Why trucks have diesel engines? A few reasons: 1) better fuel economy. Diesel contains more energy per volume than gasoline. The way diesel works is compress the fuel/air mixture until it goes boom. By definition it’s a high-compression engine, and with help of turbocharger (on most diesels, making it a turbo diesel) it makes plenty of hp for most needs. 2) less maintainence. To handle higher compression, diesel engines are heavier duty overall, have a lot less parts (no ignition system at all!) and less plumbing and 3) diesel engine is opposite of gas engines that it generates more torque than horsepower, which is needed to get heavy loads started.

It is true that diesels are noisier, and generates more “soot”, but a) that’s due to older design, and b) that’s due to sulfur in the fuel. Newest engines like CAT w/ ACERT technology are extremely clean compared to engines of 5 or 10 years ago. .

EDIT: Doug B’s right, diesels do have catalytic converters, but they serve somewhat different purpose than the cats for gas engines. For gas engines, the cat is used to convert unburned hydrocarbons to forms that won’t get turned into acid rain and such. For diesel engines, the cat is mainly there to deal with soot and sulfur. Both cats deal with Nitric Oxides and Carbon Monoxide. So they would have slightly different formulation. Both types of cats are expensive though.