FUEL CONSUMPTION

FUEL CONSUMPTION

Fig. 16-9 Specific fuel consumption

comparison.

Fig. 16-10 Afterburning and its effect on the rate of climb.

23. Afterburning  always  incurs  an  increase  in specific fuel consumption and is, therefore, generally limited to periods of short duration.  Additional fuel must be added to the gas stream to obtain the required temperature ratio (para. 19). Since the temperature rise does not occur at the peak of compression, the fuel is not burnt as efficiently as in the  engine  combustion  chamber  and  a  higher specific fuel consumption must result. For example, assuming a specific fuel consumption without after- burning of 1,15 lb./hr./lb. thrust at sea level and a speed of Mach 0,9 as shown in fig. 16-9. then with 70 per cent afterburning under the same conditions of  flight,  the  consumption  will  be  increased  to
approximately 2.53 lb./hr./lb. thrust. With an increase in height to 35,000 feet this latter figure of 2.53 lb./hr./lb. thrust will fall slightly to about 2.34 lb./hr./lb. thrust due to the reduced intake temperature. When this additional fuel consumption is combined with the improved rate of take-off and climb (fig. 16-10), it is found that the amount of fuel required to reduce the time taken to reach operation height is not excessive.