The three regulated pollutants a three-way catalytic converter treats are
Answer: A
The converter oxidises the first two and reduces the third, all on the same catalyst bed. Doing both at once is only possible within a narrow band of air-fuel ratio, which is why the device depends absolutely on closed loop fuelling control.
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The stoichiometric air-fuel ratio for petrol is approximately
Answer: B
It is the ratio at which there is exactly enough air to burn the fuel completely with none left over. A three-way catalyst works only within about one per cent of this point, which is the whole reason for the lambda sensor and its feedback loop.
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Lambda in engine control is defined as
Answer: C
A lambda 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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A lambda or oxygen sensor in the exhaust works by
Answer: D
A zirconia cell produces a large step in output as the residual oxygen disappears, which makes it an excellent switch but a poor absolute meter. The control system uses that step to hunt continuously around the stoichiometric point.
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Running an engine rich of stoichiometric increases emissions of
Answer: A
There is insufficient oxygen to complete the combustion, so carbon is left partly oxidised and some fuel escapes unburnt. Oxides of nitrogen actually fall on the rich side, because peak temperatures and free oxygen both decline.
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Oxides of nitrogen from a petrol engine peak at an air-fuel ratio
Answer: B
A little excess oxygen combined with still-high flame temperature is the worst combination for thermal nitrogen oxide formation. Going leaner still cools the flame and the emission falls away again, which is the basis of lean burn strategies.
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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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