How a Catalytic Converter Cleans Petrol Engine Exhaust

A catalytic converter is a metal canister in a car's exhaust pipe that changes three harmful gases from a petrol engine into less harmful ones. Carbon monoxide becomes carbon dioxide, unburned fuel becomes carbon dioxide and water, and nitrogen oxides become plain nitrogen. It does this with a thin coating of precious metals that speeds up the chemistry without being used up.

What is inside the canister

The core is a honeycomb block, usually made of cordierite, a heat-resistant ceramic. The honeycomb has thousands of tiny channels, so a great deal of surface area fits into a small space. Exhaust flows through the channels instead of around a solid lump.

The channels are covered with a rough coating called a washcoat, often based on alumina. The washcoat spreads the catalyst thinly across the surface so more of it touches the gas. Some designs add cerium compounds to the washcoat, which can store oxygen and release it later. A fibre mat wraps the block to hold it in place and absorb vibration, and a steel shell encloses everything.

The catalysts are metals from the platinum family. Platinum helps with both kinds of reaction described below. Rhodium mainly helps remove nitrogen oxides, and palladium mainly helps with oxidation. Which mix a maker uses varies, and U.S. converters commonly use some combination of the three, according to NAPA's technical guide.

The three reactions

Engineers call the modern design a three-way converter because it handles three pollutants at once:

• Reduction of nitrogen oxides (NOx): the converter strips oxygen from the molecules, and the nitrogen atoms pair up to form harmless nitrogen gas.
• Oxidation of carbon monoxide (CO): oxygen is added, giving carbon dioxide.
• Oxidation of unburned hydrocarbons (fuel that escaped burning): oxygen is added, giving carbon dioxide and water.

One reaction adds oxygen and another removes it, so the converter needs the exhaust to contain about the right amount. That is where the engine computer comes in.

Why the air-fuel mixture matters

A three-way converter works best when the engine burns a mixture close to the chemically exact ratio of air to petrol, called stoichiometric. If the mixture runs lean, with extra air, the exhaust has too much oxygen and nitrogen oxides are removed less well. If it runs rich, with extra fuel, there is too little oxygen and carbon monoxide and hydrocarbons are removed less well.

To hold the mixture near that point, the engine computer reads an oxygen sensor placed before the converter and adjusts the fuel supply many times a second. The mixture therefore swings slightly lean and slightly rich around the target. The oxygen-storing material in the washcoat smooths those swings: it soaks up oxygen during lean moments and gives it back during rich ones. A second sensor after the converter lets the computer check that the converter is working, and it can switch on the warning light if the two sensors' readings look too alike.

Why a cold converter does little

The chemistry needs heat. Wikipedia's article on the device says a converter needs about 400 °C to work effectively, and NAPA gives an operating range of roughly 500 to 1,200 °F (about 260 to 650 °C), noting that reaching it can take several miles of normal driving. The two sources describe the threshold differently, but both agree the converter is much less effective right after a cold start.

Because of this, makers mount converters close to the engine, where the exhaust is hottest. Some cars add a small extra converter nearby that warms up first. A few have used electrically heated converters; Alpina offered one in 1995. Wikipedia notes that much of a car's pollution is released in the first minutes of running, before the converter has warmed up.

A short history

Eugene Houdry, a French engineer who had worked on catalytic refining of oil, moved to the United States in 1930 and later developed a converter for gasoline cars in the mid-1950s. Engelhard engineers built the first production converter in 1973, and most U.S. petrol cars from the 1975 model year had one, according to Wikipedia.

Those early converters could not work with leaded petrol, because lead coats the catalyst and stops it working. Removing lead from fuel was therefore tied to the spread of converters. The three-way type came later: Wikipedia says Volvo first commercialised it on 1977 California-specification 240 cars, and that nearly all U.S. makers used it from the 1981 model year.

What damages a converter

Both sources describe two main kinds of failure.

• Poisoning: lead, sulfur, manganese, zinc and phosphorus can coat the catalyst. Zinc and phosphorus from engine oil additives are a common source, and silicon can come from coolant leaking into the engine.
• Overheating: misfires or a rich-running fuel system send unburned fuel into the converter, where it burns and can melt the honeycomb.
• Physical damage: the ceramic is brittle, and hard impacts can crack it.

A blocked or damaged converter can reduce power and fuel economy, and the usual first sign is a warning light with a stored fault code. NAPA says a working modern converter removes close to 90% of nitrogen oxides and carbon monoxide once it is hot.

Sources

• Catalytic converter, Wikipedia
• How a Catalytic Converter Works, NAPA Know How