OCR Text |
Show Hg = F.hg = (F.hr) .(hg / hr) = Hr.(hg / hr) [4] where: he is the specific enthalpy (heat of combustion) of the fuel. Equation 3 then reduces to: He . (1 - hg / he) = Hw + H. [5] At Idle, when HI = 0, and with the Idle condition defined by the zero superscript, these expressions (Eqs. 4 and 5) become,: where the parameter, fl.o0 , is the limiting Intrinsic Efficiency defined as: [6] [7] [8] Equation 7 must always be satisfied: it links the Idle Firing Rate (Ht), the Idle wall loss (HwO), and the limiting Intrinsic Efficiency defined by Eq.8, 0.00, in a critical relation. The Firing Equation, Eq. 1, is then obtained from Eq. 5 by use in substitution of two further expressions for, respectively, the specific exhaust enthalpy and the wall loss as functions of output. In past studies [3-5], the specific exhaust enthalpy (ratio) has been written as a linear function of output (HI) as a first approximation, but is now known to be more accurately described by: = [9] where: f is a parameter correction factor whose value is of the order of unity. For f = 1, the second term in the numerator vanishes. At low values of output, Hs, this expression expands to the linear form used in past studies [3-5]. For the wall loss, a quadratic dependence on output is empirically assumed, thus: [10] 5 |