Emissions vs MPG

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Wed 6 Nov 2013 13:05
Emissions vs MPG - davecooper

Should there be a close (ish) correlation between emissions (g/km Co2) and MPG? For example, would you expect a 2.0l engine in a mid size car to return a similar mpg as a 1.3l engine in say a Supermini if both had the same emissions e.g. 110g/km?

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Wed 6 Nov 2013 13:16
Emissions vs MPG - brum

There should be a perfect correlation (bear in mind that petrol has a lower calorific content than diesel, and that calorific content varies slightly between winter and summer fuels)

Unfortunately between the EU looney scientists and the sunny jim manufacturers, quoted figures have very little correleation either in theory or real life.

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Wed 6 Nov 2013 13:23
Emissions vs MPG - skidpan

All diesel cars with certain C02 figure will have exactly the same mpg e.g. 119 CO2 = 62.8 mpg combined.

Same for petrol cars, one with a CO2 of 119 will have an mpg figure which is 54.3 mpg

But note, the mpg of a petrol car with 119 CO2's is lower than that of the diesel. That is because diesel emmisions are higher

So you compare diesels and you can compare petrols but be careful comparing a petrol with a diesel.

Edited by skidpan on 06/11/2013 at 13:24

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Wed 6 Nov 2013 15:43
Emissions vs MPG - daveyjp
As I understand it the EU test is actually an emissions test and from this mpg is calculated. It is not a test of how much fuel the car uses over the run.
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Wed 6 Nov 2013 18:58
Emissions vs MPG - Andrew-T

All diesel cars with certain C02 figure will have exactly the same mpg e.g. 119 CO2 = 62.8 mpg combined.

Same for petrol cars, one with a CO2 of 119 will have an mpg figure which is 54.3 mpg

Sorry, Skidpan, not strictly true. CO2 emitted will (should) be directly proportional to the amount of fuel used. Some cars are more efficient than others in converting burnt fuel into work, so they get more miles to the gallon (or gram-molecule, or whatever) for the same emissions. Some cars, of course, burn the fuel more completely than others too.

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Wed 6 Nov 2013 21:05
Emissions vs MPG - Brit_in_Germany

Andrew, your reasoning ignores the fact that the co2 value is per km. I would agree that if one engine is emitting more unbunt hydocarbons or carbon monoxide than the other, the correlation will not be perfect but the differences are likely to be small.

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Wed 6 Nov 2013 23:24
Emissions vs MPG - Andrew-T

Yes, you're right. I can't help feeling that there is a flaw somewhere though. If the CO2/km is a solid basis for calculating VED, and bears a fixed proportion to a car's mpg, why is the 'combined' figure not an equally good basis? Is it just a fad about the magic number of 100 ?

Edited by Andrew-T on 06/11/2013 at 23:27

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Thu 7 Nov 2013 16:18
Emissions vs MPG - cmo

Do you mean the combined l/100km figure? This makes it harder to compare petrol/diesel and LPG emissions as they have different values for the different fuel.

Standardising on CO2 g/km means you can compare a petrol car to a diesel cart and see which produces the most CO2.

for example

50 mpg (5.65l/100km) diesel emissions = 147g/km

50 mpg petrol emissions = 125g/km

50 mpg LPG emissions = 84 g/km all rounded to nearest gram.

These values are based on the 2013 CO2e greenhouse emissions per litre of fuel figures published by Defra. The actual values vary slightly depending on the blend of fuel, whether it is bio diesel etc.

If anyone is interested, I wrote an app which calculates and displays CO2 emissions, economy figures and shows the tax ban the values lie in. I'm not sure if it's acceptable to mention it here or not.

If you look in the Apple app store for and app called "Eco Calc" you'll find it. If the moderators want to delete this please do so. I mention it as something relevant to the topic being discussed.

There are several apps in the store which perform a similar function if you search for CO2 or economy calc.

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Thu 7 Nov 2013 18:01
Emissions vs MPG - Andrew-T

Do you mean the combined l/100km figure? This makes it harder to compare petrol/diesel and LPG emissions as they have different values for the different fuel.

Yes, I meant the combined mpg figure.

But there seems to be a small discrepancy between your numbers and Skidpan's. Yours say that diesel has 17.6% more carbon per unit volume; Skidpan only 15.6%. If we are assuming complete combustion in both cases, that means diesel is 15 (or 17) % denser than petrol. Is that the right figure, allowing for slightly different C/H proportions? And in any case, that figure will vary through the year as refiners adjust the mixture - never mind small variations between brands.

Edited by Andrew-T on 07/11/2013 at 18:03

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Fri 8 Nov 2013 11:05
Emissions vs MPG - Andrew-T

After a bit of googling I have found the following logic and fact:

Assuming complete combustion (the first approximation), each gram of carbon in fuel becomes 3.66 grams of CO2. Diesel is denser than petrol, so a given volume contains more carbon. Typically (composition varies throughout the year and between refineries) a litre of diesel contains 705 grams of carbon, petrol 627 grams.

As we all still think in gallons: 3205 grams carbon in a gallon of diesel, or 2850 grams for petrol. We should get 11.75 or 10.45 kg of CO2 from each gallon we burn.

So to attain the magic 100g of CO2 per kilometre, a diesel must do more than 117.5 km/gallon, or a petrol car 104.5 km/gallon. In UK terms, 73.3 or 65.2 mpg. Diesels come closer because they extract more work from the combustion - higher compression.

That's assuming complete combustion. We all know about DPFs (diesel particulate filters) and we still occasionally see vehicles emitting clouds of soot. That's unburnt carbon.

Edited by Andrew-T on 08/11/2013 at 11:13

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Fri 8 Nov 2013 11:21
Emissions vs MPG - cmo

But there seems to be a small discrepancy between your numbers and Skidpan's. Yours say that diesel has 17.6% more carbon per unit volume; Skidpan only 15.6%. If we are assuming complete combustion in both cases, that means diesel is 15 (or 17) % denser than petrol. Is that the right figure, allowing for slightly different C/H proportions? And in any case, that figure will vary through the year as refiners adjust the mixture - never mind small variations between brands.

Yes as you say the conversion from a litre of fuel to the amount of CO2 produced will vary slightly with different fuel mixes and if the fuel contains a percentage of bio fuel.

I used the conversion factors in the UK government guidelines for calculating emissions for business purposes, different groups use slightly different values but they are within a couple of percent of each other. The value for 2012 was different to 2013 and is based on an average fuel from UK forecourts. The figure is also for what they call kgCO2e which is the CO2 equivalent and includes the small quantities of methane and nitrous oxide.

The carbontrust publish some info, this includes the 2013 figures from Defra.www.carbontrust.com/media/18223/ctl153_conversion_...f

Basically diesel produces about 16% more CO2 than petrol, but an equivalently sized diesel car generally travels further on a litre of diesel than a petrol car on a litre of petrol. Hence the g/km figure can be used to compare vehicles even though they have different fuel types.

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Thu 7 Nov 2013 19:57
Emissions vs MPG - coopshere
So cmo, are you saying that diesel fueled cars are far more polluting than petrol or LPG?

If so why are the Oligarch's in government and the motor industry suggesting that we should all be driving around in diesels?

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Fri 8 Nov 2013 11:27
Emissions vs MPG - cmo
So cmo, are you saying that diesel fueled cars are far more polluting than petrol or LPG?

I'm saying that a litre of diesel when burn in an engine will prduce more CO2 than a litre of petrol burnt in an engine. If you had 3 identical cars that did identical miles per gallon, but they ran on diesel, LPG and petrol then yes, the LPG car produces less greenhouse gas and the diesel the most.

But the diesel car will typically travel further on a litre of diesel than the petrol car will travel on a litre of petrol. This is why CO2 per kilometer is a way of comparing the vehicles as it takes into account the diesel car will travel further.

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