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Show '-' .0.. . u ~ u.. c ..g... ~ .-...-~.. ~.... ~ Q.) ::c "0 c ~ ~ u c Q.) 'u I.C c...., ~ Fig. 7 0.5 • 0.4 • a. -. .... • ... 0.3 e · 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1 W, MMBtulhr Efficiency and Heat Utilization factor curves for GM 3.4L V6 engine. With Firin~ Constants of: H/ = 230,000 BtuIhr, Hsm = 1.34x 106 Btulhr,ar = 0.432 . Fig. 8 compares computed exhaust gas enthalpies with the assumption of a linear variation of exhaust gas enthalpy with output. The exhaust gas enthalpy was computed by subtracting the output W and the estimated wall loss H w from the firing rate H j The comparison is Fig. 8 is poor, demonstrating that one or more of the assumptions about the variation of losses with output is incorrect. As is evident from Fig. 7, below an output of about 0.2 MMBtu/hr (90 hp) the influence of losses is decreasing faster than the analysis allows. This observation is consistent with the variation of friction horse power (fup) with engine speed. Over the range of engine speeds, frictional power losses typically vary by the square of engine speed [12]. Application of the principal equations (1) - (4) was carried out in two stages. In the first stage, the direct test of the analysis and its assumptions showed the departures from agreament demonstrated in Figs. 7 and 8. These departures were postulated as resulting from a breakdown in the assumption of linearities in the energy losses. In the second stage, the analysis was developed further to account for nonlinearities in the loss terms. From the observation that frictional losses in an engine vary with the square of engine speed, the assumption of a linear variation of wall losses with output was modified. To properly account for friction power losses, Eq. (2) must allow for a quadratic variation of 13 |
