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.

air pod

My friend and fellow transit lover David Seitz sent me the cartoon on screw-hot-air-balloonsthe right side.  It is too funny not to publish, but too irrelevant to fuel an entire post.  After all, I cannot claim with a straight face that hot air balloons are the future of travel.  We’re not living in the time of Jules Verne!  However, we are living in the time of Guy Negre, the founder and CEO of Motor Development International (MDI), which is marketing an air propelled automobile.

MDI has been mentioned before on this blog.  The company is creating engines that run on compressed air.  The company has also developed dual-energy engines that have some alternative fuel source in addition and have greater range.  The designs are fun (see above) and are incredibly eco-friendly.  Seeing them makes me think of great, efficient urban travel.  Currently, when I see smart cars, they make me smile.  They’re tiny, fuel-efficient, fit into just about any parking space, and still manage to haul the groceries.  The Air Pod could work the same way, except without fossil fuels!  While MDI is at the forefront of the air engine movement, there are a few other companies as well, such as Engine Air in Australia.

I am not claiming that air pods are a total solution, but there is no reason they cannot be part of one.  The Air Pod which runs on compressed air only is expected to travel about 220 km with the following features.

With small size, a tiny price, zero pollution, fun and futuristic design, AIRPod mark a turning point in the range of urban vehicles while renewing the idea of the automobile and transportation. You can drive with a joystick, it only costs one euro per 200 km and leaves no one indifferent in crept in traffic.

The dual power Air Flow has a 900 km range and extremely low emissions.  It has the following features

  • Two minutes refill
  • Mono-energy compressed-air in city, dual-energy on road
  • Zero Pollution with mono-energy, 30g of CO2 with dual-energy
  • 900Km road range with dual-energy, 100Km with mono-energy
  • Car body monoblock made in fiberglass and external chassis for safety

I personally hope MDI succeeds and we end up seeing air cars on the road.  They are a great idea, better than hot air balloons (though perhaps not quite as much fun).  For all those people already thinking about getting around urban areas on mopeds or in Smart Cars or other similar vehicles, this is an even more eco-friendly solution.

I have been trying to cover Edward Glaeser’s weak posts on the Economix blog regarding high speed rail investment.  Ryan Avent at DC Streets blog has been doing a wonderful job breaking down the intellectual dishonesty of Glaeser’s work.   To add to points I have already raised, he writes:

Glaeser’s analysis assumes no population growth — he bases ridership on current metropolitan populations — and no shift in mode share over time, despite the fact that both Houston and Dallas have rates of transit ridership well below similar-sized cities (suggesting that with growth, transit’s share will increase) and are rapidly constructing new systems to facilitate greater transit use.

If one adjusts anticipated ridership figures to correct for these errors, and if one uses a more realistic figure for the value of business traveler time, then benefits appear to come quite close to or exceed costs of construction.


Glaeser makes more mistakes as he goes on. He appears to use the fuel efficiency for passenger cars — 22 miles per gallon — even though nearly half of the nation’s households vehicle fleet consists of light trucks, which average only 18 miles per gallon.

If you like what you see, you can see Avent take on Eric Morris (of Freakonomics fame) as well and the assumptions he writes about regarding transportation in the Times.