This work was devoted to a simulated analysis of the blow-by (compressible flow of air-fuel fluid through the radial clearance between the piston and the cylinder) in a theoretical engine, in which the piston rings were removed and permanent magnets were added to reduce friction at the piston-cylinder interface. Using equations of mass conservation and momentum, the set of equations to be solved for a compressible and isothermal air flow was defined. The problem was represented by a flow between stationary parallel flat plates, using as reference the couette flow. In the absence of the analytical solution, ANSYS - Fluent software, a numerical resolution for different pressure differences between inlet and outlet of the channel, and a gap variation were obtained with the help of the ANSYS - Fluent software. Experimental data of indicated pressure and rotation of a real compression ignition engine were used to describe the behavior of the blow-by in the periods of compression and expansion without heat exchange. Finally, from several interactions, it was possible to size and estimate a theoretical gap, without the piston rings, in order to have the same leakage compared to the leakage of a real motor with the piston rings.