Fuel tax and CO2 (LPG)

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I've been meaning to post these figures for a while and I did so in the coffee-break section for another discussion. These are the DEFRA standard values of CO2 production for various fuels (the number is kg of CO2):

Grid Electricity (note1)
0.43 per kWh

Natural Gas
0.19 per kWh
5.43 per therm

Gas Oil
3190 per tonne
0.25 per kWh
2.69 per litre

Diesel
3164 per tonne
0.25 per kWh
2.63 per litre

Petrol
3135 per tonne
0.24 per kWh
2.30 per litre

Fuel Oil
3223 per tonne
0.27 per kWh

Coal (note2)
2548 per tonne
0.32 per kWh

LPG
0.214 per kWh
6.27 per therm
1.49 per litre

Note1: This figure is relatively low becasue of nuclear/renewable input to the grid - it is a 'standard' which has been fixed for accounting purposes until 2010 and might not be 100% accurate.
Note2: This is for industrial coal use, not power generation or home heating. If anyone would like alternative applications I'll see if I can find a definitive CO2 figure.
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Tim.

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OK now we have got some raw data let us see what it means:

The part which may be of particular interest is the CO2 produced by the three main road fuels:
LPG 0.214 per kWh
Petrol 0.24 per kWh
Diesel 0.25 per kWh
Notice that I have taken the CO2 per kWh and if I have got this right, this is the energy in the fuel NOT what you get out of the engine. If our government was to apply a tax based on CO2 emissions then LPG (while the best of the three) shouldn't get anywhere near the tax concession that it does. This is the reason I think in the long run the tax benefit of LPG will be removed.

Let us now factor in engine efficiency, that is the amount of mechanical energy you get out verses the chemical energy in the fuel. I will have to generalise but automotive diesel engines are about 40% (should be able to find a reference) more efficient at converting the fuel into useful mechanical energy. This changes the positions quite dramatically, putting diesel strongly in the lead as the fuel of choice.

It is interesting to note that a purpose built LPG engine could probably get close to diesel CO2 output. But this simply can't be done with a converted petrol engine becasue the thermodynamic efficiency is not altered much without changing (raising) compression ratio either physically or by adding a turbo/supercharger.

Discuss.
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Tim.

[iansoady]

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Ian, some good point there.
There are indeed problems with respiratory pollutants. Most of the polluting chemicals naturally break down quite quickly in the atmosphere and catalytic converters accelerate this process, but they are not perfect. The problem is caused mainly by the huge number of vehicles used for short journeys.

Some diesel vehicles are now fitted with particulate filters (or scrubbers), which seems like a good idea. I don't know if it has any detrimental effects on the engine, nor how often they need changing/emptying.

You it the nail on the head with your comment about where the hydrogen will come from. Every process I know of is inefficient in energy terms. Personally I prefer battery vehicles based on Lithuim-Ion batteries, but I have to admit bias because I've build an EV based on this technology.
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Tim.

[Privateer]

who gives a toss, i love my 4.0L on petrol or gas.