Automobile Emission Control Questions and Answers

Answer: A

The emission peaks a little lean of stoichiometric, where flame temperature is still high and free oxygen has become available. Going lean enough to fall past the peak brings its own difficulty, since a three-way catalyst then cannot reduce what remains.

Answer: B

Vapour lines are easily left disconnected or misrouted during a repair, and the canister then vents directly to atmosphere. Evaporative emissions can rival exhaust hydrocarbons in total, so the fault is far from trivial.

Answer: C

Modern vehicles derate deliberately to prevent operation without the reduction system's reagent, since the emissions would otherwise rise by a factor of several. A lambda sensor belongs to a petrol engine and has no part here.

Answer: D

A value of one is stoichiometric, above one is lean and below one is rich. Expressing the mixture this way makes the same number meaningful for any fuel, since each fuel's own stoichiometric ratio is built into the definition.

Answer: A

Below roughly three hundred degrees the conversion efficiency is very low, and a large share of a short trip's emission occurs before it is reached. Close-coupling the converter to the manifold shortens that period considerably.

Answer: B

Correct answer: regeneration

Regeneration is the periodic burning off of trapped soot, achieved by raising the exhaust temperature - usually by injecting extra fuel that oxidises over an upstream catalyst.

It is necessary because the filter traps continuously and would otherwise block. Passive regeneration occurs whenever the exhaust is hot enough of its own accord; active regeneration is commanded when it is not. Purging refers to the carbon canister, and the other two terms belong elsewhere.

Answer: C

Correct answer: carbon

The canister is filled with activated carbon, whose very large internal surface adsorbs the fuel vapour escaping from the tank while the vehicle is parked.

When the engine runs under suitable conditions, air is drawn through the canister to desorb the vapour and carry it to the intake, where it is burnt. The same adsorption principle serves organic vapours in air cleaning generally - the carbon holds the molecules until conditions favour their release.

Answer: D

Correct answer: slip

Ammonia slip is unreacted ammonia passing through the reduction catalyst and out of the tailpipe. It arises when urea is overdosed, or when the catalyst is too cold or too small to consume what is supplied.

It matters because ammonia is itself a pollutant and a precursor of secondary particulate. Systems therefore dose carefully against temperature and load, and often carry a small oxidation catalyst downstream specifically to destroy any that escapes.

Answer: A

The map runs in opposite directions for the incomplete combustion products and for the thermal nitrogen oxides. It is exactly why the three-way catalyst was developed, since it treats all three at one point rather than trading them.

Answer: B

Between 150 and 350 degrees the conversion goes from almost nothing to almost everything, and reaching that band takes a minute or two from cold. A five kilometre trip may therefore emit more than a fifty kilometre one.