If recips were made nowadays

[Shortround6]

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 modern reciprocating engine using jet fuel (a diesel) would be great engine for a modern airship, otherwise not so much.

You have a real problem with any reciprocating engine trying to make power. They are basically air pumps and the more air you can put through them in a given amount of time the more power you can make. There are three basic ways to do this, more displacement of the engine (size and number of cylinders) for air per revolution of the crankshaft. More revolutions of the crankshaft per unit of time. using a supercharger to force more air into the engine. Many engines use all three.
Each as some drawbacks or associated penalties/trade-offs.

large displacement means a large (bulky) and quite possible heavy engine. Higher rpm means a lot stress on the components, engine may be small but not as light as it might appear. Will need a reduction gear adding weight and cost. Superchargers have few problems of their own but generally allow for smaller physical engines.

Every engine is a collection of trade-offs and you wave a "proper engineering" magic wand and get the results you desire.
Many diesel engines have 3 or 4 different power levels listed on the specification sheets.
1. is for station powerplant use, electric generator, water pump or other near 24 hour a day 7 day a week use where reliability is paramount
2. is for general use like over the road trucks, for engines that are of that size range. a bit higher power can be used for increased performance being traded for a bit shorter life.
3. is for fire trucks or emergency equipment, more power for fewer hours per year in operation.
4. may be for military use. often the most power and least life, large engines in an armoured vehicle require larger armoured hull to accommodate them which can bring on large weight gains for the complete vehicle. Military also has the budget and facilities to do more frequent engine swaps for repairs than most civil operators.

even boat/marine diesels are given different rating depending on the type of boat/ship they are going in. Commercial fishing boat or work boat vs yacht or sport fishing boat.
Ratings are modified by fitting different size/capacity injectors and limiting RPM.

for aircraft use you want the highest power to weight ratio you can get which means you can throw station power plant engine life right out the window. Stationary power plants have the least constraints on weight or physical size for a given level of power.

You don't get the reliability/long life of a stationary power plant by waving a magic wand and claiming "proper engineering". Part of the proper engineering of that stationary power plant was using an engine of sufficient size/weight to get the power desired for the number of hours of operation desired.
Over the road trucks, boats and other mobile users get properly engineered engines to suit the weight and space limits of the vehicle/boat in question, the intended loads and speeds and the expected hours per month or year of operation.

Aircraft are the same. A properly engineered engine will suit the intended use of the aircraft. A 2-4 seat private plane that flies only a few hundred hours a year (or less) can make do with an engine that requires overhaul at 1000 hours or so (modern version of old engines can double that) and go years between overhauls. while even a small commuter plane hauling 12 passengers and flying 8 hours a day, 6 days a week would rack up 2496 hours a year. Having to change engines (or overhaul in place?) twice a year might be unacceptable if there is an alternative.

 

[swampyankee]

I couldn't tell you what the TBOH is for modern gas turbines, because the concept is no longer used for most large aircraft engines: FADECs and engine health monitors report a number of parameters, some in real time, and most maintenance is on-condition, possibly after borescope inspections. When TBOH was still used for large engines, the JT8D was up over 10,000 hours. Aircraft engines with pistons never beat a third of that.

I really have no idea of the service life or maintenance needs of big utility turbines, like GE's 9HA series (rated at 450 MW and up). I was unable to find any values for their simple-cycle efficiency, but in combined-cycle operation, they're exceeding 60% net efficiency.

 

[PWR4360-59B]

No need for a gear box for reduction when they have propellers that turn at crazy high rpms. I think the one was around the 10K area. Need to watch a few of those NASA videos on the props. In the old model T and A days they would never have dreamed of the 700 plus hp production car engines we have today. It is all about the out of the box thinking and good design and engineering. And wow with the high pressure ratios mentioned I can imagine all the NOx that they produce, but then of course jets are exempt from that sort of stuff. 60% efficiency? Its about 98% or so with diesels on some cruise ships.
And again with propellers did anyone watch the videos?
So along with all the turbine loving going on here, please give me some fuel burn figures for some of the aircraft that are flying nowadays.

 

[KiwiBiggles]

Its about 98% or so with diesels on some cruise ships.

A thermal efficiency of 98% would require a compression ratio of 50 to 1, and a peak temperature of about 15000 K. And that's for an idealized Carnot cycle, which is unattainable. An efficiency of 60% is remarkably high for heat engine. 98 % is bollocks.

 

[swampyankee]

No need for a gear box for reduction when they have propellers that turn at crazy high rpms. I think the one was around the 10K area. Need to watch a few of those NASA videos on the props. In the old model T and A days they would never have dreamed of the 700 plus hp production car engines we have today. It is all about the out of the box thinking and good design and engineering. And wow with the high pressure ratios mentioned I can imagine all the NOx that they produce, but then of course jets are exempt from that sort of stuff. 60% efficiency? Its about 98% or so with diesels on some cruise ships.
And again with propellers did anyone watch the videos?
So along with all the turbine loving going on here, please give me some fuel burn figures for some of the aircraft that are flying nowadays.

Low speed diesels -- which are the most efficient simple-cycle engines -- are a bit over 50%.

As for propellers turning at "crazy high rpms," that may depend on one's definition of "crazy high," but propfans aren't designed for particularly high rpm -- or, importantly, tip speeds, but to deal with mildly supersonic Mach numbers near the blade tips at aircraft speeds of M=0.8 or slightly better.

Tip speeds of 500 m/s are considered high for ducted turbomachinery; that would be about 4800 rpm for a prop with a diameter of about 78 inches; for 10,000 rpm, that would be about 1050 m/s tip speed on the same diameter, or about 3 times the speed of sound.

 

[Shortround6]

So along with all the turbine loving going on here, please give me some fuel burn figures for some of the aircraft that are flying nowadays.

You have different fuel burns. are you talking about the SFC or pounds/grams per HP hour which is between about .42lbs and .50lbs per HP?hr for most spark ignition engines when cruising (take-off and high power can be much more but fuel injected engines cut that back somewhat),


or are you talking about total fuel burn like Plane XX burns 120 gallons an hour to fly 195mph at 12,000ft?

Nobody in the west has built (in large numbers) reciprocating aircraft engines of over 500hp in the last 30-40 years. Poland kept up production of the AS 61/62 engines (copies of the Wright R-1820) for some time.

 

[PWR4360-59B]

Fuel burn per person on the plane would be the best comparison. But yes lbs/bhp hr would be great, since that it what gas or diesel recips use.

 

[PWR4360-59B]

Low speed diesels -- which are the most efficient simple-cycle engines -- are a bit over 50%.

As for propellers turning at "crazy high rpms," that may depend on one's definition of "crazy high," but propfans aren't designed for particularly high rpm -- or, importantly, tip speeds, but to deal with mildly supersonic Mach numbers near the blade tips at aircraft speeds of M=0.8 or slightly better.

Tip speeds of 500 m/s are considered high for ducted turbomachinery; that would be about 4800 rpm for a prop with a diameter of about 78 inches; for 10,000 rpm, that would be about 1050 m/s tip speed on the same diameter, or about 3 times the speed of sound.

Yeah thats old school stuff, did you watch the video? I'm not sure if rpm is mentioned there but they are way breaking sound barriers with newer props. Even with high speed turbine compressor and fans. As always it seems just because we are talking old aircraft and old piston engines, it doesn't mean there are not new and better developments that have improved on the old school ways. We are in the 2018's not the early to mid 1900's.

This is a better and longer video link. At 4:44 he mentions 12,000 rpm for the propeller system.

View: https://www.youtube.com/watch?v=51BEbZlps80

 

[wuzak]

The Spitfire XIV had a 10'5" prop rotating at ~1,400rpm, which is a 0.51:1 reduction from the engine running at 2,750rpm.

And the prop would have tip speeds greater than the speed of sound at high speed at altitude. And that's not even at the speed and altitude that airliners cruise.

 

[Shortround6]

we are in 2018 but that doesn't mean the laws of physics have been repealed.

for instance.
High rpm engines have slightly lower SPC than low rpm because friction goes up with the square of the speed. You are using more fuel just to overcome internal friction. Better piston rings and better oil/lubrication only go so far. if you want high power to weight then high rpm is one way of getting it.

Diesels need stronger construction than spark ignition engines, they operate at higher peak pressures inside the cylinders. Yes we know have strong materials and better heat treatment but then the same improvements could be used on a spark ignition engine.

There are a number of sound regulations that may not favor supersonic propellers.
older jets had to be modified.
Hush kit - Wikipedia

The shroud over the fan helps limit the noise.

A number of older business jets were close to being outlawed due to noise in 2015. I don't know how that turned out.

Noise rules have gotten stricter in the last 20-30 years.

 

[Shortround6]

This is a better and longer video link. At 4:44 he mentions 12,000 rpm for the propeller system.
View: https://www.youtube.com/watch?v=51BEbZlps80

the input to the "propeller system" may have been 12,000rpm. the Propeller was not turning 12,000rpm.

The engine/system used on that plane was pretty much a one-off using an Allison T710 power section (used as the basis for the engine in the Osprey) and the Gear box from a T-56-A-14 (P-3B & C Orion). it used a two stage reduction gear with an overall ratio of 13.54 to 1. The gear box also weighed approximately 550lbs.

The compressor section of a T-56 ran at 13,280 rpm.

One of the laws of physics that you cannot repeal is that the air at 15,000ft is about 63% of the weight it is at sea level, at 20,000ft it is 53% and at 25,000ft it is
45% the weight of sea level air.
This means the drag is much lower as your aircraft is hitting/moving fewer air molecules but it also means that your air breathing engine is falling off in power at a similar rate. Also your propeller has that much less air mass to "bite" and accelerate rearward. Going higher just gets worse of course. At 35,000ft you are down to 31%

Now you can use oversized jet/turbofans that help get you out of short airfields better and allow a steep climb angle so less area near the airport are subject to the noise and that gives you a surplus of power when cruising at altitude (also a large safety margin of power incase of an engine failure). Specific fuel consumption for turbine engines usually rises as they are throttled back. However even very large, powerful Turbofans are comparatively light. An GE CF6-90C2 turbo fan was rated at 60,000lb thrust for take-off and weighed 8,946 lbs, max cruise at 35,000ft was 11,330lbs at mach 0.8.
this is totally out of reach for any reciprocating engine.

There was an experimental diesel listed several pages later in the 1986-87 Jane's all the Worlds Aircraft I got that information from.

It was the In-Tech Merlyn. a 3 cylinder two stroke diesel of 3.47 liters that used cylinder ports for intake and 4 exhaust valves in the head, It was rated at 650hp for take-off at 4800rpm and 600hp max continuous. It weighed 580lbs, other details are scarce/not given. It hadn't reached flight status yet (or at least not sales status) with more ground testing in hovercraft and marine vehicles wanted before flight. Engine life not commented on.

Now figure what happens when you get to 15,000ft? 378hp max continuous? unless you use a bigger supercharger which affects SFC.

Extra good SFC helps if the combined weight of the engine and fuel is lower for a given length of flight so more revenue can be earned at the same gross weight.
Or if the difference in fuel prices out weigh the loss of revenue.

 

[PWR4360-59B]

So what is the tip speed for a radial engine super charger? What is the tip speed for the compressor blades and even the fan in the old non geared turbo fan jets? Altitude less air that is what turbo chargers fix, and maybe even a new spin on a certain fuel additive that I'm not mentioning could also help over come altitude sickness. Engine speed? Nothing says a recip needs to spin up like a turbine does and also cuts down on the air requirements.

 

[Zipper730]

for example when P & W stuck a two stage fan on the JT3C engine to create the JT3D not only did take-off thrust go from 12,000lbs to 18,000lbs the specific fuel consumption at max continuous dropped from 0.76lbs fuel per pound of thrust per hour to 0.53 LB/LBT/hr.

It was that little? I figured it was more like 0.80.

This may be test stand data and not real world.

How does real world compare?

 

[Shortround6]

If the tips are inside a shroud or engine cover then the noise outside is reduced.

your last sentence shows you are having a little trouble understanding what is going on.

"Nothing says a recip needs to spin up like a turbine does and also cuts down on the air requirements"

Basically the amount of power you get is directly related to the amount of fuel you burn per unit of time (second or minute or hour) for the complete engine.
And the amount of fuel you can burn depends on the pounds (or KG) of air you can put through the engine in that amount of time.
Granted diesels don't use the same fuel/air ratio as gasoline engines but they close and not as different as some other fuels (like alcohol).

Basically the only way you can cut down on the air requirements is to cut down the power.
A WW II Allison that was running 10,000lb of air through the engine was making a theoretical 1672hp in the cylinders. After you take out for internal friction and the power needed to drive the supercharger you got 1285hp to the prop of an engine using the 9.60 gears. Using lower supercharger gears, if they still supplied 10,000lbs per hour, got you higher power to the propshaft from the same fuel/air burn.

Lower the RPM and lower the boost and less air goes through the engine and it makes less power.

The turbine T-56-A-15 engine of 4910ehp moved 32.4lbs of air per second or 116,640 lbs per hour.

A P & W PT-6A-65 of 1376shp moves 9.5lb a second or 34,200lb of air an hour. It has a max cruise rating 956shp and a fuel burn at the take-off rating of 0.506lb per hp hour. The PT-6A-65 weighs 463lbs dry and that is a big reason the reciprocating engine is not coming back in commercial aviation (or military). The Allison went about 1350lbs dry. Granted it is WW II technology but the chances of making a 1300hp for take-0ff reciprocating engine that weighs less than 500lbs and will last for thousands of hours is about zero.

See: Orenda OE600 - Wikipedia
for long tortured tale of trying to bring an aviation version of the old 1970s Can-AM race car engine to aviation use. It weighed 740lb dry for about 600hp take-off in one version (at 4400rpm).
Please note that the PT-6 target engine was not standing still.
from wiki" From 1963 to 2016 power-to-weight ratio was improved by 50%, brake specific fuel consumption by 20% and overall pressure ratio reached 14:1"
Impart due to competition from newer engines.

Turbos add weight, they add complexity.
Many turbines are simply flat rated. The fuel management system is set up to provide less than full power under standard temperature and altitude conditions so the official rating (and actual power of the engine) doesn't change under hot and high conditions, at leas up until a certain point.

you can't make a reciprocating engine big enough to replace even medium turbine engines without getting into monstrosities of well over 24 cylinders.

as far as fuel additives go, I have a bottle of specially tablets, just put one in your gas tank and add 10 gallons of water and you can drive your car 200 miles,only 200 bucks per bottle of tablets.

 

[gumbyk]

Turbos add weight, they add complexity.

And engine stresses.

About the only thing turbos reduce is time between overhauls.


Regarding spinning a prop at 10,000 RPM, I seriously doubt that we have the materials to withstand the stresses involved in developing such a prop. Even then, the consequences of a failure mean that it would completely destroy anything that it is attached to if it was unbalanced (which it would be after FOD ingestion).

 

[PWR4360-59B]

If the tips are inside a shroud or engine cover then the noise outside is reduced.

your last sentence shows you are having a little trouble understanding what is going on.

"Nothing says a recip needs to spin up like a turbine does and also cuts down on the air requirements"

Basically the amount of power you get is directly related to the amount of fuel you burn per unit of time (second or minute or hour) for the complete engine.
And the amount of fuel you can burn depends on the pounds (or KG) of air you can put through the engine in that amount of time.
Granted diesels don't use the same fuel/air ratio as gasoline engines but they close and not as different as some other fuels (like alcohol).

Basically the only way you can cut down on the air requirements is to cut down the power.
A WW II Allison that was running 10,000lb of air through the engine was making a theoretical 1672hp in the cylinders. After you take out for internal friction and the power needed to drive the supercharger you got 1285hp to the prop of an engine using the 9.60 gears. Using lower supercharger gears, if they still supplied 10,000lbs per hour, got you higher power to the propshaft from the same fuel/air burn.

Lower the RPM and lower the boost and less air goes through the engine and it makes less power.

The turbine T-56-A-15 engine of 4910ehp moved 32.4lbs of air per second or 116,640 lbs per hour.

A P & W PT-6A-65 of 1376shp moves 9.5lb a second or 34,200lb of air an hour. It has a max cruise rating 956shp and a fuel burn at the take-off rating of 0.506lb per hp hour. The PT-6A-65 weighs 463lbs dry and that is a big reason the reciprocating engine is not coming back in commercial aviation (or military). The Allison went about 1350lbs dry. Granted it is WW II technology but the chances of making a 1300hp for take-0ff reciprocating engine that weighs less than 500lbs and will last for thousands of hours is about zero.

See: Orenda OE600 - Wikipedia
for long tortured tale of trying to bring an aviation version of the old 1970s Can-AM race car engine to aviation use. It weighed 740lb dry for about 600hp take-off in one version (at 4400rpm).
Please note that the PT-6 target engine was not standing still.
from wiki" From 1963 to 2016 power-to-weight ratio was improved by 50%, brake specific fuel consumption by 20% and overall pressure ratio reached 14:1"
Impart due to competition from newer engines.

Turbos add weight, they add complexity.
Many turbines are simply flat rated. The fuel management system is set up to provide less than full power under standard temperature and altitude conditions so the official rating (and actual power of the engine) doesn't change under hot and high conditions, at leas up until a certain point.

you can't make a reciprocating engine big enough to replace even medium turbine engines without getting into monstrosities of well over 24 cylinders.

as far as fuel additives go, I have a bottle of specially tablets, just put one in your gas tank and add 10 gallons of water and you can drive your car 200 miles,only 200 bucks per bottle of tablets.

Yeah pretty funny stuff telling me what I know or don't know, thanks a bunch. So lets see turbines spin at what over 12k rpms, average recip 3k max.
The power is related to the type of fuel used and how well that fuel is used and not always how much is used to fuel a given device, meaning adding more of said fuel could blow the thing apart, I'll hear some crap over that one I'm sure. When it comes to alot of inventions and ideas there is a huge lack of thinking out of the box and why when the new invention comes along everyone says I wish I would have thought about that. Its funny how some like to battle an idea rather than think of how can it be done.

 

[gumbyk]

Yeah pretty funny stuff telling me what I know or don't know, thanks a bunch. So lets see turbines spin at what over 12k rpms, average recip 3k max.
The power is related to the type of fuel used and how well that fuel is used and not always how much is used to fuel a given device, meaning adding more of said fuel could blow the thing apart, I'll hear some crap over that one I'm sure. When it comes to alot of inventions and ideas there is a huge lack of thinking out of the box and why when the new invention comes along everyone says I wish I would have thought about that. Its funny how some like to battle an idea rather than think of how can it be done.

Recip speed limit is due to acceleration forces due to having a piston that has to change direction, which is partly due why small displacement engines can turn so fast.
Turbine engines have to have gearboxes to reduce that '12,000 RPM' to something useable. Turbines don't have much torque, which is what you really need to turn a prop. Propellers have a lot of differing loads on them normally, adding supersonic stresses is just asking for trouble. Yes, it can be done, but you'll lose efficiency in some part of the blade. Best to keep it all sub-sonic.

Regarding 'battling and idea rather than thinking how it can be done': There's no battling going on here - but plenty of us have experience enough to see the pitfalls in an idea. Spinning a prop at 12,000 RPM has plenty of pitfalls - failure forces are almost unimaginable.
Diesel engines are a classic case, no-one has doubted that they are a good idea for piston aircraft, but ask Thielert about the issues in designing a gearbox for one. You would think it was an easy thing to do, but it almost destroyed the company, and only being bought out saved them.

 

[Shortround6]

There is thinking outside the box and there is thinking outside the box but without specifics it is just so much blather to say "think outside the box."

What are we thinking off?
Ceramic engine parts?
Carbon fiber or other composite engine parts?
Unnamed and unknown fuel additives?
Parts made of unobtainium?

I am sorry that you think you have demonstrated clear thinking on this subject ( maybe you are thinking clearly but just not communicating it well) when the majority of your examples are so far removed from aircraft engines.
Fairbanks-Morse Diesel in use.

Not quite as far removed from aircraft use as a container ship engine but looking at such engines for comparison to aircraft engines leads not to thinking outside the box but into thinking that is removed from reality.

from
www.dieselduck.net
Yellow engine block is Caterpillar 3306 truck engine. Grey engine is a Fairbanks-Morse. It is a total red herring.
Junkers built opposed piston diesel engines for aircraft by the hundreds before and during WW II.

6 cylinder opposed piston 868hp at 2800rpm from 1312lbs. later versions got to 1000hp with a turbo charger and powered the JU-86P & R

high altitude recon machines. They may be old school technology but at least they aren't tugboat engines.
Napier had taken out a licence for them prior to WW II and after WW II developed them into the Napier Deltic 18 cylinder railroad and MTB engine.

From WIki so...."Development began in 1947 and the first Deltic model was the D18-11B, produced in 1950. It was designed to produce 2500 hp at 2000 rpm for a 15-minute rating; the continuous rating being 1875 hp at 1700 rpm, based on a 1000-hour overhaul or replacement life.[3] By January 1952 six engines were available, enough for full development and endurance trials. A captured German E-Boat S212[citation needed] was selected as it was powered by Mercedes-Benz diesels with approximately the same power as the 18-cylinder Deltics. When two of the three Mercedes-Benz engines were replaced, the compactness of the Napier engines was graphically illustrated—they were half the size of the original engines and approximately one fifth the weight."
Another source claims a power to weight ratio of 4.2lbs per HP in the Dark Class MTB boats but this mis-states things as it includes the clutch and gear box (both forward and reverse) for the marine engine. weight of the actual engine was closer to 2.5lb per HP. (marine gear boxes are not light)

Yes modern versions could have better power to weight ratios and better engine life with modern materials.
The question is where is the limit.

And if you think that those propfans from the 1980s actually rotated at 12,000rpm please tell me why they had that 13.54 to 1 gearbox in the system?
and try doing the math, 9ft diameter times 3.14 (pi) times 12,000 rpm divided by 60 sec is 5652fps which is 5 times the speed of sound at sea level.

How bout you come up with some modern examples?
There are some private/light plane projects like
Gemini :: Gemini 125
but they aren't very forthcoming about details.

 

[Shortround6]

Found this, just started wading through it.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900002423.pdf

this showed pretty early
"The propfan drive system was the Allison Model 501-M78. It was required that it be geared and controlled so that propfan tip speeds of 183 mps (600 fps), 213 mps (700 fps), and 244 mps (800 fps)could be tested. A fourth tip speed of 256 mps (840 fps) was desired."

So either we have a crap load of typographical errors or that prop was never intended to have supersonic tip speeds, at least at low altitude.

I was in error before, the gearbox, as used, was no longer 13.54 to one.

"The reduction gearbox for the PTA drive system was adapted from the Allison T56-A-14 gearbox that was used on the Lockheed P-3 Orion. This gearbox had two stages of reduction gearing and an overall gear ratio of 13.54:1. This produced an output shaft speed of 1020 rpm, whereas propfan rotational speed of the order of 1700 rpm was needed for the PTA application. For the PTA gearbox, the first stage gears were changed to produce an overall gear ratio of 6.8:1."

 

[PWR4360-59B]

What is the tip speed of turbo jet rotating parts?