Higer rated vs lower rated engines fuel consumption

[Marlboro_2002]

Hi,

I have been reading on the subject but I still have some doubts over this subject.

I think it is of general knowledge that higherrated engines have ahigherfuel consumption. This has been recently been covered on the latest Airbus and Boeing Symposiums.

However, for those aircraff that can enable it, using de-rate climb also is said to increase fuel consumption.

Although I fully understand the later (more time in climb, more time consuming fuel, hence total fuel consumption during climb will be greater than using full climb thrust), I am having trouble figurring out why is it that a higher rated engine will have a final fuel value higherr than that of a lower rated engine when flying exaclty the same profile.

Can anyone explain this to me?


Thanks,

[Intruder]

You have to understand the concept of "specific fuel consumption" (SFC): kg/hour/kgthrust

The specific fuel consumption of a jet engine will be effectively constant throughout its usable thrust range. Modern engines are built to closer tolerances and higher temperatures to increase efficiency and reduce SFC. Since more thrust is required to overcome drag at lower altitude, reduced climb rate increases time at lower altitude, increasing overall burn.

So, if max climb thrust is used, the fuel used for that thrust will be converted into higher climb rate, getting out of the denser air more quickly. Conversely, reduced thrust will use more fuel overall.

While reduced T/O thrust significantly reduces wear on the engine, I don't believe reduced climb thrust has anywhere near the same benefit. Turbine blade creep is VERY temperature dependent, so any reduction below max T/O thrust (and EGT) is immediately beneficial, but further thrust reduction may only have theoretical benefit.

[Marlboro_2002]

Intruder,


Thank you very much for your input.

I absolutely agree with you on all points.

In fact I can, regarding engine degradation, say that for a CFM56, that the first 4° immediately below TOGA account for 99% of the savings that will be possible with a reduced thrust take-off. Those first degrees are what really counts.

However regarding fuel consumption, higher rated engines have higher fuel consumption when compared with lower rated engines (same flight, same conditions). This is something that does not make sense to me, specially knowing that a higher rated engine will reach cruise altitude sooner (lower fuel consumption) and allow a cruise climb sooner (if limitation is thrust and not the wing) to an optimum altitude.
This said, a higher rated engine should consume less fuel.

[mustafagander]

Intruder,

The engine manufacturers agree that the worst case TIT occurs during climb around the high 20,000's. The early B747 with P&W JT9's regularly demonstrated this. Hence derated climb thrust can and does reduce turbine distress and promote hot section longevity.

[Wizofoz]

Malboro- that fact that you can step-climb earlier does not mean it's the fuel efficient thing to do.

Optimum altitude is air-frame dependent. You may have a higher MAXIMUM altitude with more thrust, but the OPTIMUM will be the same on the same airframe at the same weight regardless of how much extra thrust is available.

Thus to achieve optimum cruise you need a certain amount of thrust regardless of the maximum available- and the larger engine uses more fuel to produce that thrust than does the smaller one.

[Intruder]

However regarding fuel consumption, higher rated engines have higher fuel consumption when compared with lower rated engines (same flight, same conditions).
That is not necessarily true. AFAIK, the CF6-80-C2-B1F (56,500 lb thrust) engines in the 747-400 have essentially the same fuel flow as the -B5F engines (62,100 lb thrust) in cruise. They are EXACTLY the same engines physically, but with different engine control firmware. I never noticed a significant difference between the 2 engines. However the holding tables (the only ones I can easily find that have exactly comparative figures) show the higher-rated B5F burns about 30-40 Kg/hr per engine LESS under identical conditions.

Also, the GEnx engines in the 747-8 (66,500 lb thrust) have significantly less fuel flow in cruise than the CF6-80 in the 744, which have less fuel flow than the CF6-50 (51-54,000 lb thrust) in the 747 Classic. Some may be due to less drag, but some is due to increased efficiency (decreased SFC). GE rated the -50 SFC at .368-.385 and the -80 as .307-.344. I can't find specific numbers for the GEnx, but GE claims are 13-15% less than the CF6-80.

[rudderrudderrat]

Hi Marlboro_2002,
However, for those aircraft that can enable it, using de-rate climb also is said to increase fuel consumption.

Is extra bleed air used to cool the turbine blades so they can operate in the hotter gas stream at higher rated power?
Is this extra bleed cooling air (which is not necessary at de-rate climb settings) part of the reason for the reduction in sfc?

[Intruder]

The engine manufacturers agree that the worst case TIT occurs during climb around the high 20,000's. The early B747 with P&W JT9's regularly demonstrated this. Hence derated climb thrust can and does reduce turbine distress and promote hot section longevity.
That may be true, but is not supported by the automatic operation of the CF6. Any Climb thrust derate is gradually reduced to 20,000'. Above 20K full Climb thrust is used.

[hawk37]

The specific fuel consumption of a jet engine will be effectively constant throughout its usable thrust range.

that's a rather bold statement Intruder. May I be as bold as to ask for a graph that illustrates that. Or perhaps a reference to a engine manufacturer that says the same.

IIRC, the lowest SFC (or often said as Thrust Specific Fuel consumption) occurs at SL, near max thrust. It is higher at mid N1 settings at sea level, and even increases as altitude increases. Yep, the best (lowest) SFC is NOT at high altitude cruise, as many believe.

But of course, best SFC is not what determines the range of the aircraft (ie, lbs of fuel per nm traveled, that's a totally different question).

Hawk

[mustafagander]

As I understand it, derates wash out in the mid 30,000 ft area. At least that is what the FCOM says in my outfit.

Nothing alters the stress on the turbine during late climb. Way above that during take off.

[EMIT]

Hello Marlboro,

When you speak about "higher rated engines using more fuel than lower rated engines" do you mean, figuratively speaking, a difference between an 850 cc three cilinder and a 6.5 litre V-12, or do you mean the 6.5 litre V-12 in basic form with 150 HP and the pepped up race version of 800 HP?

In aviation terms, do you mean one engine, in normal rated and derated version, or one big engine versus one small engine?

Putting a very big engine on a not so big airframe, could cause unnecessary drag, hence, higher than necessary fuel use.

Having identical engines on identical airframes, only one set derated and the other set not, there should be no difference in cruise burn.