But that doesn’t tell you:
a) the energy released and
b) how much of that energy goes into usable work.
Another thing to consider.
Take a barrel of crude oil. If I understand this correctly, you can’t turn it all into gas, nor can you turn it all into diesel. There will be a natural division of how much of each one you can make easily. If you try to force the production in either direction because you believe one or the other is “better” you are not utilizing that crude oil as well as you could. Just my vague memories here…
Yes, and already alluded to and linked to in my last post. But here it is again with quote this time for flavor.
Note however that a variety of combinations of higher fuel prices and decreased cost of the newer generations of true “drop-in” biofuels (such as Amyris’s* “No Compromise” yeast fermented biodiesel) would alter that calculus significantly.
*Disclosure: I own shares of the company.
well, diesel cars v gas cars is one thing…Gas engines are awful under heavy load. Ex
we have 2 heavy trucks on the farm. One has a 1989 417 Cubic turbo charged diesel engine, and the other has a 429 ford gas engine, they are the same year, and both ford f 600’s with a gvw of 26000. Now the gas will do about 75 down the high way just like a picup truck after all the 429 big block ford is what was in the mustangs in the 70’s they called it the “boss 429” … needless 2 say the diesel will pull more and burn far less fuel, infact the gas engine hauling harvested crop burns so much we have to fill it up every day it has a 50 gallon gas tank, the diesel has to be filled every 3-4 days, and it has a 35 gallon tank. So in my book diesel is better, and always will be. more power, and better milage with that power… its kinda like if you have 2 one ton chevyt pick ups the biggest gas you used to able 2 get was a 8.1 L gas V8 thats Huge… were as the diesel is the 6.6 or 6.1 L duramax… now while the diesle will never do better then 14 highway, it can also toe at least half its capacity and still get that kinda milage were as the gas… u can almost watch the gage go down… this is from first hand experiance. Now see what they should do is put smaller diesls in things like the f 150’s and suv’s that would start to save, and people would have the power then theyt need it… insted of that ecoboost joke ford came out with… "420 ft ilbs of toruq… at like… 4000 rpms… give me a break lol
That is partly what I was saying, but also that the volumetric energy content of a fuel is no measure of efficiency as it is ordinarily understood. The efficiency is the proportion of the heat energy liberated by the fuel that is converted into work. Your spark-ignition engine can burn methanol, which has less energy density than gasoline (and so will net you fewer mpg) but which is just as efficient. An engineering nit-pick, basically.
I believe the OP is claiming: firstly that switching from gasoline to diesel does not reduce CO2 emissions, and secondly that the higher volumetric energy content of diesel fuel gives a misleading impression of diesels being more efficient. The first claim may be true - my gut says it isn’t but I’d have to run some numbers. The second claim is partly true. Diesels really ARE more efficient, but the volumetric energy content of the fuel makes them look better than they are if you measure in mpg.
I’m not sure this is a valid point. Crude is a whole mixture of hydrocarbons that we split up into useful groupings or “fractions”. We don’t process a barrel of crude for the gasoline fraction or diesel fraction and throw the rest away - we process a barrel of crude into naptha, gasoline, kerosene, fuel oil, parrafin wax, etc. It’s a package deal - you’re left with diesel as well when you make gasoline whether you want it or not, and vice-versa.
Like I said, I have no technical knowledge of any of this stuff. I was trying to understand what the OP was claiming as a matter of clear language, not as a matter of technical detail, and definitely not about the correctness of any technical claims.
I think I succeeded, in that at least the OP agreed with my restatement (see post #54). But by that time the thread had taken off in another direction, and no one seems to be directly addressing the OPs restatement.
To sidestep the issue the OP’s adressing we use grams of CO2/km (have fun converting that to something Imperial).
For example (I quickly searched for 2 engines with ~ the same HP in the same car)
BMW 320d EfficientDynamics Edition
120 kW
109 g CO2/km
BMW 320i
125 kW
148 g CO2/km
So to the OP:
Diesel autos are a illusion of increased efficiency = FALSE
Now that one is not fair. The diesel you used is the efficientdynamics one with the mass flywheel and start stop hybrid technology. Use the the regular 320d and the comparison is to 128 g CO2/km - the same 15% better CO2/distance travelled as noted previously in other diesel petrol comparisons, not the 26% the comparison you made implies. OTOH mpg difference between the 320d and the 320i is 28% better.
Diesels are more efficient and using mpg alone gives an illusion of greater efficiency than really exists. Yes, CO2/distance traveled is one fair way of capturing that. In general by CO2 emitted they are about 15% more efficient, significantly less of a difference than comparing mpg alone would have you believe. The 15% is because of diesel engine efficiency; the rest of the mpg difference is because diesel is more energy (and carbon and cost) per unit volume.
(I should have read all the posts before writing this up, but I’ll add it to echo Librarian’s calculations in different units and with engines that are more comparable.)
I can see there have already been several evaluations of greenhouse gas emissions , but I’ll add one in emissions per mile traveled. The emissions per mile comparison is as apples to apples as you can get. There’s no sense in trying to compare emissions per gallon or some synthetic “gallon equivalent” number.
First, fuel economy from here:
We’ll look at the 6-speed automatic Jetta. It gets 34 mpg combined diesel and 25 mpg with regular gasoline. We can already see that diesel is more efficient by more efficient by 35%, which is huge.
Rather than stoichiometry, density, and other calculations, it’s best to simply go straight to well established emission factors when determining the GHG emissions of a source. When a chemical engineer tells you to skip the stoichiometry, there’s a very good reason. The stoichiometry is useless because nobody is showing both the energy from the combustion reaction and the density of the fuel. It’s far easier to use emission factors, and every GHG reporting protocol uses emission factors for such common fuels as diesel and gasoline.
I’ll take my emission factors from The Climate Registry’s general protocol (http://www.theclimateregistry.org/resources/protocols/general-reporting-protocol/). For simplicity, we’ll only look at CO2. N2O and methane typically make up less than 3% of mobile emissions. Diesel emits 10.15 kg CO2 per gallon and gasoline emits 8.81 kg/gallon. That yields an efficiency of 0.2941 kg CO2/mile for diesel and 0.3524 kg CO2/mile for gasoline. This gives diesel a huge edge.
On to NOx and particulate matter (PM), the real drawback of diesel technology. Gasoline simply doesn’t generate the PM that a diesel generates, and they have the NOx cleaned up by a catalytic converter. That technology doesn’t work on a diesel engine because the catalyst spoils quickly due to the higher sulfur content of diesel. (The sulfur is removed from the gasoline during refining.) But wait! The US recently switched to ultra-low-sulfur diesel, which is less than 10% of the sulfur content of low sulfur diesel. The much lower sulfur content has opened the door to treatment technology for diesel engines. While it’s still not as good as the technology in cars, current diesel cars have much lower emissions than old diesel cars.
I didn’t do any calculations, those numbers come straight from BMW.
@ Dseid:Yes, from an enginering point of view those two engines are miles apart, I selected them because they are similar in power and (suprisingly) price (here in Holland anyway). Anyway, with those two different engines my point still stands, less pronounced, but still clearly
Hooker
You may just want to check out what VW says their CO2 emissions are. Best match I can find for performance overall comes to a 16% improvement in CO2 with the diesel.
So it doesn’t. Why, anyone might think that all I was addressing was that part of the discussion which touched upon CO[sub]2[/sub] versus H[sub]2[/sub]O emissions!
What use is that comparison?
Right. The higher cost of diesel fuel mostly offsets the increased mpg, economically, which is a primary consideration of consumers.
Do the US pollution standards lower the efficiency difference even more?
It seems so. The mpg difference drops to 17 to 19% and once presumes a significant decrease in difference in CO2/distance as well.
But that doesn’t address it. Who cares about the ratio of H2O to CO2? What matters is the total CO2 released per mile not per reaction. Sure it would be better to use a fuel that only combusted into H2O, but both diesel and gas release some CO2, the only question is which releases more. According to every source provided in this thread, mile for mile gas produces more because diesel engines can put more energy into motion than gasoline engines. The OP is correct to point out that mpg comparisons are overstating the difference due to the higher energy density of diesel, but the difference is still there.
Bumping up to add this recent analysis by the US National Research Council on the issue of cost efficiency.
Real world right now we have this for a reality check
So their price difference seems to be right on so far.
I am assuming that “fuel savings” means estimated fuel cost savings. If not then add an additional $0.23/gallon (or call it 6-7%) to the cost for the diesel fuel based on most recent pricing in America.
For a mere $100 more per year than a diesel the NRC estimates a hybrid will save you an additional 6% in fuel costs/yr. At $4/gallon that pays for itself in about a year even using 30 mpg as your base ICE.
Comparatively that base of a 30 mpg spark-ignition ICE travelling 12000 miles a year at $4/gallon costs $1600/year in fuel, the diesel $1008, and the hybrid $912. True, starting with a 30 mpg base means that the hybrid doesn’t pay for itself until almost 9 years out under these assumptions, and the diesel doesn’t until almost 10 years. (And if they meant just fuel volume, not cost, then the diesel would be about $1070/year and over 11 years for ROI.) Numbers of course vary if one assumes a different cost of fuel, or drive more or less than 12K/year, or start with a different assumed base mpg. And diesels perform better for certain purposes. I’ve seen other sources estimate that hybrid ROI is generally 5 years, so I can only assume that they have started with other, perhaps more realistic, assumptions. But if so, the hybrid still is a better ROI than the diesel is and becomes only more so if fuel prices increase more dramatically over the ownership period.
One important caveat to the above analysis: a very large portion of the actual actual cost of owning a vehicle is its depreciation and both hybrids and diesels have much better retained value than do spark-ignition ICEs.
Those high retained value rates may indeed make the ROI much shorter for both of them.