Q

Q Carnot +

1,188 1,038 3,648

2,763 2,414 8,485

985.1 860.4 14,360 2,291 2,001 33,402

992.2 866.6 8,927 2,308 2.015 20,765 Btu/lb Btu/lb Btu/lb kJ/kg kJ/kg kJ/kg ratio'', fs, see Table 4.1. When the heat of combustion and the fuel/air ratio are multiplied together the result is the Brayton cycle heat addition, that is the energy added per unit mass of air. For the Brayton cycle heat addition there are essentially two families of values of heat addition using conventional fuels: hydrogen and acetylene, at 3,498 kJ/kg, and hydrocarbons at 2,954 ± 92 kJ/kg. There are indeed some exotic fuels at higher values, but these are very unstable or spontaneously ignite on contact with air. Since the total energy of the air (energy per unit mass of air) plus the square of the speed is a constant, there comes a speed when the energy of the air equals the energy added to the air by burning fuel. So, the faster the aircraft flies, the smaller the fraction fuel heat addition becomes of the kinetic energy: the ratio of the total enthalpy to the fuel heat addition ratio increases, as shown in equation set (4.3) for the fuel combustion energy (without any losses):

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