Carbon Monoxide in Car Exhaust

[ArkiStan]

So I've heard that most of a car's exhaust is carbon dioxide and there may be a small amount of carbon monoxide. Correct me if I'm wrong, but my understanding is that carbon monoxide is released in the case of incomplete combustion, and I've heard that incomplete combustion occurs when you push the engine to high RPMs. So would it be correct to assume that when the car is idling or driving very slowly (low RPMs), there is the least amount of carbon monoxide being released?

[InfiniteNothing]

It makes sense that you'd get the most carbon dioxide when you are rich and cool. When you first start up your car is going to be the worst offender. Being heavy on the gas petal will also likely induce rich conditions.

[mechmike0034]

So I've heard that most of a car's exhaust is carbon dioxide and there may be a small amount of carbon monoxide. Correct me if I'm wrong, but my understanding is that carbon monoxide is released in the case of incomplete combustion

You are correct to this point. If the combustion process in a gasoline-fueled internal-combustion engine was 100% efficient (which it is not) then the only byproducts (exhaust) would be carbon dioxide and water.


, and I've heard that incomplete combustion occurs when you push the engine to high RPMs.

High engine speeds by and of themselves do not cause incomplete combustion.

On the other hand, high engine speeds and high engine loads can cause, for example, a weak ignition coil to misfire. Misfire (among many other things) in turn can and will cause incomplete combustion.

In this example, no misfire and therefore no incomplete combustion occurs when the engine is running more slowly or with less load applied because under these conditions there is less ignition system demand. The coil is only weak under high ignition demand, which is also when the engine is, you guessed it, either at high speed or under high load, or both.


So would it be correct to assume that when the car is idling or driving very slowly (low RPMs), there is the least amount of carbon monoxide being released?

Sort of. Assuming the engine is at operating temperature and that the engine and emissions control systems are all working as designed it is a pretty safe bet, but not a blanket assumption. Incomplete combustion can have many different causes, and they aren't all engine speed related. Your question does not take into account the effects of the emission control system and devices nor the engine's operating temperature and state of tune.

As IN nored in his reply, the CO emissions are greatest at cold start, for two reasons. One, as he stated, is because the engine control computer typically richens the fuel mixture when the engine is below opertaing temperature to ease starting and improve cold driveability. This is compounded by the catalytic converter being cold as well. Converters do not begin "cleaning" the exhaust until they get to a certain temperature. The converter has to "light off" for any catalytic action to take place.

More info on the chemistry of internal engine combustion can be found here: http://www.autoshop101.com/forms/h55.pdf

[ArkiStan]

Well, I'm not too concerned about the dioxide, which in an immediate sense is much less harmful than monoxide. I'm trying to figure out the scenarios that would yield the most monoxide and whether idling would be one of them.

[InfiniteNothing]

Most carbon monoxide: starting your car
Likely honorable mention: flooring it (ecu will usually run very rich in order to protect the engine)
Idleling's probably fine provided the car (the cat) is up to operating temp and it's a newish car. Except that per mile, at idle, you're emiting infinite carbon monoxide.

[ArkiStan]

...snip...

Wow. Great info!

I was hoping that at idling it would emit the least, but as IN mentioned, per mile, I guess it's pretty bad. I'm trying to design an apartment that has the parking space intimately integrated into it. Many of the design provisions will likely be determined by the carbon monoxide emissions at idling and ignition.

Thanks to both of you.

[Napoleon54]

I thought it was all daisies and warm apple pie.

[ArkiStan]

I thought it was all daisies and warm apple pie.
You're getting it confused with Jessica Alba's poo.

[Daedalus]

I think O2 sensors play a larger role on cold start than anything else, since they don't work when cold. Cats won't clean up CO or excess HCs even when hot, unless there is O2 present. But if the car's running rich, there is not enough excess O2 to begin with. Air pumps are a rare thing on modern cars.

[mechmike0034]

I think O2 sensors play a larger role on cold start than anything else, since they don't work when cold.

Not really. They've just about all been heated since 1996 (the advent of OBD II) and most are active within 60 seconds of a cold start, sometimes less.


Cats won't clean up CO or excess HCs even when hot, unless there is O2 present.

True, if the vehicle in question is running pig-rich for a long enough period, but also keep in mind that a modern cat is an "oxygen sponge" that absorbs extra O2 when the mixture's lean and releases it when the mixture's rich.


But if the car's running rich, there is not enough excess O2 to begin with. Air pumps are a rare thing on modern cars.

Many air pumps are now electric, and only run for a short period at cold start to help light the cat off quickly. Fuel control has gotten to the point where air pumps in many cases are not necessary.

[InfiniteNothing]

Wow. Great info!

I was hoping that at idling it would emit the least, but as IN mentioned, per mile, I guess it's pretty bad. I'm trying to design an apartment that has the parking space intimately integrated into it. Many of the design provisions will likely be determined by the carbon monoxide emissions at idling and ignition.

Thanks to both of you.
Parking lot? Yeah you're screwed because people are just starting their cars in the morning and letting them idle. My suggestion, and this is not my forte, is to put the parking lot no more than 1/2 under ground and have it open on the sides (well except where it's under ground.

[johnnymk]

Not really.
Many air pumps are now electric, and only run for a short period at cold start to help light the cat off quickly. Fuel control has gotten to the point where air pumps in many cases are not necessary.

Wow, when did electric air pumps come into existence? Do they draw many amps? Air pumps have been known to sieze. What happens if they do?

[MikeD]

It makes sense...when you are rich and cool.

As are most of us here on G/A. :hihi:

[mechmike0034]

Wow, when did electric air pumps come into existence? Do they draw many amps? Air pumps have been known to sieze. What happens if they do?

I don't know exactly, but my '94 Caddy (which has LT1 Chevy power) has one.

They blow the fuse if they seize. '96 and up (OBD II) vehicles monitor the air pump for proper operation; a code will set and the check engine light will come on if the system isn't working.

[mechmike0034]

Wow. Great info!

I was hoping that at idling it would emit the least, but as IN mentioned, per mile, I guess it's pretty bad. I'm trying to design an apartment that has the parking space intimately integrated into it. Many of the design provisions will likely be determined by the carbon monoxide emissions at idling and ignition.

Thanks to both of you.

In short, either leave the car outside, or have a helluva ventilation system in place.

[InfiniteNothing]

Wow, when did electric air pumps come into existence? Do they draw many amps? Air pumps have been known to sieze. What happens if they do?
I'm guessing they just aren't needed with modern ECUs's control over the "ratio"

[stufine]

I'm trying to design an apartment that has the parking space intimately integrated into it. Many of the design provisions will likely be determined by the carbon monoxide emissions at idling and ignition.


I would say Building Codes will be the main factor.

[mechmike0034]

http://www.techniciantraining.com/patts_demo/samples/sample8.aspx

[ArkiStan]

thanks again for all the great info.