor sits before the catalyst and usually another after it, to check that the catalyst is doing its job.
A ceramic or metal honeycomb, its channels coated with platinum-group metals, in a steel housing. It starts working at about 250–300 °C and runs best at 400–800 °C, turning carbon monoxide, unburnt fuel and nitrogen oxides into carbon dioxide, water and nitrogen. Its metals are valuable, which is why catalytic converters get stolen.
Particulate filter (DPF / GPF)
A ceramic honeycomb whose channels are plugged alternately, so the gas is forced through the walls and the soot stays behind. When the filter fills up, the car burns the soot off at 600 °C or more — regeneration — and you may notice a higher idle or a running fan afterwards. Short trips can stop regeneration from finishing and clog the filter.
Selective catalytic reduction. A small tank carries AdBlue, a solution of urea in water, and a metered amount is sprayed into the hot exhaust, where it turns into ammonia. On the SCR catalyst the ammonia converts nitrogen oxides into nitrogen and water. The tank needs refilling every ten thousand kilometres or so, and NOx sensors check the result.
Inside are chambers, perforated pipes and sound-absorbing wool that make the pressure pulses of the exhaust cancel out and fade, turning a roar into a hum. Resonators remove particular droning frequencies. It is a trade-off: a freer-flowing exhaust is louder, a more restrictive one costs a little power. Condensation collects inside, which is why mufflers tend to rust from within.