From manpower to horse power (staying inline or going radi(c)al?)

[wuzak]

The top one's debateable and it depends on what you are trying to do and how high you are trying to do it at. The bottom one, what you're not saying is that that "40 percent efficiency" compares favourably to piston engines.

Yes, I should have mentioned that 40+% efficiency is much better than the vast majority of piston engines.

In my mind I am comparing to car engines, as some of them are quite efficient. Though the very best are only 40% efficient, excluding specialised engines like current F1 engines (~50% efficiency).

I'm not sure about general aviation piston engines. Aren't a lot of them 1950s designs?

Let's look at a real world example using a small gas turbine; the PT-6 weighs around 300lbs give or take, yet the most powerful variants can produce power outputs equivalent to 1,900hp, compare that with your average 1,900 hp piston engine. The PW Twin Wasp for example weighs more than three times the PT-6, so using that specific example, make a comparison between the Bazler Turbo 67 and a regular DC-3. The PT-6 powered version is faster in the climb, cruise and maximum speeds, it can carry a heavier load across a greater distance at a higher height than the regular DC-3. Now some of that efficiency comes from modern props etc, but it plainly illustrates that small gas turbines are very efficient, more so than their equivalent in similar power output piston engines. Again though, its about application. Gas turbines are far more efficient at altitude.

When I said small turbines, I meant those equivalent to a regular car engine i.e. in the 200-300hp range. Technically, I suppose, these would be microturbines.

In terms of big WW2 era piston engines, the Merlin was ~20% efficient at maximum power. Much less than the PT-6 of equivalent power (and weighing about 6 times as much).

 

[pbehn]

Yes, I should have mentioned that 40+% efficiency is much better than the vast majority of piston engines.

In my mind I am comparing to car engines, as some of them are quite efficient. Though the very best are only 40% efficient, excluding specialised engines like current F1 engines (~50% efficiency).

I'm not sure about general aviation piston engines. Aren't a lot of them 1950s designs?




When I said small turbines, I meant those equivalent to a regular car engine i.e. in the 200-300hp range. Technically, I suppose, these would be microturbines.

In terms of big WW2 era piston engines, the Merlin was ~20% efficient at maximum power. Much less than the PT-6 of equivalent power (and weighing about 6 times as much).

I think it depends on what they are doing. Concorde was quite efficient when cruising at mach2, it used much of its fuel getting up to Mach 2 at 60,000ft. Same for a wide body jet, it can cruise at much higher speeds and altitudes than a piston engine can dream about.

 

[nuuumannn]

When I said small turbines, I meant those equivalent to a regular car engine i.e. in the 200-300hp range. Technically, I suppose, these would be microturbines.

The problem we have here is equivalent units of measurement. In turboprop and turboshaft engines you can make such comparisons, but with jet engines, since their output is measured in pounds thrust, a force as opposed to horsepower, and I for the life of me don't know how to convert the two, it involves velocity of the object as well, so too hard off the top of my head. What I do know is that small turboshaft engines powering light helicopters have similar power outputs to around 250 to 300 shp, like Allison 320s and so forth, but there aren't very many light gas turbines developed for aeroplane use in this size on the market, there are a few under development, though. The lowest power outputs in the PT-6 family are about 500 shaft horsepower. Considering that the RR Trents can put out up to 100,000 lbs thrust, the PT-6 is a small engine.

This page on the Trent from RR states an equivalent of 50,000 horsepower for each Trent XWB in the Airbus A350.

Trent XWB

Trent XWB

www.rolls-royce.com

 

[nuuumannn]

I think it depends on what they are doing.

Correctamundo. The RR/Snecma Olympus 593 is perhaps not the best example to illustrate efficiency, but at height and cruise, far more efficient than on take-off, and yes, what gas turbines offer in terms of performance, range and height compared to pistons is beyond compare, particularly when time is factored in as an efficiency. I read briefly that big diesel motors aboard ships are very fuel efficient when it comes to load factors and range etc, but factor in speed/time and they are left behind in terms of overall efficiency.

 

[GregP]

About the bore an stroke limits, I have to chime in.

Consider the Wartsila-Sulzer RTA-96C Marine Engine. It has a bore of 37.9 inches (96.27 cm) and a stroke of 98.518 inches (250.23 cm). The 14-cyliner version has a displacement of 1,556,002 cubic inches (25.499.9 l). It makes 108,920 hp at 102 rpm. It is 3 stories tall, and many are running as you read this on the world's oceans, driving really big ships. This is a 2-sroke diesel engine. Weight is 4,600,000 pounds (2.086.514 kg).

You can see two people on the lower yellow railing above, below the diagonal ladder. We have a 3-story tall engine here.

For airplane engine, the words biggest piston engine was the Lycoming XR-7755. Bore was 6.4 inches (162 mm), stroke was 6.8 inches (171 mm). It had 36 cylinders and displaced 7,755 cubic inches (127 l). It made 5,000 hp at 44" MAP at 2,600 rpm. Dry weight was 6,050 lbs (2.744 kg). It was basically nine 4-cylinder engines connected to a common crankshaft in one case. As I said above, this was the largest aircraft radial engine.

XR-7755 above.

If you just want the largest radial, you have the Zvezda M503 42-cylinder radial for tractors. It has a bore of 6.299 inches (16 cm), a stroke of 6.693 inches (17 cm), and displaces 8,760.5 cubic inches (143.6 l). If makes 6,035 hp at 2,500 rpm. Weight is 8,378 pounds (3.800 kg).

Wish there was a man in there for scale. They DID make a 56-cylinder version, the M520, but they only made one as far as I know.

I'm not saying pbehn's post above was wrong, because the vast majority of aviation engines follow what he said about bore and stroke above. But the limits he mentioned are likely for aviation-related piston engines only. If it doesn't have to fly, it can be a big as a small house and still be a viable engine. The aircraft engine have to be concerned with lightness, unlike nonflying engines.

 

[SaparotRob]

Does one of those engines listed above really produce over 100,000 hp at only 102 rpm?

 

[pbehn]

About the bore an stroke limits, I have to chime in.

Consider the Wartsila-Sulzer RTA-96C Marine Engine. It has a bore of 37.9 inches (96.27 cm) and a stroke of 98.518 inches (250.23 cm). The 14-cyliner version has a displacement of 1,556,002 cubic inches (25.499.9 l). It makes 108,920 hp at 102 rpm. It is 3 stories tall, and many are running as you read this on the world's oceans, driving really big ships. This is a 2-sroke diesel engine. Weight is 4,600,000 pounds (2.086.514 kg).

View attachment 649168
You can see two people on the lower yellow railing above, below the diagonal ladder. We have a 3-story tall engine here.

For airplane engine, the words biggest piston engine was the Lycoming XR-7755. Bore was 6.4 inches (162 mm), stroke was 6.8 inches (171 mm). It had 36 cylinders and displaced 7,755 cubic inches (127 l). It made 5,000 hp at 44" MAP at 2,600 rpm. Dry weight was 6,050 lbs (2.744 kg). It was basically nine 4-cylinder engines connected to a common crankshaft in one case. As I said above, this was the largest aircraft radial engine.

View attachment 649169
XR-7755 above.

If you just want the largest radial, you have the Zvezda M503 42-cylinder radial for tractors. It has a bore of 6.299 inches (16 cm), a stroke of 6.693 inches (17 cm), and displaces 8,760.5 cubic inches (143.6 l). If makes 6,035 hp at 2,500 rpm. Weight is 8,378 pounds (3.800 kg).

View attachment 649170
Wish there was a man in there for scale. They DID make a 56-cylinder version, the M520, but they only made one as far as I know.

I'm not saying pbehn's post above was wrong, because the vast majority of aviation engines follow what he said about bore and stroke above. But the limits he mentioned are likely for aviation-related piston engines only. If it doesn't have to fly, it can be a big as a small house and still be a viable engine. The aircraft engine have to be concerned with lightness, unlike nonflying engines.

Thats basically what I meant, there was a convergence for all sorts of reasons around a similar bore and stroke for that type of engine, the XR 7755 was only slightly larger, cooling was one issue and the XR 7755 had banks of 4 not six as in most V engines.


My father drove Napier Deltic trains a few times. They were originally a marine engine cant really be measured like others.
The Napier Deltic had a 5.125 in (130 mm) bore and a 7.25 in (184 mm) stroke (x2). This gave each cylinder a displacement of 299 cu in (4.9 L), and the 18-cylinder engine displaced 5,384 cu in (88.2 L).

 

[pbehn]

Does one of those engines listed above really produce over 100,000 hp at only 102 rpm?

You can literally climb inside the cylinders, and if the head was on many people would struggle to touch the valves while stood on the piston at the bottom of its stroke.

 

[Howard Gibson]

Let's look at a real world example using a small gas turbine; the PT-6 weighs around 300lbs give or take, yet the most powerful variants can produce power outputs equivalent to 1,900hp, compare that with your average 1,900 hp piston engine. The PW Twin Wasp for example weighs more than three times the PT-6, so using that specific example, make a comparison between the Bazler Turbo 67 and a regular DC-3. The PT-6 powered version is faster in the climb, cruise and maximum speeds, it can carry a heavier load across a greater distance at a higher height than the regular DC-3. Now some of that efficiency comes from modern props etc, but it plainly illustrates that small gas turbines are very efficient, more so than their equivalent in similar power output piston engines. Again though, its about application. Gas turbines are far more efficient at altitude.

I am discussing thermal efficiency. How much of the chemical energy in the fuel is converted to useful work? You are talking about power to weight, which of an advantage of turboprops. This is a different form of efficiency.

I was told in college that very large steam turbines have thermal efficiencies of around 40%. On large turbines, the clearances over the blades/nozzles is relatively smaller, resulting in fewer losses. Someone above noted a turbofan with 90% bypass. I accept that turbofans are fairly efficient, but I don't see how bypass directly affects efficiency.

I was surprised when I saw the 30% efficiency for a WWII piston engine. The Engines of Pratt & Whitney: A Technical History, by Jack Connors, has a pie diagram showing about 20% efficiency for a piston engine at military power. I was told that cars are down around 15%. There are advantages to making a piston engine, or any other engines, at a steady state, at some predictable power output.

 

[Howard Gibson]

The problem we have here is equivalent units of measurement. In turboprop and turboshaft engines you can make such comparisons, but with jet engines, since their output is measured in pounds thrust, a force as opposed to horsepower, and I for the life of me don't know how to convert the two, it involves velocity of the object as well, so too hard off the top of my head.

Power equals force times velocity. One horsepower equals 550ft.lb/sec or 33000ft.lb/min. One wall equals 1N.m/s. Run away screaming from metric horsepower.

 

[pbehn]

I am discussing thermal efficiency. How much of the chemical energy in the fuel is converted to useful work? You are talking about power to weight, which of an advantage of turboprops. This is a different form of efficiency.

I was told in college that very large steam turbines have thermal efficiencies of around 40%. On large turbines, the clearances over the blades/nozzles is relatively smaller, resulting in fewer losses. Someone above noted a turbofan with 90% bypass. I accept that turbofans are fairly efficient, but I don't see how bypass directly affects efficiency.

I think in thermal efficiency, a high bypass engine produces a large volume of air with a lower average temperature and speed for the same thrust as an axial flow no bypass engine that produces a lot of hot air and noise as well as thrust, this makes the high bypass engine more efficient at the speeds used by commercial airlines.

 

[SaparotRob]

You can literally climb inside the cylinders, and if the head was on many people would struggle to touch the valves while stood on the piston at the bottom of its stroke.

Make a great form of capital punishment!

 

[pbehn]

Make a great form of capital punishment!

I just looked it up on wiki, a piston weighs 5.5tons and at 102 RPM it is accelerating from 0 to 30MPH then coming to rest again more than 3 times per second.

 

[SaparotRob]

It amazes me how much stress engine components take and still function.

 

[brianwnz]

Did they have an inkling about hollow turbine blades or did that come out of trial and error?

The attached paper seems to actually concern piston-engine turbocharger blades, rather than gas turbines, but the same technology is involved I imagine.

Interestingly, this paper seems to point to weight (from rotating centrifugal force) being partly behind the push for hollow blades.

Haven't read it all, but sharing it for those that want it....... I stumbled across it in the never ending search for diesel aircraft engine obscurities

 

[nuuumannn]

I am discussing thermal efficiency. How much of the chemical energy in the fuel is converted to useful work? You are talking about power to weight, which of an advantage of turboprops. This is a different form of efficiency.

Yeah, but in the real world of air operations and their returns, which is going to matter more? Your average aeroplane operator couldn't give a rats about the thermal efficiency of their powerplants, only whether or not their aircraft selection made financial sense. Depending on the operation, aircraft choice is a careful consideration and type of powerplant which determines performance, which affects the bottom line, and let's face it, in the cut and thrust of the air operator industry that's what really matters, impacts everything.

As mentioned earlier, the type of operation determines the aircraft/powerplant choice. Gas turbines offer enormous operational efficiencies that pistons can't touch in terms of the bulk of air operations these days.

 

[nuuumannn]

was told in college that very large steam turbines have thermal efficiencies of around 40%. On large turbines, the clearances over the blades/nozzles is relatively smaller, resulting in fewer losses. Someone above noted a turbofan with 90% bypass. I accept that turbofans are fairly efficient, but I don't see how bypass directly affects efficiency.

Let's keep it simple and stick to gas turbines powering aircraft.

Regarding the bypass ratio, it contributes to efficiency because of the work done producing thrust. I mentioned that 90 percent of all thrust produced by a RR Trent hi bypass engine is produced by the fan at the front. That represents an enormous efficiency in terms of fuel load.

 

[Howard Gibson]

Let's keep it simple and stick to gas turbines powering aircraft.

Regarding the bypass ratio, it contributes to efficiency because of the work done producing thrust. I mentioned that 90 percent of all thrust produced by a RR Trent hi bypass engine is produced by the fan at the front. That represents an enormous efficiency in terms of fuel load.

You burn fuel, heat air and expand it through a turbine. The turbine drives the compressor out front, and a fan if there is one. You get thrust out the rear nozzle. The fan uses power from the turbine, which means it is not available to the nozzle at the rear. Ducted fans are not magic, and there is no free lunch. It sounds like the fan is more efficient than the jet nozzle, but how much?

 

[nuuumannn]

Ducted fans are not magic, and there is no free lunch. It sounds like the fan is more efficient than the jet nozzle, but how much?

Who said anything about free lunches? Yum yum!

Using the fan to produce thrust makes far more sense and is far more fuel efficient than burning raw fuel to achieve the same amount of thrust. RR, P & W and GE have invested billions into figuring this stuff out so we don't have to. The current line of experimentation is variable pitch 1st stage fan blades.

Tell ya what, do some research if you don't believe me. Go on to Google Scholar, read Rolls-Royce's book The Jet Engine, go buy some gas turbine texts.

 

[pbehn]

You burn fuel, heat air and expand it through a turbine. The turbine drives the compressor out front, and a fan if there is one. You get thrust out the rear nozzle. The fan uses power from the turbine, which means it is not available to the nozzle at the rear. Ducted fans are not magic, and there is no free lunch. It sounds like the fan is more efficient than the jet nozzle, but how much?

from wiki Turbofan - Wikipedia


Efficiency

Propulsive efficiency comparison for various gas turbine engine configurations

Propeller engines are most efficient for low speeds, turbojet engines – for high speeds, and turbofan engines – between the two. Turbofans are the most efficient engines in the range of speeds from about 500 to 1,000 km/h (270 to 540 kn; 310 to 620 mph), the speed at which most commercial aircraft operate.[12]​[13]​

In a turbojet (zero-bypass) engine the high temperature and high pressure exhaust gas is accelerated by expansion through a propelling nozzle and produces all the thrust. The compressor absorbs all the mechanical power produced by the turbine. In a bypass design extra turbines drive a ducted fan that accelerates air rearward from the front of the engine. In a high-bypass design, the ducted fan and nozzle produce most of the thrust. Turbofans are closely related to turboprops in principle because both transfer some of the gas turbine's gas power, using extra machinery, to a bypass stream leaving less for the hot nozzle to convert to kinetic energy. Turbofans represent an intermediate stage between turbojets, which derive all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases (typically 10% or less).[14]​ Extracting shaft power and transferring it to a bypass stream introduces extra losses which are more than made up by the improved propulsive efficiency. The turboprop at its best flight speed gives significant fuel savings over a turbojet even though an extra turbine, a gearbox and a propeller were added to the turbojet's low-loss propelling nozzle.[15]​ The turbofan has additional losses from its extra turbines, fan, bypass duct, and extra propelling nozzle compared to the turbojet's single nozzle.[clarification needed]​