The term heating value is used to define the amount of heat created during the combustion of a fuel. The term higher heating value is determined by bringing all the products of combustion back to the original pre-combustion temperature. The term lower heating value is determined by subtracting the heat of vaporization of the water in the by-product from the higher heating value results. The lower heating value is what is typically used for vehicle engine analysis.

The calorific value is the measurement of the amount of heat or energy produced, and is measured either as gross calorific value or net calorific value. The difference being the latent heat of condensation of the water vapor produced during the combustion process. Gross calorific value assumes all water produced during the combustion process is fully condensed. Net calorific value assumes the water leaves with the combustion products without being fully condensed.

Since most gas burning appliances cannot utilize the heat content of the water vapor, gross calorific value is of little interest. Fuel should be compared based on the net calorific value. This is especially true for natural gas, since increased hydrogen content results in high water formation during combustion.

The Wobbe Index, a critical factor when analyzing Propane-Air plant requirements, is a function of gas quality and allows matching one gas (in this case, natural gas) to a replacement gas (in this case, Propane-Air). If the two different gases have an identical Wobbe Index, they will produce an equal amount of heat and combustion products and will require the same amount of combustion air. Burners, adjusted for a specific calorific value and fitted with a replacement orifice to match a lower Wobbe Index, result in minor combustion changes. Substituting a gas for one with a higher Wobbe Index, generally allows a narrow acceptance range. Flame characteristics determine the acceptance range for the replacement gas.

The Wobbe index is defined as:

            Wobbe Index = Gross Heating Value / (Square root of Specific Gravity)

and is used to compare the energy input into an appliance with different composition fuel gases. If two fuels have identical Wobbe indices, then for a given burner pressure, the energy input will also be identical.

See also Energy Heat