Disclosed is a method of controlling the transition between an economy mode and a performance mode of an internal combustion engine. The method comprises 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-air ratio limit. The method allows for more sable transition between engine operating modes. An engine system using the method is also disclosed.
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