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Alternative Fuels: Diesel

By Carey Russ (c) 2003

Ten or twenty years in the future, cars and trucks are likely to be 

powered by hydrogen fuel cells. Until then, expect internal 

combustion (IC) to continue to provide power, but it will not be 

business as usual. While gasoline will continue to be used, it will 

continue to evolve as it is reformulated to lower pollutants. Natural 

gas is already in limited use for commercial fleets, and specially-

modified gasoline engines have successfully run on hydrogen, at 

least experimentally. But, although both natural gas and hydrogen 

burn very cleanly, with minimal exhaust pollutants, they have a 

much lower energy density than gasoline. Simply put, it takes more 

of either fuel to provide as much energy as an equivalent amount 

of gasoline. And, since both are gases at normal temperatures, they 

need to be compressed. Hybrid vehicles, with a natural gas, 

hydrogen, or gasoline-electric drivetrain, are one way to lessen the 

disadvantages of range. Gasoline-electric hybrids are already 

available, but there is another fuel that, when used in cars, can 

produce fuel economy equal to that of today's hybrids.  

That fuel is diesel.

Pick yourself up off the floor and stop laughing. It's not the 1970s 

any more. Today's diesels are not what they once were, especially 

modern European diesels. If you associate the word ``diesel'' with 

``smelly,'' ``dirty,'' and ``sooty,'' think again. Because of their 

inherently efficient design, diesels produce lower levels of 

unburned hydrocarbons, carbon monoxide, and carbon dioxide 

than gasoline engines. Diesel problem areas are in particulates and 

nitrogen oxides, but those problems are being solved. According to 

the Diesel Technology Forum, today's diesel trucks and busses 

produce only one-eighth the emissions of their counterparts of the 

late 1980s. By 2007, diesel emissions will be virtually eliminated. 

Emissions from construction equipment and even railroad 

locomotives are also decreasing. 

Technology that has cleaned up gasoline engines can also be 

applied to diesels. This includes improved design and materials, 

precise electronic controls, and exhaust catalysts and particulate 

filtration. Also, as with gasoline, diesel fuel will evolve to lower 


Unlike other internal combustion engines, in which an electric 

spark ignites the fuel/air mixture to produce power, diesels work 

by compression ignition. The fuel/air mixture is compressed to a 

much higher degree than in a gasoline (or alcohol, natural gas, or 

hydrogen) engine, to the point where it ignites due to the heat of 

compression. The more a fuel/air mixture is compressed, the more 

power can be produced. A 10-to-1 compression ratio is common 

for gasoline engines. Alcohol and hydrogen are around 14:1. 

Diesels have compression ratios from 18:1 to 21:1.

Because of their high compression, diesels produce more torque at 

lower engine speeds than other IC engine designs. Low-rpm torque 

is what provides acceleration at everyday speeds, and it also 

contributes to the towing abilities of diesel trucks. The high 

compression and high operating temperature of a diesel contribute 

to its fuel efficiency, typically 20 to 40 percent better than a 

comparable gasoline engine. When properly constructed, diesel 

engines last longer than other types. Yes, they're heavier because 

of the torque and temperatures that need to be handled, but modern 

materials are making current diesels lighter than their counterparts 

from the past. Today's automotive-sized diesels are not huge, 

heavy pieces of machinery.

Although diesel cars are few and far between in this country, diesel 

pickups are popular, particularly at the larger end of the size and 

duty spectrum. And diesel cars are coming here. The Volkswagen 

Golf and Jetta TDI models will soon be joined by a Passat TDI, 

and the Mercedes-Benz diesel is rumored to be coming back to 

America. If these cars succeed in the American marketplace, 

expect American and Japanese manufacturers to follow.

In Europe, emissions requirements are different and fuel is much 

more expensive, so more diesel cars are sold. One-third of 

European cars are diesels, with 61 percent in Austria and over 50 

percent in Belgium and France. And these are not just small 

economy cars. Forty-four percent of European luxury cars are 

diesel powered.

Diesel luxury car? Believe it. At the recent Challenge Bibendum 

alternative fuels event, I had the opportunity to take short drives in 

an Audi A8 TDI, BMW 5-Series diesel, Volkswagen Passat TDI, 

and Fiat Stilo diesel. The Audi and BMW were true luxury cars, 

and as smooth and quiet as their gasoline counterparts. The only 

clue to diesel power was a different exhaust note - but in both it 

was very subdued and quiet, not the raucous rattle associated with 

old diesels. Although the VW and Fiat were a little lower in the 

socioeconomic spectrum, they were no less refined. Power was not 

a problem, even up the steep hills at Sears Point. If fuel economy is 

a concern for you, check out a diesel next time you're in the market 

for a new car or truck. You're likely to be pleasantly surprised.