I'm curious about how much thrust is produced by a turbocharger relative to horsepower? I do remember the XP-67 and XP-72 had such an arrangement where the turbocharger was pointed directly behind the engine and the exhaust would blow through the turbocharger and, what was left would shoot out the back.
Turbocharger Thrust
- Thread starter Zipper730
- Start date
Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
Till the turbo compound engines, the turbo charger only sufficed to keep the MP of the engine up to a desired value but did not provide shaft HP per se. The more energy extracted by the turbo charger to feed the engine, the less that could contribute to direct "thrust". Certainly variable via altitude, temperature and engine requirements. Probably not a huge addition.
Shortround6
Major General
A turbocharger is going to hurt thrust, this assumes the non-turbocharged engine had an efficient exhaust system in the first place..
Thrust is the weight of the exhaust gases times the velocity of the escaping gases. The weight will stay pretty much the same for a given engine and manifold pressure. The velocity of the escaping gases will depend on the air pressure in the system (higher altitude means lower pressure which means higher velocity) and how long it takes to get the exhaust to the nozzle/s.
The exhaust gas pressure also depends on temperature, as the gasses cool they shrink in volume and have less pressure. Long pipes between the exhaust port and the nozzle with give room for the gasses to expand and drop pressure and the exposure to the cooler pipe walls will lower the temperature/pressure.
As the gases go through the turbine of the turbocharger they do actual work and drop in temperature, this could be several hundred degrees F. It is one way the turbine works, higher pressure on the inlet side of the turbine vs lower pressure on the exhaust side. Exhaust temperature can be hundreds of degrees lower at the discharge of the turbo (with the consequential drop in pressure/velocity) than the exhaust gas temp/pressure at the end of a short (6-8in) stack.
Thrust is the weight of the exhaust gases times the velocity of the escaping gases. The weight will stay pretty much the same for a given engine and manifold pressure. The velocity of the escaping gases will depend on the air pressure in the system (higher altitude means lower pressure which means higher velocity) and how long it takes to get the exhaust to the nozzle/s.
The exhaust gas pressure also depends on temperature, as the gasses cool they shrink in volume and have less pressure. Long pipes between the exhaust port and the nozzle with give room for the gasses to expand and drop pressure and the exposure to the cooler pipe walls will lower the temperature/pressure.
As the gases go through the turbine of the turbocharger they do actual work and drop in temperature, this could be several hundred degrees F. It is one way the turbine works, higher pressure on the inlet side of the turbine vs lower pressure on the exhaust side. Exhaust temperature can be hundreds of degrees lower at the discharge of the turbo (with the consequential drop in pressure/velocity) than the exhaust gas temp/pressure at the end of a short (6-8in) stack.
The negative temperature changes of a gas when they are under reducing pressure are due to expansion. The gasses are undergoing what we call "Adiabatic" cooling. A way to think about temperature is a certain number of molecules are banging around in say a cc of volume. More molecules banging around, the temp will be higher. Of course unless they are somehow confined, they will tend to disperse and cool. Confinement might be in an exhaust system or having many thousands of feet of atmosphere sitting on top of them. This adiabatic lapse rate is why the thinner higher altitudes continue to cool till the tropopause. Charles and Boyles Gas laws a good reference. Also with regards to work and temperature changes examine the Carnot cycle.
Of course the value of the exhaust thrust loss by running through the turbo is made up by the lack of direct power loss from the shaft running through auxiliary super charger stages. In High Blower a R2800 "B" engine might use 400 HP in high blower, which was why it was rated at 1600 hp in high blower and not the SL 2000.
Of course the value of the exhaust thrust loss by running through the turbo is made up by the lack of direct power loss from the shaft running through auxiliary super charger stages. In High Blower a R2800 "B" engine might use 400 HP in high blower, which was why it was rated at 1600 hp in high blower and not the SL 2000.
swampyankee
Chief Master Sergeant
- 4,036
- Jun 25, 2013
In general (as in always), it is much better if one can utilize that exhaust energy to drive the propeller. This is why the Wright turbo-compounded the R-3350. Granted, that energy was unavailable without a blowdown turbine, so exhaust thrust was a way to utilize energy that would otherwise go to waste in the exhaust of a highly supercharged engines.
- Thread starter
- #7
I was just curious how it compared. For example I remembered that the thrust level on a supercharger at low altitude was about 1/10th the horsepower in pounds of thrust (1000 hp = 100 lbf), and around 1/5 at altitude (200 lbf). From what I remember the P-51B was producing around 150-300 pounds of thrust at different altitudes and speeds.
I'm curious how the reduction would compare from 1/10 to 1/20, or to 1/50 or so.
Good point, the exhaust system has to be properly designed.
Which lowers the temperature a few hundred degrees and also lowers the pressure/velocity in the process. If you were to estimate how much of a loss would occur compared to a turbo, assuming the turbine and exhaust nozzle went straight out the back, what would you get?
wuzak
Captain
The XP-72 did not have a turbocharger. It had a very large variable speed auxiliary supercharger mounted in the rear fuselage and driven by an extension shaft.
Republic XP-72 Super Thunderbolt / Ultrabolt Fighter
The XP-72's R-4360 engine used two-stage supercharging. The first stage was a mechanically-driven, variable-speed, remote supercharger positioned behind the cockpit, where the turbosupercharger was located on the P-47. To power the remote supercharger, a covered shaft extended from the unit, through the lower cockpit, and connected to the engine via a fluid coupling. The remote supercharger's impeller was around 3 ft (.9 m) in diameter []. The second stage was the standard supercharger that was integral with the engine.
Turbocharged R-4360s used two B-series turbos, as far as I can tell. In aircraft such as the XF-11, XF-12 and B-50.
The XP-47J had the turbocharger positioned so that the exhaust added thrust. Not sure if it made it into production versions, maybe the M and N?
The XP-67 definitely had the turbos mounted so the exhaust pointed rearwards, as well as the wastegate pipe.
Most turbo installations designed mid war onward had their waste gate and turbine exhaust pointing aft. The VS-326, AJ, XB-35, B-36, B-50, XF-12, XP-71 and C-97 all did. You can get thrust by doing this but it's mostly about reducing drag.
The XP-47J supposedly could make 100lbs of thrust at its critical altitude for WER power. The P-47J switched the positions of the turbo and intercooler so the turbo could be mounted more on its side.
Most R-4350 installations did have two turbos but the VS-326, B-50. C-97 and the XP-71 only had one.
Shortround6
Major General
A Hawker Hurricane II could get over 100lbs of thrust at anywhere near it's critical altitude in high gear at 12lbs (or close to it boost) from a smaller mass airflow from it's Merlin XX engine. Getting thrust from the turbo exhaust was doable, just don't expect large amounts of thrust even with big engine.
- Thread starter
- #11
100 lbf/2800 hp, and I assume that's at maximum speed
The Hurricane II could get about 10-12% of it's horsepower in pounds of thrust: Not sure the exact horsepower figures but for 1000 hp that would be 100-120 pounds of thrsut; 1300 hp would yield 130-156 pounds; If speed was increased to 350-375 mph you would see thrust figures go up to 103-132 for a 1000 horsepower engine; and 134-172 with 1300 horsepower.
The R-2800 would produce around 3.6% of it's horsepower in pounds of thrust, and that's at 490-505 mph, which would produce 2.6-2.8% of horsepower in thrust if slowed down to 350-375 mph (at this speed 1 lbf = 1 hp).
yulzari
Tech Sergeant
It is only the (inevitable) inefficiency that has turbochargers eject hot gas at high speed. It's task is to capture waste energy from the exhaust gas and use it to enhance the intake. If you can improve the efficiency it will have less energy left for thrust. In principle one could inject fuel into the turbocharger exhaust as reheat thrust but that is frighteningly wasteful of fuel. Modern jets try for unreheated thrust 'super cruise'§for this reason. Modern Formula One GP engines use the waste kinetic energy of the hot exhaust to run their pressure charged intakes and use the waste heat of the hot exhaust gases for conversion to electrical power for their battery power packs whist using the waste energy of braking for the same purpose.
So what I am saying is that thrust is not the way to go with turbochargers. Yes use it if you have spare energy but better to reduce thrust by better using the exhaust energy. In an unsupercharged or normally aspirated engine then the exhaust energy is being thrown away to rear facing ejector exhausts are free energy without the cost, weight and complexity of turbocharging. The ultimate (?) surely being the 2 stroke Rolls Royce Crecy.
So what I am saying is that thrust is not the way to go with turbochargers. Yes use it if you have spare energy but better to reduce thrust by better using the exhaust energy. In an unsupercharged or normally aspirated engine then the exhaust energy is being thrown away to rear facing ejector exhausts are free energy without the cost, weight and complexity of turbocharging. The ultimate (?) surely being the 2 stroke Rolls Royce Crecy.
YF12A
Airman
According to Wiki ?!?!, on the Republic XF-12 Rainbow, each R-4360 exhaust system from the turbos produced about 250 HP per engine, not too shabby. Beautiful plane, wish A/Airlines had bought them as my Dad would have had even more airline stories to tell me. As for the R-3350 turbo compound, I still remember the word "quill shaft" being repeated many, many times by Dad, can't remember why! Also, IIRC, the radiator design on the P-51 was very efficient as well.
wuzak
Captain
As altitude increases and propellers become less efficient, exhaust thrust becomes more important.
- Thread starter
- #17
I thought they used some of the exhaust thrust to also help lower the pressure on the aft side of the radiator and produce downforce too?yulzari said:
The Caproni-Campini design. From what I was told it wasn't a new idea, but the problem was that it wasn't a real substitute for a jet-engine
- You're burning fuel twice instead of once: First to drive the piston engine and propeller (which acts kind of like a compressor), followed by additional combustion in a dedicated combustion-chamber (which was really more of an afterburner).
- A jet only burns fuel once: In the combustion chamber, the turbine extracts power leaving a surplus to push the plane along, and driving the compressor, repeats the cycle
Which would amount to a little under 219 to a little bit over 234 pounds (a pound of thrust is greater than a horsepower at higher speeds), which comes out to around 7.3-7.8% thrust to horsepower at 400 mph @ 40,000 feet, and a horsepower addition of 8-1/3%. It's way better than the XP-47J but looking at the Hurricane IIB's (mentioned by Shortround6) 342 mph maximum speed: You would be left with around 187-204 pounds of thrust (about 6.23-6.81% thrust/hp) and around 170.6-199.7 horsepower (around 5.69-6.66% horsepower addition), though this presumes you're at 40,000 feet; go down to 20800 feet (the Hurricane IIC's critical altitude), you'd end up with about 77.9-85.1 lbf (around 2.6-2.84% thrust/hp), with 71.1 to 83.2 horsepower (2.37-2.77% horsepower addition) roughly produced.
With the R-4360's 3000 horsepower output producing roughly 10-12 percent thrust/horsepower you'd see 300-360 pounds of thrust which would correspond to around 262.5-337.5 (8.75-11.25% horsepower addition) horsepower at 342 mph provided it was geared for a critical altitude of 20800 feet.
Which means that for the same horsepower, the supercharger is more efficient because of the exhaust augmentation. At high altitude you would basically see a 20% boost in thrust/horsepower which means that using a well-designed twin-stage supercharger and exhaust system, using the R-4360's horsepower figures and the Merlin's high altitude performance figures
- Horsepower levels are approximately
- Military Rated: 3000 in low blower / 2540 high blower
- Horsepower and thrust additions to come for it (altitude/supercharger-setting/horsepower/thrust/thrust-percentage)
- 0'...........Low-Blower...3000...300.0...10.0%
- 5000'.....Low-Blower...3000...360.5...12.0%
- 10000'...Low-Blower...3000...436.5...14.6%
- 15000'...Low-Blower...3000...531.8...17.7%
- 25000'...High-Blower.-2540...683.6...26.9%
- 29800'...High-Blower.-2540...848.4...33.4%
Last edited:
The reason turbochargers had a limit to the amount of boost and exhaust extraction they could extract was heat and equally importantly, RPM. Lubricating and even constructing very highly balanced mechanisms that turn at very high RPM's has it's limits. Years ago I spent some time shuttling P&W 4000 series engines from the factory to Europe. The reason? Bearing failures. Very high RPM mechanisms such as turbine engines require special precision bearings. The engines were delivered a short distance away from the factory and loaded carefully on the plane in special shock resistant cradles. The issue had been that truck transport over bumpy roads had flat spotted these close tolerance bearings, leading to early failure.
Props can be designed for relatively high speed and altitude (Mach limits being regarded) but will, (constant speed features not withstanding) be optimized for some Reynolds number and advance ratio.
To achieve a given IAS a certain thrust to overcome drag is necessary. Eventually speed equilibrates as the two values equal. The various 747's I flew cruised along nicely at about the same IAS at SL (Sydney once requested 350 kts on downwind...) as at altitude. The only reason we were efficient at all was that the drag and thrust, though decreasing about the same with altitude, let us have a much higher TAS with much lower fuel burn.
That plane was critically optimized for a limited range of Mach cruise at altitude. M.86 (for a 400 or -8) was much more efficient than say M.84! At M.90 it would shake a little and you could see the shockwave standing up about ten feet inside the winglet.
It was because of their turbocharger work that GE was selected to do early jet engine work in the US.
Props can be designed for relatively high speed and altitude (Mach limits being regarded) but will, (constant speed features not withstanding) be optimized for some Reynolds number and advance ratio.
To achieve a given IAS a certain thrust to overcome drag is necessary. Eventually speed equilibrates as the two values equal. The various 747's I flew cruised along nicely at about the same IAS at SL (Sydney once requested 350 kts on downwind...) as at altitude. The only reason we were efficient at all was that the drag and thrust, though decreasing about the same with altitude, let us have a much higher TAS with much lower fuel burn.
That plane was critically optimized for a limited range of Mach cruise at altitude. M.86 (for a 400 or -8) was much more efficient than say M.84! At M.90 it would shake a little and you could see the shockwave standing up about ten feet inside the winglet.
It was because of their turbocharger work that GE was selected to do early jet engine work in the US.
- Thread starter
- #19
So they didn't run as hot as early gas-turbines?fliger747 said:
Sure, and once thrust and drag equal, acceleration stops.
I'd like to hear the story behind that one...
And I'm guessing the combination of increased ram compression and the aircraft's speed and exhaust velocity coming closer together (as well as the ratio of the exhaust temperature to the outside air temperature) yield the differences in maximum speed at altitude and at sea-level?
I didn't know the 747-400 had a different optimum cruise than the 747-100/-200/-300.
While this probably sounds redundant: The L/D ratio is higher at 0.86 than 0.78 for the same indicated airspeed? I know that L/D ratios sharply drop when supersonic, but at high subsonic speeds you start seeing supercritical flow along the wings. There seems to be suction in this supersonic zone as long as a shockwave hasn't formed ahead of it. If the shockwave is small, there would be no real drag-divergence.
I do remember a person who flew 747-200's
I flew every 747 type except the XP 100-200-300-400 intercontinental-8 and the Dreamlifter. Hard to generalize about the 200's with different engines and avionics setups. All of the 400's and -8's I flew had a common cockpit setup. The main change between the 200 and 400 series besides engines, avionics, nav capability and on and on was what I believe was a change in the angle of incidence of the wing. The 400 had a much flatter deck angle in cruise and on landing approach. The wing was much different and an advance, but with the flatter cruise attitude I believe that there was less mach related drag at cruise from the fuselage. We one time flew from Alamaty to JFK nonstop (over Moscow) with low winter temps (low fuel temp warning) and ended up flying at a M.87 cruise rather than the flight planned M.84 and cut 45 mins off the flight time and saved maybe 10 tons of fuel? I think the "hump" was quite a drag producer at higher AOA values.
The 200 always felt doggy in cruise and the 100 very much so. The one I flew we called "Miss Piggy". A little more power can really make an airplane!
The 200 always felt doggy in cruise and the 100 very much so. The one I flew we called "Miss Piggy". A little more power can really make an airplane!
