How are diesel engines better for the environment than gasoline?

Question

I understand that diesel engines have become much more efficient at controlling emissions over the last couple decades. Various websites like this one describe the technological improvements that have brought this about, such as:

• Direct injection controlled by computers
• Particulate filters to reduce soot
• Catalytic converters that reduce other pollutants
• Improvements to diesel fuel itself such as ultra low sulfur diesel (ULSD)

Combined with the tendency for diesels to be more fuel efficient than similarly-sized gasoline engines (according to fueleconomy.gov, they can get 20-35% more distance out of one gallon of fuel) and that they are required to meet the same emissions standards as gas, they must be at least comparable to gasoline engines on total emissions.

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However, there is still a public perception, at least in the United States, that diesels are dirty, even if we accept that they might be more fuel efficient. This is due in large part to the history of diesel where we used to see diesel trucks belching out soot on the highways and leaving black streaks behind the exhaust pipe. Many of these trucks are still on the road, so they have not seen much of the new technology that has improved this situation. I'm sure the attention drawn to the Volkswagen emissions scandal is not helping any either.

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My research is also suggesting that, in spite of the technological advances, diesel still produces up to 15 times as much emissions per liter burned than gasoline. This is a huge difference and quickly eats up the 35% maximum fuel efficiency boost you get from using diesel. A slightly older study seems to suggest that, even under the stricter emissions guidelines, diesel contributes more to global warming than gasoline.

This seems to run against the idea that diesels are cleaner overall. Though they are hard to compare since different pollutants are allowed to escape into the atmosphere, total effect on global warming is a good measure. So how are diesels cleaner than gasoline engines? Am I missing something?

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As a secondary question, why aren't diesels used in electric hybrids as generators instead of gas? It seems that would be the perfect place for them since generators run at a constant speed most of the time, so emissions controls could be tuned to specific RPM. Note that I am excluding hybrid vehicles where both the electric and diesel engines are connected to the drive train. This article outlines why those are not necessarily the best option.

Answer 1

They aren't better for the environment

But efficiency is often confused with emissions. The average person would think that because diesel engines are more efficient and burn less fuel, they must be less harmful to the environment.

I think few can be more authoritative than the folks at Bosch on why diesel engines are so prevalent (emphasis my own)¹:

No other internal-combustion engine is as widely used as the diesel engine. This is primarily due to its high degree of efficiency and the resulting fuel economy.

The same chapter goes on to highlight the emissions challenges associated with diesels. It's wrong to say that it's a cleaner fuel than gasoline (emphasis own):

Improving emission characteristics will continue to be a major challenge for diesel-engine developers in the future. Consequently, further innovations can be expected in the area of exhaust-gas treatment in years to come.

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So why are they so commonplace?

Because less fuel consumption = less cost.

And in most societies, reducing cost is considered a good thing. It all boils down to the moolah.

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¹ - Diesel Engine Management: Systems and Components - Areas of use for diesel engines: Suitability criteria, page 12

Answer 2

Locally worse, globally better

Diesel exhaust versus gasoline:

• about 15-20 % less CO₂
• about 4-100 times the NO_x (highly temperature dependent, mitigated by proper use of diesel exhaust fluid, ref the VW scandal)
• and a fair amount of soot (esp. before common rail, mitigated by a particle filter).

What this means for the local environment:

• Local CO₂ is the least of problems, just good for the plants.
• NO_x is the primary concern for lung illness in smoggy areas. It also creates acid rain (which can be a bit less local).
• People don't like to inhale soot either.

What this means for the global environment:

• Less CO₂ = save the climate
• NO_x breaks down methane, a strong greenhouse gas. So much so that ship emissions are estimated to have a net global cooling effect.
• Soot is probably not such a problem on the global scale yet.