When Vanessa and I visited France in the fall of 2010, we spent a week driving around in a rental car. I didn’t think too much about the car when renting it — one broken thing about the rental car industry is it’s really hard to know what kind of car you’re going to get until you pick it up — so I just reserved something cheap, and when we picked it up, it turned out to be a Citroën C4 Picasso with a diesel engine. Nondescript enough, at first, but after a week on the road with it, I noticed impressively high fuel mileage — somewhere above 50 mpg (approximate since I was doing the calculations in my head and converting from liters and kilometers).

This 50+ mpg result was while driving around on the good highways where the speed limit is 130 kph (80 mph). That got me thinking. The only mainstream cars in the US that gets gas mileage like that are gas-electric hybrids (e.g. Prius), and even those not at 80 mph; meanwhile, compared to the Prius the C4 has more usable space, and is more fun to drive thanks to its manual transmission. A car that beats the Prius on performance, packaging and mileage — why hasn’t this caught on in the US?

Fast forward most of a year to when Vanessa and I found out she was pregnant, and I decided that’s enough reason to sell my 2-seater convertible and get something more practical, i.e. with back seats and a hard top. And thanks to that Citroën C4, I decided it should be a manually-shifted diesel.

Once I started shopping, I realized I had a pretty short list to choose from. The US auto manufacturers think diesel is for trucks; there are no US-made diesel passenger cars sold here. The Japanese manufacturers make diesel passenger cars for sale elsewhere but don’t import them to the US. That leaves European companies: Audi, BMW, Mercedes and Volkswagen all sell diesel cars here. But only Volkswagen offers even a single stick-shift model (shame on you, Audi and BMW — I get pretty annoyed surfing audi.de or bmw.de and looking at the drivetrain combinations available in Europe and not here, but that’s a story for another time).

What I would have chosen if it were available here, or if we lived in Europe: either an Audi A3 or BMW 3-series with all-wheel-drive, stick shift and turbodiesel engine. What’s available here in the US combining a stick shift with a diesel engine: Volkswagen Golf, Jetta and Passat.

So I’m now driving a 2011 VW Golf TDI. But before I made that decision, I realized I needed to understand whether and why diesel vehicles are really more efficient. Actually, I had 3 questions: – What is diesel fuel, actually, and does refining it from crude oil come at the expense of usable gasoline? – Why do diesel engines get better fuel economy than gasoline engines? – Why does diesel generally cost more at the pump, compared to unleaded gasoline?

which combined into a suspicion that perhaps diesel is effectively condensed gasoline, and that it costs more and drives you farther because it’s just using up more of the original crude oil energy.

Off to Wikipedia: I needed to read the articles on Diesel fuel, Diesel engines and petroleum and crude oil. It turns out my half-baked hypothesis was well-intentioned but completely wrong. You can read the article yourself, but to summarize and answer my questions: crude oil is a mix of hydrocarbons of various chain lengths, ranging from heavy and less volatile (diesel) to light and more volatile (gasoline, kerosene, etc). Refining oil is the process of separating out these different hydrocarbons; when you refine a given amount of crude oil you get some fixed quantity of diesel, gasoline, and various other products. It’s not the case that there’s a tradeoff between producing diesel vs. the other fuel products; in fact when you produce one (in a modern refinery) you produce them all, and you want to use and sell them all for best efficiency. (Contrast this with the situation in the 1800s, when the only petroleum product we knew how to use was kerosene, so they’d separate that out and throw the rest away.)

This also means there’s no fixed price relationship between diesel and gasoline; the prices are set by market forces and supply and demand; that explains why diesel (in California in 2012) generally costs more than even premium unleaded, but in other places and times it’s been cheaper than regular unleaded.

Finally, the efficiency advantage: it turns out diesel is denser than gasoline, both in terms of mass per volume and energy per volume (this falls out from the fact that they’re roughly equivalent in energy per mass). Diesel’s energy content is about 15% higher than gasoline, by volume. This translates directly to a 15% miles-per-gallon for diesel engines. Additionally, the Diesel cycle is inherently about 20% more efficient than the gasoline powered Otto cycle, apparently due to higher compression ratio.

What this means is that a diesel engine will typically get 30-40% better fuel mileage than a comparable gasoline engine, about half of this advantage due to the energy density of the fuel and half due to the higher compression ratio.

So if your goal is to use less of a scarce natural resource and generate less polluting emissions, diesels do have a real advantage over gasoline engines. A pure diesel powertrain is cheaper and lighter than a hybrid gas-electric powertrain like that of the Prius, and compares favorably for highway mileage, though not so favorably for stop-and-go driving. A hybrid diesel-electric should be substantially better than gas-electric for both city and highway driving, I would imagine (diesel-electric hybrids exist in transit buses, but at this point, there are no such passenger vehicles).

As for my Golf TDI: I get about 42 mpg door to door on my standard commute (37 miles, most of which is 75 mph highway driving). I can also typically go 500 miles on a tank of fuel, which is a real convenience advantage over most cars. I wouldn’t call it a sports car, but it is zippy and fun to drive. Compared to other high-mpg options available in the US, it strikes a pretty good balance between performance and fuel economy — I’d like to see more diesel options available here including diesel-electric hybrids, though this may be a passing phase as pure electric cars become more widely available. (Note that the Citröen C4 I started this post with was significantly more efficient, probably due to a smaller engine. I already bemoaned the lack of powertrain options that the Europeans deign to import here, and chose Volkswagen because they alone import manually shifted diesels, but they’re not immune to the curation effect either; in Europe they sell a smaller diesel engine that gets 50+ mpg, but the US gets only the 2 liter version for which mpg figures in the low 40s are typical.)