Automobile Emission Control Questions and Answers

Answer: C

Correct answer: Lean exhaust prevents reduction

Read the table's duty column. Oxidation and filtration both work in oxygen-rich exhaust, but nitrogen oxide reduction does not - reduction requires the removal of oxygen, and a diesel's exhaust is full of it.

A three-way catalyst avoids the problem by working at stoichiometric, where oxidation and reduction can proceed together. A diesel cannot run there, so it needs separate devices: one to oxidise, one to filter, and either recirculation or urea to deal with nitrogen oxides.

Answer: D

A tenth of the vehicles carries nearly a third of the emission, and repairing them is far cheaper than tightening standards on vehicles not yet built. Standards and inspection address different parts of the same problem.

Answer: A

Correct answer: Aftertreatment technology and fuel quality had to advance together

Every row pairs a fuel change with a technology that could not have been introduced without it. Lead destroys catalysts, sulphur poisons them and adds sulphate particulate, volatility drives evaporative losses, and aromatics promote soot and benzene.

Each device was therefore waiting on a refinery investment, which is why standards and fuel specifications were legislated in step. Neither could deliver alone, and the durability of engines was a secondary benefit rather than the motive.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the reason. The catalyst must oxidise and reduce simultaneously, and those chemistries want opposite exhaust conditions - oxygen for one, its absence for the other.

Only a narrow band around stoichiometric satisfies both, and holding a carburettor there is impossible across the operating range. A lambda sensor and closed loop control became necessary, which is why electronic fuel injection and the three-way catalyst arrived together rather than by coincidence.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is why the change was mandatory rather than desirable. Lead compounds deposit on the precious metal sites and block them permanently - a single tank of leaded fuel ruins a converter.

Unleaded fuel therefore had to be available before catalytic converters could be fitted, and the two were legislated together. It is the clearest example of the general point that aftertreatment and fuel quality advanced in step.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the reason. A diesel runs lean, so its exhaust carries substantial oxygen - which suits oxidation perfectly and prevents reduction entirely, since a reducing reaction cannot compete with the oxygen present.

Separate devices are therefore required: an oxidation catalyst, and either exhaust gas recirculation to prevent nitrogen oxides forming or a urea system supplying ammonia to reduce them selectively. A petrol engine at stoichiometric needs only one device because both chemistries can proceed together there.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the mechanism. The catalyst converts almost nothing below light-off, so the first ninety seconds or so passes essentially untreated exhaust - and that exhaust is itself worse, the engine running rich while cold.

The quantity is roughly fixed per start, so a short journey carries the whole of it over very few kilometres. Close-coupled converters, low thermal mass substrates and electrical heating all attack the same interval.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second explains the scheduling. Recirculated exhaust displaces fresh charge and slows the burn, so too much of it costs power, worsens drivability and can raise particulate and hydrocarbon emissions.

The rate is therefore mapped against speed and load: high at part load where nitrogen oxides matter and power does not, reduced or cut at full load and during warm-up. It is a trade managed continuously rather than a setting applied once.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the reason such programmes repay their cost. Emissions across a fleet are heavily skewed: a small fraction of vehicles, poorly maintained or with equipment defeated, produces a disproportionate share.

New vehicle standards cannot reach them, since those vehicles were compliant when new. Testing in service finds them and repair fixes them, so the two instruments address different parts of the problem and are complementary rather than alternative.

Answer: D

Correct answer: A is false but R is true

The reason is correct as far as it goes, but incomplete - and the omission is what makes the assertion false. Thermal formation needs available oxygen as well as a high flame temperature.

A rich mixture is hot but oxygen-starved, so nitrogen oxide formation falls. The peak occurs slightly lean of stoichiometric, where both conditions are met. That is why the emission curves for nitrogen oxides and for carbon monoxide peak on opposite sides of stoichiometric, and why no single mixture minimises both.