Q: Why Do Diesel Engines Give Off so Much Smoke?

The smoke you see is made up of the visible particulates that are emitted as a normal part of the combustion process of a diesel engine. There are other non-visible emissions in the exhaust also, like nitrous oxide. Diesel fuel has more energy potential for a given volume of fuel than gasoline, which is one of the reasons why the fuel economy of a diesel powered vehicle is better than a gas engine vehicle. The energy potential is visible when you compare samples of diesel fuel and gasoline. Gasoline flows very thinly, while diesel feels oily and heavier. A very rough analogy is it’s kind of like burning big pieces of wood in your fireplace instead of smaller branches. Because of Diesel’s heavier nature, essentially what you’re seeing is the residue leftover from combustion, similar to the ash left after a wood fire goes out. The reason you don’t see smoke from modern diesel powered cars and light-duty trucks is because manufacturers are required to add exhaust particulate traps and catalytic systems downstream of the engine to process particulates and other emissions into less toxic gases.