Internal-combustion engines are heat converters for the conversion of heat energy into useful mechanical work. They can be divided according to various parameters, according to the number of tacts on two-stroke (2T) and four-stroke (4T) engines, according to combustion mode and propulsion fuel on Diesel, Otto and Wankel engines, by cooling mode on air-cooled or fluid-cooled engine etc. Internal-combustion engines consists of about one thousand smaller parts, however the main parts of the combustion process are the piston mechanism, the head and the engine block and the engine units. From the fluids it is important to mention the oil from the engine crankcase, which reduces friction and lubricates elements that are in contact, tank fuel that, with a certain amount of air, stimulates combustion and cooling liquid that maintains engine operating temperature and prevents overheating. Differences in the design of vehicles with Diesel engines and electric vehicles are big, primarily because electric vehicles have significantly less parts, consisting of batteries, electric engine and controllers. They use renewable energy, they do not have exhaust gases, and can generate energy themselves, for example, with regenerative braking, which saves kinetic energy in the rechargeable battery. Maintaining vehicles with internal combustion engines is more demanding than maintenance of electric vehicles because there are more parts that may be malfunctioning, but unlike electric vehicles, maintenance can be done on multiple locations and parts are far more accessible. With the aim of reducing exhaust emissions and the consumption of non-renewable energy sources, through the ban of entering Diesel into centers of large cities and stopping of production in the coming years, and with the incentives for electric vehicles, it can be said that the Diesel engines are slowly going into the past and electric vehicles are just coming, still developing and providing a cleaner future.