This disclosure relates to a method of controlling an engine and more particularly for controlling the transition between operating modes of an internal combustion engine such as between an economy mode and a performance mode. The method comprising the steps of determining a current fuel-air ratio at which the engine is operating and comparing it with a predetermined fuel-air ratio limit. The time duration for which the current fuel air-ratio is below the fuel-air ratio limit is determined and compared with a predetermined waiting time threshold value. A count is triggered, which is based on a difference between the current fuel-air ratio and the fuel-air ratio limit when the engine is operating at a current fuel-air ratio which is below the fuel-air ratio limit and this is compared with an intensity threshold value. The operating mode is shifted from the performance mode to the economy mode when both the time duration and the count exceed the waiting time threshold value and the intensity threshold value respectively. The operating mode is automatically shifted back to the performance mode when the current fuel-air ratio reaches or exceeds the fuel ratio limit. The method alternatively comprises using the air-fuel ratio, instead of the fuel-air ratio, and the switch from performance mode to economy mode only occurs when the current air-fuel is above a predetermined air-fuel limit and it remains above the predetermined air-fuel limit until two other predetermined thresholds have been reached.