PowerSwitch

Is there a straight correlation between vehicle fuel consumption and CO2 emissions? In other words, could you have a high-mpg vehicle with high emissions or a low-mpg vehicle with low emissions?

Basically yes. Diesel contains more carbon per gallon than petrol, but this is usually more than offset by being more energy dense and diesel engines being more thermodynamically efficient, Otherwise yes.

I have never understood why the figures do not match exactly between different makes and models. I guess it is down to measurement errors or slight variations in the grade of petrol used in the tests.

Of course, if you use organically grown biodiesel you could argue that your net emissions are zero, but I don't think it would wash with official regulations.

Plug in hybrids get complicated, as the carbon content of the electricity from the grid is not included in the official measurement of tailpipe emissions, so CO2 figures for such cars are fairly meaningless.

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of course CO2 is simply the biggest of a car's emissions. There are nitrous oxides, sulpher oxides, unburnt hydrocarbons, particulates, and lots of nasty polycyclic and aromatic chemicals from partially burnt fuel, and compounds of additives in the fuel. Historically Lead was the most toxic, but it's replacement is far from benign.

Thanks Ralph.

An 'no' to the second question, then? Meaning, every fossil-fuel vehicle that gets, say, 60 mpg, emits the same weight CO2 per gallon?

A further complication is that the MPG figure quoted is usually a weighted average between two different official test cycles, and I am not sure how the CO2 test is done.

But basically a diesel car which returns 73 mpg (real world) will emit about 100g of CO2 for every kilometre.

For petrol cars, somewhere between 63 and 68 mpg emits about 100g CO2.

Also official MPG figures are little more than fiction, because car companies do their own tests and cheat, more and more as the years go by because that cuts the car tax people have to pay when buying them.

emordnilap wrote:Thanks Ralph.

An 'no' to the second question, then? Meaning, every fossil-fuel vehicle that gets, say, 60 mpg, emits the same weight CO2 per gallon?

Yes, the burning of a given weight of a given grade of fuel will produce a set amount of carbon dioxide.
Virtualy all the carbon in the fuel will be reacted with oxygen from the air, a very little carbon might be emiited unburnt, as soot or smoke, or patialy burnt as carbon monoxide, but this is normally negligable.

Talking of emissions,

http://www.theguardian.com/environment/ ... en-dioxide

Diesels are the primary culprit. Real world emissions are far worse than official figures, especially for 'clean' diesel cars in stop start traffic.

PS_RalphW wrote:Talking of emissions,

http://www.theguardian.com/environment/ ... en-dioxide

Diesels are the primary culprit. Real world emissions are far worse than official figures, especially for 'clean' diesel cars in stop start traffic.

Carbon dioxide emisions CAN NOT be greater than the amount of carbon contained in the fuel being burnt.

Other pollutants such as nitrogen dioxide are not directly related to the volume of fuel burnt and are a cause for concern.

PS_RalphW wrote:Diesels are the primary culprit. Real world emissions are far worse than official figures, especially for 'clean' diesel cars in stop start traffic.

Interesting, maybe the Americans were right all along to shun 'dirty' diesel in favour of cleaner burning petrol. Maybe we should all be driving small, light, clean, petrols - especially if a lot of driving is in urban areas. Save the diesels for long, open country drives?

PS_RalphW wrote: Real world emissions are far worse than official figures, especially for 'clean' diesel cars in stop start traffic.

My diesel, a Citroen C3 seems to give us about what the official figure promised, though it's not been quite so good in cold weather. Is that normal, even after a few miles and the engine is at normal operating temperature? Will cold air coming in make a reduction in performance?

Yes, cold air will require the burning of a greater weight of fuel in order to do the same work, but the difference is minute and unlikely to be noticable.
And since fuel is sold by volume and not by weight, this should be counterbalanced by the fact that a gallon of diesel sold in the winter should weigh more, provided that the chemical make up of the fuel remains the same.
A noticable increase is more likely to be due to other factors such as

Slightly different driving style in the winter.
Different and slightly more energy intensive routes taken in winter.
Greater weight of persons or payload carried in the winter
More electricity used for lighting, heated rear window, or heater blower in winter
Greater rolling resistance in tyres, wheel bearings, and gearbox when cold
Winter blend of diesel contains less energy per gallon than summer fuel.

Air is more dense in cold weather. At higher speeds wind resistance is the majority of the energy expended. In a recent trip I got 60mpg into the wind, and 75mpg with a lesser wind behind me coming back again, and driving faster.

All interesting stuff guys, thanks.

clv101 wrote:Interesting, maybe the Americans were right all along to shun 'dirty' diesel in favour of cleaner burning petrol.

Not really wishing to malign ordinary Americans again (!) but some of them are certainly fruitloop crazy bonkers kooky March hare non compos mentis.

Any idea of the emissions of this vehicle: http://youtu.be/EoTVvTTiT6w And what, precisely, is its purpose?

clv101 wrote:Maybe we should all be driving small, light, clean, petrols - especially if a lot of driving is in urban areas.

Due to tightening EU emissions regulations, 2015 should see better mpg from new petrol vehicles (with more improvements mandated by 2020). For instance, they're talking about the next VW Polo petrol doing over 60mpg, though that'll depend on the style of driving. We've had three petrol Polos and the later model (2009) was defo. the worst of the three - only 45mpg with very careful driving, compared with 55-ish from a 2001 model.

The choice between diesel and petrol is not all that obvious. All crude oils are a mixture of hydrocarbons, and distilling them will produce a range of products, from very light to very heavy, with petrol towards the light end and diesel a bit heavier. The exact proportion of diesel and petrol extracted depends on the nature of the crude oil the design of the refinery, and the extra chemical processing employed. As conventional crude oil production has peaked, so the global supply of diesel relative to petrol has begun to decline. Very heavy oils like tar sands produce less, and very light oils like US shale oil produce very little. The middle fraction is also used to make jet fuel and heating oil. These products will all be fighting for a declining supply before the supply of petrol peaks.

Also diesel is used in more economically important areas, like food production, distribution and emergency services. The price and availability of diesel will become difficult and will probably be rationed before petrol.

So, if buying new today, a small petrol hybrid makes more sense as a long term investment than an efficient diesel. The latest hybrids are very good, and soon plug in versions will be on the market at more affordable prices. However these are complex machines which will be difficult to maintain in a powerdown environment - it depends on how much and how long you think BAU will stagger on.

I am thinking of going all electric myself. Not 4 wheels yet, just 2. And hybrid electric / leg power at that. The longer commute into these strong so'westers has been taking its toll on my energy levels

For tractors diesel is a clear winner. Not only is the fuel more energy dense but the turbocharged engines matched to multispeed transmissions(24 speeds are common) allow the operator to run the engine at its most efficient rpm regardless of varying loads. They run tractors through a Nebraska tractor test to compare them and record the fuel consumption in hp-hours/gallon.
For old gas tractors they run about 12 HP-hours /gallon and for modern 200HP tractors they can get above 17 HP-hours/gallon.
You can't extend that to an automobile though as the heavier weight of the diesel engine becomes a factor and eats into the fuel and engine efficiency advantage that diesel has.