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Diesel me this, Diesel me that.

One of the many perks of being a professor is that we get to choose where we spend our summers. I often take my family to the home of soccer, beer, and renewables – Germany. The place has changed quite a bit from 40 years ago. The potato fields I used to traverse on my BMX bike are covered in Chinese made Solar PV installations, there are windmills everywhere and the smell of high sulfur coal wafting across the iron curtain is absent.

While cruising down the Autobahn, you notice the extraordinary share of Diesel powered passenger vehicles. 48.1% of new passenger vehicles sold in Germany were Diesels, up from 14.6% in 1995. The corresponding Diesel share of new vehicle sales in the US is below 1%. For most Americans the only experience they had with Diesel technology is maybe their uncle’s 1970s Mercedes 300D, which smelled of fryolator grease and had Grateful Dead Stickers on it. Oh boy has that technology come a long way.

Diesels have several advantages over traditional (Otto powered) combustion engines. Most notably, they are more fuel efficient, have lower emissions of the global pollutant CO2, and they have a tremendous amount of torque. For those of you who don’t read Car and Driver every night, torque is loosely correlated with how fast your car accelerates. A little Diesel powered VW Golf GTD has the same torque as a V6 Ford Mustang, at more than 40+ mpg on the highway. In short, you get a more fuel efficient and more fun car by simply switching from a regular combustion engine to a Diesel engine.

Downsides you ask? Diesels, all else equal, have a higher price tag. For a VW Jetta expect to pay about $4,000 more, which is stiff. In Germany Diesel is taxed less heavily than regular gasoline, which gives it a pretty significant cost advantage per gallon. In California the price of Diesel and Gasoline are almost the same according to the EIA.

Further, Diesels used to have a bad reputation for their high emissions of soot and other particulates. The “new Diesels”, which are equipped with relatively expensive urea filter technology and powered with ultra low sulfur Diesel do not suffer from this disadvantage. A quick survey of some EPA testing data on representative vehicles indicates that they are not worse and in many cases better in terms of emissions of local pollutants.

I am cautiously optimistic about the role of Diesels in improving the overall fleet fuel economy. They provide another alternative to those drivers who do not want to be seen in a Prius, enjoy Mustang-like torque, and seek good fuel economy.

Why am I cautious? We have not seen how these engines hold up in terms of emissions of local pollutants, which cause things like asthma and other pulmonary diseases. Smog checks are only required for vehicles older than 4 years. The “new Diesels” have not been allowed in California for long enough for us to see Smog Check results. Further, if these cars are driven for a decade or more, it would be interesting to see how the trajectory of local pollutant emissions compares to that of regular combustion engines. Finally, if the US market went the way of the European market with half of new cars sold being Diesels, we would likely run into refining capacity constraints for Diesel.

But personally, I am going to Diesel down the road once the lease on my current Otto engine is up.

Diesel vs Gas comparison by Ryan Kellogg, Associate Professor, University of Michigan, Department of Economics
Diesel vs Gas comparison by Ryan Kellogg, Associate Professor, University of Michigan, Department of Economics



Maximilian Auffhammer View All

Maximilian Auffhammer is the George Pardee Professor of International Sustainable Development at the University of California Berkeley. His fields of expertise are environmental and energy economics, with a specific focus on the impacts and regulation of climate change and air pollution.

14 thoughts on “Diesel me this, Diesel me that. Leave a comment

  1. This is a 4 year old blog now, but interesting and well presented and comments are considered. I value this sense of collegiality 🙂
    some comments of my own.
    I think that energy density of diesel and petrol fuel is only half significant, as the difference is only 10% while the consumption figures are usually greater than 20% in diesels favour.
    The remaining benefit is due to increased compression ratios and concomitant higher maximum temperatures.
    I privately bemoan the fact that diesel is surely a better match to a hybrid solution than petrol, and yet precious few companies have developed one. I usually bemoan the fact that the general design misses out on diesels advantages. Its power band and efficiency peak approaches a thermal efficiency close to 50%, where as spark ignited engines only approach 40%(usually much less). Toyota boasts a 38% thermal efficiency for a 1.3l engine (which is exceptional).

    Diesel engines optimally run for extended periods with near maximum loads and moderate RPM levels. Their average efficiency could be optimized in serial hybrid designs to only operate near their high optimal thermal efficiencies by only operating when the optimal load is provided by a generator. In a serial design, they can compensate their increased engine weight with removal of transmission components.
    What are the problems with this admittedly “naive” analysis? Am I missing some other more critical flaws?

  2. The very smart Chuck Mason points out that:

    1) England is the home of soccer. To that I add that the UEFA champions’ league trophy resides in Munich currently.
    2) Diesel may slush at very cold temperatures, making it a less favored fuel by folks in very cold climates.

  3. Nice article and one close to my heart. I drive a BMW 335d and love the car. This car has massive torque and accelerates so quickly especially at typical driving speeds between 20 and 85 mph. It is unphased by steep hills and gets 39 mpg highway and 30 mpg average. I typically can travel over 500 miles on a tank. The price premium was about $1500 (probably subsidized by BMW). With diesel and gasoline prices at near parity, the decision for me was easy. Beyond the lower rumble diesel sound at idle it is difficult to tell it is a diesel car, certainly not from the exhaust. After the 2009-11 model years, BMW has discontinued this model in the US market due to disappointing sales. As you said, diesel has evolved. Hey, I hate being behind an older diesel Mercedes, they stink! So it will take time for Americans to modernize their notion of diesel. I wish my wife’s VW Tiguan was available in a diesel model.

  4. I quibble with your description of the impact of torque. The higher torque of diesels at lower rpms is what gives diesels better mileage. Higher torque at lower rpms doesn’t mean that the diesel accelerates faster(is that what you mean by more fun?) than the gasoline engine with the same or greater displacement. Depending on how the vehicle is geared, it might and usually does have less acceleration than the gasoline engine with the same displacement.

    • For the same displacement this is certainly true. What I was thinking is two cars which get 30 mpg, one Diesel one Otto. On average the Diesel would beat the Otto off the line.
      I also tried to be clear that torque is not acceleration, but most people do not know what torque is.

  5. I personally do not think switching to diesel cars suits California. Not in a state where more people drive. Diesel would have been the choice for large buses that carry more people but the LNG powered buses serve the same purpose with better environmental advantages than diesel. Diesel therefore would be great in stationary plants where the exhaust can be completely harnessed and controlled.

    Diesel, I guess emits many more particulate matter than just the CO2 and NOx that seem to be under control. We need to hold the state or even nationwide health paramount to individual economic gains in terms of MPG savings etc. What shall it profit a person to save money in driving only to come out of the vehicle and suffer congestions that may require more than the savings to cure?

    The above observation not withstanding, I think we need research into everything that is emitted. The NOx harnessing technology may require quicker yet expensive replacements even more than catalytic convertors do in gasoline vehicles.

  6. I carpool to work with co-worker who drives a 2012 Golf TDI and I think it is a phenomenal car. He get’s 40-50mpg and the vehicle is spacious, quiet, maneuverable and responsive. I think as more Americans end up behind the wheel or in the passenger (or even spacious back) seat of these TDI VWs, they will see a dramatic increase in sales. In light of the recent article about hybrid vehicles in IEEE Spectrum, it seems one of these small TDIs might be the most environmentally friendly vehicles available!

  7. I used to be bullish about diesel cars, but it seems to me an increasingly tougher sell going forward as emissions rules become more stringent and the standard gasoline engines in most cars are getting better.

    From casual observation, gasoline and diesel cars are in some ways converging and other ways the opposite. Example of the latter: the next edition of VW’s diesel will be sold with an SCR and a urea tank to further clean up NOx, something not found in the version sold today, and possibly bumping up the price tag even more when compared against the gasoline model that employs relatively much simpler emission controls to achieve the same result. On the other hand, that gasoline engine to be sold in future VWs is full of advancements over the current unit, that will make it more efficient, and likely more expensive. I’m curious about the difference in price between the two, whether it will increase or decrease.

    Either way, my prediction is that it will continue to be hard to justify the additional initial cost to buy a diesel car that can achieve MPG in the 40s over a gasoline car with MPG in the 30s.

    And since I’m commenting on an energy blog, here’s a final piece of information on energy density of the two fuels, from wikipedia:
    Gasoline 34.8 MJ/L
    Diesel 38.6 MJ/L

    Is it any wonder then, that Diesel cars always get better MPG?

    • I agree with your assessment. The more relevant comparison might be a hybrid to a Diesel?

      • Maybe not hybrid-diesel comparisons. Even though there don’t seem to be many such offerings, electrification can be added to diesel cars jus the same as gasoline, or for that matter, natural gas. I want to see more head-to-head gasoline/diesel comparisons, I suppose you look to Europe for that. I just went to VW’s UK website for info on the same car described above, sold there as a Golf Estate wagon. One can chose from no fewer than 4 gasoline and 3 diesel options. Comparing the fuel consumption and carbon emissions numbers listed yields some interesting numbers. I picked two engines of comparable horsepower (105):

        Diesel 1.6L 70.6 MPG 104gCO2/km

        Gasoline 1.2L 56.5 MPG, 115g CO2/km

        Aside from the mix of units and the very optimistic euro fuel economy testing, the take-away here is this: Diesel is about 20% better in MPG, but only about 10% better with CO2, owing to the higher density of the black oily fuel. They are 1600 English pounds different in price, but that’s with the old Euro5 emissions regs that are not even close to what VW must comply with to sell cars in CA.

        Finally, the two engines above have the same torque numbers, so there’s no argument about fun-to-drive (other than the gasoline car being 100kg lighter than the diesel model).

  8. Count me a German. Just got 2 German diesels, over hybrids (replacing 2 gas German cars).
    The cost difference is made up in gas savings. The drive of a diesel is just as umph’y as that of the VW hybrid, and a lot better than the Prius.

  9. Thank you for this well framed and balanced article. Your caution about long term emissions is prudent and appropriate given that typically in any fleet of light-duty motor vehicles more than half of the emissions of smog precursors are from less than ten percent of the fleet, so-called high emitters. If even a few percent of diesel vehicles turn into high emitters, as likely based on historical experience, their emissions could overwhelm any perceived air quality benefits from diesels. You are correct it will take at least a decade or more of experience with the new generation of diesel LDMVs to understand how this plays out.

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