Diesel fuel: why aren't there more diesels?
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Diesel engines generally have far higher gas mileage than equivalent gasoline engines. They also tend to last longer, which can make them cheaper in the long, long run. Busses and trucks tend to have diesels because of their massive torque and economy; however, American taxis and fleet cars do not, partly because very few cars are available with diesels.
The U.S. government subsidizes ethanol despite that fuel's terrible economics and impact on the environment as a whole (ethanol takes a great deal of energy to produce). However, biologically created biodiesel can be better for the environment than standard gasoline, and can be a greater boon to farmers, since it is much less energy-intensive to create. It also does not require modifications to diesel engines, as far as research can tell.
So what stands in the way of diesels?
First, in terms of the environment, diesel emissions have a high percentage of particulates per gallon burned - particulates suspected to be carcinogenic. However, we have not seen any research that considers the impact per mile driven and compares it with the impact of gasoline engines. If one vehicle has, say, 1% noxious emissions in its exhaust, and another has 1.5%, which is better for the environment? Clearly the one with 1.5%, if it gets double the gas mileage - but the U.S. government doesn't see it that way. Everything works by proportion, so that a thrifty Toyota Corolla is judged by the same proportion standards as the gas-hog Cadillac - and the even-bigger-hog heavy-duty Yukon gets a break!
In terms of sales, many people are reluctant to pony up the extra thousand (or three thousand) for a diesel, partly because gasoline is relatively cheap, and partly because most people do not keep their cars nearly as long as European drivers do. (In Europe, diesels are very popular, but cars are kept for a long, long time due to their relatively high cost - taxes are one factor, and small markets are another).
Just as there is a fleet average mileage law, there is a (less publicized) fleet average emissions law. Why do you think VW sells those turbo diesels like hotcakes, then "runs out" in the middle of the year? They have used up their quota.
Here in California, it is even worse. There is a strict quota on the number of 1 ton diesels the Big 3 can bring into the state each month. Do you think Dodge is going to cut back on high profit Ram diesels so it can sell diesels in Caravans?