Abstract: In a combustion system, the economic optimum efficiency is achieved by periodically computing a relative index of combustion efficiency using the combustion chamber as a real-time, on-line calorimeter. This is accomplished by first determining the combustion chamber inputs and outputs required to calculate a relative index of performance (including measuring the amount of heat released or work performed at the present air/fuel ratio including the stack losses, without oxygen and/or carbon monoxide sensors), sequentially comparing the latest relative index value with the previous value, then adjusting the air/fuel ratio to achieve an improved index value.