BMW’s been running a series of “if we all switched to diesel” ads lately, a bid to make their cars (in general) seem more environmentally friendly. The mileage they are advertising is actually “good but not great”, as their roadsters are advertised at 36 miles per gallon of diesel (diesel is slightly more energy-dense to start with, so there’s actually no increase in engine efficiency for the car itself, and as we’ll see below, diesel is a much less plentiful resource than gasoline). That “36 mpg” is also a highway measurement, and the real-life city measurement drops precipitously because diesel engines have a much narrower power band than gasoline engines. Diesel engines are made for “long haul, steady use” applications, such as trains, heavy truck shipping, and electric generators. They’re much more inefficient than gasoline engines when it comes to stop-and-go traffic.

The major conceit in the advertising is a ridiculously false basic premise. As I discussed somewhere a while back, the split of various products produced when you “crack” (distill/refine) a barrel of crude oil is more-or-less set. It can be massaged by maybe 2-3 percentage points through advanced distillation methods, but there’s only so much of each given type of hydrocarbon in the barrel, and you get what you get. It’s theoretically possible to convert diesel to gasoline or vice versa, but the problem is that once you start doing this, you are spending more energy than you will get back and thus, you’re just wasting fuel.

The underlying conceit of BMW’s ad – that it would be possible for every single person to switch to a diesel vehicle – falls flat. The ratio in a given barrel of gasoline to diesel/heating oil is approximately 2:1. Diesel vehicles already have to compete with demand for heating oil; that’s part of why diesel prices rise around October, when homes above the 40th parallel generally begin turning off their air conditioners and turning on their oil-based heaters for winter.

Indeed, it seems that the secondary assumption of BMW’s ads – that it would be a good idea for even a small chunk of the market in cars to switch to diesel, say 10% or so – doesn’t work out. A few years ago, the price of diesel fuel went above the price of gas, and the price of jet fuel, heating oil, and other “associated products” went up. Or rather I should say, they didn’t “go up” so much as they were available in less quantity. The factor was that ethanol replaced MTBE as the fuel stabilizer/octane-booster of choice for gasoline blends in the US, aided and abetted by some very junk science the EPA used to mandate corrosive ethanol-laced gasoline in certain major metroplex markets such as Chicago, New York, Houston, and Los Angeles. The “hidden” factor was that the gasoline blends went from using ~2% MTBE to 10% (or sometimes even sneakily greater with severe consequences) ethanol. Instantly, in other words, ~8% more “gasoline” was being produced per barrel of oil and shipped out. Faced with a relatively static demand for gasoline (the primary product from the barrel of oil), the oil refiners scaled back, and so diesel, heating oil, and everything else suddenly had an ~8% supply availability drop with a corresponding rise in price.

Now look to modern time. Decrease the demand for gasoline by 10%, and increase the demand for diesel/heating oil by a corresponding amount. How does the idea of $7/gallon diesel fuel sound to you? Essentially, a shift in demand for diesel is going to disproportionately punish diesel usage, and as was just shown above, BMW’s diesel vehicles are not any more energy-efficient (in terms of joules per mile) than their gasoline counterparts.

Of course, I’m sure BMW doesn’t care. They get an ad campaign claiming their cars are “clean” and “better for the environment”, and most of the population is too stupid to understand what’s really going on. The real fright will come if/when other car companies begin mimicing BMW, and there’s enough of a shift in usage patterns to make diesel and heating oil cost-prohibitive.


Category: Road

About the Author

Guy Webster (web) is an IT specialist at Southern Tech University, where he and Will Truman attended college.

5 Responses to Irresponsible Greenwashing

  1. Linus says:

    My understanding is that turbo- and super-chargers help diesel’s power-band issues considerably. Obviously they add some cost (and complexity), but don’t they really help city mileage and drivability?

    Interesting to hear about the relatively set gas-to-diesel ratios you get from oil. I’d suspected this, but it’s good to get the specifics.

  2. Kirk says:

    I wrote a paper on this. I’m quite interested about diesel, so please bare with me….

    Although crudes differ, there is no exact “set” mix of fuel that you get from a barrel of crude. Rather, all that link is showing you is the current division of mixes. Gasoline is actually harder to make than diesel, as it requires higher boiling. (And gas might even require cracking–which is a really high-temp process–though don’t quote me on that.)

    You can get more diesel from a barrel of crude than you can get gas. And as you noted, diesel has more energy. (Though for some reason you seem to discount this.) Diesel engines run leaner (at full throttle, around 1-20 fuel-air mix vs. a constant 1-14 for gas). This improves their mileage further.

    Diesel is more expensive than gas, but only due to artificial factors: being a commercial fuel, it’s taxed higher in the U.S., and there’s less “economy of scale”, as with gas. Without these factors, diesel would actually be cheaper than gas. (Interesting side note: in the U.S., only the Red Cross is entitled to use “blue diesel,” which is tax-free.)

    Diesel engines last longer. A diesel car with 250k miles on the odometer is about the equivalent of a gas engine with 100k miles. (Commercial diesel engines last far longer. One of the very first Caterpillar truck diesels was torn down around the 600k mile mark. It was fine, to the point where the original “hone” marks on the cylinder walls were still visible.)

    “Stop and go” driving has no more effect on diesels than gasoline engines. In fact, diesels are non-stochiometric, meaning that, unlike gas engines, they can “lean out” far more when idling. (Typically at idle, a diesel will run around 1/80 parts fuel-air mix. Even big commercial diesels at that point use only a pinprick of fuel on each power stroke. Gasoline engines, by comparison, need a constant fuel-air mix, which means they need a constant 1-14 ratio. Even with its throttle plate mostly closed at idle, the gas engine is still comparatively hogging fuel.)

    As for turbochargers…like in a gasoline engine, they do force air into the cylinder on the intake stroke, increasing power. Unlike gas engines though, the only limit on the boost is the engine’s stength. (Gas engines will need higher and higher octane. And past a certain boost point, nothing will work. This is because where a diesel compresses only air, a gasoline engine compresses a fuel-air mix. The gas can’t take the compression.)

    However, a diesel goes further, using its turbo to “scavenge” out the exhaust gases….

    Near the top of the exhaust stroke, a turbodiesel will open its intake valve early, allowing the air forced in by the turbo to force any remaining exhaust gases out through the open exhaust valve. This ensures that the cylinder is completely void of exhaust on the start of the intake stroke…

    A gasoline engine cannot do that. The air coming into a gasoline engine already has gasoline mixed with it, so having both the intake and exhaust valves open at the same time would only let gasoline go out the exhaust…

    So, a gasoline engine always has an amount of exhaust in it. This is ineffecient, as exhaust gases cannot be used to facilitate combustion. (This is why no engines–other than lawnmowers–have side-valve designs any more.) So, you need a bigger, (and heavier) motor to do your work….

    And of course diesel is a lot safer. It’s about as flammable as vegetable oil. This is one of the reasons the military uses it for everything. (They even have diesel-powered motorcyles.)

    Compare that to a mostly-empty gas can sitting in someone’s garage. It’s a friggin’ bomb.

  3. Kirk says:

    Now that I think about it, I do seem to remember that crude does have a natural breakdown, achieved through boiling. However, to get more gasoline than boiling provides does require “cracking,” which is a high-temp, high-energy exercise. To get less gasoline (or more diesel) involves some sort of recombination. I’m betting recombination is more efficient than cracking, but I have yet to see a breakdown on this.

    Regardless, the world is slowly shifting to heavier crudes (like Venezuelan), which naturally yields more diesel and less gas. As the lighter crudes get used up, diesel will be a more natural choice.

  4. trumwill says:

    Man, I’ve learned more about Diesel today than I have learned in the other many thousand days of my life combined.

  5. Kirk says:

    Man, I’ve learned more about Diesel today than I have learned in the other many thousand days of my life combined.

    Well, thank you. 🙂

    At one point, I seriously considered becoming a diesel mechanic. I even bought the ASE Certification textbook. However, there are no diesel schools around here, and it was easier to just keep going to college at that point. (It takes just as long to train a diesel mechanic as it does to train an x-ray tech.)

    Who knows though? I’ve been out of college for six months now, and I’m getting bored. I haven’t tried to find a job using my degree, but I’m not sure I’d want an office job, anyway.

    It would be kind of cool, to work on semis…or locomotives…or ships…

    This is an okay site that explains diesels. The guy doesn’t go into a whole lot of detail though.

    http://auto.howstuffworks.com/diesel.htm

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