The Willans line represents the relationship between fuel energy input and engine output. Extrapolation to a zero line of fuel input provides a useful approximation of the mechanical losses. Linearity of these curves is assumed in the range from low load downward to a hypothetical zero value of the indicated work. A critical analysis revealed that these lines must have a continued curvature and also that the location of the zero point of indicated power does not coincide with zero fuel input. The paper suggests corrections and defines a directional field which may improve the accuracy of the loss analysis.