A | Mass air to fuel ratio | R | Moisture content of fuel: mass water/mass fuel |

C | Specific heat air = 1.01, kJ/K (at constant pressure) | Rθ | Total thermal resistance of AHX from working gas to ambient, K/W |

C | Specific heat combustion gasses. (at constant pressure) | Rθ | Effective thermal resistance of the combustion process |

C | Specific heat of fuel, kJ/K (at constant pressure) | Rθ | Total thermal resistance of HHX from combustion gas to Stirling working gas, K/watt |

C | Specific heat steam = 1.93, kJ/K | T | Ambient temperature, Kelvin |

| Higher calorific value of fuel, kJ/kg | T | Temperature of the cold heat exchanger (AHX), Kelvin |

L | Latent heat of evaporation of water = 2264 kJ/kg | T | Entry temperature to the heat engine, Kelvin |

| Mass flow rate of air, kg/s | T | Ideal flame temperature, assuming no dissociation of component gasses |

| Mass flow rate of fuel, kg/s | T | Temperature of the hot heat exchanger (HHX) in Kelvin |

| Mass flow rate of combustion gasses, kg/s | W | Work output from the engine, W |

| Mass flow rate of water, kg/s | X | Mass ratio of air to stoichiometric air flow |

| Heat available from combustion, watts | η | Efficiency: work out/heat in |

Q | Amount of heat supplied to the Stirling engine, W | η | The new efficiency of the Stirling engine: work out/heat from combustion |

r | Otto cycle compression ratio | γ | Ratio of specific heats at constant volume to constant pressure |