If recips were made nowadays

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According to one book the Boeing 377 could cruise at 315mph at 25,000ft using 467 gallons an hour. Max fuel was 7790 gals and range was aprox 4600 miles.
it carried 55-80 passengers.
An early 707 had a max fuel of 13,475 gallons and a max range (with allowances for climb and decent) of 4,650 miles max cruise at 25,000ft was 571 mph and econ cruise was 549mph height not given. 707s had widely varying passenger capacities depending on seat arrangements. but one typical arrangement was 36 first class and 95 coach class. Please note that this for the early 120 model with jet engines, when refitted with turbo fans (and the engines themselves could be converted) and a wing modification, the range jumped to 6,215 miles with the same fuel, cruise speeds went up just a bit.

707 could carry easily 50% (or higher) more passengers using 73% more fuel and make more trips per week (or even per day) and required less maintenance.

Please note that the JT8-D was coming into service in 1959-60 and modern turbo fans can do much better.

A modern diesel reciprocating engine might very well be able to equal the old R-4360 in power and fuel economy and even power to weight. But it is just not big enough to challenge the modern jets/turbofans and offers no economic advantage.
 
Since most of the thrust of a modern turbofan is generated by the fan, I did a search for how much power does the fan drive consume. One result I found pointed me in the direction of aero-derivative power generators.

The Trent 60 power generator has 2 versions, one rated at 54MW (72,415hp) and the other at 66MW (88,510hp). These are based on the Trent 700/800 turbofans, the 700 having similar thrust ratings as the Trent 1000 in the Boeing 787 Dreamliner.

https://www.energy.siemens.com/hq/p...s-turbine-industrial-trent-60-brochure-en.pdf

The RB211 was used in Boeing 747, 757 and 767 as well as the Lockheed Tristar.

The aero derivative power generator delivers between 25MW (33,525hp) and 32MW (42,915hp).
https://www.energy.siemens.com/br/pool/hq/power-generation/gas-turbines/RB211/industrial-rb211.pdf

The Pratt & Whitney JT8D was a low-bypass turbo fan, but its power generator derivative, FT8, still produces ~30MW (40,230hp).
http://www.pwps.com/wp-content/uploads/2017/09/FT8-MOBILEPAC-Gas-Turbine-Package.pdf

The Pratt & Whitney PW4000 has been used in the Airbus A300, Airbus A310, Airbus A330, Boeing 747-400, Boeing 767, Boeing 777, Boeing KC-46 and McDonnell Douglas MD-11.

The FT4000 derivative power generator produces 60-70MW (80,460 - 93,870hp).
http://www.pwps.com/wp-content/uploads/2017/09/Aero-derivative-Gas-Turbine-Packages.pdf

The General Electric LMS100 is based on the CF6 turbofan which powered the Airbus A300, Airbus A310, Airbus A330, Boeing 747, Boeing 767, Lockheed C-5M Super Galaxy, McDonnell Douglas DC-10 and McDonnell Douglas MD-11.

It produces 117MW (156,900hp) @ 44% efficiency. That efficiency would tough to match with a piston engine.
https://www.ge.com/content/dam/gepo...Sheet/2018-prod-specs/LMS100-power-plants.pdf

Those would be some pretty big piston engines!
Let's take the Pratt & Whitney R4360 for comparison.

The R-4360-51 VDT version was rated at 4,300hp (3.2MW). Assume a modern version would have twice the efficiency (it would be around 40-45%), so the output would be 8,600hp (6.4MW). You would need 5 of those to equal the lowest power in the list above. The R-4360-51 weighed in at 1,760kg. Let's say we can bring that down to 1,500kg each, that still means you have 7,500kg weight. The RB211 weight 5,000-6,000kg, so the 5 x R4360 is only 25%-50% more. But that's not factoring in the weight of the gearboxes required and the weight of the fan section itself.

The R-4360-51 is around 2.45m long and 1.4m in diameter. Depending on variant, the RB211 is between 4.5m and 5.2m. The fan diameter is 1.88m - 2.19m in diameter. Imagine the installation of all those R4360s to drive one fan!

And imagine the maintenance.
5 x 28 x 2 (twin fan aircraft) x 2* (plugs per cylinder) = 560 spark plugs to change!

looks like a winner!
 
I would like to see fuel consumption numbers with those 30K plus HP figures. Also remember at cruise the power levels are backed off, as well as the speed. Also comparing a modern swept wing to old school prop airframes is not a fair comparison. There is an industrial engine that is at the 50% efficiency mark it is a diesel recip. I have seen chatter on some sites about electric motor power for aircraft, that is where an extremely efficient recip would shine as competition. When it comes to comparing maintenance of an old recip to a modern jet, where is the fairness of that? We are talking engines built 60 to 70 years ago against the latest and greatest. The parts for a recip will always be less costly than a jet / turbine, the manufacturing techniques as well as the exotic materials needed for turbines will always keep the costs up high. Turbines are sensitive to dust etc. and volcanic ash, birds and anything else that they should not be ingesting. A recip due to the lower air demand can be equipped with inlet filters to keep the bad stuff out.
Turbines are gas guzzlers and that won't change.
 


Those engines are low-speed diesels with bores of 500 mm and strokes up to 3 meters. When I worked at Lycoming, the AGT-1500 beat the sfc of many gasoline engines except the turbocompounds (its sfc was about 0.42); it was a small engine, and larger gas turbines tend to be more efficient. In comparison to the diesel engine it was competing with, it had a far longer MTTF and MTBR, was much lighter, and took up less volume, including its air cleaner. The recuperator actually worsened the sfc at high power conditions, because it increased the pressure loss between the compressor outlet and turbine inlet and the need for variable geometry hot end geometry to match the compressor and turbine over a broad range of conditions reduced the maximum turbine efficiency.

Aircraft are very weight-sensitive. As an example, an engineer at Boeing claimed that reducing the weight of the landing gear from 4000 lb to 3000 lb would pay for itself if the landing gear could be made out of gold, vs steel, when gold was $35/oz; in the early 1960s.
 
PWR4360-59B said:
I would like to see fuel consumption numbers with those 30K plus HP figures.
Click to expand...

They are 40-45% efficiency, which is high for a 4 stroke piston engine .


PWR4360-59B said:
Also remember at cruise the power levels are backed off, as well as the speed.
Click to expand...

They also have to take-off at full power and fly on one engine (for a 2 engine airliner)


PWR4360-59B said:
Also comparing a modern swept wing to old school prop airframes is not a fair comparison.
Click to expand...

I don't think I did that. I assumed a modern airframe driven by the fan section of turbofans driven by piston engines.

Previously I compared the dH Comet with contemporary piston engine airliners.


PWR4360-59B said:
I have seen chatter on some sites about electric motor power for aircraft, that is where an extremely efficient recip would shine as competition.
Click to expand...

Not sure that electric airliners are coming anytime soon.

I have seen some light planes with electric power.


PWR4360-59B said:
When it comes to comparing maintenance of an old recip to a modern jet, where is the fairness of that? We are talking engines built 60 to 70 years ago against the latest and greatest.
Click to expand...

They still have to have 2 spark plugs per cylinder which would have to be changed frequently.


PWR4360-59B said:
The parts for a recip will always be less costly than a jet / turbine, the manufacturing techniques as well as the exotic materials needed for turbines will always keep the costs up high.
Click to expand...

Less costly and wear out more quickly.



A piston engine needs the filter if it wants to keep running without excessive wear or damage.
 
[QUOTE="wuzak, post: 1417007, member: 42564]

They still have to have 2 spark plugs per cylinder which would have to be changed frequently.




Less costly and wear out more quickly.




A piston engine needs the filter if it wants to keep running without excessive wear or damage.[/QUOTE]

They would probably be diesels, so no spark plugs.

A piston engine, to have even a vague hope of replacing the engines on a commuter, like an ATR-72, would need to be around 2,500 shp before the greater power required by the increases in take-off weight due to a much heavier engine, heavier gearbox, increased sound insulation, vibration damping, etc.
 
Sorry but there is a small lack of knowledge of the state of the art diesels here. I'm not talking about a huge 3000 ton sulzer. The engine is from Fairbanks Morse it is an opposed 2 stroke, that boasts 50% efficiency it is light weight in comparison to the ultra large engines. There is no reason the technology can not be sized down and even better efficiency numbers could be attained, the use of power recovery techniques could help bring that number higher, amongst a few other additions. Diesels power to weight, you act like there are no diesels used in aircraft or ever has been used. Just do a little study on the topic. I also liked the recips wear faster, well it depends on what a turbine ingests how long they last, give them a nice sand storm or dirt / rocks on the runway, and a wrench or two. Then you got a multi million dollar pile of scrap.
 
How about a few sources and/or links to help educate us and bring us up to speed.

And I, for one, knew about the Packard Diesel aircraft engine, Guiberson radial aircraft engines, Junkers Diesel aircraft engines (opposed pistons) and a few others a long time ago.
I also knew about the Napier Nomad
Napier Nomad - Wikipedia

Great fuel economy, however the weight of 3,580lbs for 3,476hp take-off wet and a max continuous of 2,392 hp doesn't look so good for an aircraft engine in the first 1/2 of the 50s.

RR had the Tyne Turbo prop of 1935lbs making 4020 shp for take-off, 3325hp max continuous and 2270hp shp (shaft horsepower) + 150lb thrust at 25,000ft at 425mph cruise. CLAIMED specific fuel consmption was 0.41 lb/eshp/hr.
Some other turbo props were not far behind.

A Propeller driven airliner is going to be slower than a jet or fan jet. Propellers start losing efficiency a bit below where pure jets start to get efficient.

assuming reciprocating engine propellers show the same fall off in efficiency as turbo-prop propellers we can see that any reciprocating engine has some serious difficulty over 450-500mph and once you get to 550mph the efficiency of the propeller is pretty much in the toilet while the jets were coming into their own.

Comparing specific fuel consumption as the primary criteria in selecting an engine disregards too many other things.

Most reciprocating engines only used air filters when moving on the ground or at very, very low altitudes. They rob power and most aircraft could cut the air filters in and out. Not sure what would happen to a reciprocating engine that ran into a "cloud" of volcanic ash. How thick does the cloud have to be for the pilot/s to notice/recognize it and switch to filtered air and loose power/altitude? How much does it take to score cylinder walls and ruin piston rings?
You could wreck a piston engine with a rag, no wrench needed.
 

The speed at which prop efficiency is "pretty much in the toilet" also reduces with altitude.

Most jet airliners cruise 30,000ft-40,000ft. Not the greatest place for prop efficiency.
 
wuzak said:
Most jet airliners cruise 30,000ft-40,000ft. Not the greatest place for prop efficiency.
Click to expand...

To cruise at such altitudes (or even over 25,000ft) with a reciprocating engine calls for a sophisticated supercharger set-up. Unfortunately, while good for power to weight ratios, sophisticated supercharger set-ups usually are expensive to build and maintain and cost specific fuel consumption. Power for the extra compression of the ambient air has to come from somewhere.
 

Sorry, but you won't get a multi-million dollar of scrap for a turbine engine ingesting dirt and rocks on the runway (these can also scrap a propeller, in any case), a sand storm, or a wrench or two. In case of a bird (which can take down an airplane without damaging the engines -- a B-1 was lost to a bird hitting the leading edge of one wing): all those things are designed for. Boeing 737s were (are?) routinely operated off gravel runways. There was increased engine maintenance, but there was also a fix.
 
Speed is no longer important there is info on the net showing that jet planes have slowed down since the old days of the 707, for what ? Fuel savings.
Propeller ? Like they say there is more than one way to skin a cat. What is needed is some out of the box thinking for propellers. Maybe this
View: https://www.youtube.com/watch?v=Cew5JF8q6eY
, or maybe this, View: https://www.youtube.com/watch?v=c0Icfopt8Yw
. And I'm sure there are more great ideas out there. Very likely a poor slob like me that has no $ to put the great ideas into operation.
 
There were several things that killed off the profan, and its GE semi-clone, the UDF, among which were increased maintenance costs, greater weight, and greater noise, both interior and exterior.

High-power gear boxes are non-trivial, as seen in the A400M, where the gearbox has been a chronic problem.
 
They may have backed off a bit from the days of the 707 but they have not backed off to point of making propellers really viable for medium/long distance flight.

Some airlines are advertising 5.5 hours from LA to New York and 6 hours 10 minutes from New York to LA. 2451 miles. Time is take-off to landing so cruise is higher than the 420mph average (both ways) would suggest.

And as the chart above shows the turbofans are coming into their element compared to props at even 500mph.

One thing is the turbofan (or jet) can give us a real readout on fuel burned per lb of thrust/hour.(if you can find it, it does exist) With a reciprocating engine we know the fuel burned per lb of hp/hour but then we have to figure the efficiency of the propeller to get the fuel burned per lb of thrust/hour. and if the thrust is a good match for the desired speed and conditions. In reciprocatine engine days bombers and transports often used a different reduction gear than fighters/small combat planes so they could swing larger diameter but slower turning propellers to better match the volume and velocity of the propeller stream to the speed of the aircraft.

Turbo props still work well for short haul airlines where the distances are too short to allow climbs to high altitude and the difference in travel time is small (do you really want to spend another 2-3 hours inside the plane on a cross country flight with a prop plane?)

But here the turbine engine has displaced reciprocating engines from all aircraft needing more that about 400hp per engine. Most crop dusters have switched to turbo props because while they are much more expensive to buy, they last 3-5 times longer between overhauls and the price averages out.

From Wiki as an example "The 715 shp TPE331-6 used in the Beech King Air B100 have a 400-hr. fuel nozzle cleaning interval, 1,800-hr. hot section inspection interval and a 5,400-hr. time between overhaul; approval is possible for 3,000-hr. HSIs and 6,000-hr. overhauls and engine reserves are cheaper than for the PT6A with $130,000 HSI checks and $190,000 overhauls"

Please note that it can cost around $50,000 to overhaul a Lycoming or Continental flat six if you are using one of the higher powered, more complicated versions. Like fuel injected and using a turbo. For Continental having an engine that uses a reduction can add $10,000 on top of other costs.
Engine Overhaul

For Lycoming overhaul times as per the factory are here
https://www.lycoming.com/sites/default/files/SI1009BB TBO Schedule.pdf

Please note that the O-540 can vary from 2000 hours down to 1200 hours depending on exact model.
Also note that the Garrett TP#331 is hundreds of pounds lighter than the high powered O-540/541 engines which means the the aircraft can either haul more fuel to feed the thirsty engines or on short flights, carry more cargo.

This is the problem faced by any attempt to bring back over 400hp reciprocating engines, spark ignition or diesel. The Turbines are much lighter for more power and have much longer established overhaul times. The Diesel reciprocating engine is going to need a huge advantage in fuel economy to even a peak under the tent and it doesn't have it. Many turbines since the late 50s and 60s using a pressure ratio (think compression ratio) of over 10 to 1 (some are around 15 to 1) so their thermal efficiency is actually pretty good.



 
There are so many more ways to improve recip efficiency and fuel economy, it is just not attainable for jets or turbines. And why all the gear box talk about the props I showed? I was not suggesting to hook them to a turbine that turns in the 10,000 plus rpm range.
Overhaul? Some industrial diesel type engines are in the 16000 to 20000 hour range, so again with proper engineering desired effects can be obtained.
 
A direct-drive reciprocating engine would be far too heavy, so it will have a gearbox. Indeed, just about all the aircraft diesels being produced now have reduction gears.

I was one of the engineers involved in propfans, toward the end of the program, when HSD was running tunnel tests on counter-rotation propfans. Gearboxes were a big concern of the airlines. Fuel was also less tha 10% of the aircraft DOC. Remember, gas turbines killed off recips on airliners when aircraft gas turbines had less than 15 years of non-experimental service.
 
Last edited:
Shortround6 said:
Many turbines since the late 50s and 60s using a pressure ratio (think compression ratio) of over 10 to 1 (some are around 15 to 1) so their thermal efficiency is actually pretty good.
Click to expand...

The new Rolls-Royce Trent 7000 has a pressure ratio of 50:1
Rolls-Royce Trent 7000 - Wikipedia

The Turbo-Union RB199 that first ran in 1972 had a pressure ratio of 23:1
Turbo-Union RB199 - Wikipedia

The Rolls-Royce Avon had a pressure ratio of 7.45:1 in the '50s
Rolls-Royce Avon - Wikipedia


The Olympus used in the Concorde had a pressure ratio of 15.5:1
Rolls-Royce/Snecma Olympus 593 - Wikipedia
 
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