VIN WAS THE DIESEL ENGINE OF
THE PLOT VEHICLE
Is Diesel Performance Superior Overall Compared to
Gasoline Power?

At one time, diesel cars were a relatively common sight in the United States. In Europe, they still are, accounting for about 40% of new cars sold each year. Diesel fuel has higher energy content than gasoline, and diesel performance is usually better in terms of engine output. These factors also mean that vehicles running on diesel put less global-warming pollution into the air. Thus, it might seem that in today's world of rising temperatures and fuel prices, diesel is ripe for a surge in popularity in North America.

Ah, ah, ah. Not so fast. Good diesel performance may mean better fuel economy and less carbon dioxide, but what about diesel's problems with other types of air pollution? What about the higher up-front cost of diesel technology? There's also a catch when it comes to comparing mileage estimates for diesel and gasoline vehicles.

Today's article, adapted from materials made available by the Union of Concerned Scientists (UCS), discusses the pros and cons of diesel vehicles and compares them to their gasoline counterparts.

~    ~    ~

DIESEL OR GASOLINE?
Fuel for Thought
by By Patricia Monahan and David Friedman, UCS

DIESEL PERFORMANCE —
EXTRA POWER, HIGHER EFFICIENCY, BUT ...

If you or your parents owned a diesel car 20 years ago, you may have some bad memories of the experience. American drivers have steered clear of diesel since the early 1980s because many of the cars were unreliable, noisy, and polluting. Though today's diesel cars have overcome most of their past performance problems, they account for only a few percent of new automobile and truck sales in the US.

In Europe, on the other hand, about 40% of new cars sold are diesel, amounting to more than five million vehicles each year. The demand for diesel in Europe is fueled by the high cost of gasoline. (Unequal taxation of the two fuels results in diesel costing about one dollar less per gallon in most European countries.)

Over the past few years, diesel's popularity as an automotive fuel has grown significantly. Thanks to its higher energy content and its efficient combustion process, diesel performance enables cars to travel at least 30% farther on a gallon of fuel than comparable gasoline models.

The improved efficiency of diesel engines can also help reduce oil consumption. It should be noted, however, that it takes about 25% more oil to make a gallon of diesel fuel than a gallon of gasoline, so we should really look at how a vehicle does on fuel efficiency in terms of "oil equivalents." Thus, we need to adjust the mileage claims for diesel vehicles downward by about 20% when comparing them to gasoline-powered vehicles.

DIESEL — EXTRA POLLUTION

There are three size categories of soot particles:

• Large soot particles (>10 microns) deposit from the air into your nose, throat, and lungs, causing coughing and irritating your throat, and are ejected from your body through sneezing, coughing, and nose blowing.
• Coarse soot particles (~10 microns) are inhaled into your windpipe and settle there, causing more irritation and more coughing.
• Fine and ultra-fine soot particles (less than 2.5 microns) are the most successful at invading your body—they're small enough to travel deep into your lungs. Once there, these soot particles can irritate and mutate the most sensitive tissues in your lungs, your alveoli. These air sacs exchange oxygen and carbon dioxide from the air you breathe with blood in your capillaries, thus allowing your circulatory system to carry oxygen to the rest of your body. Soot particles, however, make this task more difficult because they cause inflammation and scarring of the alveoli. This also strains your heart because it must work harder to compensate for oxygen loss.

Soot also finds other ways to harm your body, including:

• chronic bronchitis,
• asthma,
• reduced ability of respiratory system to fight infections and remove foreign particles, and
• cancer.

All this means that diesel pollution can be deadly, causing premature mortality through cancer or heart and respiratory illnesses. The California Air Resources Board has concluded that diesel soot is responsible for 70% of the state's risk of cancer from airborne toxics. In the population as a whole, studies have shown a 26% increase in mortality in people living in soot-polluted cities.

THE DIESEL POLLUTION SOLUTION — AND ITS LIMITATIONS

To address diesel's emissions problems, tougher emissions rules are coming into effect. To meet the tougher pollution standards, high-tech diesel engines need low-sulfur diesel fuel. Unfortunately, US Department of Energy modeling has shown this fuel to be more oil- and carbon-intensive than reformulated gasoline.

Making a gallon of diesel fuel requires 25% more oil and emits 17% more heat-trapping greenhouse gases than gasoline reformulated with MTBE. Similarly, diesel requires 17% more oil and emits 18% more heat-trapping gases than gasoline reformulated with ethanol. This means that diesel fuel's advantages from its higher per-gallon energy content and better performance on greenhouse gases are partially offset by the impact of diesel's fuel-production process.

Still, future diesel vehicles, though perhaps not as cost-effective as gasoline, may have a role to play in reducing oil consumption and global warming pollution. Of the vehicles evaluated in the UCS report "The Diesel Dilemma: Diesel's Role in the Race for Clean Cars," full hybrid-electric diesels offered the maximum improvement in fuel economy as well as the greatest reduction in heat-trapping emissions. But a key challenge remaining is whether diesel vehicles will ever be able to deliver the same progress on other air pollutants that we've seen in today's gasoline-power technologies.

SO WHICH IS BETTER OVERALL — DEISEL OR GASOLINE?

Technologies are being developed that can make diesel much cleaner and more fuel-efficient. But those advances have to be compared to continuing advancements in gasoline-powered vehicles. The chart below compares diesel performance to gasoline performance for a variety of characteristics. The more dark blue (+ signs), the better.

Table 1. Comparison – Diesel vs. Gasoline

	CATEGORY DIESEL GASOLINE NOTES Initial cost – ok   Range + ok   Extreme towing capability + ok   Infrastructure availability – + +   Tested tailpipe pollution – +   In-use pollution – – – a Maximum potential oil reduction + +   Maximum potential global warming benefits + + + b Cost-effectiveness for oil reduction + + +   Cost-effectiveness for global warming benefits + + +   Net consumer savings + + + c KEY:         Excels in this area + +   Does well in this area + Performs adequately in this area ok Does less well in this area – Performs poorly in this area – – NOTES: a. Assumes diesel emission controls fail at the same rate as those for gasoline vehicles, resulting in higher in-use pollution. b. If diesel soot proves to be an important heat-trapping gas and is difficult to control, the potential global warming benefits from diesel will be muted. c. It should be noted that different gasoline and diesel formulations can affect cost, oil demand, and greenhouse gas emissions. Nevertheless, our general finding that gasoline is more cost-effective than diesel appears to hold true among common gasoline and diesel formulations.

The chart above helps us determine which automotive choice is best when considering the following factors:

• petroleum usage;
• greenhouse gas emissions (global warming pollution);
• toxic air contaminants (including soot);
• total cost to purchase, own, and operate.

Diesel does have a slight advantage over gasoline in the first two categories. But UCS modeling suggests that diesel's tough pollution-control challenges and the high up-front cost of engines and emission controls for diesel vehicles gives gasoline technology the edge overall. That means there is no mandate to bring back diesel in a big way—gasoline-powered cars, particularly gasoline-electric hybrids, are likely the best way to go.

POLICY RECOMMENDATIONS FOR GASOLINE
AND DEISEL VEHICLES

Here are a few broad recommendations that can ensure the role diesel plays in our future vehicles will be a positive one:

RESEARCH AND INSPECTIONS: Emission-control systems deteriorate with age, are sometimes tampered with, may be improperly maintained, or may have been poorly engineered in the first place. To ensure that actual emissions for vehicles—all vehicles, including diesel—match predictions, we should do more to monitor vehicles' real-world pollution performance. Inspection and maintenance programs can help and should be expanded to include diesel vehicles, particularly if diesel becomes more popular in the light-duty vehicle sector. We should also study the public health effects of non-regulated emissions to determine whether emissions standards are effective enough.

MILEAGE STANDARDS AND LABELING: To enable consumers to properly evaluate fuel-efficiency claims for new cars, mileage standards should compare gasoline and diesel on an energy-equivalent basis; that is, how efficiently they use a gallon of oil, not how far they go on a gallon of gasoline or diesel. Window stickers on new vehicles should be expanded and made more accurate to give consumers more information so they can evaluate the impact of their purchase on air quality, global warming, and energy security.

INCENTIVES: Protecting public health and improving vehicle fuel economy can and must be complementary goals. Incentives for vehicles with higher fuel economy and lower pollution—regardless of fuel type—can help increase production volume, lower costs, and raise consumer awareness of better vehicle technologies.

Whether we're talking about diesel or gasoline, improved vehicle technologies have the potential to cut oil usage and global warming pollution by more than 50% while saving consumers money and protecting public health. It just takes the will to make it happen!

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Resources:

• Full UCS report, "The Diesel Dilemma: Diesel's Role in the Race for Clean Cars"
• Find fuel-efficient vehicles at FuelEconomy.gov
• Find low-emissions vehicles using the US EPA's Green Vehicle Guide

Publish date: 12-APR-2005

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