Abstract
The rotational speed of crankshaft is the main factor influencing on starting possibilities of diesel engine, especially at low temperature conditions. On its value depend the thermodynamic parameters of the air compressed in combustion chamber, fuel pulverizing quality and further pre-combustion and combustion processes. The influence of the rotational speed on diesel engine starting abilities is very complicated. At every temperature there is a minimum value of the rotational speed of crankshaft, which makes possible to start the engine within the determined time period. The increase of rotational speed of crankshaft during engine starting is profitable for engine start-up only to its special value, which is called the starting optimum speed. Further speed increasing causes the worse diesel engine starting abilities. So the maximum speed of crankshaft can be indicate. In paper the main points of this starting rotational speeds existing substantiation is done. There is the temperature, where the minimum, optimum and maximum rotational speeds are equal. The author as a triple starting point names this point. There are presented the investigation results, which allow introducing the bordering speed of engine start-up. It is the lowest rotational speed value of crankshaft, which makes possible to start diesel engine with the starting support means, for example, thermostart. The surface in coordinates temperature - rotational speed can be divided on the areas where engine start is impossible, it is possible with the help of start support means and it is possible the so called natural engine start.