CO2 Emissions explanation...
Discussion
All this budget nonsense has got me angry, and I was hoping someone could explain the following question regarding CO2 emissions...
I've been looking at various cars and their emissions, as below..
Car MPG (Combined) CO2 emissions (g/km)
BMW M3 22.8 295
Monaro VXR 24.9 384
Impreza WRX 27.4 244
WRX Sti 25.9 257
Assuming that the above figures are correct (taken off internet), how is it possible for an M3, to drive say 1 mile and burn more fuel than a Monaro, yet produce less CO2?
Any explanations welcome..
Thanks,
Stu
I've been looking at various cars and their emissions, as below..
Car MPG (Combined) CO2 emissions (g/km)
BMW M3 22.8 295
Monaro VXR 24.9 384
Impreza WRX 27.4 244
WRX Sti 25.9 257
Assuming that the above figures are correct (taken off internet), how is it possible for an M3, to drive say 1 mile and burn more fuel than a Monaro, yet produce less CO2?
Any explanations welcome..
Thanks,
Stu
Emmissions tests are different to MPG.
IIRC the emission test are conducted on a cold engine within x minutes of startup. MPG are conducted on a warm engine.
Its one of the reasons that Euro 4 engines are less efficient than euro 3 - they need to heat up so quickly in order to meet the emissions tests that it impacts fuel economy.
The old rover 1.8 in an elise could reach a combined fuel over 40mpg but the new Toyota euro4 engine struggles to get into the mid 30's.
IIRC the emission test are conducted on a cold engine within x minutes of startup. MPG are conducted on a warm engine.
Its one of the reasons that Euro 4 engines are less efficient than euro 3 - they need to heat up so quickly in order to meet the emissions tests that it impacts fuel economy.
The old rover 1.8 in an elise could reach a combined fuel over 40mpg but the new Toyota euro4 engine struggles to get into the mid 30's.
Edited by ads_green on Thursday 13th March 22:01
it to do with the efficiency of petrol combustion process in the engine.
Petrol engines are most efficient when under load and not light cruising (diesels are the opposite). This is the main reason why a smaller engine is proportionately more efficient than larger engines.
so a smaller engine can burn the same amount of fuel but if it's doing it under more load the combustion is better and less CO2 produced.
(well thats how I understand it but could be wrong)
Petrol engines are most efficient when under load and not light cruising (diesels are the opposite). This is the main reason why a smaller engine is proportionately more efficient than larger engines.
so a smaller engine can burn the same amount of fuel but if it's doing it under more load the combustion is better and less CO2 produced.
(well thats how I understand it but could be wrong)
twistedsanity said:
isnt it the catalytic converters that the governbent made us fit which ensures that all our exhaust gases are converted into CO2 anyway?
yeah - converts CO and NO to CO2 and NO2.
Now if somebody could invent a cat converter that could change them into something else they would be rich beyond wildest dreams of avarice.
ads_green said:
it to do with the efficiency of petrol combustion process in the engine.
Petrol engines are most efficient when under load and not light cruising (diesels are the opposite). This is the main reason why a smaller engine is proportionately more efficient than larger engines.
so a smaller engine can burn the same amount of fuel but if it's doing it under more load the combustion is better and less CO2 produced.
(well thats how I understand it but could be wrong)
Rot I say Petrol engines are most efficient when under load and not light cruising (diesels are the opposite). This is the main reason why a smaller engine is proportionately more efficient than larger engines.
so a smaller engine can burn the same amount of fuel but if it's doing it under more load the combustion is better and less CO2 produced.
(well thats how I understand it but could be wrong)
By weight fuel combusts as follows
So more fuel is more CO2, simple. The only possible way this is not the case is if HC or CO is emitted, as you have rightly said cats convert these incomplete combusted byproducts to CO2 as far as possible. So assuming working cats, exactly the same amount of CO2 will be produced by each vehicle burning the same amount of fuel. Of course if one vehicle belches out C, CO and HC then thats where the less amount of CO2 is. Thats very bad and eith Euro3/4 standards less likely as complete combustion to H2O and CO2 is always the goal of any emissions system.
Have to agree with ads_green here; the products of combustion depend entirely on the energy conversion (i.e. combustion) efficiency.
So, whilst your equation may be correct ringram; it is only true if the amount of hydrocarbons (C2H18 molecules) are in the same ratio to oxygen that you have stated (i.e. 2:25).
Thus, the VXR engine is probably operating with incomplete combustion for a greater amount of time than the M3 engine, thus the M3 engine appears to produce less Carbon Dioxide. It'll all be to do with the different fuelling of each engine, because of the different masses of each car (the M3 being around 300-400kg lighter than the VXR).
Calum.
So, whilst your equation may be correct ringram; it is only true if the amount of hydrocarbons (C2H18 molecules) are in the same ratio to oxygen that you have stated (i.e. 2:25).
Thus, the VXR engine is probably operating with incomplete combustion for a greater amount of time than the M3 engine, thus the M3 engine appears to produce less Carbon Dioxide. It'll all be to do with the different fuelling of each engine, because of the different masses of each car (the M3 being around 300-400kg lighter than the VXR).
Calum.
Combustion of petrol in cars isn't efficient - thats the fundamental reason why tubos actually do anthing. They utilise the latent energy in the still expanding exhaust gases that go otherwise unsed.
As far as I know the CO2 figure is not induced from the mpg figures as these are often calculated rather than taken from imperical experimentation. The accuracy of the figures vary from manufacturer to manufacturer. For example, if you can find any RX8 owner gettin near the official figures then you're doing very very well.
There is a specific process for obtaining the CO2 with strict regulations on the engine rpm, temperature relative to the outside air and suchlike. Similar methods are used during the MOT process for subsequent comparison.
As far as I know the CO2 figure is not induced from the mpg figures as these are often calculated rather than taken from imperical experimentation. The accuracy of the figures vary from manufacturer to manufacturer. For example, if you can find any RX8 owner gettin near the official figures then you're doing very very well.
There is a specific process for obtaining the CO2 with strict regulations on the engine rpm, temperature relative to the outside air and suchlike. Similar methods are used during the MOT process for subsequent comparison.
I have to agree with Ringram, the CO2 HAS to be directly proportional to the fuel burnt. Especially if the cat converts any unburnt/incompletely burnt fuel as well.
Also take the other point made by adsgreen; the quoted fuel consumption and CO2 emmisions are measured under totally different conditions, so don't correlate properly. IMO the CO2 figure [b]should[b] be calculated from the consumption figures, which in turn should reflect real world use.
Also take the other point made by adsgreen; the quoted fuel consumption and CO2 emmisions are measured under totally different conditions, so don't correlate properly. IMO the CO2 figure [b]should[b] be calculated from the consumption figures, which in turn should reflect real world use.
tim the pool man said:
I have to agree with Ringram, the CO2 HAS to be directly proportional to the fuel burnt. Especially if the cat converts any unburnt/incompletely burnt fuel as well.
Also take the other point made by adsgreen; the quoted fuel consumption and CO2 emmisions are measured under totally different conditions, so don't correlate properly. IMO the CO2 figure [b]should[b] be calculated from the consumption figures, which in turn should reflect real world use.
CO2 is calculated from fuel burnt. Fuel consumption figures though are not intended nor purport to reflect actual driving conditions. They are obtained under very strict conditions meant to provide an accurate way to compare vehicles. See http://www.vcacarfueldata.org.uk/information/fuel-... and http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?... for example.Also take the other point made by adsgreen; the quoted fuel consumption and CO2 emmisions are measured under totally different conditions, so don't correlate properly. IMO the CO2 figure [b]should[b] be calculated from the consumption figures, which in turn should reflect real world use.
Unburnt fuel isn't converted by the cat. It's actually incredibly dangerous and I know of more than one car going up in flames from unburnt fuel getting into the cat (800c+) and combusting creating even more heat.
Agree that a given amount of fuel burnt will cause x amount of co2 but in engine terms its not that simple. Alot of engines have "exhaust gas recirculation" that allows some of the exhaust to be fed back into the induction to be burnt again to further reduce emissions. Now if you had two cars one with this on and one without then they would both appear to burn the same amount of fuel but one would output more CO2 than the other. This is just one example of differing engine technologies causing unexpected results.
Agree that a given amount of fuel burnt will cause x amount of co2 but in engine terms its not that simple. Alot of engines have "exhaust gas recirculation" that allows some of the exhaust to be fed back into the induction to be burnt again to further reduce emissions. Now if you had two cars one with this on and one without then they would both appear to burn the same amount of fuel but one would output more CO2 than the other. This is just one example of differing engine technologies causing unexpected results.
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