Audi TDII know admittedly very little about fuel efficiency or internal combustion engines.  My engineering background is nonexistent.  However, I have been quite intrigued by auto manufacturers – in particular Audi and Volkswagen – promoting diesel engines for cars.  They are not only promoting diesel as an alternative, but a more efficient one.

Wikipedia does its traditionally thorough job of explaining how diesel works.

Unlike petroleum ether and liquefied petroleum gas engines, diesel engines do not use high voltage spark ignition (spark plugs). An engine running on diesel compresses the air inside the cylinder to high pressures and temperatures (compression ratios from 15:1 to 21:1 are common); the diesel is generally injected directly into the cylinder near the end of the compression stroke. The high temperatures inside the cylinder cause the diesel fuel to react with the oxygen in the mix (burn or oxidize), heating and expanding the burning mixture in order to convert the thermal/pressure difference into mechanical work; i.e., to move the piston. (Glow plugs are used to assist starting the engine to preheat cylinders to reach a minimum operating temperature.) High compression ratios and throttleless operation generally result in diesel engines being more efficient than many spark-ignited engines.

This efficiency and its lower flammability and explosivity than gasoline are the main reasons for military use of diesel in armoured fighting vehicles like tanks and trucks. Engines running on diesel also provide more torque and are less likely to stall as they are controlled by a mechanical or electronic governor.

Diesel-powered cars generally have a better fuel economy than equivalent gasoline engines and produce less greenhouse gas emission. Their greater economy is due to the higher energy per-litre content of diesel fuel and the intrinsic efficiency of the diesel engine. While petrodiesel’s higher density results in higher greenhouse gas emissions per litre compared to gasoline,[10] the 20–40% better fuel economy achieved by modern diesel-engined automobiles offsets the higher-per-litre emissions of greenhouse gases, and produces 10-20 percent less greenhouse gas emissions than comparable gasoline vehicles.[11][12][13] Biodiesel-powered diesel engines offer substantially improved emission reductions compared to petro-diesel or gasoline-powered engines, while retaining most of the fuel economy advantages over conventional gasoline-powered automobiles. However, the increased compression ratios mean that there are increased emissions of oxides of nitrogen (NOx) from diesel engines. This is compounded by biological nitrogen in biodiesel to make NOx emissions the main drawback of diesel versus gasoline engines.

Many periodicals have taken interest in the “new” technology, including Green Tech Media.  The article stated that Audi and VW believe that diesel can have 25% of the auto market.  It also noted that the cars aren’t quite as efficient as a Toyota Prius, but are more efficient than most other car models.  What holds diesel back are its higher cost than traditional gasoline and the continuing problem of sulfur emissions, even if the newer engines are much cleaner than their predecessors.

Popular Mechanics recently ran a great overview of all the new diesel cars, including those made by Honda and Cadillac.  However, what I truly appreciated was the longevity inherent in diesel engines:

Meanwhile, diesel’s core virtues remain unchanged. The fuel contains more energy per unit volume than gasoline, and diesel engines operate at higher compression ratios than gasoline engines—typically 14:1 to 25:1, compared to 8:1 to 12:1. (The compression ratio is the relationship between the volume of the cylinder when the piston is at the bottom of its stroke and the volume when it’s at the top.) The higher the compression ratio, the more mechanical energy an engine can squeeze from its fuel/air mixture. So each time the mixture in a diesel engine’s cylinder ignites, the car gets a slightly bigger push than it would in a gasoline engine. That means it takes less fuel to move the car down the road. It also means that the engine generates a lot of power even when it isn’t cycling fast—and that’s the source of the beefy low-end torque these vehicles are famous for.

The high compression ratios also explain why diesel engines tend to last so long. “Diesel engines need to be built stronger,” explains Tony Molla, author of Chilton’s Diesel Engine Service Manual. “The crankshaft and connecting rods are quite a bit heavier than those in gasoline engines.” And because diesel has a low coefficient of friction, it also happens to be a good lubricant that provides protection to the cylinder walls. The result? It’s not unusual to see diesel engines still chugging along at 250,000 miles.

I personally would like to see Americans less dependent on fossil fuels in general, but that doesn’t mean the diesel car does not have its  place.  After all the car is more efficient and uses less oil than current internal combustion cars.  Moreover, if it lasts longer it is inherently more sustainable as we need to make fewer cars.  One of the elements of transportation sustainability frequently overlooked is the production of new cars and all the energy and raw material that goes into their production.  If each American kept his or her car for 250,000 miles worth of driving, this country could be a lot different (though GM would probably be bankrupt).

The other advantage for diesel is that as opposed to new technologies like electric cars, there is already a standard fueling system that is widespread across the country.