Almost all types of internal combustion engines are in use in forestry, therefore we will be concentrating mainly on engines instaled in tractors. Modern tractor engines have been more or less the same throughout their evolution, except that they were upgraded with sistems that increase performance such as the turbocharger, intercooler, common rail. All the main engine components and sistems are listed and described, with special emphasis on air intake components and fuel distribution and injection components. A part of this paper is intended for the description of engine characteristics such as power, torque and specific fuel consumption. Main fuels and their parameters are also listed. By comparison of the latest and previous types of engines with same displacement it can be established that fuel consumption decreased and that power and torque increased, the amount of exhaust gasses also decreased.