Catalytic converters

A catalytic converter, or cat, is a metal box in the exhaust, usually close to the engine, that cleans up the harmful gases before they leave the tailpipe. How an engine gets rid of exhaust gases showed it at work. Here we look inside it, and at why it needs the mixture so precisely right.

About 1% of a petrol engine’s exhaust is harmful, in three groups:

• Carbon monoxide (CO), a poisonous gas from fuel that only half burnt.
• Hydrocarbons (HC), fuel that didn’t burn at all.
• Nitrogen oxides (NOx), made when nitrogen from the air reacts in the heat of the burn.

A catalyst is something that helps a reaction happen without being used up itself. In the converter, thin layers of precious metals help these gases react with each other and turn into carbon dioxide, water and nitrogen. A warm one cleans up about 98% of them.

Converters became law on new cars in the US from 1975, and on every new petrol car sold in the UK and Europe from 1993.

The demo shows exhaust gas running through a converter, with the harmful gases as dark dots. The slider sets the mixture, and the graph shows how much of each group the converter cleans up at each one.

The converter has two jobs that pull in opposite directions:

• Burning the carbon monoxide and unburnt fuel. They need to take on oxygen to become carbon dioxide and water.
• Breaking up the nitrogen oxides. They need to give their oxygen away, and the only things in the exhaust that will take it are the carbon monoxide, unburnt fuel and a little hydrogen.

Try each mixture:

1. Just right, 14.7:1. There’s exactly enough oxygen to go round. The nitrogen oxides hand their oxygen over to the carbon monoxide and unburnt fuel, so both jobs get done, and 98% of all three are cleaned up.
2. Rich, 14.3:1. There isn’t enough oxygen to burn all the carbon monoxide and unburnt fuel, so only about half of them are cleaned up. The nitrogen oxides have plenty to react with.
3. Lean, 15.1:1. Now the spare oxygen burns up all the carbon monoxide and unburnt fuel first, leaving nothing for the nitrogen oxides to give their oxygen to. Almost all of them go straight through.

The shaded band on the graph is the window, where the converter cleans up at least 90% of everything. It’s tiny: within about 1% of 14.7:1. That’s why the ECU spends nearly all its time holding the mixture right there, checking it with the oxygen sensors, as the next lesson explains.

Now press Cold start, which runs six times faster than life. The converter starts at 20°C (68°F) and does almost nothing until it reaches about 250°C (482°F), called its light-off temperature. Step through ten seconds at a time: after 40 seconds it’s only just starting, and after about a minute it’s cleaning up nearly everything.

The model is simplified, but the numbers match the earlier lessons.

Cut a converter open and you’ll find:

• The substrate. A block of ceramic honeycomb with 400 to 900 tiny channels in every square inch, all running the length of the converter. The walls are thinner than a credit card, so the gas can flow through easily while touching as much surface as possible. Some performance cars use a honeycomb of thin metal foil instead, which is tougher and flows a little better.
• The washcoat. A rough coating on the channel walls that gives them a huge surface area. Spread flat, the inside of one converter would cover a few football pitches.
• The precious metals. Just a few grams, spread through the washcoat in tiny specks. Platinum and palladium help the carbon monoxide and unburnt fuel burn. Rhodium helps the nitrogen oxides break up.
• Oxygen storage. The washcoat also holds cerium, which soaks up oxygen when the exhaust is lean and gives it back when it’s rich. The ECU can’t hold the mixture perfectly still, so it swings slightly either side of 14.7:1 about once a second. The stored oxygen evens out those swings, keeping the converter in its window.
• A mat and shell. A heat-proof mat holds the brittle honeycomb tightly inside a stainless steel shell, with a heat shield round the outside.

The reactions give off heat of their own, so a working converter runs hotter than the gas going into it, typically 400°C (752°F) to 800°C (1,472°F). Above about 1,000°C (1,832°F) the coating starts to wear out quickly, and the ceramic can melt.

The sooner a converter lights off, the less pollution comes out on a short trip. So most modern cars have two:

• A close-coupled converter, bolted right after the exhaust manifold, or onto a manifold cast into the head, as in Exhaust manifolds. It gets hot gas straight away and lights off within seconds.
• An underfloor converter further back, which is bigger and cleans up whatever’s left.

After a cold start, the ECU also fires the sparks late and may raise the idle speed, sending extra heat down the exhaust to warm the converter faster.

A diesel always runs lean, so a three-way converter can’t clean up its nitrogen oxides. Diesels use a set of parts instead:

• An oxidation catalyst, which uses the spare oxygen to burn the carbon monoxide and unburnt fuel.
• A diesel particulate filter (DPF), which traps soot and burns it off on longer runs.
• Selective catalytic reduction (SCR), where AdBlue, a liquid of urea and water, is sprayed into the exhaust. It turns into ammonia, which a special catalyst uses to turn the nitrogen oxides back into nitrogen and water.

Direct-injection petrol engines make some soot too, so since about 2018 most new ones have a gasoline particulate filter (GPF) as well, often built into the converter.

A converter should last the life of the car. When one fails, it’s usually because something upstream went wrong first.

• Melting. A misfire sends unburnt fuel into the converter, where it burns and can push it well past 1,000°C (1,832°F). The ceramic melts and blocks the exhaust, so the car loses power, especially uphill, and may glow red underneath. This is why a flashing engine management light means stop driving hard: it’s warning of a misfire bad enough to damage the converter.
• Poisoning. An engine that burns oil, or leaks coolant into the cylinders through a failing head gasket, coats the metals and stops them working. So does leaded fuel, and some silicone sealants.
• Wearing out. After many years the coating slowly loses its oxygen storage. The rear oxygen sensor notices, and the ECU stores fault code P0420, catalyst efficiency below threshold.
• Rattling. A knock to the underside or a sudden soaking from a puddle when hot can crack the honeycomb. The pieces rattle, especially at idle, and can block the flow.
• A rotten-egg smell. The converter stores sulphur from the fuel and releases it as hydrogen sulphide when the mixture runs rich. An occasional whiff after hard driving isn’t unusual; a constant smell means the engine is running rich.
• Theft. The precious metals are worth a lot, and thieves can cut a converter off in under a minute. Hybrids such as the Toyota Prius are a favourite target. Marking it, or fitting a cage or a guard, makes it less attractive.

• Sports converters have fewer, wider channels, about 100 to 200 per square inch, so they flow more easily on a tuned engine. They clean up less and may not pass the emissions test, so look for one that is approved for road use.
• Decats, taking the converter off, free up a few per cent of power on a turbo engine, but they’re illegal on the road. The rear oxygen sensor then shows the converter has gone, so people hide it with a spacer or a software change. That’s illegal too.
• Changing the manifold or the downpipe on a turbo car often moves or replaces the close-coupled converter, so check the new part includes one that is approved.
• Running rich on a tuned engine keeps it cool, but the unburnt fuel overheats the converter. A tune needs to look after the converter as well as the engine.