exhaust temperature

[klb76]

Does anyone have any numbers on how hot the exhaust gas is coming out of the engine? I know they make pyrometers for this but I don't need an exact number... just a good estimate...

thanks
klb

 

[Twin_Turbo]

Depends on the load. On idle or normal cruising it will be around 500-900F, on heavy acceleration or under load (hauling some serious weight) it might get as high as 1400+ F.

Marck

 

[klb76]

so theoretically speaking about 1400F is going to be the max temp? and I would guess as it passes through the pipe it cools considerably.... if anyone was wondering I am just doing some research for school...

klb

 

[Stingray6974]

Actually the exhaust under cruise conditions is closer to 2500 degrees. The cat converter has to reach 2200 degrees before it will "lite off" to combat Nox.

Mike

 

[78SilvAnniv]

I was going to ask about the cat...

I knew that the catalytic converter has a catlyst inside to react with gases to neutralize them, and I also understood that they needed a higher temperature for optimum performance, but...

...what makes the cat get so much hotter?
I thought exhaust was exhuast, unless it had a super-heater.
Heidi

 

[Twin_Turbo]

The catalytic converter operates at a much higher temperature than the exhaust gas. The converter generates it's own heat.

The converter has 2 sections (monolithic blocks, sort of honeycomb), an oxydizing section and a reduction section. The oxydizing section uses oxygen to oxydize (that's why the AIR system pumps air into the cat)

Exhaust gas consist of:

* Nitrogen gas (N2)
* Carbon dioxide (CO2)
* Water vapor (H2O)

The above one are harmless (well, CO2 is believed
to contribute to global warming)

* Carbon monoxide (CO)
* Hydrocarbons (unburnt fuel)
* Nitrogen oxides (NO and NO2, together called NOx)

The 1st stage of the covnerter is the reduction:
2NO => N2 + O2 or 2NO2 => N2 + 2O2
Here platinum & rhodium are the catalysts

The 2nd stage is the oxydation:
2CO + O2 => 2CO2
here platinum and palladium are the catalysts.
this section also burns the hydrocarbons. If there are too many hydrocarbons it can damage (clog) the converter. (that's why pushing a car that won't start or towing it is a bad idea!)

The converter does need heat to start becoming effective, this is done by pumping air into the headers (newer cars) so that the exhaust gas heats it up faster when the engine is cold.

The reduction step needs a certain temperature to start the reaction. This temp is around 250-300degC

The monoliths can only withstand about 1200F (higher temps will melt it and render the cat useless, can be caused by unburnt fuel in exhaust system, wich will burn on the hot monolith)

Marck

 

[Nut]

Marck,

That's the best description of how a cat works that I've seen in a long time. Simple, straight forward and accurate. Well done.

............ Nut

 

[Twin_Turbo]

Thank you