Automobile Emission Control Questions and Answers

Q: Three of the following are diesel aftertreatment devices. Which belongs to a petrol engine? Three-way catalyst Diesel oxidation catalyst Particulate filter Urea reduction catalyst

Correct answer: Three-way catalystA three-way catalyst works only in exhaust near stoichiometric, which is a petrol engine's condition. A diesel runs lean, so the oxygen-rich exhaust prevents the reduction half of the chemistry entirely.The other three are diesel devices: an oxidation catalyst for carbon monoxide and hydrocarbons, a filter for soot, and a urea reduction catalyst for nitrogen oxides. The diesel needs three because no single device can do in lean exhaust what the three-way catalyst does at stoichiometric.

Q: Burning off the soot trapped in a diesel particulate filter is called _____.

Correct answer: regenerationRegeneration 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.

61.

Three of the following are diesel aftertreatment devices. Which belongs to a petrol engine?

Three-way catalyst

Diesel oxidation catalyst

Particulate filter

Urea reduction catalyst

A.Three-way catalyst
B.Diesel oxidation catalyst
C.Particulate filter
D.Urea reduction catalyst

Answer: A

Correct answer: Three-way catalyst

A three-way catalyst works only in exhaust near stoichiometric, which is a petrol engine's condition. A diesel runs lean, so the oxygen-rich exhaust prevents the reduction half of the chemistry entirely.

The other three are diesel devices: an oxidation catalyst for carbon monoxide and hydrocarbons, a filter for soot, and a urea reduction catalyst for nitrogen oxides. The diesel needs three because no single device can do in lean exhaust what the three-way catalyst does at stoichiometric.

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62.

A vehicle used only for short urban journeys repeatedly reports a blocked particulate filter.

What is the most likely explanation?

A.The urea dosing system has failed
B.Exhaust temperature never rises enough for the filter to regenerate
C.The filter element is faulty
D.The fuel sulphur content is too low

Answer: B

Correct answer: Exhaust temperature never rises enough for the filter to regenerate

A filter traps soot continuously and must periodically burn it off, which needs an exhaust temperature that short urban journeys never sustain.

Soot therefore accumulates until back pressure forces the engine management to warn or derate. The usage pattern is the cause, and the remedy is a periodic longer run at load, or a forced regeneration in a workshop. A faulty element or a dosing failure would not correlate so exactly with journey length, and low sulphur helps rather than hinders.

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63.

A petrol vehicle fails its test on carbon monoxide and hydrocarbons but passes on nitrogen oxides.

What does the pattern suggest?

A.The exhaust gas recirculation valve is stuck open
B.The particulate filter has blocked
C.The mixture is running rich of stoichiometric
D.The mixture is running lean

Answer: C

Correct answer: The mixture is running rich of stoichiometric

The pattern is diagnostic. Rich running means incomplete combustion, so carbon monoxide and unburnt hydrocarbons both rise; and it means an oxygen-starved flame, so nitrogen oxide formation falls.

Two failures and one pass, in exactly those pollutants, points to fuelling rather than to any single component. A lean mixture would give the opposite signature, high nitrogen oxides with low carbon monoxide, and neither recirculation nor a particulate filter would produce this combination.

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64.

A fleet operator finds that ten per cent of the vehicles account for a third of the total emission.

What does this argue for?

A.Tighter standards for new vehicles
B.Reducing the fleet size proportionally
C.Improving fuel quality
D.An inspection and maintenance programme targeting the worst performers

Answer: D

Correct answer: An inspection and maintenance programme targeting the worst performers

The distribution is the argument. If a tenth of the fleet produces a third of the emissions, attention paid to that tenth is worth several times attention spread evenly.

Such vehicles are typically poorly maintained or have defeated equipment, and both are found by testing and corrected by repair. New vehicle standards do nothing for a vehicle already in service, better fuel helps every vehicle equally rather than targeting the problem, and reducing the fleet does not address the skew at all.

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65.

An engine is retuned to run lean to improve fuel economy, and nitrogen oxide emissions rise rather than fall.

What has happened?

A.The mixture has moved to the slightly lean peak of nitrogen oxide formation
B.The catalyst has failed
C.The exhaust gas recirculation has been disabled
D.The fuel sulphur has increased

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.

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66.

A vehicle's evaporative emissions rise sharply after a fuel system repair.

What should be checked first?

A.The urea injector
B.The carbon canister and its purge and vent connections
C.The particulate filter
D.The exhaust gas recirculation valve

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.

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67.

A heavy vehicle's engine derates itself and a warning appears after a long journey.

Which cause fits best?

A.The carbon canister is saturated
B.The compression ratio has fallen
C.The urea solution tank has run empty
D.The lambda sensor has failed

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.

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68.

The ratio of the actual air-fuel ratio to the stoichiometric value is called _____.

A.the cetane number
B.the octane number
C.the equivalence factor
D.lambda

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.

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69.

The temperature at which a catalytic converter becomes effective is its _____ temperature.

A.light-off
B.flash
C.dew point
D.sintering

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.

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70.

Burning off the soot trapped in a diesel particulate filter is called _____.

A.backflushing
B.regeneration
C.purging
D.scavenging

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.

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