When crude oil is refined, it can be separated into several different kinds of fuels, including gasoline, jet fuel, kerosene and, of course, diesel. Clean diesel fuel evaporates much more slowly than gasoline, but has a higher energy density than gasoline. This, combined with the improved efficiency of diesel engines, explains why clean diesel engines get better fuel economy and mileage than equivalent gasoline engines.

Both clean diesel engines and gasoline engines covert fuel into energy through a series of small explosions or combustions. The major difference between clean diesel and gasoline is the way these explosions happen. In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a clean diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it’s compressed, the fuel ignites. Using direct fuel injection, diesel fuel is injected directly into the cylinder. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist.

Clean diesel engines collect soot from the exhaust fumes through its particulate filter. The filter captures the soot so it isn’t emitted through the tailpipes. Some engines also spray the remaining exhaust gas with a urea-based substance that helps convert harmful nitrogen oxides into harmless nitrogen gas and water vapor.