A development of the "Effect of Speed on economy" thread, though a different point:
This is a little simplistic and does not account for appropriate gears etc however an engine does not run at it's most efficient at very low revs, i.e low road speeds, therefore while increased drag and rolling resistance will generally cause it to consume more fuel at higher road speeds the engine is likely to be operating nearer peak efficiency at higher road speeds and therefore will be more effectively burning the fuel it is using and thus be emitting much less in the way of hydrocarbons albeit while producing a little more in the way of CO2.
So perhaps:
CO2 is not the best measure of emissions.
MPG is not the best measure of a vehicles enviromental credentials.
Reduced average speeds do not deliver a corresponding reduction in harmful emissions.
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Cheddar,
Your engine has to be efficient at low engine speeds otherwise it would never pass the MoT idle test.
HC levels and CO are monitored by the engine and mixture adjusted accordingly, whatever the speed. Otherwise you'd have lots of taxis and 'town' cars requiring new catalysts quite frequently.
Emissions are inverely proportional to MPG.
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Your engine has to be efficient at low engine speeds otherwise it would never pass the MoT idle test.
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It is all relative, an engine is much more efficient at peak torque than it is at idle.
The MOT idle test is simple measuring emissions at idle and bears no direct relation to MPG or emissions underload whatever the engine rpm or road speed.
Emissions are inverely proportional to MPG.
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CO2 maybe, unburnt hydrocarbons no, because the latter is a matter of how completely the fuel is burnt as opposed to how much fuel is burnt.
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If the cat's working there aren't any unburnt hydrocarbons, though granted they are still wasted.
Unfortunately if you increase your speed by 20% the energy required to overcome the additional drag increases by 44%, more than negating any slight improvement in engine efficiency.
Back to the drawing board.
In the same vein though, it has been said that if you accelerate briskly, but changing up early, you make the engine operate more efficiently in the getting-up-to-speed phase, thereby saving fuel overall. This seems to work - I can severely damage my economy by using higher speeds and revs through the gears, but accelerating quickly in the 2000-3000 range seems to have no adverse effect. The same theory is used in diesel electric locos, which increase torque demand rather than revs when more power is needed, to optimise efficiency.
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It is all relative an engine is much more efficient at peak torque than it is at idle.
True, I was thinking of effectiveness in terms of fuel conversion.
CO2 maybe unburnt hydrocarbons no because the latter is a matter of how completely the fuel is burnt as opposed to how much fuel is burnt.
When warm, the ECU will do its upmost to keep incomplete combustion to a minimum. It does this by knowing what proportion of oxygen should be in an exhaust gas, checking this is correct by the lambda sensor, and adjusting the fuel mixture accordingly. It also adjusts ignition timing in comparison to throttle opening.
The engine designers will do their bit by adjusting the valve timing, amount of swirl, and compression ratio to achieve optimum performance and emissions over all rpm, with a bit of compromise.
The catalyst takes care of the rest by converting HCs into water and CO 2 using the remaining oxygen in the exhaust gas. Likewise with CO.
So you really shouldn't have many HCs in the exhaust whatsoever.
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"In a hurry are we sir?"
"No officer, I'm saving the planet"
Let us know if it works!
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