2 engine vs. 4 engine fuel burn
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2 engine vs. 4 engine fuel burn
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.
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.
This may be apples and oranges, but I am just curious.
Thanks for your help.
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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
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
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small but crucial differences
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?
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?
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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.
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.
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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??
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??
Originally Posted by Bolty McBolt
What would the fuel order be on same sector??
No it doesn't stand close scruitiny but it's a reasonable generalisation.
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Originally Posted by 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:
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.
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.
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.
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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)
In round numbers the B777 takes 80tons in tanks and the B747 takes 120tons. (but the B747 takes more folks)
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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.
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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
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
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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
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
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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 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
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Originally Posted by 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
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
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...
Originally Posted by chornedsnorkack
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.
