Measuring Combustion Performance
Combustion efficiency quantifies how completely chemical fuel energy converts to useful heat. It is distinct from thermal efficiency (which includes work extraction) and overall system efficiency. The primary losses are sensible heat in hot exhaust gases, latent heat in water vapor, and chemical energy in unburnt fuel (CO, hydrocarbons, soot). Modern combustion analyzers measure O₂, CO, and flue gas temperature to compute efficiency in real time.
The Equivalence Ratio Curve
Plotting efficiency against equivalence ratio reveals a characteristic shape: efficiency rises as φ approaches 1 from the lean side (less excess air dilution), peaks just lean of stoichiometric (φ ≈ 0.95), then drops sharply on the rich side due to incomplete combustion. This single curve guides the design of fuel-air control systems in boilers, furnaces, and engines.
Exhaust Heat: The Largest Loss
In most combustion systems, the dominant efficiency loss is sensible heat in exhaust gases — hot products carrying thermal energy out of the system. Reducing exhaust temperature from 500°C to 150°C can recover 10–15 percentage points of efficiency. Condensing boilers go further by also recovering latent heat from water vapor in the flue gas, achieving efficiencies above 95%.
Balancing Efficiency and Emissions
Maximum combustion efficiency and minimum emissions often conflict. Peak flame temperatures at stoichiometric produce the most NOx. Lean combustion reduces NOx but increases CO at very lean ratios. Rich combustion produces soot and CO. Modern systems use staged combustion, flue gas recirculation, and catalytic aftertreatment to navigate this complex trade-off space.