CO2 - Mike Williams
Can anyone please help me with the following query?

Which produces the most CO2 when burned in an internal combustion engine, a gallon of petrol, or a gallon of Diesel?

Is there any difference in the CO2 output of petrol and Diesel depending on how they are burned, or is the figure always the same?

Thanks ........... Mike
Re: CO2 - John Slaughter
A galon of diesel produces more CO2. It's 10% more dense than petrol, but with a similar CV. This alone accounts for some of the economy benefit - you buy by volume, but get a greater mass. It's also got a slightly different H/C ratio - lower than petrol. I'll see if I can find some more accurate figures, but someone will probably beat me to it!


Re: CO2 - Andrew Gordon
Er, yes but you burn less of it. If this is a CO2 argument in favour of petrol, forget it.
Re: CO2 - John Slaughter

Yes, a diesel does have a slightly greater thermal efficiency than a petrol engine, but that's not the whole story. The point here is that we buy fuel by volume, but in that gallon you get 13.5% greater mass of diesel (I've now got the accurate figures) because it's more dense. The CV of diesel and petrol are effectively identical, so you get 13.5% more energy in a gallon of diesel. In two engines of the same efficiency, one diesel, one petrol, you'd get 13.5% more miles/gallon from the diesel because of this alone. This therefore accounts for quite a lot of the fuel consumption benefit of a diesel - diesel buyers get more energy for their money!

I've also confirmed that the hydrogen/carbon ratio of diesel and petrol are the same (slight error in my memory before), so equal volumes of the two fuels will, assuming complete combustion, produce equal amounts of carbon dioxide. However, the question was based on gallons. A gallon of diesel contains more carbon than a gallon of petrol, because it's a greater mass, so a gallon of diesel produces more carbon dioxide than a gallon of petrol.
This is independent of the efficiency of the engine.


Re: CO2 (Correction to last posting) - John Slaughter

Just spotted an error! Last but one para should say-

... so equal MASSES of of the two fuels will, assuming complete combustion, produce equal volumes of carbon dioxide...

Sorry about that!

Re: CO2 - Chris
If you're concerned about the CO2 emissions at the tailpipe you have to take into account the total amount of CO2 emitted including the process of making the fuel. As I understand it diesel requires far less energy in the refining than petrol and is actually a byproduct of making LPG. Taking into account the fact that diesels are roughly twenty percent more fuel efficient than equivalent petrol engines, they are a much more CO2 friendly choice overall. I'm not even going to mention the many benefits of biodiesel. Diesels are so good now that from an environmental point of view there really is no excuse for a new petrol engine unless you are on a race track (and I include the so-called fuel efficient hybrids).
Re: CO2 - Neil
Unfortunately, the Biodiesel referred to by Gordon Brown for a reduction in fuel duty does not include Biodiesel manufactured from fuel-crops. Although there will be CO2 emissions in fuel-crop farming the crop itself absorps the CO2 it produces later in the combustion cycle. If you're after a solar-powered car then Biodiesel is, in essence, an incredibly effective fuel.
Re: CO2 - Jon Shaw
Sorry for the late followup but I doubt diesel is a by product of LPG manufacture as the volume of diesel/fuel oil sold must much greater than LPG for fuel/road use. Further more Low Sulpher diesel would require additional energy to clean it up.

Deisel cars may produce less CO2 than other fuels but unlike the chancellor I tend to think that its the other pollutants that are important for road vehicles, as all cars produce lots of toxic pollutants just where people are living and breathing. So from a an environmental view concentrate on reducing these and so I say steer clear of diesel and choose LPG check out for stats. If you want to reduce the amount of CO2 you use, turn the heating down, put on a jumper and cancel that package holiday to the Med.

To declare my intrest (and bias) I have a power shift funded LPG Mondeo. It produces 13% less CO2 per km on LPG than petrol. All powershift literature quotes fuel economy in DEL (Diesel Equivalent Litre) to compensate for the different densities menitoned by John.

Best regards

Value my car