Energy in form of heat is obtained when fuel is burnt in air. The release of this heat energy can be slow or can be very rapid.
When fuel oil is sprayed as a fine mist in boiler burners, it is able to burn at a relatively slow rate. When fuel is sprayed into cylinders of diesel engines, fuel burns in such a rapid rate that explosions occur. Fortunately, these explosions are protected from persons as these engines are called internal combustion engines.
Whatever type of combustion, it is a chemical reaction between carbon, hydrogen, sulphur and oxygen.
C + O2 = CO2 2CO + O2 = 2CO 2H2 + O2 = 2H2O S + O2 = SO2 2S + 3O2 = 2SO3
Air consists of 77% Nitrogen and 23% Oxygen by mass. For a particular design of combustion air, theoritical oxygen multiplied by 100/23 will give theoritical air required.
How do you measure a good combustion. The percentage of Oxygen or Carbon Dioxide will tell us whether combustion is good or not good.
The lower Oxygen content in exhaust gas, better combustion. It means that Oxygen has been fully utilized for burning. It also means that fuel air ratio is set properly. Too much excess air is no good because heat generated will be lost through exhaust trunking.