How an engine gets rid of exhaust gases

Every time the fuel burns, it leaves a cylinder full of hot, burnt gas. Before the engine can breathe in again, that gas has to go. The exhaust system gets it out, and on the way it has two more jobs: cleaning up the harmful gases in it, and making it quiet.

The parts that take the gas away are:

• Exhaust valves. Doors in the top of each cylinder, opposite the intake valves, that open to let the burnt gas out.
• Exhaust manifold. A set of pipes bolted to the engine that collects the gas from every cylinder into one.
• Oxygen sensors. They measure what’s left in the gas, so the ECU can check the fuel is right.
• Catalytic converter. A box with a honeycomb inside that cleans up the harmful gases.
• Silencer. One or more boxes that quieten the noise. In the UK, the last one is often called the back box.
• Tailpipe. Where the gas leaves the car, at the back, well away from the people inside.

Everything that goes into the engine comes out again. A 2.0-litre engine flat out at 3,000 rpm pushes out about 64 grams of gas every second: all the air it took in, plus the fuel it burnt. And because the gas is so hot, the same amount takes up three or four times the room the air did.

Step along the exhaust with the numbered buttons, and try different pedal positions.

1. Exhaust valve. It opens near the bottom of the power stroke. The gas is still at several times outside air pressure, so most of it rushes out on its own; the piston rising on its exhaust stroke pushes out the rest. Because each valve opens in turn, the gas leaves in pulses.
2. Manifold. The pipes from each cylinder join into one. The gas leaves the engine at anything from about 350°C (662°F) at idle to about 850°C (1,562°F) flat out, so the manifold is one of the hottest parts of the car. It has a metal heat shield over it to protect everything nearby.
3. Oxygen sensor. Also called a lambda sensor, it sits in the gas and measures how much oxygen is left. Plenty of oxygen means the engine ran short of fuel; none means too much. The ECU reads it many times a second and trims the fuel to keep the mixture just right. A second sensor after the catalytic converter checks that the converter is working.
4. Catalytic converter. Inside is a ceramic honeycomb with thousands of tiny channels, coated with a thin layer of precious metals: platinum, palladium and rhodium. As the gas passes over them, the harmful gases react and turn into harmless ones.
5. Silencer. Each pulse of gas is a bang of pressure, and with four cylinders there can be a hundred of them a second. Inside the silencer, chambers, baffles and holes make the pulses bounce about and cancel each other out, so the gas leaves the tailpipe as a steady flow and the bangs become a hum.

Try Cold start. A catalytic converter does nothing until it reaches about 250°C (482°F), so for the first minute or two after you start a cold engine, the harmful gases go straight through. On a short trip, much of the pollution comes out in that time. The ECU helps by firing the spark a little late, so more heat goes into the exhaust and warms the converter faster.

Then try No silencer, and you’ll see each pulse leaving the tailpipe on its own. A silencer cuts the noise by around 30 decibels, so without one the exhaust sounds about eight times as loud.

Most of the exhaust is harmless. The air going in is about four-fifths nitrogen, which mostly passes straight through. The fuel burns into carbon dioxide and water. That water is why you see a white cloud from the tailpipe on a cold morning, and sometimes drips of water underneath.

About 1% is harmful, and that’s the part the catalytic converter is there for:

• Carbon monoxide, from fuel that didn’t burn completely. It’s poisonous, with no smell or colour.
• Unburnt fuel, called hydrocarbons, which help to make smog.
• Nitrogen oxides, made when the nitrogen in the air gets hot enough to react in the cylinder. They harm people’s lungs.

A petrol engine’s converter is called a three-way converter because it deals with all three at once. It only works properly if the mixture is just right, which is why the oxygen sensor is so important. Catalytic converters looks inside one.

Diesels burn with lots of spare air, so they need a different setup. A diesel particulate filter, or DPF, traps the soot, then burns it off now and then on a long run. Many diesels also squirt a liquid called AdBlue into the exhaust, which turns the nitrogen oxides back into nitrogen and water.

Carbon dioxide can’t be cleaned up: it’s what burning fuel makes. The only way to make less of it is to burn less fuel.

• Blowing exhaust. Pipes and boxes rust from the inside, because the water in the exhaust collects in them on short trips. A hole or a cracked joint makes the car louder, often with a rattle or a puffing sound. A leak at the manifold gasket gives a ticking noise that’s loudest when the engine is cold.
• Fumes in the car. A leak under the car can let carbon monoxide into the cabin. If you smell exhaust inside, or feel drowsy or get a headache while driving, open the windows and get it checked straight away.
• A failed or blocked catalytic converter. The engine warning light comes on, and the car can smell of rotten eggs. A blocked one chokes the engine, so it loses power. A misfire is a common cause: unburnt fuel burns inside the converter and can melt the honeycomb.
• A failed oxygen sensor. The ECU can no longer check the mixture, so the warning light comes on and the car uses more fuel.
• A blocked DPF. Diesels driven only on short trips never get hot enough to burn the soot off. A warning light comes on, and a long run on a fast road can often clear it before it needs a garage.
• Catalytic converter theft. The precious metals inside are worth a lot, and thieves can cut a converter off in under a minute. If your car suddenly sounds like a racing car when you start it, check underneath.

The exhaust is one of the most popular things to change, mostly for the sound.

• A cat-back exhaust replaces everything after the catalytic converter with bigger, smoother pipes and freer-flowing silencers. It changes the sound a lot but usually adds only a few per cent more power on an engine without a turbo.
• A sports catalytic converter has fewer, wider channels, so gas flows through more easily. It cleans up less, and can fail the emissions test.
• Tubular manifolds, often called headers, are shaped so the pulses from each cylinder help pull the gas out of the next one. Exhaust manifolds explains how.
• Bigger isn’t always better. Pipes that are too wide slow the gas down, so it can’t help pull the next pulse out, and an engine can lose power low down.
• Turbo engines gain more. The exhaust gas spins the turbo, so a freer exhaust after it lets the turbo spin up more easily. That’s covered in the turbo and supercharger lesson.

Changes to the exhaust usually need a new map in the ECU to get the most from them.

Every car has to meet limits on its emissions and noise, and the parts that keep it within them must stay working.