to 24 cause the variable count to return to 45 000 over the subsequent 90 second period provided the variable count is not decremented to zero repeated power boost operation is immediately available although not illustrated in fig3 to 6 the steps 17 19 and 13 permit the amount of power boost to be varied during power boost operation varying the percentage of power boost requested causes the rate at which the variable count is decremented to be varied by the step 18 it is also possible to modify the operation of the controller 3 to take into account other factors such as ambient temperature which affect the permissible maximum period of power boost operation although the rate at which the variable count returns to its maximum value after power boost operation is fixed in the illustrated embodiment this may also be varied for instance by varying the amount of the increment in the steps 22 and 27 for instance this rate of return may be varied in accordance with the ambient temperature the time spent in power boost operation the rate of decrementing in the step 18 or by any combination of such factors it is thus possible to provide a controller for power boost operation which automatically prevents overheating of the engine by limiting power boost operation when necessary the variable count may be considered to.