Exhaust gas recirculation reduces oxides of nitrogen by
Answer: C
Recirculated exhaust absorbs heat without taking part in combustion, so the peak temperature falls and thermal formation with it. Too much of it costs power and drivability, so the rate is scheduled against load and speed.
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A diesel engine emits more particulate than a petrol engine principally because
Answer: D
Fuel burns as it is injected into hot air, so there are always regions richer than stoichiometric where soot forms. Later injection strategies and higher injection pressures both aim to shorten the time such regions persist.
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A diesel particulate filter is regenerated by
Answer: A
Soot accumulates in the wall-flow channels until the back pressure calls for a burn, which is achieved by late injection or a fuel burner. Frequent short journeys prevent the filter reaching regeneration temperature, which is a common cause of blockage.
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Selective catalytic reduction on a heavy diesel vehicle uses
Answer: B
The urea decomposes to ammonia in the hot exhaust and the catalyst then reduces the oxides to nitrogen and water. Carrying a second consumable is the price of allowing the engine itself to be tuned for efficiency rather than for low emissions.
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An oxidation catalyst on a diesel engine treats
Answer: C
A diesel runs lean, so there is ample oxygen for oxidation but no possibility of reducing nitrogen oxides on the same bed. That is precisely why a diesel needs separate devices for the two duties while a petrol engine does not.
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Evaporative emissions from a petrol vehicle arise principally from
Answer: D
Correct answer: Fuel vapour escaping from the tank and fuel system
Evaporative emissions are hydrocarbons that never pass through the engine at all - petrol vapour escaping from the tank, the fuel lines and the injectors, driven off by ambient warmth and by the day's temperature cycle.
They are controlled by a carbon canister that adsorbs the vapour while the vehicle is parked and releases it to be burnt when the engine runs. The exhaust, crankcase and catalyst are all sources of emissions, but of combustion products rather than raw fuel vapour.
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A carbon canister in the evaporative system
Answer: A
Vapour is held on activated carbon while the engine is off and drawn into the intake once it is running warm. The purge must be scheduled carefully, since an uncontrolled purge upsets the fuelling and drives the mixture rich.
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Positive crankcase ventilation exists to
Answer: B
Gases forced past the piston rings carry unburnt fuel and oil vapour, and venting them was once a substantial source of hydrocarbon emission. Routing them back to be burnt was among the earliest and cheapest control measures adopted.
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Lead was removed from petrol chiefly because it
Answer: C
Lead deposits coat the precious metal sites and destroy the catalyst irreversibly, so unleaded fuel was a precondition for the whole technology. Its own toxicity, particularly to children, made the case unanswerable in any event.
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Low sulphur diesel is required for modern aftertreatment because sulphur
Answer: D
Correct answer: Poisons catalysts and forms sulphate particulate
Sulphur attacks aftertreatment twice over. It adsorbs onto the precious metal sites and suppresses catalytic activity, and it is oxidised to sulphate which condenses and is weighed as particulate.
Both effects made low sulphur fuel a precondition rather than a refinement: particulate filters and reduction catalysts could not be introduced until the fuel supply was cleaned up, which is why sulphur limits and emission standards advanced in step. It does not affect atomisation or cetane appreciably.
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