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tahir
Joined: 28 Oct 2004
Posts: 45665
Location: Essex
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jema
Downsizer Moderator
Joined: 28 Oct 2004
Posts: 28231
Location: escaped from Swindon
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Treacodactyl
Downsizer Moderator
Joined: 28 Oct 2004
Posts: 25795
Location: Jumping on the bandwagon of opportunism
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Maxwell Smart
Joined: 29 May 2005
Posts: 607
Location: London Town
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Treacodactyl
Downsizer Moderator
Joined: 28 Oct 2004
Posts: 25795
Location: Jumping on the bandwagon of opportunism
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jema
Downsizer Moderator
Joined: 28 Oct 2004
Posts: 28231
Location: escaped from Swindon
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Mad Dad
Joined: 12 May 2005
Posts: 407
Location: Nowhere near where I want to be
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dougal
Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
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 Posted: Sun May 29, 05 6:09 pm Post subject: |
 
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| Treacodactyl wrote: |
| Most diesel cars are more expensive to buy than a petrol and you tend to need to do quite a high mileage to brake even. Does anyone know if diesel engines are that much more expensive to make, compared to a similar petrol? It's often £500-£1000 more for a similar diesel. |
A diesel engine has more precision engineering in its injection system. The more efficient ones have a turbocharger.
Diesel engines run at compression ratios over 20:1 - which is about double the compression rato of a petrol engine. They therefore have to have more/thicker metal in the engine block and cylinder head, higher tension bolts holding the head down, stronger crankshaft, bigger bearings, etc...
But diesels don't have a high voltage ignition system.
So yes, diesels tend to cost slightly more. And at any mileage, their used ("residual") value is going to be higher. And in general, they are usually more efficient.
Take one relevant specific vehicle comparison:
The Audi A2. 1.4 litre non-turbo petrol and 1.4 litre turbo-diesel.
The list price of the turbo-diesel is 7.7% higher.
But the diesel emits 22.7% less carbon dioxide per unit distance travelled.
But a super efficient petrol vehicle offers the very lowest CO2 emission figure of all production vehicles. The Honda Insight only puts out 2/3 of the CO2 of the "green" A2 turbo diesel...
CO2 emission reflects the amount of **carbon** being burnt as fuel - whether from diesel, petrol, LPG or CNG, biodiesel or ethanol. And therefore gives a reasonable, and useful, comparison of vehicle efficiencies.
Comparisons based on miles per gallon are perfectly valid for financial comparisons - because we are taxed on fuel's volume not weight.
Diesel/petrol **efficiency** comparison is complicated by the fuels different densities - diesel is "heavier" than petrol. Its something like 720 grams per litre for petrol to 860 grams for a litre of diesel.
How much energy is there in the two fuels?
If you look at a litre of fuel there's 8.7 units of energy in the petrol - but 9.7 units in the litre of diesel. This fact alone explains much of the claimed "efficiency" of diesel vehicles - when that efficiency is looked at in distance per unit volume of fuel.
But if you compare the energy in a kilogram of fuel - surprise - there's 12.7 units in the petrol and only 11.6 units in the diesel. Thats one reason you don't see diesel engined aircraft.
These energy "units" btw are kWh - kilowatt hours, otherwise known as electricty units!
So take note that just **one** kilo of petrol or diesel stores as much energy as a "Windsave" turbine might generate in 12 hours of a Force 6 gale! Or to light a 60 watt lightbulb, 24 hours a day, for over 8 days! These are *very* compact stores of energy. |
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dougal
Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
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dougal
Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
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| Posted: Sun May 29, 05 7:58 pm Post subject: Re: Diesel vs Petrol |
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| tahir wrote: |
| ... but what about the type of emissions? There are a lot more particulates in diesel exhaust tham petrol... |
There's two big aspects to particulate pollutant emissions...
- medical effects and climate effects.
It seems that particulates actually counter global warming, by stabilising (reflective) clouds. More clouds, less incoming energy, cooling.
But breathing them has gotta be a negative. They are very tiny and so can penetrate very, very deep into the (tubules? of the) lungs. But its not so much about the sooty particulates themselves that are the problem. More likely detremental are active chemicals adsorbed onto the large surface of these tiny carbon snowflakes. Things like sulphuric and nitric and nitrous acids... which are formed from the sulphur and nitrogen oxides also in the exhaust...
Now, about particulate formation. They arise because of incomplete burning of the fuel. A problem that afflicts the carbon-richer diesel. And especially, older, cruder, less efficient ones. And the problem **is** lessened with biodiesel, because it actually has bonus Oxygen right in the fuel molecules, (which is why its less dense...) which means more complete, thus cleaner, combustion.
And Biodiesel is Ultra Ultra Ultra Low on Sulpur. So there are less chemical nasties to be adsorbed onto the particulates as well.
So, IMHO, if your diesel is a modern, efficient one, and you drive it on Biodiesel, you needn't feel too guilty. |
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Maxwell Smart
Joined: 29 May 2005
Posts: 607
Location: London Town
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dougal
Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
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| Posted: Sun May 29, 05 11:51 pm Post subject: |
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| Maxwell Smart wrote: |
In regards to biodiesel being a better fuel than fossil diesel, I had seen a few people claiming that it is better for the engine. Here is one:
https://www.surrey.ac.uk/eng/InfoPoint/mddp/mddp03/group-1a/biodiesel.htm
| University of Surrey {student project} website wrote: |
Benefits of Biodiesel
i. Smoother running - biodiesel is a natural fuel lubricant, and as a blend with ordinary diesel will improve fuel flow around the engine and consequently lengthen the useful life of the engine. |
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Fuel lubricity is indeed better. But its only the fuel pumps and injectors that see the benefit - not the bearings, and it doesn't seem to greatly impact cylinder bore wear. So I'd say it was overclaiming to suggest that the engine "runs smoother".
| Quote: |
| Quote: |
| ii. Reduces particulate emissions... |
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Yes, as I stated above.
| Quote: |
| Quote: |
| iii. The increased levels of oxygen in biodiesel allow it to burn more freely and completely than traditional diesel. This can lead to a reduction in fuel consumption in cars using the blended fuel |
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I noted the increased Oxygen in the fuel molecule, and that this is the reason for the particulate reduction. Yes the carbon monoxide is also reduced, as more of the fuel is completely burnt to CO2. However that is usually masked by the exhaust catalyst, one of who's jobs is tidying up the CO.
BUT any "reduction in fuel consumption" might only be true in terms of *weight* of fuel. You should expect a *higher volume* of fuel to be used - which most folk would report as *increased* consumption or "reduced mpg". Its only a few percent - but its in that direction.
I think the Surrey students got that one wrong.
6% worse consumption is the official expectation in the USA, see for example https://www.state.hi.us/dbedt/ert/atfv_hi.html (an Hawaii State Govenment info page)
The experience of running a specific truck in Yellowstone National Park for 180,000 miles was that they achieved 16.3mpg ("about 1 mpg worse" than expected for fossil. Thats 6.1% worse.)
https://www.nps.gov/renew/yellbio.htm
Its slightly worse than fossil, but remember this is based on fuel volume...
There's a published Government/University research study of some Florida buses - which I think demonstrates that even gathering reliable data is tricky. At the end of their year-long study, they found they had results ranging from 180mpg to 0.1mpg (this on buses returning about 8mpg!). I can explain such results by the misrecording of *which* bus the fuel went into... doh! They still produced charts and statistics where *negative* mpg was within 3 standard deviations of the mean... uh-oh! (I think they might have got meaningful results for "whole fleet" rather than individual bus statistics...
Their preamble mentions that a 10% (by volume) *increase* in *consumption* would be expected on changing to Bio, but that previous studies had only found "only two or three percent" difference.
The paper gives lots of figure for emissions reduction.
https://www.dot.state.fl.us/research-center/Completed_Proj/Summary_PTO/FDOT_BC137_51_biodiesel_rpt.pdf
The Yellowstone study also did "rolling road" dynamometer testing and measured a 6% drop in power output when running on Bio, which they attributed to the "lower fuel density". And recorded that most drivers did not notice the power loss.
| Quote: |
| What makes you say that fossil diesel is better as a vehicle fuel{than SVO}. My understanding (and I am still quite ignorant on this subject - but learning) is that the only reason fossil diesel took off is due to the fact that it was cheaper and more readily available than peanut oil at the time. |
Fossil Diesel is 'better' than Straight Veg Oil not in the sense of moral good, but in the sense of more efficient, convenient, etc.
Biodiesel lubricates pumps and injectors even better than fossil; but SVO gums them up.
Biodiesel vapourises at least as well as fossil. SVO is more reluctant. This has two consequences. *Less* complete combustion, so more soot and even unburnt fuel in the exhaust - hence the "chip shop" smell. The second consequence is difficult starting. Cold SVO doesn't want to form a vapour, so it doesn't explode in the cylinder to start and run the engine! This is why conversion kits are centred on pre-heating the fuel. Some start the engine on fossil and change over once engine heat can warm the SVO ("changeover" means two tanks...). Changing over *can* mean changing back each time before *stopping* the engine, so that the injector system is primed with fossil ready for the restart.
All this is, frankly, a pain.
Biodiesel combines the practical advantages of fossil diesel with the ecological advantages of other biofuels.
The only advantage of SVO is the greater possibility of evading Fuel Duty by not making full disclosure to Customs & Excise. Thats illegal and silly.
Ask any manufacturer if running on SVO would invalidate your warranty. I doubt any would allow this, whereas there should be no such problem with using commercial (ie German DIN spec) Biodiesel. As previously noted European ULS Diesel can contain a significant proportion of Bio, without even requiring notification of that fact.
There's a lot of "Internet Myth" about Rudolph Diesel and Peanut Oil. My understanding is that he originally used native german veg oils for his engines for agricultural machinery. The *imported* peanut oil was thinner and less prone to gumming up the works - ever noticed how *sticky* a chip pan gets? Rudolph used the "best" he could get. Fossil diesel is "better", Biodiesel is better still.
If Rudolph himself had had the chance of running on Biodiesel, he'd never have stuck (literally) with SVO. We've had a bit of progress in the last hundred years or so! |
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