Emissions tech question: NOx and HC levels with dirty catalytic converter
06-15-2007, 11:33 PM
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Staging Lane
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Emissions tech question: NOx and HC levels with dirty catalytic converter We were going over the review questions on our chapter about Emission Control Sytems when we had a nice long discussion about one of the answers to a questions.
The question reads:
A dirty catalytic converter can cause all of the following except
A. Stalling after the engine starts.
B. Decreased Hydrocarbon emissions from the tailpipe.
C. A drop in engine Vaccuum.
D. Decreased production of NOx (Oxides of Nitrogen) in the cylinders.
The instructer (who is teaching this class for the second time) went with the answer on his master key which was B. He is right because a dirty catalytic converter will cause increased Hydrocarbon emissions from the tailpipe, not decreased. A and C can be caused by a dirty converter and are ruled out.
Now onto the controversy:
Some of the other students and I thought that answer D was also a correct answer. He told us the master key says the answer is B but on two other occasions in other courses, our class has proved the book was wrong.
We base our answer on the assumption that the engine will have to work harder to overcome any extra backpressure caused by the dirty cat, slightly increasing temperature and raising NOx production in the cylinders which forms when the combustion temperatures are elevated over 2,500 degrees.
Also, the converter and exhaust will heat up because of the restriction and conduct heat into the head, raising temperatures slightly.
The instucter spent close to 45 minutes at the end of the day searching for a way to prove that B was the only correct answer and was only able to say that if the engine can't get the air out because of the converter, it can't get the air in and the NOx will drop.
I know there are a few tech gurus on here so lets see if I can get some good educated opinions on this.
The question reads:
A dirty catalytic converter can cause all of the following except
A. Stalling after the engine starts.
B. Decreased Hydrocarbon emissions from the tailpipe.
C. A drop in engine Vaccuum.
D. Decreased production of NOx (Oxides of Nitrogen) in the cylinders.
The instructer (who is teaching this class for the second time) went with the answer on his master key which was B. He is right because a dirty catalytic converter will cause increased Hydrocarbon emissions from the tailpipe, not decreased. A and C can be caused by a dirty converter and are ruled out.
Now onto the controversy:
Some of the other students and I thought that answer D was also a correct answer. He told us the master key says the answer is B but on two other occasions in other courses, our class has proved the book was wrong.
We base our answer on the assumption that the engine will have to work harder to overcome any extra backpressure caused by the dirty cat, slightly increasing temperature and raising NOx production in the cylinders which forms when the combustion temperatures are elevated over 2,500 degrees.
Also, the converter and exhaust will heat up because of the restriction and conduct heat into the head, raising temperatures slightly.
The instucter spent close to 45 minutes at the end of the day searching for a way to prove that B was the only correct answer and was only able to say that if the engine can't get the air out because of the converter, it can't get the air in and the NOx will drop.
I know there are a few tech gurus on here so lets see if I can get some good educated opinions on this.
06-16-2007, 01:08 AM
#2
If exhaust backpressure goes up, and residual exhaust gas is left in the chamber, then NOx should go down. Basically, it's another form of EGR. EGR will actually cool the combustion temperature, because the exhaust gas is, for the most part, non-reactive. Therefore, it will just absorb energy from the combustion process. The resulting drop in combustion temperature will decrease NOx production. Don't confuse charge temperature with combustion temperature.
I would agree with your instructor's answer guide. None of the answers are as simple as they are stated, but A/C/D are correct.
I would agree with your instructor's answer guide. None of the answers are as simple as they are stated, but A/C/D are correct.
06-22-2007, 01:27 PM
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Right on Formulated. Gotta remember that exhaust gas is cooler than what is actually happening in the combustion process. The part about the engine working harder is wrong. The only time an engine can do more work is if more air and fuel enter in at the proper ratio. And then you would have more work, which would equal more heat, which would equal more NOx. Nice try, but the book is right. Sorry
