I heard here that a catalytic converter needs to operate within a certain "chemical range" otherwise it will generate unnecessary heat.

  • What exactly is a catalytic converter?
  • How does it work?
  • What is their purpose on a car?
  • Are they necessary (i.e. do all cars have them)?
  • Why does it need to operate within a certain chemical range?

Catalytic Converter

It is a funny looking thing. This is one of those things that I often hear about but still don't really understand what it does.


4 Answers 4


All of this comes from: http://auto.howstuffworks.com/catalytic-converter2.htm

Ok, attacking each of these in order:

What exactly is a catalytic converter?

Catalytic converter location
It is a device in the exhaust path of your car that scrubs unwanted gases from the exhaust stream.

How does it work?

Interior of the catalytic converter

In chemistry, a catalyst speeds up a chemical reaction without actually taking part in it. In the case of the one in your car the catalyst is platinum, rhodium, palladium, and today even gold. There are two types of catalyst in your car, reduction and oxidation catalysts. The general idea is that you create a box that is parked in the exhaust stream that exposes as much surface area of the catalyst as possible, while keeping the amount of the catalyst small as they are very expensive. Today they use gold in some because it is less expensive than the others.

Catalyst Cross Section

The reduction catalyst is the first stage of the converter and is used to scrub NO or NO2 molecules from the gas stream. When those molecules hit the catalyst they are converted into N and O2. N and O2 are benign, NO and NO2 not so much.

The second stage is there to reduce the emissions of hydrocarbons and carbon monoxide by burning them on the catalyst honeycomb screen seen above.

What is their purpose on a car?

See above. They remove unwanted toxic gases from the exhaust stream.

Are they necessary (i.e. do all cars have them)?

In the US at least every car after 1975 was required to have one. You cannot register a car here if it has been removed.

Why does it need to operate within a certain chemical range?

As for why does the chemistry only work within a narrow band of fuel air mixture, I have no idea.

One more thing, and I think you got this idea from me so I apologize for not being clear. It's not a case of the converter causes excess heat when operating outside its chemical design range. If the engine is running too lean, it's the running lean that causes the heat, not the converter.

  • As for temperature it's probably of the excitement of the molecule that allows it to efficiently work; or non excitement depending on the reaction desired.
    – Dee
    Jan 26, 2016 at 21:59

A catalytic converter incorporates a structure that is coated in precious metals. When catalytic converters first came out that structure was ceramic balls coated in the metals. This didn't work so well and the structure was changed to either a honey comb or a monolith, both ceramic. The tubes of the honey comb run the length of the converter. A monolith is similar to the honey comb but squares instead of the hexagons.

The four metals in a catalytic converter are platinum, rhodium, palladium and cerium. These metals react with the four offending pollutants when the temperature of the converter is sufficiently high. The expression "the converter lit off" refers to the temperature reaching sufficiently high to start the chemical reaction.

The offending emissions are oxides of nitrogen (NOx), hydrocarbons (HC) and carbon monoxide (CO). These pollutants are broken down during different conditions. CO and HC are broken down when the fuel mixture is lean. The lean mixture contains more oxygen and that excess oxygen is used to oxidize the CO into CO2 and break the HC into CO2 and H2O. NOx is broken down during a rich mixture when there is very little oxygen. NOx is broken down into N2 and O2 releasing oxygen. These processes in the converter are preformed by the platinum, rhodium and palladium.

The cerium in the converter is used as an oxygen storage device. When the mixture is lean the cerium stores up oxygen. When the mixture is rich the the cerium releases the oxygen to help convert the pollutants that need oxygen.

Because the conditions that break down the pollutants are opposite of each other the car needs to switch back and forth from lean to rich and back again continuously to properly convert all the emissions. This leads the vehicle to operate in a tight band around stoichiometric or 14.7:1, dashing back and forth across it.

These emissions are bad for you (there is lots of info available on google). They are converted to not harmful things by the catalytic converter.

In the US all highway vehicles require a catalytic converter. The exceptions are very old cars, cars over 10,000 pounds, diesel vehicles (they have something else), there are others but these are most common.

  • 3
    I hope you don't mind @vini_i, I edited your answer to change "The four meats in a catalytic" to the four metals.. Although the four meats was much more fun :) I so love typos. I have made some real beauties myself..
    – cdunn
    Jan 26, 2016 at 22:22
  • @cdunn no problem :)
    – vini_i
    Jan 27, 2016 at 1:27
  • Wow, a really amazing invention. It converts fuel-driven cars from very toxic to just a CO2 emitter.
    – cst1992
    Jan 27, 2016 at 10:46

• Why does it need to operate within a certain chemical range?

The gasoline engine catalyst feedgas (exhaust gasses) must remain in a very small window of fuel mixture because the chemical reactions the that reduce NOx and oxidize hydrocarbons are mixture dependent and mutually exclusive. NOx can only be reduced in a fuel rich environment and HC can only oxidized in a lean, oxygen available, environment.

The most important governing equations for HC oxidation are:

H2 + 0.5O2 -> H2O (i)

CO + 0.5O2 -> CO2 (ii)

C3H6 + 4.5O2 -> 3CO2 + 3H2O (iii)

C3H8 + 5O2 -> 3CO2 + 4H2O (iv)

The most important governing equations for NOx reduction are:

H2 + NO -> H2O + 0.5N2 (v)

CO + NO -> CO2 + 0.5N2 (vi)

C3H6 + 9NO -> 3CO2 + 3H2O + 4.5N2 (vii)

C3H8 + 10NO -> 3CO2 + 4H2O + 5N2 (viii)

The Cerium reaction: Ce2O3 + 0.5O2 -> 2CeO2 (ix)

The only fuel mixture that meets the requirements of both set of reactions is mixture very near stoichiometric. The catalyst will reduce about 80% of the input NOx and HC as long as the mixture is kept within this very small window of fuel mixture, the operating window is AFR’s from 14.55 to 14.69. (Lambda .995 to 1.005). That is a mixture that does stray from stoichiometric by more than ½ of 1 percent.

A graphic representing this concept.

enter image description here

A common misunderstanding is that the mixture is required to cycle from rich to lean for the reactions to complete. This is not true; it needs to stay in the window. In older systems it was required to cycle the mixture to keep it in the window because a narrowband oxygen sensor only reports mixture at stoichiometric. Wide band sensors do not cycle the mixture; they hold steady mixture in an even smaller window which improves pollutant conversion to rates above the 80% that was achievable with the narrowband cycling system.

  • Love your answers. +1 Jan 27, 2016 at 8:41
  • 1
    @DucatiKiller Thanks, This is my favorite kind of question because it involves one of the most misunderstood parts of the fuel control system. Jan 28, 2016 at 4:40

The short answer.

It's part of your exhaust that helps clean out exhaust fumes. It usually has a sensor right after it (or in it) that monitors how it's doing. I think in newer cars it can help with the air to gas mixture if your car is running lean or running rich.

You don't necessarily need it but if you do happen to remove it you should at least add some regular exhaust pipe to take its place. It will help with noise and back pressure .

  • In most US states you are legally required to have it, but functionally you are correct, it is not required. Mar 9, 2017 at 23:06

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