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.