Five Gas Test Questions

Let's start out with some simple questions on 5-Gas Exhaust Analysis. Let's assume the catalytic converters on the cars in the test are not real good, maybe only 50% efficient. If we had really good cats, no matter what the problem, the cat could clean it up for a while, and we wouldn't see any emission problems coming out the tail pipe. And please keep in mind that in real life these emission readings would vary a lot depending on the exact vehicle being tested. You may have seen one in the workshop yesterday that was different. Just think about the general concept or theory involved in the test question. Write down you answers on piece of paper and compare to the answer at the bottom of this page. NO CHEATING :-))

1. A simple problem like a spark plug wire that fell off will likely cause lots of which pollutant to come out the tailpipe? Let's assume the engine is a non-feedback engine, or it stays in open loop.

A: NOx
B: CO2

2. If an EGR valve functioned properly and came open, but the passage was clogged with carbon, which pollutant would likely be high in a loaded mode test on a dyno?

C: NOx
D: CO2

3. High numbers of NOx come out the tailpipe when Vehicle A had it's dyno smog test. Which of the following conditions could cause this?

A: An ignition misfire from a shorting spark plug wire
B: Too much carbon inside the combustion chamber
C: A bad thermostat that causes the engine to run hotter than normal
D: Both B and C

4. Let's say an engine puts out these readings at the tailpipe: HC 459 ppm, CO 4.7%, CO2 10.3% and O2 0.1%. What do you think is going wrong with this engine?

A: Air-fuel ratio too lean
B: Air-fuel ratio too rich
C: Normal, nothing wrong
D: Too much air

5. Which exhaust gases are measured in percentage (%)?

A: HC, CO, CO2
B: CO, CO2, O2
C: CO, CO2, NOx, O2
D: HC, NOx

6. Let's say an engine puts out these tailpipe readings: HC 537 ppm, CO 0.05%, CO2 9.7%, and O2 4.5%. What do you guess is wrong with this engine, if anything? (Hint: there is no air injection.)

A: Air-fuel ratio too lean
B: Air-fuel ratio too rich
C: Normal, nothing wrong
D: Plugged exhaust

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Answers and Explanations 5-Gas test

1. D, HC or raw gas that went in will come out when it isn't ignited.

2. C, when EGR isn't flowing, the combustion chamber gets hotter and creates more NOx.

3. D. Extra carbon causes more pressure in the chamber, which causes more heat, which leads to more NOx formed. An engine that overheats will do the same thing. An ignition misfire will create much less heat, so much less NOx.

4. B. The high CO and high HC show a rich mixture. Notice the O2 is very low and the CO2 came down too. Without enough oxygen, not all the CO becomes CO2 and not all the HC can burn. The HC may only come from the rich condition. So first get the air-fuel ratio correct, and then retest to see if the HC are still too high. If it had been running too rich for too long, you may have to clean the carbon out of the combustion chamber with a top engine cleaner to get the HC to lower.

5. B. CO, CO2 and O2 are measured in percentage, HC and NOx are measured in parts per million.

6. A. This is a lean mixture when the HC and O2 are high, (but there is no air injection) and the CO is low and the CO2 is lower than our normal 13-14%. With too much oxygen, there is some left over and the fuel is thinned out so it can't all be burned. A plugged exhaust often makes the system richer. The CO being so low and the O2 being high is a clue that the HC comes from a lean condition, not an ignition misfire. With an ignition misfire you will see excess O2, but not as much and the CO won't be quite so low.