Automobile Emission Control Questions and Answers

Q: Which statement about nitrogen oxide formation is false?

Correct answer: It is highest when the mixture is richThermal formation needs high temperature and available oxygen. A rich mixture is hot but oxygen-starved, so formation falls rather than peaking.The other three statements are true, and together they place the peak: formation rises steeply with flame temperature, requires oxygen, and therefore peaks slightly lean of stoichiometric where both conditions are satisfied. It is the interaction of the two requirements, rather than either alone, that shapes the curve.

Q: Which of the following is not a source of hydrocarbon emission from a vehicle?

Correct answer: The cooling systemThe cooling system carries coolant, not fuel, and is sealed from the combustion and fuel systems. It contributes no hydrocarbons.The other three all do: the exhaust carries unburnt fuel that escaped the flame, the crankcase carries blow-by past the piston rings, and the fuel tank evaporates vapour directly. Each has its own control - the catalyst, positive crankcase ventilation and the carbon canister respectively - which is why all three are counted separately.

Q: Which of the following does the carbon canister not do?

Correct answer: Treat exhaust gas leaving the engineThe canister is part of the evaporative control system and sees no exhaust at all. It sits between the fuel tank and the engine intake.The other three are exactly what it does: adsorb fuel vapour from the tank onto activated carbon while the vehicle is parked, release it when the engine controller commands a purge, and so reduce evaporative hydrocarbon loss. Exhaust treatment is the catalyst's work, on a quite separate path out of the vehicle.

Q: Running an engine very lean always reduces every regulated pollutant. True or false?

Correct answer: False, misfire raises hydrocarbonsLean running does reduce carbon monoxide, and beyond the nitrogen oxide peak it reduces those too. But taken far enough the mixture becomes too weak to ignite reliably, and a misfiring cylinder passes its entire charge unburnt into the exhaust.Hydrocarbons therefore rise sharply at the lean limit. A warm catalyst helps but cannot convert that volume, and the lean exhaust also prevents nitrogen oxide reduction - which is why lean-burn engines need their own aftertreatment.

Q: Which of the following would not reduce cold-start emissions?

Correct answer: Increasing the engine's compression ratioCompression ratio is a fundamental engine parameter affecting efficiency and knock limits, and it does nothing to shorten the interval before the catalyst lights off. A higher ratio in fact extracts more energy from the gas, leaving the exhaust slightly cooler.The other three all attack that interval directly: close-coupling puts the converter where the gas is hottest, electrical preheating warms it before the engine starts, and retarded ignition deliberately wastes energy into the exhaust to heat it faster.

91.

A three-way catalyst works well on a diesel engine because diesel exhaust is clean. Is this correct?

A.Yes, provided the fuel is low in sulphur
B.No, because diesel exhaust is too hot
C.No, its oxygen-rich exhaust prevents nitrogen oxide reduction
D.Yes, diesel exhaust suits the catalyst well

Answer: C

Correct answer: No, its oxygen-rich exhaust prevents nitrogen oxide reduction

A diesel runs lean, so its exhaust carries substantial oxygen throughout. Oxidation proceeds readily in it, but reduction cannot compete with the oxygen present.

The catalyst's third function therefore fails, and the device is not used on diesels. Low sulphur helps every aftertreatment device but does not create the missing chemistry, and diesel exhaust is generally cooler than a petrol engine's rather than hotter - which is a separate difficulty for light-off.

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92.

Which statement about nitrogen oxide formation is false?

A.It rises steeply with flame temperature
B.It requires oxygen to be available
C.It peaks slightly lean of stoichiometric
D.It is highest when the mixture is rich

Answer: D

Correct answer: It is highest when the mixture is rich

Thermal formation needs high temperature and available oxygen. A rich mixture is hot but oxygen-starved, so formation falls rather than peaking.

The other three statements are true, and together they place the peak: formation rises steeply with flame temperature, requires oxygen, and therefore peaks slightly lean of stoichiometric where both conditions are satisfied. It is the interaction of the two requirements, rather than either alone, that shapes the curve.

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93.

Fitting a catalytic converter to a vehicle running on leaded petrol is acceptable if the converter is oversized. True or false?

A.False, lead poisons the catalyst regardless of size
B.True, if the converter is at least twice the normal size
C.True, provided the engine runs lean
D.False, only because the converter would overheat

Answer: A

Correct answer: False, lead poisons the catalyst regardless of size

Lead deposits coat the precious metal sites and block them permanently, so a larger converter simply provides more sites to be destroyed - it delays the failure without preventing it.

A single tank of leaded fuel is enough to ruin a converter, which is why unleaded fuel had to be available before catalytic converters could be mandated. Mixture strength does not protect the catalyst, and overheating is a different failure mode altogether.

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94.

Which of the following is not a source of hydrocarbon emission from a vehicle?

A.The crankcase
B.The cooling system
C.The exhaust
D.The fuel tank

Answer: B

Correct answer: The cooling system

The cooling system carries coolant, not fuel, and is sealed from the combustion and fuel systems. It contributes no hydrocarbons.

The other three all do: the exhaust carries unburnt fuel that escaped the flame, the crankcase carries blow-by past the piston rings, and the fuel tank evaporates vapour directly. Each has its own control - the catalyst, positive crankcase ventilation and the carbon canister respectively - which is why all three are counted separately.

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95.

Exhaust gas recirculation reduces nitrogen oxides by supplying extra oxygen. Is this correct?

A.Yes, the recirculated gas is oxygen-rich
B.No, it works by raising the compression ratio
C.No, it works by diluting the charge and lowering peak temperature
D.Yes, extra oxygen completes combustion

Answer: C

Correct answer: No, it works by diluting the charge and lowering peak temperature

Recirculated exhaust is largely inert, having already burnt. It absorbs heat as combustion proceeds without releasing any, so the peak flame temperature falls.

Since thermal formation of nitrogen oxides rises steeply with temperature, that reduction cuts the emission substantially. Supplying more oxygen would raise them, not lower them, since oxygen is the second requirement for formation. Compression ratio is unaffected.

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96.

Which claim about diesel particulate filters is incorrect?

A.They trap soot in porous wall-flow channels
B.They must be regenerated periodically
C.Repeated short cold journeys can block them
D.They need no maintenance or intervention over their life

Answer: D

Correct answer: They need no maintenance or intervention over their life

A filter accumulates incombustible ash from lubricant additives, which regeneration cannot remove. Ash builds up over the vehicle's life and the filter must eventually be cleaned or replaced.

The other three are true: soot is trapped in porous wall-flow channels, regeneration is periodic and necessary, and repeated short cold journeys prevent it and block the filter. The device is maintenance-intensive by the standards of the catalyst it accompanies.

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97.

A vehicle passing an idle emission test is necessarily clean over a full driving cycle. True or false?

A.False, idle conditions omit load, transients and cold start
B.True, idle is the worst case
C.True, provided the catalyst is warm
D.False, only because idle speed varies

Answer: A

Correct answer: False, idle conditions omit load, transients and cold start

An idle test measures the engine at one condition - warm, unloaded and steady - which is the least demanding it ever sees.

A driving cycle includes cold start, when the catalyst is inactive; acceleration, when the mixture may be enriched; and load, when temperatures and flows are highest. A vehicle can pass at idle and emit heavily in every one of those. Idle is the cheapest test rather than the worst case, which is why cycle testing replaced it for type approval.

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98.

Which of the following does the carbon canister not do?

A.Reduce evaporative hydrocarbon loss
B.Treat exhaust gas leaving the engine
C.Adsorb fuel vapour from the tank
D.Release stored vapour when purged

Answer: B

Correct answer: Treat exhaust gas leaving the engine

The canister is part of the evaporative control system and sees no exhaust at all. It sits between the fuel tank and the engine intake.

The other three are exactly what it does: adsorb fuel vapour from the tank onto activated carbon while the vehicle is parked, release it when the engine controller commands a purge, and so reduce evaporative hydrocarbon loss. Exhaust treatment is the catalyst's work, on a quite separate path out of the vehicle.

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99.

Running an engine very lean always reduces every regulated pollutant. True or false?

A.True, provided the catalyst is warm
B.False, only because power output falls
C.False, misfire raises hydrocarbons
D.True, lean running reduces all pollutants

Answer: C

Correct answer: False, misfire raises hydrocarbons

Lean running does reduce carbon monoxide, and beyond the nitrogen oxide peak it reduces those too. But taken far enough the mixture becomes too weak to ignite reliably, and a misfiring cylinder passes its entire charge unburnt into the exhaust.

Hydrocarbons therefore rise sharply at the lean limit. A warm catalyst helps but cannot convert that volume, and the lean exhaust also prevents nitrogen oxide reduction - which is why lean-burn engines need their own aftertreatment.

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100.

Which of the following would not reduce cold-start emissions?

A.Mounting the catalyst close to the exhaust manifold
B.Electrically preheating the catalyst
C.Retarding ignition briefly to raise exhaust temperature
D.Increasing the engine's compression ratio

Answer: D

Correct answer: Increasing the engine's compression ratio

Compression ratio is a fundamental engine parameter affecting efficiency and knock limits, and it does nothing to shorten the interval before the catalyst lights off. A higher ratio in fact extracts more energy from the gas, leaving the exhaust slightly cooler.

The other three all attack that interval directly: close-coupling puts the converter where the gas is hottest, electrical preheating warms it before the engine starts, and retarded ignition deliberately wastes energy into the exhaust to heat it faster.

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