The functioning of the engine in closed loop with lambda sensor and catalyzer makes the excess air factor to be maintained as close as possible to the value λ=1. Based on an own model, the authors realized an analytical calculation of the indicated diagram. The indicated diagram was also raised on an experimental stand in p-V coordinates, and the real indicated diagram in p-α coordinates. The equations for the thermal processes that take place in the engine have been determined and, after that, these were introduced in a computer. Using these equations, the state parameters in the characteristic points of the engine cycle were calculated.