The Diesel Dilemma

The Diesel Dilemma

When I bought my first car about a year ago I honestly believed I was making a choice that at least partially minimized the impact of driving on the environment. I considered getting a hybrid, but for several reasons I chose a Jetta TDI. The TDI means it's a diesel. Diesel's have long had a reputation for being noisy, smelly, and polluting, but those opinions always seemed rather ill-informed. I knew, for example, that diesel passenger cars are far more popular in Europe than in the US, and Europe tends to have more progressive environmental laws. It's also a fact that diesel engines are more efficient than gasoline engines. This is an undisputed truth that comes as a result of the way diesel engines function; that is, by compression rather than spark ignition. Due to the diesel engine's popularity in Europe, car manufactures there have been modernizing and improving the design of the diesel engine for decades. Americans tend to associate diesel engines with industrial vehicles like busses and construction equipment, which spew out thick clouds of black smoke, but the modern European diesel passenger car emits almost no visible smoke whatsoever.

Being the proud owner of a new car I often found myself in a position to talk about it. I would mention that it's a 2005 model year (the redesigned Jetta was introduced partway through the 2005 model year, so my car really is officially a 2005.5) and that it's a diesel and therefore gets almost 50 mpg (also true). If I wanted to be even more of a tree-hugging environmentalist I could run it, without modification, on bio-diesel. When I tell people it's a diesel, I usually get one of three reactions. People either don't care that it's a diesel because it means nothing to them, think it's cool that it's a diesel and therefore efficient and environmentally friendly, or say something about how awful diesel emissions are. The fact that environmentalists would give me two conflicting responses says something about the misinformation out there concerning the environmental impacts of diesel exhaust. If given the third response I would usually try to defend it by saying that it emits far less greenhouse gasses but more of the smog causing compounds, and since I'm using less oil to run it, the net effect is slightly positive.

This friendly debate, plus the recent press around low-sulfur diesel fuel and "clean diesel" technology inspired me to try to get some facts so I'd know what I was talking about when it came to diesel emissions. The truth, whatever it is, is not easy to find. Do a web search for "diesel exhaust" or "diesel emissions" or "clean diesel" and all the results are organizations with agendas. Obviously the TDIclub.com site makes diesel engines out to be the earth's best friend. Obviously, hybridcars.com makes diesel out to be the biggest polluter this side of coal-fired power plants. So which one is correct? Like most things the answer seems to lie somewhere in between, but is still very unclear.

I finally found an article that mostly spelled it out like it is. I'm still curious about some of their claims. In an effort to find an unbiased opinion I turned to the website for the Union of Concerned Scientists. I often respect this organization for being unbiased and fair. They readily admit to having an agenda, which is sound environmental stewardship, but they back up their claims with hard, scientific fact. They published a paper in 2004 called "The Diesel Dilemma: Diesel's Role in the Race for Clean Cars." The paper's authors, Patricia Monahan and David Friedman, try to objectively analyze the best and cheapest technologies for achieving high-efficiency and low-emission vehicles. In their executive summary, linked to above, they basically say that gasoline-powered vehicles, gasoline-electric hybrids especially, are the technology worth investing in for a variety of reasons and that diesel will never be as clean or efficient as the best gasoline-powered cars. I think most of their points are valid, but I would like to offer some counter-arguments, not because I think I know better, but just to illustrate my questions and hopefully spark some more debate.

One of the paper's major arguments against diesel engines is that they are more expensive than gasoline engines, especially when emission controls are taken into account. They argue that the increased upfront costs of the diesel, combined with the emissions control technology necessary to make diesel exhaust as clean as gasoline exhaust make diesel impractical. They argue that there are more gains to be made investing this money on improvements to gasoline engines. Here's an excerpt from the paper:

Today’s diesel cars are just one
of many strategies available to increase fuel economy.
A variety of technologies—more efficient
engines, better transmissions, improved aerodynamics,
and high-strength materials—can
give today’s diesel a run for its money.
It should be noted that the fuel economy advantage
of diesel does not translate into equivalent
reductions in oil usage and heat-trapping gas emissions.
Differences in the production, energy content,
and formulation of low-sulfur diesel fuel
and federally reformulated gasoline come into
play. Relative to conventional gasoline vehicles,
the energy security and global warming benefits
from diesel are lower than gains in fuel economy,
even accounting for diesel’s higher energy density.

Here they list technologies that can be used to make gasoline-powered vehicles more efficient, and thus catch up to the inherent efficiency of diesel; more efficient engines, better transmissions, improved aerodynamics, and high-strength materials. Well, these are all technologies that can make the already more efficient diesel car even more efficient. Take two otherwise similar cars, one with a diesel engine, one with a gas engine. The diesel starts out with an efficiency advantage because it's diesel. Now, give them both a better transmission, improved aerodynamics, high-strength materials, and more efficient engines. The diesel is still way more efficient.

The second point, about the efficiency of diesel not translating into equivalent reductions in oil usage and heat-trapping gas emissions seems suspect. Later in the article they go into more detail about this point, normalizing a gallon of diesel to "gallon of gas equivalent". They do this by itemizing all the components and energy that go into making a gallon of gas vs. a gallon of diesel. They rightly take into account that a gallon of diesel takes less energy to refine than a gallon of gas, but also that it takes slightly more crude oil per gallon of diesel than gallon of gas, due to diesel's higher energy content. Where they go astray, I believe, is in accounting for the ethanol content of gasoline. In their tabulation of the energy requirements for formulating gasoline, the ethanol seems to suddenly appear magically, fully formed. No mention is made of the energy requirements of producing said ethanol, in the from of petroleum products needed to fertilize the corn, refining it into ethanol, or shipping it. Even after all this calculation, they still come to the conclusion that diesel engines use less oil and produce less greenhouse gasses than a gasoline engine, just that the difference isn't as extreme as simple comparing miles per gallon of the two vehicles might lead one to believe.

As an aside, I'd like to point out that part of the up front costs of a diesel engine translates into higher durability. Diesel engines last longer than gasoline engines because they are built more robustly to withstand the compression ignition cycle that defines a diesel engine. This translates directly into less waste; fewer cars need be built and fewer cars ending up in junkyards.

But what about emissions? So a diesel engine lasts longer and is more efficient than a gasoline engine, but everyone can see that gross, black smoke that come out of a diesel tailpipe. That must be bad, right? Well, the short answer is yes, it is. There seems to be plenty of information out there saying that diesel exhaust is bad. The sooty particulates that you can see are a known lung irritant, and diesel exhaust is known to contain NOx, a major producer of smog. But burried deep in the Union of Concerned Scientist's report is the following,

While the toxicity of diesel has been well
researched, there is relatively little information on
the toxicity of gasoline. New studies indicate that
today’s average gasoline and diesel vehicles have
the same toxicity per unit of mass (Seagrave et al.,
2002), but the exhaust from high-emitting gasoline
engines may be more toxic.

and,

Recent European measurements indicate
that gasoline spark-ignited engines running at high
speed and load may release as many or more nanoparticles
as typical diesel engines (Kittleson et al.,
2003). Unfortunately, because it is not yet clear
whether today’s technology can accurately measure
nanoparticles and there is no accepted testing
method to ensure consistent measurement, comparisons
between different studies are nearly impossible
(Andersson, 2001). Different transient
cycles, operating conditions, and exhaust temperatures
also affect nanoparticle generation.

So, what's the conclusion? Diesel exhaust is toxic, smog forming badness. But gasoline exhaust is toxic, lung irritating badness, too, which may be much worse than diesel exhaust. The jury is clearly out. A major theme running through the paper is that hybrids should be encouraged because the technology is a stepping stone to zero-emission vehicles that need no fossil fuel whatsoever. I completely agree. Like I said at the beginning of the article, I was close to getting a hybrid, but chose not to for a variety of reasons, one of which was their availability at the time. My only vehicle for several years prior to getting the Jetta was my bicycle, and when I decided to finally buy a car I didn't want to be on a waiting list for several months for a true hybrid. I wish I could have supported the development of cleaner, more efficient hybrid technology with my consumer dollars, but it just wasn't a good fit for me. Until then, I will continue to drive my Jetta TDI, which gets 50 mpg on the highway, produces fewer greenhouse gasses, and may or may not have more harmful emissions from a public health perspective. Purchasing the TDI in the first place might not have been as environmentally friendly as I thought it was at the time, but clearly there are worse choices I could have made.

If anyone knows of a good source of information concerning the harmfulness of diesel exhaust vs. gasoline exhaust I'd be very interested in hearing about it. As I mentioned earlier, almost all information available on the web has an agenda attached to it and any unbiased information is hard to come by. Thanks!