Governing Equations
Thermal efficiency: $\eta_{th} = W_{net} / Q_{in}$, Net output: $W_{net} = \eta_{th} \cdot \dot{m}_f \cdot LHV$
Specific fuel consumption: $SFC = \dot{m}_f / W_{net} = 3600 / (\eta_{th} \cdot LHV)$ [g/kWh]
Carnot limit: $\eta_{Carnot} = 1 - T_C / T_H$ (T_H: combustion temp ≈ 1200–1600 K, T_C: exhaust temp ≈ 400–700 K)
$$CO_2\,[\text{g/kWh}] = \frac{CO_2\,[\text{g/MJ}] \times 3600}{(\eta_{th} \times 1000)}$$Student 🧑🎓: Why do engineers use LHV instead of HHV for engine efficiency?
Professor 🎓: Because in most engines and turbines the exhaust gas leaves at temperatures well above the water dew point (~60 °C), so the water stays as vapour and the condensation latent heat is never recovered. Reporting efficiency on the LHV basis avoids claiming energy that the system can't actually use. Condensing boilers are the exception — they deliberately cool the flue gas below the dew point to reclaim that latent heat, so they can exceed 100% efficiency on an LHV basis.