In this method a graph between the rate of fuel consumption (Kg.hr) taken on Y-axis and the break power (kW) on X-axis is drawn when the engine is run at constant speed as shown in Fig. The graph is extrapolated back to zero fuel consumption which cuts on the negative X-axis at point A.

The Negative intercept on X-axis represents the frictional power at that speed on the engine. The reasoning is as follows:

When B.P. = 0, the fuel consumption at this power represents the fuel consumed to overcome the friction losses.

Since the variation in fuel consumption rate and B.P. is linear, it implies that the extrapolation of the almost straight line part of the curve as negative intercept on X-axis at zero fuel consumption will represent the friction power.

Fig. Willan’s Line Method