Same engine model-more thrust .Why?

[BMM389EC]

What differences in the same engine model ensure that it produces more thrust? For example, the JT8D-15 as apposed to the -17? More cans? More compressor stages?
While I'm on the subject of thrust, why do these engines use EPR to set the thrust and not N1?

[Al Weaver]

?more cans??

Nope, but they did manage to squirt more fuel into the existing cans. This has the effect of raising the turbine inlet temp and getting it to squeeze in a higher amount of air by also increasing normal operating RPM a tad. More air = more thrust, more temp can only be tolerated by higher temp materials in the turbine of sophisticated cooling air.

Re: EPR vs RPM vs Thrust. Well EPR just refers to the pressure differential that make up thrust. RPM typically directly relates to these pressures, but not always. I seen either EPR or RPM give off false values for thrust when the engine is operated in a damaged mode, however EPR is typically more reliable.

[Bally Heck]

It is sometimes a materials issue.

If you can make the hot bits hotter, then on otherwise identical engines, the hottest will produce more power, more efficiently. If you make the hot section blades with a more heat durable material, or design them so as to be able to withstand greater heat by the use of cooling gallerys. You will have a more powerful and efficient engine.

[Intruder]

...then if you miscalculate the turbine blade creep on the hotter engine, you get an engine that tends to eat itself, like the upgraded -408 engine in the US AV-8B Harrier.

[Eff Oh]

We fly the B757-200 and the B757-300. The -200 has the RB211-535E4. The -300 has the RB211-535E4B. The latter has an extra 7% thrust. This is accomplished by raising the Exhaust Gas Temperature (EGT) limit of the engine, hence the extra thrust!
Suppose Rolls Royce know what they are doin!? :D

Eff Oh :D

[Denti]

We're using the CFM56-3 on our 733 with the 22K-Rating. I understand there are some 733s with the same CFM56-3 engine out there which can only deliver up to 20K lbs of thrust. According to our maintenance staff it's only an extra sum you have to pay to Boeing (or maybe CFM) to get the extra 2K of thrust.

[Intruder]

Eff Oh:

I purposely left out the name of the engine mfgr, lest I be accused of "bashing" someone. The fact is, the mfgr miscalculated in this case.

I fly the 747-400 with the CF6-80-C2-B5F engines. They used to be -B1F engines until a new electronic controller plug gizmo was installed in each of them, raising the thrust substantially. All engine mfgrs do it, generally after analysis of the lower-rated engines in operations over a reasonable period. if operational experience validates their engineering analysis, they can extrapolate and uprate the engines.

Now I wish I had saved all the -B1F stuff from the FHB, so I could compare max EGT, N1, and thrust...

[GotTheTshirt]

There is also a "life" issue.
The hotter the engine the more power (until it melts !)
Also the hotter the engine the shorter the life on turbine components.
Many engines can be uprated and de-rated without any physical change to the engine.
Some engines use the same turbine part numbers in the higher and lower power engines but the higher temp (power) parts have a shorter life.
Aircraft operate in different enviroments, (hotter/higher) so rather than penalise the guy who doesn't need the higher perfomance the same engine has different ratings

[fernando]

GotTheTshir and got the answer.

If there is a change in the design of a component of the engine, either in material or blade cooling, the engine will not be the same engine and needs a new certification.
All they do is allow the engine to be operated at a higher temperature. Some one said that you pay for that, also overhaul might be sorter.


tell me if I am wrong

[GotTheTshirt]

Fernando,
Yes the certification is also a consideration.
But all turbine engines have life limited parts (LLP's) usually discs and blades but some engines have other items.
The LLP has a finite life, no overhaul,no rework just throw it away.
Engines with variable ratings have different lives for the same part number part dependant on the rating.
The higher the rating the lower the life.
As many of these part are in the middle of the engine, particularly turbine parts so that replacing a LLP can be quite expensive in work scope, apart from the cost of the part itself

[Al Weaver]

Fernando

You point is taken

The idea concerning certification (limitation section of the manual) and uprated same engine model is to control interchangeability of parts between models. Obviously if there are the same model with two different overhaul lives there would be a big problem in sorting this out.

For various reasons (to avoid extra certification testing) the engine manufacturers try to avoid changing the model number for even the smallest changes in the power curve. But the arguments are examined thoroughly by the CAA/FAA when it comes to commonality.

[delarocha]

Dear forum readers

The basic thrust setting parameters for modern high bypass engines are Engine Pressure Ratio EPR used by Pratt & Whitney and Rolls Royce, and Fan speed N1, used by GE and CFM. The engine manufacturers have documented the reasons for their choice but in fact neither of the 2 parameters gives a proper indication of engine thrust. Direct thrust management is a continuing research topic.
Early turbojet engines used rpm as the operating parameter to establish thrust, while more modern turbofan low bypass engines like the JT8 use EPR. EPR is the ratio of the total pressure at the front of the compressor to the total pressure at the rear of the turbine, which may be thought as thr product of pressure and cross sectional area. On a hot day, compressor rpm for given thrust will be higher than on a cool day. Furthermore, a dirty or damaged compressor will reduce thrust for a given rpm. Using EPR as the thrust indicator means that on a hot day it is quite possible for the engine rpm to exceed 100%.
On newer high-bypass ratio engines, such as GE CF6, the fan speed N1 is used as the primary method of setting power because most of the total thrust is generated by the fan. Engine control is acomplished by keeping N1 constant for a selected throttle lever position, regardless of the ambient conditions, and (with Autothrust disconneced)let the pilot adapt to the non-linear relationship between rpm and thrust. The pilot is selecting not thrust but a proportion of the maximum thrust that can be developed by the engine at a particular altitude and Mach No.
The advantages of using N1 parameter for control are:
-rotational speed can be measured easily with good accuracy
-measurement is independent of aircraft altitude
The problem with monitoring thrust through rpm is that thrust changes greatly for the last few percent of engine speed before 100% rpm is reached. The system must be very sensitive to small changes in rotational speed.
Although EPR is also a reliable indicator of thrust the main disadvantage is that pressures cannot be measured as exactly as rotational speed. In a high bypass-ratio engine the EPR method would be more complex, because the thrust of both the fan and the core engine streams would have to be calculated separately. On the Rolls-Royce RB211 engine the parameter used to indicate and manage thrust is the (IEPR) integrated engine pressure ratio. This parameter is the integrated average of the fan and core exhaust pressures divided by the inlet total pressure. RR feels that because IEPR is based on both fan and core presssure ratios, provides the most accurate indication of engine thrust compared to other parameters.
On modern FADEC engines thrust control is enhanced. On the PW4000 series engine power is controled by both EPR and N1 rpm. The EPR is the rated or normal mode, while N1 is the alternate or fault mode.

Regards

[Eff Oh]

INTRUDER:- I ment nothing sinister in my post. What I implyed is not that RR got it wrong, but that they know a lot more about it than I do! Please forgive me if you thought I was being underhand. :)

It was an intentional raise in EGT on the RB211-535E4B. It is true that our engines get absolutely thrashed, and I doubt if they will last very long. However I believe that this is also the case on the B767-400.

Eff Oh
;)

[GotTheTshirt]

delarosa,
what you say is correct and this is a means of establishing the fact that the engine is producing enough power to haul the aircraft into the sky.
What is also important is the condition of the engine to produce that power and that is where the EGT (TGT) comes in.
The engine can produce the power be it EPR or N1 but at an EGT price.
The lower the EGT the less the engine is working. When engines leave the test cell they have an EGT margin. That is the EGT it takes to produce the required power and the max EGT. As the engine wears the EGT gets higher for the same power. Once you get to max EGT you have used the margin ( and the engine is removed). Especially in low cold places the engine will still make power at max egt. That's why the life on the LLP's ( see above) is rated on EGT.