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So would we find similar carbon emissions per mile from petrol and diesel engined vehicles? I think we don't. Why's that?

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I think the same power output diesel vs petrol on the bench does give similar carbon emissions. I believe the difference is that typically a far greater proportion of drivers of Diesel cars do so for economy than their petrolhead counterparts.

A few reasons:

1) I'm not sure of the chemical composition of diesel and petrol - the ratio of H to C might be different.

2) Diesels emit more carbon in the form of solid carbon (soot)

3) Catalysts on petrol engines concert CO to CO2.

Cheers
Peter

If there is a reason and a difference, I think this is it. (I've been thinking about it since my last post.) We get energy from breaking molecular bonds, and as I recall there's lots of energy in carbon-carbon bonds.

Acetelyne is C2 H2, and since there are 4 bonds sticking out of each carbon atom, acetelyne must have triple carbon bonds - that's where the energy comes from to do our gas welding.

Methane (CH4) lacks the triple (or even a double) bond and is less energetic.

Petrol and diesel are both soups of mixed hydrocarbons, and unless there's a difference in the proportion of carbon-carbon bonds, I'm having trouble understanding what the difference is.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

In a sane world we wouldn't be worried about CO2 emissions - we'd be more worried about particulate and other harmful emissions, in which case petrol would win hands-down over diesel. In fact, I doubt there would be many diesel cars around.

Cheers
Peter

All you need to do is look at the co2 of a car to see where is stands.

Golf 115hp diesel 138 g/km
Golf 1.4L petrol 161 g/km

I suspect the co2 is probably related more or less to fuel consumption.

I agree on particulates...they are the killer (literaly)

You can soon tell if you are behind a diesel when they put the pedal to the metal...BLACK SMOKE...cough, cough.

No matter how much technology you put into emissions control you still get smoke.

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Speed camera policy Kills

Hell, yes. Exactly!

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Yes I stand by what I say - that the cost of fuel in a diesel at 7p per mile is pretty cheap. Doing an average 10000 miles per year that would be about £700 a year in fuel. Compare that to the depriciation that a new or nearly new car suffers it's by no means the most expensive part of driving. Personally I can't see the justification in lowering the cost of diesel anymore then that, exactly how cheap do you want it? As cheap as other countries? Well I'd hardly think that was likely since the government wants to introducing road charging upto an insane £1.35 a mile. There are far easier ways to cut the cost of motoring then to be worried that diesel is a few pence more the petrol. Buying older cars where the sting has been taken out depriciation is an excellent place to start.

Of course all that relates to private motoring - yes I do consider there to be an issue with foreign hauliers filling their twin tanks with 1000 litres of cheaper foreign diesel and undercutting British hauliers. Perhaps the solution if it was possible(after all this is brainstorming) would be to have blue diesel, perhaps where the price was harmonised accross Europe to give everyone a level playing field. Or maybe not, I don't know it's just a thought.

California has the toughest air polution laws in the world

This is what they say about DIESEL.........

http://www.epa.gov/ttn/atw/urban/dieselfs.pdf#search='california%20diesel%20emissions'

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Speed camera policy Kills

Now you're entering into a debate about the moral ethics of road fuel taxation. Also, I still do not agree with your assessment that "diesel production costs are, if anything, higher than those of petrol" - unless you're incorporating oil company profit into the "production cost".

The diesel that I put in my car at 90p/ltr is, it has to be said, not that different from heating oil. In fact I could probably put heating oil in my car and get reasonable performance. So I had a poke round to see if I could get a heating oil price. I found a .PDF document produced by the States of Jersey government. Last year (2004) the average price of heating oil was 25p/ltr. So I feel confident that the oil companies would be capable of producing genuine diesel fuel for about the same price, or not more than 30p/ltr. States of Jersey document (.PDF): http://www.gov.je/statistics/content/pd ... gjun04.pdf If you still maintain that the tax on fuel is only 47.1p, then diesel would cost about 70p not 90p. Do your figures include VAT?



It might have something to do with the compression ratio. Please refer to this analysis and comparison between diesel and petrol engines: http://www.iea.lth.se/people/evs20_karin_jonasson.pdf I have not read all of this document, but it makes clear that as well as diesel fuel having more energy per litre (as you stated correctly - the actual figure I have seen is 17%) the other factor is the diesel engine's much higher compression ratio. The document indicates that a petrol engine cannot operate at more than ~33% efficiency, while a diesel engine can operate at ~45% efficiency.

No, I'm deliberately not doing that as it's a separate issue.


Unless you subscribe to the "oil companies are evil greedy bastards" school of thought, I fail to see why they would deliberately apply higher mark-ups to diesel than to petrol. The total production and distribution costs of each are going to be fairly similar.


No, 47.1p per litre is just fuel duty. VAT is 7/47ths of the selling price. So, for a litre selling at 90p, duty is 47.1p, VAT is 13.4p, so the oil company (including all stages of the distribution chain) gets 29.5p, which is consistent with the figures you quote above, taking into account the fact that the price of crude has risen significantly since 2004.

IIRC there is no VAT in Jersey.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

Hi there, PeterE

I’ve tried to acquire an analysis of fuel prices, but the best I could do was from the AA website which I’ll discuss in a moment.

You’re probably right about the relative production costs of diesel/petrol. I think the thing was 90 years ago – the era when, according to that Gigantic Machines TV programme, diesel cost one fifth of petrol, apart from there being no swingeing taxes on motorists, there were no real concerns about vehicle emissions, so no additional processes were carried out in the diesel fuel production process to make it more environmentally friendly.

Some other interesting material I’ve been able to Google up:

• One of Rudolph Diesel’s early engines ran on peanut oil – the original bi-diesel.
• One link suggested that 25% more crude oil was needed to produce diesel than petrol.
• With reference to the above, America (especially California) is hyper sensitive to the emissions issue, and I suspect the reason for the extra cost of diesel there is to make it comply with emission regs. This could be the reason why America is one of very few countries where diesel costs more than petrol, but even then it’s price is only about 34p/ltr.
• "California Diesel" costs 3-4 cents a gallon more than diesel in other states - because of the extra processing to get it to meet environmental standards.
• It is suggested that the particulate emission from diesel engines using biodiesel will be 26% less than those using normal hydrocarbon diesel.
• One report I found suggests that Rudolph Diesel committed suicide by throwing himself overboard a ship - a reaction to criticism of his engines!

Thanks for explaining how the UK tax works! So the VAT is not just on the base price of the fuel, but on the duty as well. Tax on tax! I wonder which government introduced that.

Table of Fuel Prices Around the World

The ones in blue show where diesel costs more than petrol. Source: http://www.theaa.com/onlinenews/allabou ... ay2005.doc

I'll leave it to the reader to decide how much we're being ripped off for diesel in Britain.

Technical differences between petrol and diesel engines:

Diesel engines run more efficiently at part-'throttle' than petrol engines. The reason being to control the power of a diesel engine more or less fuel is injected. The same amount of air is drawn in per stroke irrespective of the power generated. The fuel/air ratio is not critical for combustion with diesel.

Petrol engines, on the other hand have to vary the amount of fuel AND the amount of air drawn into the engine, as the fuel/air ratio required for combustion is more critical. (Look at a carburettor and you'll see butterfly valves to restrict the air intake). This means at part throttle on a petrol engine the pistons have to act like pumps trying to suck in air through the partly closed off air intake, using up some of the energy that would otherwise go to turning the wheels.

This means that on average diesels run more efficiently for the same energy input.

(edit...)

After a bit of research on the net from the following sites:
http://www.simetric.co.uk/si_liquids.htm
http://www.engineeringtoolbox.com/fuels ... 9_169.html

Diesel has lower energy/kg than petrol but as it is also denser its energy/litre is slightly higher.

Based on the figures from the above pages 1 litre of petrol contains 97% of the energy 1 litre of diesel contains. Not a big difference!

The vast majority of current diesel engines are turbocharged which uses otherwise wasted exhaust energy to push air in.

Petrol engines with a turbo also get this advantage, but tend to be designed as high-performance engines rather than for economy. Building in the turbo complicates the build and increases the expense.

I was pleasently surprised by how fuel efficient my petrol car with a large turbo on it can be when driven with a very light right foot on longer journeys, but it can also be very uneconomic when driven with "enthusiasm"... I expect that many high-performance diesels also have poor economy when driven in in a similar style, and they certainly beltch out large amounts of particulates when pushed hard.

Diesels appear to geneate particulates when pushed hard because the increase in airflow through the engine and out the exhaust tends to clear particulates that have accumulated in the exhaust system when the engine hasn't been revving so high.. If a diesel is 'thrashed' all the time then particulates wouldn't build up in the exhaust so much and there wouldn't be the black smoke. I know this for a fact, having driven a small diesel car that needed to be driven hard to maintain progress . There was no visible black smoke even under the hardest acceleration.

That would have been about 20 years ago.