I don’t get it. Packing the same amount of useable energy into a smaller volume of fuel is pretty much the definition on that fuel being more efficient.
Are you saying that it takes more crude oil to produce one gallon of diesel than it does to produce one gallon of gasoline?
If you can cite and quantify THAT, there may be basis for a discussion.
I think to measure the efficiency of an engine the way the OP might be attempting, you have to consider how much energy is required to construct and maintain the cars, plus the amount of crude oil needed to keep the car going as fuel and lubricant. Then you have to operate the cars under a wide variety of conditions through the life of the engine. For given conditions you could determine the difference in efficiency to a reasonable degree of accuracy. That would be another statistic ignored by people when purchasing a car, or used as an excuse to spend more than you should on a car.
I think the point he’s trying to make is that while how much energy goes into a gallon is something to consider (ie total range of a car and such), if diesel takes more resources to produce/buy/etc. to offset its energy density advantage then the per-gallon advantage it has as a fuel would be illusory. Or to take another example, let’s say we could somehow condense gasoline so that there’s double the energy per gallon. Now you could say “wow, amazing, the doublegas engine can drive twice as many miles on the same gas!”, but if doublegas took twice the resources to produce and cost twice as much it wouldn’t really have an efficiency edge on regular gas, except that it’d take up less volume. To say in this scenario “doublegas is twice as good as regular gas!” ignores the differences in cost/production difficutly/etc. It would be twice as volume-efficient, but not necesarily anymore cost/energy/production/etc efficient.
Now, I have no idea if his idea has any basis in reality. Is creating diesel harder than creating gasoline? Does it take more resources to refine it, and does that offset the higher energy density it has? Do you have to use more raw materials in creating it - that is, does more of the stuff coming out a well go into making a gallon of diesel than a gallon of gas?
I suspect he’s not really making a valid point, but I don’t know enough about the subject to judge the merits, and I thought I’d try to at least clear up a miscommunication.
I don’t really agree with that. Diesel and gasoline engines are both heat engines. They can be modelled thermodynamically as taking heat from a “hot reservoir”, converting a fraction of it into “work” (the form of energy that does useful things like apply a sustained push in one direction over a distance) and exhausting the remaining heat into a “cold reservoir”. The “hot reservoir” is simulated by the heat released by oxidising the fuel and the cold reservoir is the outside world, but that doesn’t affect the theory. Heat engines are fundamentally limited from being 100% efficient, and practical limits of materials means that they generally don’t even get close.
Diesels convert a greater proportion of their fuel’s heat into work - it’s that simple. The fact that diesel fuel contains more energy than gasoline per unit volume makes their efficiency in mpg look somewhat better than the simple engine efficiency would merit, but the engine is still fundamentally more efficient than a spark ignition.
Technically you could make a diesel engine that ran on gasoline - the trouble is that gasoline burns slower than diesel (more on this in a moment) so the gasoline diesel would have to spray its fuel droplets very much more finely to get the same burn rate.
We’re not used to thinking of gasoline being a slow-burning fuel and diesel being fast-burning because their flashpoints are so different. At room-temperature, a pool of gasoline produces a layer of vapour that’s within its flammability limits so gasoline is readily easily ignited. Diesel at room temperature has too low a vapour pressure to ignite without a wick, so we think of diesel as the slowpoke fuel. As a spray of droplets in a cylinder in hot air compressed to several hundreds of degrees C, diesel is the fast burner and gasoline is too slow, which is why gasoline in a current diesel runs with low power and produces lots of smoke. A diesel-cycle engine designed to run on gasoline would however give a better mpg than a spark-ignition engine even though they used the same fuel.
The hybrid comparison isn’t exactly cheating, except there’s no practical reason why you couldn’t have a diesel hybrid and beat both gasoline and diesel economy. The main reason why it isn’t done is probably the expense for relatively little payback. Electric hybrids lose a fair bit of energy in the charging-discharging cycles of the battery and have a lot of additional weight which makes the regenerative braking less useful than it might be. A diesel hybrid would reveal the fundamental weaknesses of the whole electric-hybrid concept. Hydraulic-hybrids with a short-term accumulator for efficient regenerative braking and monster acceleration, that’s a vehicle I’d like to see!
What I think the OP is trying to do is to figure out the efficiency of gasoline versus diesel in terms of miles per amount of energy. And because diesel has more bang for the gallon, all other things being equal, this would give a vehicle with diesel a higher mpg. Which, for the OP, seems unfair for some reason. So, using the numbers kindly provided by Martin Hyde, I have done the following calculation: If gasoline has 115,400 btu/gal and diesel has 128,700 btu/gal, taking everything to be proportional, which is probably not the case, this implies that 1 gallon of diesel is the energy equivalent of 1.12 gallons of diesel. So, if we wanted to continue in this vein this would mean that a diesel car that gets 40 mpg would be “equivalent” to a gasoline car that gets 35.9 mpg. (It’s really (miles/btu)×(some constant), but whatever). So, if a diesel engine is more than 12% more efficient than a gasoline engine, which I believe has been shown to be the case above, then the higher energy content of diesel is not the cause of this efficiency, but rather something else is.
Those two reactions aren’t talking about comparable amounts of fuel in the first place, so it’s not appropriate to compare their outputs as if it were a competition.
Unless, that is, Diesel manages to pack 48 carbon atoms and 92 Hydrogen atoms into exactly the same volume as gasoline only manages to fit 16 Carbon atoms and 36 Hydrogen. If that were the case, then a gallon of diesel would weigh nearly twice as much as a gallon of gasoline. Does it?
Or to put it another way:
Gasoline weighs ~.75 kg/l
Diesel weighs ~.83 kg/l
It’s not physically possible that any volume of diesel contains about three times as many of both carbon and hydrogen atoms as the same volume of gasoline.
What a strange OP. As already pointed out, the only way it would make sense is if the OP would try to compare not only the fuel consumption, but also the cost of producing diesel vs. gasoline. But he’s not.
Yeap, and as pointed out before, this "something else" is the higher efficiency of the [Diesel cycle](http://en.wikipedia.org/wiki/Diesel_cycle) compared to the [Otto cycle](http://en.wikipedia.org/wiki/Otto_cycle). From the previous links:
[QUOTE=Wikipedia]
Comparing the two formulae it can be seen that for a given compression ratio (r), the ideal Otto cycle will be more efficient. However, a Diesel engine will be more efficient overall since it will have the ability to operate at higher compression ratios. If a petrol engine were to have the same compression ratio, then knocking (self-ignition) would occur and this would severely reduce the efficiency, whereas in a Diesel engine, the self ignition is the desired behavior. Additionally, both of these cycles are only idealizations, and the actual behavior does not divide as clearly or sharply. And the ideal Otto cycle formula stated above does not include throttling losses, which do not apply to Diesel engines.
[/QUOTE]
The links provide formulae for efficiency calculation, so the OP can have fun with some numbers. 
I am still waiting for an engine based on the Carnot cycle:
[QUOTE=Wikipedia]
It is the most efficient existing cycle capable of converting a given amount of thermal energy into work
[/QUOTE]
Argh, before my brain explodes from your idiocy.
I fill my jetta with diesel, I get 43 MPG and can go roughly 640 miles before i need to tank up again. If I take my almost identical gas jetta, it gets roughly 34 MPG, and it can go roughly 460 miles before i need to tank it up.
Do the freaking math. Which would I be better off driving? Seems to me that 43 MPG is better than 34 MPG … it is not apples vs oranges, it is both liquid petroleum based fuels with different witches brews of light distillates taken from the same source liquid. The difference is in energy output due to where in the distillation stack the fuel is taken from.
Apples and oranges is petroleum based fuels vs coal generated electricity.
I really don’t think that the OP has any deeper point about diesel than wanting cars to be measured in miles per BTU. All the questions about cost of diesel refining and so on seems to be an afterthought.
OK, This may be enough. If both cars get the same mpg’s, the diesel one needed 13300 more btu’s to do that distance, or about 10% more. So my OP seemed to stand that to compare mpg’s on a equal basis you have to deduct 10% from the diesels mpg’s.
There are allowances for a single person to drive in HOV lanes if they drive a auto that is over a certain MPG because it saves energy. Diesel is a loophole in this, as a diesel auto surpassing 45 mpg is consuming more energy then a car surpassing 45 mpg. If the reason behind allowing a single occupant vehicle is based on consuming less energy the standard for diesel’s making it should be about 10% higher or diesels mpg’s reduced by 10%.
If that is not taken into account then all one has to do is switch fuels and keep driving larger more energy hungry vehicles and pretend everything is good.
I’m interpreting what you say as that it is not necessarily the fuel that is more efficient, but the engine. That may be true, but it does not necessarily exclude the fuel ALSO being more efficient.
However, I’m not claiming that, because I just don’t know enough about fuels or engines. I was stating what I understood from the OPs claim, and perhaps I misstated it. It wouldn’t surprise me if I did, because lots of people here are not getting it either.
The other thing I did in an attempt to remove some perceived diesel “bias” was to ask about the amount of crude oil needed to produce the same volumes of gas and diesel. If, for instance, diesel fuel gets 10% better mpg than than gas, but takes 20% more crude oil to produce, the OP’s claim would make a lot more sense to me. But I don’t know if this interpretation is correct either.
This is also along my lines of the OP. I do know that a diesel engine can be run on some forms of crude directly, perhaps poorly, but it works. (more of the old tractor then the modern diesel, so try that at your own risk ) So one gallon of crude produces perhaps 1 gallon of diesel, but looking at the weight and energy the one gallon of crude would produce about 1.1 gal of gasoline.
Or to put in another way, if the car refined crude into fuel it uses then with a diesel you could put 10 gallons of crude in, refine that into 10 gallons of diesel and at 33 mpg go 330 miles. Or you could put 10 gallons of crude in the gasoline car, produce 11 gallons of gas and at 30 mpg’s go 330 miles.
Same input same output, though the diesel looks better on paper because it gets better mpg’s, but all in all they are identical.
We tout “energy” efficiency but it is inexact to directly compare MPG of a diesel and gasoline car and suggest that one is more energy efficient without accounting for differences in energy per gallon.
One would never try to compare the efficiency of a natural gas vs oil fired boiler on a gallon to gallon basis, since fuel oil has about 1000x the energy density of NG, they measure the true fuel efficiency by comparing the amount of fuel energy in vs fuel energy converted to useful heat.
I suspect by this type of measure, the diesels still come out on top, but it should be accounted for. As well, if you can pack 10% more energy into a gallon, it reduces cost to transport the fuel.
Thank you. I understand your point better, though I don’t understand enough to know if it’s correct or not. I just wanted to clarify the issue, and I look forward to reading how others will address it.
I think the OP has a point if people buying cars were concerned about efficiency. Largely we are not. Many may be concerned about energy consumption, but only relative to how many dollars it extracts from their wallet when they fill the tank.
For that purpose mpg regardless of whether it is diesel or gasoline works well. Easy to do the math and figure what your commute or a tank will cost you.
That’s a hell of a price premium for the diesel! Here’s some numbers from the UK I put together in the other diesel thread:
BMW 120d SE Coupe
2.0 litre turbo-diesel
173 bhp
53 mpg
£24795
Diesel price £1.42/litre
BMW 120i SE Coupe
2.0 litre petrol
168 bhp
43 mpg
£23965
Petrol price £1.35/litre
I only do about 12,000 miles/year but even then it would only take a little over 3 years to recoup the difference in starting price.
I think for the individual, the cost per mile+upfront cost is the best way to look at things.
But from a public policy perspective (HOV stickers, tax breaks, fuel taxes) more needs to be taken into account. How much pollution is released, how much crude is required for the whole production chain (including shipping if the sources are different), etc. If that is what the OP is recommending, I agree.
Looking at the EPA Fuel Economy site, I selected 2011 Jettas, with the same size engine and transmission, one gas and one diesel. They had the same air pollution rating. Diesel costs about 24% less per mile to drive and uses 15% less petroleum for the same distance driven. Also diesel produces 17% less CO2. So, I would say it looks like diesels are better for the environment, but not as much better as just looking at mpg would suggest, as the diesel does 36% better there. There may be other factors that are not considered on the EPA site, but I am not sure where to look for those.
Nevertheless, Mangetout, it’s plain from the balanced equations given that gasoline produces proportionately less CO[sub]2[/sub] and more H[sub]2[/sub]O in complete combustion. As should be obvious anyway, since the diesel molecules (dodecane) contain a higher proportion of carbon to hydrogen than does octane…