Traffic is known in the literature for its harmfulness and leaving greater consequences not only on humans but also on the environment. Air, water and ground are most affected, which further causes harm to people and their health. In addition to the harmfulness itself, traffic also creates side effect such as noise, vibration and land degradation. In traffic, diesel engines contribute the most to this, as their internal combustion emits carbon dioxide, carbon monoxide, hydrogen and other substances into the atmosphere. At the same time, soot and smoke are the biggest problems of diesel exhaust. This paper will examine the mode of operation and use of diesel engines, make a detailed analysis, see which are their biggest consumers and how they affect the harmfulness of not only air but also other flora and fauna. The research, except by reading the literature will be conducted by comparing the harmfulness of a gasoline engine with a diesel engine. Legislation on emissions will also be mentioned, and the aim of this paper is to show ways to reduce the harmfulness od diesel engines, in addition to the use of eco-driving, the use of alternative fuels, electric vehicles or hybrid propulsion.