3.1-3 Olympus engine efficiency still very good

NVhen comparing engines we have tocompurc installed engine efficiencies (tip.1- although mostly specific fuel consumption (SFC) is used.

SFC divides the fuel flow (i. c an energy flow) by the thrust force; so physically it is not a meaningful value and can only be compared at the same speed (v * flight velocity)). In contrast Пр is the amount of energy flow provided by engine thrust divided by the energy flow provided by the fuel (H = calonfic value of fuel). Пр and SFC are connected via the equation

H a V/8FC (31)

When not using a consistent system of units like SI. the respective unit conversions have to be applied For Kerosene H is given by

H = 42.817 MJ/kg (32)

Now we can compare engines. At Mach 2 0-cruise. Olympus (401 and a proposed new supersonic engine arc given, as well as a modem high bypass transonic engine used for wide – body aircraft [41):

Olympus. M =* 2.0

SFC = 1.19 kg/daN/h

1E * 0.41

(33)

New engine, M « 2.0

SFC = 1.13 kg/daN/h

rij. ■ 0.43

(34)

CF6-80-C2.M » 0 85

SFC = 0 56 kg/daN/h

rip = 0 37

(35)

We see that Olympus is still very good. The improvements of subsonic engines, mainly achieved by strung!) increased bypass ratio, did not yet reach Olympus’ efficiency. Indeed, at supersonic cruise a modern optimized engine w ith very low bypass ratio would provide slightly better values than the one given above But this engine like Olympus will never meet the strin­gent noise criteria at take-off and landing which new SCT have to fulfill Probably a bypass ratio of about 2 and extensive noise suppression (damping plus cjectori will be applied to meet noise criteria; this will decrease engine efficiency at supersonic cruise to the values indicated above.

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