The intake stroke is performed when the piston moves away from the upper center. to n.m.t. Since the volume of the cylinder above the piston at this time increases, the pressure in it becomes less than atmospheric and air fills the cylinder through the open intake valves. However, due to resistance in the pipeline and in the air cleaner, where the air is cleaned from dust, and a number of other reasons, the average pressure in the cylinder at the intake stroke will be less than atmospheric by 0.1-0.25 kgf / cm², which means that the air mass entering the cylinder will be slightly less than the maximum possible.
The mass of air decreases, in particular, with a decrease in the density of the outside air, which changes with increasing altitude and temperature. To ensure the most complete combustion of fuel, it must be supplied to the cylinder in an amount determined based on the actual mass of air in the cylinder at the end of the intake stroke.
Increasing the amount of fuel burned in a cylinder per cycle is the primary means of increasing engine power. The required increase in the mass of air entering the cylinder is carried out by installing a supercharger on the intake line, which supplies air to the cylinder at a pressure exceeding atmospheric pressure.