2 engine vs. 4 engine fuel burn

[admiral ackbar]

Hello all,

professional SLF here, I just read an article in the April 1st issue of The Economist about Boeing and Airbus and a sentence in there caught my eye:

Meanwhile, the European firm is losing out in the next size up, where Boeing sold 155 of its 777 long-haul planes last year, compared with only 15 Airbus A340's, which suffers from having four engines in a time of high oil prices.

Is that true? Sorry of it sounds like a stupid question but my gut tells me that the fuel use is sensibly the same between two. I do understand that maintenance and all that is cheaper with 2 engines, but fuel burn must be about the same no? The Economist is usually pretty good when it talks about aviation.

This may be apples and oranges, but I am just curious.

Thanks for your help.

[18-Wheeler]

I read in Flight International (not the most reliable tech source) that the 777 burns about 8% less than the A340.

[Old Smokey]

Any sector profile is essentially made up of 3 segments, Climb, Cruise, and Descent. If the aircraft is flown at or close to optimum altitude, optimum cruise speed, and optimum engine Thrust Specific Fuel Consumption (TSFC), there's little to differentiate between the 2 and the 4 engined aircraft in the cruise. The major differences lie in the Climb and the descent.

The Climb is far and away the most fuel expensive segment of the flight, and the degree of climb performance (angle or rate) lies in the amount of excess thrust available, and, following from this, the best rate which is proportional to the excess thrust multiplied by the speed (Thrust X Speed equals Power). The 2 engined aircraft has far greater excess thrust in all phases of flight than does the 4 engined aircraft. This arises from the fact that both aircraft must achieve APPROXIMATELY the same performance with One Engine Inoperative, i.e. the 2 engined aircraft must achieve on 1 engine what the 4 engined aircraft achieves on 3. On the 99.999% of occasions that an engine does not fail (normal operations), the 2 engined aircraft now has a 100% increase in thrust above minimum requirements, whereas the 4 engined aircraft has a mere 33% increase in thrust above minimum requirements. Normal climb performance on the 2 engined aircraft is therefore far superior to it's 4 engined counterpart, and with it, a much shortened climb time. Thus, Climb fuel is much less, and climb is the most fuel expensive phase of flight.

Although I discounted cruise performance in general terms earlier on, there is a further consideration here, and again a greater degree of excess thrust will allow for climb to a higher, more fuel efficient altitude for cruise. Thus, cruise fuel performance is improved somewhat.

In the descent phase, all engines are operating at idle thrust. Whilst the flight idle thrust certainly extends the descent 'cheap ride' distance, the fuel expenditure for the idle thrust is very high due to poor TSFC at the much lower engine speeds. A lot of fuel for a little thrust in short. The 4 engined aircraft must tolerate twice as much fuel flow for little in return during the descent, than does the twin.

That's the comparison in a nutshell. It's a bit more complicated than that, but that's the simplified version.

Regards,

Old Smokey

[enicalyth]

Matching the engine and pylon to the airframe is an engineering art too. Four55000lb slst engines might promise 0.56lb fuel burn per lb thrust per hour. Two more modern 110000lb slst engines might promise 0.55lb instead of 0.56lb which is a 2% improvement.
Not only that but despite being physically larger it may be possible to integrate the two more cleanly than the four into the airframe and sneak economic cruise up by 0.01M from 0.83 to 0.84 or 0.84 to 0.85 or something like it.
There does of course come a point when a large engine becomes so large as to impose so much drag that there is no point in having it.
As far as maintenance goes it is to be hoped that the amount of time spent up ladders and poking things is about the same regardless of size. Or at least roughly the same. Maintenance costs are then lower because the time spent peering down holes with poles is halved. Engineers can tell you if this is spurious or not because some engines are more fiddly than others. But bean counters just see engineers up ladders hinging up covers and then folding 'em back down again. Fewer engines less cost, Carruthers old chap.
It would be a strange world if things were equal.... cats v dogs, blue v red, GE v RR, Company "A" v Company "B", twin v quad. I think it's the work of the devil sowing discontent in the amiable fraternity/sorority where never a fractious opinion is offered.
Who's for a whopping single then? Or better still, none at all but gravity waves wafted up from generating stations?

[HCB]

Generally speaking, more engines do burn more fuel for a given total thrust requirement. I could not give you a definitive reason why that is so, but according to all the data I've seen, it is true across nearly all engines (of comparable age).

Hard figures for the A340-500HGW and 777-200LR are as follows (the figures for the A346 and 773 are comparable):

Airbus A340-500HGW
engines: Rolls-Royce Trent 556
engine's specific fuel consumption: 0.568 lb. of fuel per lb. thrust per hour
engine's maximum thrust: 56,250
engine's cruise thrust: 11,000 lb.
typical seats: 313

Boeing 777-200LR
engines: General Electric GE90-110B1L
engine's specific fuel consumption: 0.530 lb. of fuel per lb. thrust per hour
engine's maximum thrust: 110,000 lb.
engine's cruise thrust: 19,000 lb.
typical seats: 301

As you can see, the GE90 has about 7% lower SFC (0.530 v 0.568) compared to the Trent 500. This would give the 777 a small advantage alone (the A340 has only 4% more seats than the 777), but the A340's problems are compounded by its higher total thrust requirements, due to its higher weight. Why is it heavier? Primarily because both the wing and the fuselage were pressed into service outside the limits of their original design. The fuselage for example is narrower than that of the 777, so it has to be longer; this increases weight in the same way that bicycle frames with narrow tubes are heavier than those with "fat" tubes (i.e. the walls have to be disproportionally thicker in the narrow tube for equal strength), and also because the longer fuselage must be reinforced to counter bending moments.

The wing's cross section and sweep were likewise optimised for a lighter plane travelling at slower speeds. So the A340 has higher SFC, lower speed (therefore more time in the cruise burning fuel), higher weight resulting in a higher total thrust requirement, and only slightly more seats to counteract these disadvantages. It is no mystery why Boeing sold ten times more 777s than Airbus did A340s last year. The only question that remains is whether Airbus made the right decision in massively reducing development and manufacturing costs by using an existing fuselage cross section. With hindsight it increasingly looks like that was a mistake (because of sharply rising oil prices), and that A340 sales will slow to an inevitable halt in the near future.

On a related note, it looks like Boeing's gamble with the all-new 787 is going to pay off handsomely in the sales battle against the derivative - if highly evolved - A350. Time for Airbus to get some A380s out the door and face the task of developing a new widebody fuselage if you ask me! Fortunately the hot-selling A320 family gives Airbus enough breathing space to get its momentum back, provided we don't see any more harmful turf battles inside Airbus management.

[Bolty McBolt]

This may be comparing apples and oranges but for those in the know some rough figures would be help full..

A boeing 747-400 RR eng in a 3 class config 300 pax flying Sydney - Singapore

7h:45 min flight time

Fuel load approx 88,000 kgs
With this amount of fuel I assume there is also extra cargo other than pax bags but not sure how much cargo weight to assume.

A 777 in 3 class config same pax (cargo) etc

What would the fuel order be on same sector??

:ok:

[18-Wheeler]

What would the fuel order be on same sector??
:ok:

A very rough comparison is that a 777 burns about six tonnes per hour and a 747-400 is up around ten.
No it doesn't stand close scruitiny but it's a reasonable generalisation.

[False Capture]

Bolty,

A B777-200 operating with 280pax on a 7hr45min sector would burn about 51,000kg. Depending on alternates etc., total fuel would be in the order of 58,000kg to 60,000kg.

[CONF iture]

So, for anyone flying both type, I would be interested to know what kind of fuel burn difference, in similar conditions on a same leg, between a 330 and a 340 ?
Thanks

[jai6638]

Hello all,
professional SLF here, I just read an article in the April 1st issue of The Economist about Boeing and Airbus and a sentence in there caught my eye:
Is that true? Sorry of it sounds like a stupid question but my gut tells me that the fuel use is sensibly the same between two. I do understand that maintenance and all that is cheaper with 2 engines, but fuel burn must be about the same no? The Economist is usually pretty good when it talks about aviation.
This may be apples and oranges, but I am just curious.
Thanks for your help.

LOL! I had exactly the same question after reading that article :)

On the 99.999% of occasions that an engine does not fail (normal operations), the 2 engined aircraft now has a 100% increase in thrust above minimum requirements, whereas the 4 engined aircraft has a mere 33% increase in thrust above minimum requirements.

how?!

[False Capture]

jai6638, because the "minimum requirements" to which Old Smokey refers are based on one engine being inoperative.

[Swedish Steve]

I fill up B777-200 (Trent) and B747-400 (PW4000) for the same 12 hour sector.
In round numbers the B777 takes 80tons in tanks and the B747 takes 120tons. (but the B747 takes more folks)

[admiral ackbar]

Thank you so much for the detailed yet very clear explanations!

I'm going to go to sleep less stupid tonight! :D

[False Capture]

It's a bit unfair comparing the fuel burn of a B747-400 with that of a B777-200. The B777 engines (being more modern in design) are far more efficient than the comparatively older engines on a B747. Therefore, the difference in fuel burn is largely down to technology and not the "2 engines vs 4 engines" topic of this thread.

[Bolty McBolt]

Thanks everyone for your posts.

Does anyone have fuel figures for the 777-300

And figures on 3 class config numbers.

Please excuse the dumb questions but my employer never purchased 777 so I only have the claimed figures from boeing web site..

Regards
MBolt

[Old Smokey]

jai6638, as False Capture has indicated, all Transport category aircraft must meet APPROXIMATELY (Exactly if obstacle limited) the same performance with one engine inoperative. That is, the 2 engined aircraft meets it's requirements on 1 engine, the 3 engined aircraft meets it's requirements on 2 engines, and the 4 engined aircraft meets it's requirements on 3 engines. (OK purists, in non-obstacle limiting circumstances the 3 and 4 engined aircraft do slightly better).

Now, in normal operations where an engine doesn't fail, all of the aircraft are "given back" the engine that was assumed failed. The 2 engined aircraft now has 2 instead of 1 (100% increase), the 3 engined aircraft now has 3 instead of 2 (50% increase), and the 4 engined aircraft now has 4 instead of 3 (33.3% increase). The 2 engined aircraft in NORMAL operations now has 3 times the thrust excess than does the 4 engined aircraft. Apart from the economics, safety in circumstances such as wind-shear recovery is dramatically increased.

Bolty McBolt, a fair question to compare the B777-300 to the B747, but it still falls a little short. Perhaps a better comparison would be between the B777-300ER and the B747, now we're talking approximately equal commercial uplift / distance capability.

I'm doing a 7 hour sector on a regular B777-300 tonight, I'll try to convince the dispatcher to give me a comparative fuel figure for the B747-400.

Regards,

Old Smokey

[Bolty McBolt]

SMOKEY

I appreciate your good work.
Correct me if I am wrong the difference between 777-300ER and 777-300 is additional fuel tanks and an uprated engine = extra range...

Regards
MBolt

[chornedsnorkack]

SMOKEY
I appreciate your good work.
Correct me if I am wrong the difference between 777-300ER and 777-300 is additional fuel tanks and an uprated engine = extra range...
Regards
MBolt
I think the wing, too.
Thus the bigger wing to carry more weight, bigger engine for the same (is there any change in landing gear?) - those distinguish -300ER from -300 and -200LR from -200ER.

One wonders how the Boeing 747-800 is to be competitive with 777-300ER?

Or perhaps it is not... no one wants Intercontinental, months after "launch".

Anyway, Boeing tried, and failed, to design a 747 trijet after DC-10 and Tristar turned out to be in some respect cheaper to fly than 747. The upper deck bump disturbed airflow into the inlet, so they had to keep 4 engines on the SP. How exactly does the fuel burn of SP compare with DC-10?

Since 747-800 does not extend the upper deck all the way to tail, centre engine is out of question. Or perhaps it would be better to fly 747 as a twinjet, like 777? The engines seem almost big enough...

[18-Wheeler]

Anyway, Boeing tried, and failed, to design a 747 trijet after DC-10 and Tristar turned out to be in some respect cheaper to fly than 747. The upper deck bump disturbed airflow into the inlet, so they had to keep 4 engines on the SP.
Like this ->
http://www.billzilla.org/tri_747.jpg

[XPMorten]

..it seems as if the A330 has lower FF for the same weight.
The A330 cruises M0.02 faster though, anyone know why?

http://www.xplanefreeware.net/morten/DOCS/A330.jpg

http://www.xplanefreeware.net/morten/DOCS/A340.jpg

Cheers,

M

[Voeni]

damn, that's the best thread in terms of detailed answers to questions... why can't any thread be as informative as this one??

keep goin' guys, that's how pprune has to be!

[swh]

It's a bit unfair comparing the fuel burn of a B747-400 with that of a B777-200. The B777 engines (being more modern in design) are far more efficient than the comparatively older engines on a B747. Therefore, the difference in fuel burn is largely down to technology and not the "2 engines vs 4 engines" topic of this thread.

Correct,

That is why the 748i is more efficient than the 777-300ER, and maybe why Boeing has not sold a 777 this year. The GEnx/Trent 1000 family on the new aircraft either on a twin or a quad are far more efficient than the equivalent current models.

The 787-10 is better than the 777-200, 747-800 better than the 777-300ER, nothing to do with the number of engines, just the type.

[CONF iture]

A friend of mine who is working for an airline who operates both types of the same series, 330 and 340, gave me a few very interesting figures:

If that airline has to send a 340 instead of a 330 on a route that is not ideally suited to that 4 engines, let's say a 6 or 7 hours flight, on an identical typical payload, the 340 will use about 15% more fuel !

To partially explain that extra fuel burn, the 340 suffers from an extra 8% on the OEW.

[ok1]

It might be somewhat off-topic, but anyway, this is my question:

What are/used to be the reasons for the practice of shutting down one or two engines right after vacating the runway on 2 or 4 engine aircraft respectively? Is there any fuel benefit in doing so, or the only reason is to enhance engine life? I think that this practice is not that common anymore, why is that?

Thanks,
OK

[admiral ackbar]

It might be somewhat off-topic, but anyway, this is my question:
What are/used to be the reasons for the practice of shutting down one or two engines right after vacating the runway on 2 or 4 engine aircraft respectively? Is there any fuel benefit in doing so, or the only reason is to enhance engine life? I think that this practice is not that common anymore, why is that?
Thanks,
OK

You should check out http://www.pprune.org/forums/showthread.php?t=219191

[ok1]

admiral ackbar: merci:ok:

[Old Smokey]

Upon re-reading many of the well thought out inputs here, I have to agree that comparison between aircraft such as the B777 and B747 is unfair, the B747 does not have the advantage of the much later refined super-critical wing of the B777, and has good, but older technology engines. The only fair comparison would be to build a 2 engined B747 with comparable technology engines, or alternatively to build a 4 engined version of the B777 with comparable technology engines.

Fortunately for this argument, Airbus has done just that for us in building the very similar technology A330 and A340, and CONF iture's post has well illustrated the fuel efficiency differences between the essentially similar 2 engined A330 and 4 engined A340.

It all gets down to excess thrust allowing for much improved climb, and the ability to cruise at higher levels at all phases of flight at similar weights.

Regards,

Old Smokey

[Bolty McBolt]

A Fair comparison. I am asking a Fare comparisson .

The 777 was marketed as a replacement for airlines flying clapped out old 747 classics.
I for one could not see the comparison between the 2 when the 777-200 emerged as it was a (3 class config) 300 seater but the 777-300 is a 350 seat (3 class config) with more cubic cargo space than a 747.

Lets assume the aircraft will be based at the ass end of the earth (to quote a former prime minister) and many of your airlines cash earning routes are 6 - 8.5 hour flights to asia which would be well suited to the 777.

These routes for years have been serviced by 747 classic and 747-400 due to the fact that no new/efficient type aircraft were purchased in the 90s.

I am asking difference in fuel burn from a 777 to 747 as a 747 classic burns over 100K fuel to Japan and a 744 around 90K.

It would appear but I can not confirm that a 777-300 full pax and cargo over these distances would burn about 30K kgs less fuel

The Fair comparison is about commercial reality with the price of fuel souring thru the $70 US and more.

Can anyone help with these figures?

Bolty

[XPMorten]

All the data I have goes in favor of the 777 vs the 744 in terms of FF compared with weight.

Fuel planning data (x1000 lbs);
777 landing weight 400 lbs
744 landing weight 500 lbs

...............777......744
4000 nm....118....174
3000 nm......87....127

Also BADA data and long range cruise tables show a similar picture.

How this compares pr SEAT however, I have no clue...:O

Cheers,

M

[Dani]

If that airline has to send a 340 instead of a 330 on a route that is not ideally suited to that 4 engines, let's say a 6 or 7 hours flight, on an identical typical payload, the 340 will use about 15% more fuel !

Yes, that's a fair comparison. Someone has a comparison between a 330 and a 777?

One has to understand that Airbus designed its A340 before Boeing began boosting its Etops limitations. Before that, lots of flights were economically not useful with a twin. That's why Airbus should have started selling the A330 as the standard long haul type, whith an A330-500 and even -600.

Dani

[The SSK]

Drifting off-topic, but I used to have a series of graphs that plotted fuel burn against sector distance for several aircraft types. Obviously the figure was very high for ultra-short hauls, and descended steeply thereafter before levelling out and … starting very gradually to rise again (burning fuel in order to … carry fuel).

The consequence was that, for any given type, the optimum stage length in terms of fuel burn was significantly shorter than the capability of the aircraft. For a typical longhaul widebody this might be about 5500km (say, London-Dubai).

So two 5500km sectors (4x taxi, 2x takeoff, climb, landing…) would burn less fuel IN TOTAL than one 11000km (say, London-Singapore).

Seems counter-intuitive, but a tech stop very close to the Great Circle could be a fuel-saving measure.

[N1 Vibes]

All

have we overlooked the fact that with a 2 engined aircraft you only have to overhaul 2 engines at say $3-4 million a piece on a 777, when on the A340 you have to shop visit 4 engines at say $2-3 million. B777 = $7 million, A340 $10 million. Approximate figures used, terms and conditions may apply, the value of your A340 may go up as well as down!

And, with the CFM haridryer's having just enough thrust and EGT margin to pull the skin off a rice pudding you will be shop-visiting more often than say it's bulletproof GE90 big-brother.

So perhaps the comments were leaning more towards the bean counters who love twin engined a/c because of the above. My current operator, spend 1/3 of the engineering budget on the noise generators.

Thoughts?

N1 Vibes :8

[chornedsnorkack]

Yes, that's a fair comparison. Someone has a comparison between a 330 and a 777?
One has to understand that Airbus designed its A340 before Boeing began boosting its Etops limitations. Before that, lots of flights were economically not useful with a twin. That's why Airbus should have started selling the A330 as the standard long haul type, whith an A330-500 and even -600.
Dani
Which they could not quite have done... Airbus started with shorthaul widebody twin (A300) and continued with a somewhat longer haul twin (A310). ETOPS was very much pioneered by Airbus... but they felt that it still limited their routes and range, which is why A340 was designed.

Problem: A330 seemed to be hard to stretch. I think that the standard MTOW for A330 is 230 tons (both -200 and -300) and the MTOW for A340-200/300 is 275 tons.

I have heard rumours that it may be the landing gear/engine clearance of A330/A340 - could it be the case? That A340 could have enlarged wing and stretched body for the -500/600 version while 330 would run out of the available underwing space for expanding the two (big to begin with) engines?

A350 quotes MTOW of 245 tons to begin with. Can they expand it?

B777 has been successfully expanded to 350 tons -200LR and -300ER - with a new wing. But the old wing -200ER and -300 have appreciably lower MTOW... How does B777-200 (non-ER) OEW and fuel burn compare with A330?

[XPMorten]

According to BADA, the A333 has far better cruise FF than the B772 for
the same weight. The B772 has however much bigger MTOW and range.
I guess it would be more fair to compare the B772 with the A343.

Compare the below with the A333 and A343 tables on the end of the previous page.

Cheers,
M
http://www.xplanefreeware.net/morten/DOCS/772.jpg