Optimal Engine Operation During A Flight?

[AeroTech]

Hi,
Modern turbine engines are designed to operate optimally at a single flight condition, and are compromised at other mission points. Currently pilots must use the throttle to match thrust when conditions are not optimal. This results in diminished fuel efficiency and performance
a-If this statement is true regarding commercial aircraft, at what phase of flight the engines will operate optimally?
b-If this quote is true, does it apply to all aircraft gas turbine engines (turbofan, turbopropeller,…)? What about aircraft piston engine?
Your feedbacks are appreciated. Thank you.

[barit1]

To some extent this statement is true of all engines. However in the case of later engines the compromises are fairly minor.

Consider the following for a typical jet transport mission:

Highest thrust required is at top of climb. Takeoff is usually reduced thrust if there is excess runway available.

Lowest SFC should be at cruise; as fuel is burned off, the thrust required is less, and the new optimum altitude is higher; thus thrust is reset to match the new cruise condition after step climb.

In descent, the engines are wasting fuel, so the lowest possible thrust and fuel use is desired.

ATC restrictions often mean you cannot fly an optimum climb/cruise/descend profile. BUT, All things considered, the modern turbofan covers these mission points pretty well.

[taperlok]

Read this somewhere once. A turbo fan optimum N1 +/- 93%.
If you aren't cruising at that setting you are not at optimum altitude for that specific configuration. ie not at your most economical.
well said Barit1:ok:

[twistedenginestarter]

You could say the same about a car engine. In practice, engines are optimised for a series of modes. A jet needs high power for at-the-limit take-off which determines things like max payload, range, runway length. Here the forward speed is low and the ambient temperature high. It also needs to deliver (less) thrust at altitude where speed is high and temperatures very low. For any given set of conditions there is an optimum something or other (eg SFC) but I guess the design is a compromise mapped onto typical flight profiles. As regards general rules - higher compression gives better efficiency, as does higher engine temperature. The lower the ambient temperature, the better for a jet.

[barit1]

Don't take the 93% N1 as optimum for all engines. Some engines will cruise at well over 100% N1 indicated. Go by the book.

[Checkboard]

The efficiency of a heat engine is largely governed by the compression ratio. A heat engine is one that converts heat in a gas (added by a fuel) to do work. All combustion engines, like the Otto cycle (four stroke) engine, Diesel engine and Brayton Cycle (gas turbine) engine are heat engines.

How does the amount the air/fuel mix is compressed effect the efficiency? Imagine a piston travelling up a cylinder and compressing the gas in that cylinder (like pushing in the handle of a bike pump with your thumb over the exit hole). If released, the piston would now travel back down the cylinder until the pressure inside equalled the atmospheric pressure. The work done in compressing the gas is recovered as the piston moves back down the cylinder under the pressure of the compressed gas (less friction losses etc.)

If the piston compresses the gas to half its original volume, then it has a compression ratio of 2:1, as the piston recovers it can do work based on the compression it started with. The more the air is compressed, the more work can be done as it recovers. Otto cycle piston engines have a compression ratio of around 8:1.

Now lets introduce some combustion after the piston has compressed the air, as happens in an Otto cycle engine (a "suck, squeeze, bang, blow four stroke). If the charge was only compressed at a 2:1 ratio, then the effect of the combustion can only do work until the piston recovers to ambient pressure. The more the piston compresses the charge, the further away from ambient pressure it is and thus more work can be done by the piston as it recovers. So the higher the fuel/air mix is compressed, the more efficiently that fuel can expend its energy.

Why not compress the charge more in a four stroke, and get even better efficiency? Because as the charge is compressed it heats up, until at higher compressions it heats enough to explode at the wrong time ("known as "pinging", "knocking" or detonation) which may create enough pressure to exceed the strength of the engine components. Typically you put holes in pistons or cylinder heads - not considered a good thing.

Diesel engines actually take advantage of this effect and use "compression ignition" instead of a spark plug, so they can achieve double the compression of Otto engines and are consequentially more efficient. An aviation diesel engine, burning kerosene (Jet fuel) is being developed and should be available on the Socata Trinidad in a year or so. It is lighter, has less moving parts and burns less fuel with more power than your current Lycoming or Continental.

All this can be proved mathematically from first principles through the laws of thermodynamics, a branch of Physics usually studied at second year degree level in Mechanical Engineering. I won't bore you with the proof, but if you are really interested you can click here.

The same principle applies in gas turbine engines, although technically you don't talk about compression ratios, but rather cycle pressure ratios. Known as the thermal efficiency, or internal efficiency of the engine the higher the pressure ratios, the higher the efficiency of combustion. As the compressor is not a cylinder travelling a fixed distance, but a "fan blowing air into a small space" the pressure ratio is governed by the engine RPM.
So gas turbine engines like to run at high RPM.

Next you need to look at the propulsive efficiency, or external efficiency of the engine (the study in Physics known as mechanics). The amount of thrust provided by an engine (propeller or jet) is related to the amount of air they throw out the back, and the speed at which they throw that air. In symbols, if m kilograms is the mass of air affected per second, and if it is given an extra velocity of v meters per second by the propulsion device, then the momentum given to the air per second is mv, so Thrust = mv (per second).

A propeller engine uses a large m and a small v, a gas turbine engine uses a small m and a large v. 10 kg of air given a velocity of 1 m/s has the same thrust as 1 kg/s of air given a velocity of 10 m/s. Which is most efficient? Well, the rate at which kinetic energy is given to the air (the work done) is ½mv² watts. So the first case requires 5 watts of work and the latter requires 50 watts of work. Clearly the piston is more efficient. Problems occur as the speed increases, and the propeller efficiency breaks down.

An easier way to think of it is by considering the waste energy in the flow. Stand behind a propeller engine at takeoff, and it will knock your hat off. Stand behind a jet giving the same thrust and it will knock you off your feet! Waste energy dissipated in the jet wake, which represents a loss, can be expressed as [W(vj-V)²]÷2g (W is the mass flow, vj is the jet velocity, V is the aircraft velocity, so (vj-V) is the waste velocity). As the jet exhaust leaves the gas turbine at roughly the same speed whether it is standing still or moving, the faster the engine is moving the less waste energy lost. Assuming an aircraft speed of 375 mph and a jet velocity of 1,230 mph the efficiency of a turbo-jet is approx. 47%. At 600 mph the efficiency is approx. 66%. Propeller efficiency at these speeds is approx. 82% and 55% respectively.
So gas turbine engines like to fly at high TAS.

The problem is that at high RPM at sea level turbo-jets are sucking in very thick air, so when they add a heap of fuel and burn it they make tremendous amounts of thrust - good for take-off but way too much for level flight at low level (with the associated high IAS).
Aircraft generate too much drag at high Indicated Air Speed and keeping the engine turning at an efficiently high RPM at sea level would quickly exceed the aircraft speed limitations.

As you increase altitude, however, the amount of thrust the engine can produce reduces (as it is sucking in "thinner" air) even though it is still operating at high compression ratios (for good thermal efficiency). Also you can have a high TAS (for good propulsive efficiency) at a low IAS (for lower drag on the airframe). There are a few other advantages, like the cold temperature, which keeps the turbine temperatures down as well.

So jet engines like to fly at high power and at high TAS, while aeroplanes like a moderate IAS, and the regime were all this can be achieved together is at high altitude.

(I wrote this a while ago in Why are jet engines more fuel efficient at high altitude? (http://www.pprune.org/forums/showthread.php?t=10655&highlight=efficiency)

[GlueBall]

Rollers use EPR as primary thrust settings, N1 secondary

[airbus757]

All engines are designed to be most efficient at a particular RPM. For a modern jet that is somewhere around 90% n3 or n2 as the case may be. This means that on the thrust/fuel flow graph 90% RPM would be the most efficient..

Now lets level off at twenty thousand feet and set 90% and see what happens. The airspeed will soon increase above limits which in turn causes more drag and decreases efficient operation. So what do we do, we climb up to an alttitude where that same 90% RPM sets us up at the airframe's optimum speed somewhere around .80 mach. We also get the benifit of increased TAS as compared to levels below somewhere around 26000 ft.

Then we say to our selves if climbing is good lets keep going, but then more thrust is needed to maintain .8 mach which increases the N3 above its 90% optimum. So this explains why optimum is lower than max altitude (still air of course). As we proceed we get lighter and less than optimum 90% RPM is the result so we climb to where we get that nice 90% again. This explains the fact that optimum altitude rises as the aircraft gets lighter.

7

[411A]

What about aircraft piston engine?


Looking back many years, I flew the DC-6B for awhile, and this airplane, equipped with Pratt&Whitney R2800CB16 engines, was always cruised at a constant power setting, which was 1100BHP, 'round about 46% of takeoff BHP.
This provided superb fuel efficiency and enhanced engine longevity, without question.

[mitzy69]

engines with variable vanes and FADEC , can be trimmed to get the N2 rotor to turn at the design speed and get lower SFC in cruise as did KLM on CF6 - 80 engines by adjusting the programming in the FADEC.

[wondering]

Highest thrust required is at top of climb. Takeoff is usually reduced thrust if there is excess runway available.

I reckon, that´s not really the case. At TOC the thrust is a fraction of what is available at Sea Level. At TOC thrust only needs to match drag. Excess thrust is only needed when accelerating or climbing. Reduced thrust T/O is a 'luxury' on long runways.

[airbus757]

Correct.

7

[barit1]

Sorry - I am used to thinking in "corrected thrust" terms. By that I mean at TOC is the highest pressure ratio, trying to meet the required thrust in a very thin atmosphere. In engineering terms this is the highest corrected thrust point in a flight cycle.

Of course, at/near SL, much more physical thrust is available because of the dense air. :O

[enicalyth]

Slide 1 [3 graphs] illustrates near linearity and non linearity in a particular example of long range cruise under ISA conditions.
http://i28.photobucket.com/albums/c220/enicalyth/FUEL.jpg
Slide 2 collates these into a fuel burnout plot.
http://i28.photobucket.com/albums/c220/enicalyth/FUELFLOW.jpg
The tsfc graph should be read right to left. At FL310 tsfc is high but the characteristic is flat and has actually dipped to minimum and started to rise before step up. At FL350 the pilot sees only the rising portion of the tsfc curve in this case which has a much steeper gradient. Then a further step to FL390 offers a chance of further improvement including a slight dip before tsfc begins to climb relentlessly again.

[AeroTech]

Thank you for your posts.
Since it seems that turbine engines operate optimally at cruise (fuel efficiency), this will be beneficial for long-haul aircraft.
a-What about short-haul jets: how they will operate optimally since they have short cruise?
b-Since short-haul jets will fly daily more flights (compared to long-haul jets), does it mean that short-haul jet engine will have less MTBF (mean time between failure) or mean time to overhaul/maintenance?
c-If there is a trade-off between short and long-haul jet engine regarding fuel efficiency, reliability, maintenance cost,...how would a design of a short-haul jet engine will be different than the long-haul jet engine?
Your feedbacks are very appreciated. Thank you.
Regards

[barit1]

Generally speaking, turbine engines are scheduled for maintenance based on flight cycles rather than hours; one cycle means one startup, one takeoff, one cruise segment, one landing, one T/R cycle, and one shutdown. Rotating parts are certified for xxxx cycles, and hot sections see hardware deterioration as a function of cycles.

So you can see that whether the cruise segment is 14 minutes or 14 hours, it makes little difference in engine life. :)

[AeroTech]

Barit1, thanks for your post.

Generally speaking, turbine engines are scheduled for maintenance based on flight cycles rather than hours
Are there certain cases or engines where engine maintenance is based on flight hours?

So you can see that whether the cruise segment is 14 minutes or 14 hours, it makes little difference in engine life
Let’s take your statement (quote) as an example:
Assume we have similar engines installed in a short-haul and long-haul jet and the TBO (time between overhaul) is 3000 cycles. We assume also that both jets have equal flying time in other segments.
The short-haul jet engine will totalize 700 hrs (14 minutes X 3000) cruise time and the long-haul jet engine will totalize 42000 hrs (14 hours X 3000) cruise time.
Can you please explain me how could this wide difference on cruise time with engine operating in relatively high power will make only little difference in engine life?
Your feedback is appreciated.
Regards

[barit1]

I've obviously overstated the case, AeroTech, and I don't mean the two examples are really equivalent.

But the takeoff/climb portion of a flight cycle is the 800# gorilla consuming parts life. Derated or reduced (flex) thrust is a huge cost-saver to the overall maintenance budget, and far outweighs the cruise portion of the maintenance budget.

Plus - life-limited parts (discs, shafts etc.) will force the engine into the shop at xxxx cycles, even if the engine is still running fine on wing. This is the insurance policy against metal fatigue and uncontained failures.

So - flight hours are of much less significance than cycles, and you'll almost never see a time limit employed for engine removal. (I have seen some exceptions, mostly for a new engine model introduced to a very conservative country, and a regulatory agency decrees a one-time special inspection.)