Ideally when thermal energy= Potential energy of the fuel. Inefficiency in combustion is due to poor mixing which may lead to formation of CO. Poor combustion efficiency leads to

• Wastage of fuel
• Environmental pollution
• Expensive to run

For any given type of fuel, some amount of CO is always observed in POC even at stoichiometric amount of air in all practical combustion systems. Presence of CO in POC denotes incomplete combustion and combustion efficiency will be low. So, excess air has to be used to increase combustion efficiency. Increase in excess air drastically decreases CO in POC but at the same time increases amount of POC due to increase in nitrogen and oxygen (at stoichiometric air no oxygen is present in POC). The additional nitrogen and oxygen in POC due to excess air will carry heat of combustion with them. Thus, control of excess air is important. The following plot illustrates the relationship between excess  and heat losses.