Turbo Compound Piston Engines. Almost magic tech.

greetings this is Greg the World War II era ushered in some incredible piston engine technology stuff that had been considered very exotic in the previous decades ended up in mass production during the war Paving the way for it to enter mainstream applications today a lot of this technology is commonplace things like superchargers turbochargers intercooling double overhead cam shafts use of four valves per cylinder fuel injection both direct and throttle body types and much more in fact even the most boring car company in the world uses most of this stuff however some World War II Aerotech that did work never made it into the mainstream and or faded away with the dawn of the jet engine one such bit of Technology was turbo compounding which utilized a power recovery turbine this device was so amazing that at first glance it appears to defy the law of physics of course it doesn't and I'll get into that but it really was a near Miracle device increasing horsepower without an increase in fuel consumption this allowed aircraft using this technology to take off at higher weights and a greater percentage of that weight could be Revenue generating payload not just extra fuel needed for a long flight that was a big deal and to understand why and to really understand the need for this we have to back up in time a bit during the second world war the best L hul airplane was probably the Douglas DC4 I suppose there could be some debate about that but the DC4 was certainly the most successful plane of this type from World War II there were of course military versions like the c-54 seen here during the Berlin airlift which took place in 1948 and 1949 now the DC4 was a fine airplane it was a huge step forward from the DC3 Not only was it much bigger as you see here but it was far more capable in terms of speed payload and range the DC4 could cross the Atlantic well sort of I I need to clarify that a bit what does it mean exactly to fly across the Atlantic would a flight from Greenland to Ireland qualify well maybe but it wasn't going to be commercially viable and nobody was going to offer prize money for being the first to fly from Greenland to Ireland in fact it's still not a commercially viable routee at the time of this video's production there are no Direct commercial passenger flights from Greenland to Ireland Charles Lindberg was very famous for flying non-stop New York to Paris not specifically for crossing the Atlantic by some definitions Atlantic Crossings had been accomplished by airplanes quite a few times at that point Lindberg won the ortigue prize for his famous flight a prize offered by a New York Hotel owner not for crossing the Atlantic but for the first nonstop flight from New York to Paris Raymond ortigue the man who offered the prize seen here with Lindberg understood the commercial significance of that route after all again he owned a New York hotel and he was very plugged into the aviation world when he offered prize money for the first non-stop flight New York to Paris he knew exactly what he was doing back to our DC4 yes it could cross the Atlantic but it was an unpressurized airplane which meant it was down below 10,000 ft most of the time and often stuck in the clouds and the associated turbulence in bad weather rather than above it in order to carry a decent payload it usually depending on the winds had to make at least one stop eastbound to Europe and at least two stops coming back to the United States after World War II Air France had a Paris to New York Route using the DC4 it took two stops and 23 hours and that was if nothing went wrong the US Air Force was flying from Frankfurt Germany to New York and back the westbound legs required two stops as a minimum the route was Frankfurt Germany to Shannon Ireland from there over to Gander newfinland Gand exists at least exists as a town in an airport largely because it's located on the great circle route from New York to London thus was a common refueling point in the old days from there it's on to New York's Idle Wild airport which is of course now called JFK the fuel critical leg on this trip was the Shannon to Gander leg often based on weather they had to Route the flight from Shannon to kfic Iceland instead and then on to Gander making this a a three-stop affair this would add a lot of time but would not increase the plane's payload because the alternate airport requirements at the time meant that the plane had to carry enough fuel to fly all the way back to Scotland if it couldn't land at keik due to weather or for whatever reason the point of all this is to make it very clear that crossing the Atlantic in a DC4 was a complicated Affair and the plane's payload was compromised because it had to carry so much fuel to do it I want to point out that fuel requirements for long overwater flights are a big deal in aviation it's not just a question of calculating time and route or range or whatever I'm going to simplify this but you have to have enough fuel to get to the destination accounting for headwinds or whatever you're dealing with and then to an alternate airport and still have some amount of Reserve fuel additionally this has to be planned for a worst case type of system failure like the loss of an engine which will reduce the plan's range so planes have to take a lot of fuel when crossing the ocean in most cases more fuel means less Revenue generating payload there's a pretty good movie that deals with this exact topic called the high and the mighty it's an interesting movie for a number of reasons first of all it's surprisingly accurate in regards to Aviation stuff second it stars John Wayne at a time when he was a big star yet he isn't exactly the star of this movie he's just another cast member I'm sure there has to be a story behind that because movie stars of his magnitude don't normally agree to roles where they're not clearly the main character anyway the movie is about a DC4 flying from Hawaii to San Francisco the plane suffers an emergency which puts them into a critical fuel situation if you're into vintage Airline stuff it's worth watching just for all the DC for action but it's also a pretty decent movie in most most other respects as well it's also worth noting that the plane in the movie has a crew of five and only 17 passengers that's historically correct for that plane on that route PanAm had 70 seat DC 4S and there were some airlines that packed in even more but on that route Hawaii to sanfran with the fuel requirements they couldn't put a lot of people on and baggage in the case of PanAm they Norm used a 30 seat DC4 for these sort of routes and very often they were weight limited and thus couldn't fill all of those seats so what was needed for these longer routes was a bigger airplane and with pressurization so it could operate above the weather Along Came the Douglas dc6 and Lockheed Constellation both of which were quite a bit faster than the older DC4 and these newer planes were pressurized as a general rule in the North Northern Hemisphere eastbound Plains will have a Tailwind West bounds will have a headwind so going up high allowed the new pressurized airliners to get up into those Tailwinds on flights from North America to Europe although on the return trip they would usually have a headwind the result was that they could now usually fly from the US to Europe nonstop but would still need to land and refuel at Gander on the return trip crossing the United States was a similar story these planes could normally fly non-stop from the west coast to the east coast but going the other direction headwinds would require a fuel stop the dc6 and the early locked constellation were fairly comparable the new version of or I should say the version of the constellation I'm talking about right now is the L 649 version this was the first version built specifically as a civil airliner and both this plane and the dc6 went into into production in 1946 maybe early 47 for the Douglas plane the constellation was a very expensive airplane to build its fuselage was shaped in such a way that its diameter was constantly changing along its length this complicated construction but was also aerodynamic the triple tail was designed to allow it to fit into existing Airline hangers at the time that's quite different than the reason for twintails often seen on bomber aircraft I have a video on that subject ironically the constellation looks a lot like the original DC4 prototype seen in this picture Douglas determined that this design would be too costly to produce and thus couldn't succeed commercially they sold this plane to the Japanese in 1939 Nakajima ended up with it and created the shinsen bomber from it there's also a bit of Cloak and Dagger to the story too the Japanese pretended that the plane crashed to remove suspicion that they might be using it to develop a foreign engine bomber the US not only believed the story but bragged about it in a sort of maob way back to our main topic Douglas went forward with another design the one which was to become the DC4 we know today back to the dc6 and early constellation they were very capable compared to the DC4 and way ahead of the even earlier DC3 but more capability was needed especially for the highly desired non-stop East Coast to West Coast us routes enter the Douglas dc7 along with the Lockheed 1049 super constallation and later the 1649 Starliner I talked a bit about the Starliner in an earlier video you can see here I'll put links for the other videos I mention in the description but for this video I'll focus more on the dc7 what Douglas needed for their new dc7 and what locky needed for their newest constellations was some way to increase fuel efficiency and increase it a lot that would not only improve range allowing West Coast to East Coast us trips Non-Stop and easier Crossings to Europe or Hawaii it would allow for higher payloads due to greater power available for takeoff and reduced fuel requirements remember fuel weighs 6 lounds a gallon that's a lot every gallon you don't need to take means more Revenue generating payload in most cases now increasing payload via conventional methods of increasing horsepower was possible bigger engines more manifold pressure those sort of things can get the plane to lift more but on a long flight The increased fuel consumption will partially offset that benefit Along Comes turbo compounding and it does exactly what they needed it gave an increase in power without an increase in fuel consumption or you could throttle back and enjoy even more fuel savings to do this the system takes energy from the exhaust and directs it to a turbine now that part isn't new turbo superchargers had been used on airplanes for a long time by this point the turbo supercharger usually called a Turbocharger these days uses exhaust energy to spin a turbine which is connected to a compressor which is used to raise manifold pressure that increase in manifold pressure can allow for a tremendous increase in power but at a cost in fuel consumption and with quite a bit of additional stress on the engine a Turbocharger is a wonderful device I love them but it's not something that can help with fuel consumption in the type of application we're talking about right now the solution was turbo compounding which is totally different from turbocharging turbo compounding was used on the later constellations and the dc7 as well as some other planes you can see here to explain how this works I'm going to have to go through it from start to finish I'll be using two book booklets from Curtis Wright as well as this one from Delta Airlines and a report from NAA Now Delta Airlines love the dc7 in fact they still have one at their Museum in Atlanta I was hoping to get to it before I made this video but it just never happened uh if this is the type of content you enjoy please like And subscribe also please consider joining my patreon supporters there get access to all the manuals I used in creating these videos which in this case means the Delta dc7 book the Curtis W manuals the US Army Air Force c54 pilot manuals a relevant NAA report and much more back to our story you might notice that Curtis Wright refers to these setups as turbo compounding systems that term is technically correct but easily confused with compound turbochargers which are another thing entirely so to be clear we are talking about turbo compounding here's how it works in this case the engine itself is a wri r 3350 duplex cyclone in World War II an earlier version of this engine without turbo compounding was used on the b29 super Fortress by the time the dc7 came along the r-3350 was a pretty good engine it's air cooled with 18 cylinders and is of course supercharged in the dc7 each R 3350 has three power recovery turbines each one is driven by the exhaust from six of the 18 cylinders those groups of six are manifolded into pairs thus two cylinders provide exhaust for one of the three entries into each of the three turbines the diagram that you see here is from the Delta book I'm going to clear off the text and put numbers on it to make it easier to go through I'll go clockwise starting at the arrow location one is one of the 18 cylinders on the r 3350 location two is the exhaust pipe going from that cylinder to the power recovery turbine remember we're just looking at one cylinder and one turbine here but this cylinder is one of six driving that turbine and one of two blowing in at the point shown location three is the power recovery turbine itself exhaust velocity at this point is making that thing spin more on that later that rot ational force is transmitted via a shaft to the bevel gears at location four at position five we have a fluid coupler pinion gear and crankshaft drive gear I'll come back to that fluid coupler in a moment at position six we have the crankshaft as you can see the entire system is set up to take energy from the exhaust and mechanically put it back directly into the crankshaft now the fluid coupler is there for several reasons exhaust reaches the turbine in pulses causing a torsional vibration in the turbine drive system the fluid coupling absorbs this vibration and prevents it from being delivered to the crankshaft but vibration can go both ways so the fluid coupling also prevents engine vibration from reaching the turbine the fluid coupling does two more things it handles inertial loads during changes in engine speeds and it lightens the load on the engine starter motor the fluid coupler is operated by engine oil which drains out of the coupler after the engine is shut down thus the power recovery turbine is essentially disconnected until oil pressure refills it that way the starter motor doesn't have an extra load on it from the turbine during engine start all of this is great but of course there is some price to pay for that fluid coupler but it's not much the fluid coupler causes about a 1 to 2% loss as compared with the director drive that loss is of course due to slippage so how well does all this work well here's a chart from Curtis wri the three turbines per engine combine to add over 450 horsepower on takeoff actually a bit more and about 200 horsepower each at Cruise that's remember that's per engine that's a lot of extra horsepower especially when you consider that the engine doesn't burn any extra fuel to deliver it I want to stress that it's not just the increase in power that's important here it's the increase in power without an increase in fuel consumption and with no or minimal additional stress on the engine itself so how does it manage this at first glance this would appear to be taking power from the engine via exhaust pressure turning it into rotational energy via the turbine and then putting that into the crankshaft the mechanism itself has to have some inefficiency in it so how does this result result in a net gain well just for a moment for a moment think of this thing as if it's a Turbocharger or at least the exhaust side of a Turbocharger in a Turbocharger there would be high exhaust pressure at position two plus there would be the resulting heat built up between the turbine and the exhaust valve at the cylinder this exhaust back pressure costs the engine power hopefully if you deal with turbochargers this isn't a surprise to you but but it does in fact take power to drive a Turbocharger in fact it takes quite a bit NAA has reports on this specifically nothing is free including the energy needed to spin a Turbocharger I cover this topic with the evidence from NAA in this video let's get back to this picture for a moment at position two we have the exhaust pipe for this cylinder if this was a turbocharged engine then pressure in that pipe would typically be at least equal to manifold pressure and in all cases it will be above atmospheric pressure if Boost is being created 20 lbs of boost in the intake manifold would typically mean at least 20 lbs of exhaust pressure in this pipe very often it's much higher there are turbocharged cars out there with exhaust back pressure more than double that of intake pressure and this exhaust back pressure cost horsepower of course the increase in manifold pressure you get from the turbo gives you back far more power than it costs to drive it via the ability to burn more fuel turbochargers are actually very efficient devices in that respect so if the pressure is higher at this point with our power recovery turbine how are we getting more back than we're putting in well that's the trick pressure isn't higher this type of turbine is called a blowdown turbine it's very different from the type of turban used in a Turbocharger which is correctly called a pressure turbine with a blowdown turbine as we we have here the pressure at location 2 is essentially just ambient atmospheric pressure it's not higher as it would be with a regular turbocharger a blowdown turbine gets its name from the fact that the pressure before it is allowed to blow down to the level of atmospheric pressure or whatever pressure exists after the turbine atmospheric in this case the blowdown turbine is something that's not too common anymore in fact it was becoming uncommon back then at least on gasoline powered engines this was a relic of the steam locomotive days sometimes it's a good idea for engineers to look to the Past for answers and that's true today and it was true back then as well the way a blow down turbine works is by harnessing the exhaust velocity not via a pressure differential from one side to the other or heat energy or anything else it's just the velocity the increase in exhaust pressure on the Upstream side is negligible thus there is no meaningful power loss from an increase in exhaust back pressure I'll include some Snippets from NAA as we go along feel free to pause and read them if desired at position seven in this picture they're trying to show you how a blowdown turbine Works let's switch to this image from the actual Source document from Curtis that's a picture of an undershot water wheel like what you might find in a stream powering an oldtime Mill now that I think about it this is probably where the steam locomotive guys got the idea again it often pays to look to the past the mill does not increase pressure or raise the level of the river Upstream by any measurable amount it's just using the velocity of the river okay so we're not causing an increase in back pressure but we must be giving up something right remember nothing is free well we are and what we're giving up is exhaust V velocity as it exits the cylinder exhaust is moving with Incredible speed that velocity can be and was being exploited in piston engine aircraft during World War II by turning it into exhaust thrust back during the war this was almost a science unto itself with aircraft and engine manufacturers trying to maximize the thrust from exhaust NAA did an entire study on thrust from the Spitfire exhaust and designed spefic specific exhaust Stacks to optimize it I have a video about that now doing this with a radial engine is more difficult but it was still done the fw190 has an interesting looking exhaust system and this was all about maximizing that exhaust thrust when you put a turbine into the exhaust you lose a lot of this thrust and if the turbine is doing a lot of work you effectively lose all of the thrust so the next question is are you better off with the thrust from the exhaust or or the extra horsepower being delivered to the propeller well the answer to that depends almost entirely on the speed of the aircraft at the point at which you want to make the comparison the lower the speed the better off you are with horsepower to the propeller that's the main reason that you see turbine engines driving propellers in certain types of aircraft rather than just propelling the aircraft with jet thrust alone turboprop airplanes like this C130 have what are essentially jet engines just set up to deliver their power to a propeller why add that complexity well the reason is because the airplane is primarily intended to operate in a speed range where props are advantageous to understand this it's important to have some idea of the relationship of thrust and propulsive horsepower and this is a subject that really confuses a lot of people including some people that should know better now referring to aerodynamics for Naval aviators we can see that there is a relationship here involving speed for any given amount of thrust power goes up with speed and vice versa note that at 325 knots one pound of thrust is equal to one propulsive horsepower so that number 325 knots is pretty important here now let's take a look at this chart for a moment it's for a turbojet and it's clear that the faster you go the more propulsive horsepower you get now let's compare this to a propeller driven airplane if we have a prop plane with 4,000 horsepower and a jet with 4,000 lbs of thrust the prop plane will have more power anywhere below 325 knots the jet will have more anywhere above that speed it's important to know that power and thrust are two different things and affect aircraft performance in different ways for examp example maximum angle of climb in a jet aircraft is a function of thrust available in excess of thrust required climb rate in that situation is a function of power available in excess of power required let's go back to this chart for a second but now let's say that's a jet engine with 8,000 lbs of thrust I'll add in a line in blue for the 4,000 horsepower propeller airplane the scale on the left is for both hor power and thrust notice thrust also varies a bit with speed but not much how will that Turbo Jet with 8,000 lb of thrust compared to that 4,000 horsepower prop airplane I added a line to remind us where 325 knots is on this chart I've circled the crossover point which is now at 163 knots so anywhere below that speed the 4,000 horsepower prop airplane will have more power than the Jet Engine with 8,000 lb of thrust for reference the dc7 typically had 3400 horsepower available for takeoff and the deavin comet 2 which came out about the same time has engines with 7,000 lbs of thrust so this comparison I've drawn here is somewhat meaningful so when does the dc7 need its full 3400 horsepower per engine it needs it on takeoff and take off is a relatively low speed Affair which means that the extra power to the prop is of more value than the extra thrust from the exhaust it also needs it during the climb and remember climb rate is a function of Excess power not excess thrust piston engine airliners typically climb at relatively low rates and at relatively low speeds in this video they were climbing out at about 300 ft per minute now I'm certain they were being being gentle with the engines I would be interestingly I actually know this gentleman here I flew with him in a DC4 in 1988 haven't seen him since but I remember him very well he's a very relaxing Captain to fly with and really knows his stuff to this day I strive to treat new co-pilots the way he treated me way back when the normal climb rate for the dc7 at Max takeoff weight is listed in Delta's booklet as 750 F feet per minute I don't have the pilot or flight engineer manuals for the dc7 so I'm afraid that's the best I can do for you it's likely the numbers in the Delta book are really best case scenarios and actual performance was probably a bit less but either way the dc7 had very good performance still it's going to take quite a bit of time to climb to its Cruise altitude of 20,000 ft or more so all during that time extra power is far more beneficial than extra thrust regarding the trade of exhaust thrust for horsepower I can't really qualify that for you or quantify it in this specific application because I don't know just how much thrust they gave up here but obviously the engineers at Curtis Wright Douglas and locked did the math and determined that the trade-off was worth it that makes sense to me because I know that climb rate was always an issue with these types of planes and more power helps plus I know that at lower speeds it takes a lot of thrust to equal a given amount of horsepower the dc7 would spend quite a bit of time climbing to altitude at relatively low speeds all piston engine airliners would I'm guessing that speed would be around 160 knots in this case now once it got up to altitude it could Cruise pretty darn fast faster in fact than any other piston powered airliner that made it into production it could reach a maximum speed for a short dash of over 400 milph I doubt any really did that in Airline service but it was possible the average Cruise speed was about 365 mph or 3177 knots that's incredibly fast again these numbers are from the Delta book I suspect that in actual practice they throttled back a bit for engine life and ran a bit slower when in actual service still fast though they make a point of showing that the Delta dc7 passengers are already on the beach in Miana when the Connie is still in route speed matters because passengers or Freight what an airline sells is not travel it isn't Transportation it's time for example you can ship a box for a lot less money via truck ship rail literally anything else other than by air and it will cost less if it's going by air someone is paying a premium in an effort to save time thus the more time your airline takes takes to deliver the item the lower the value of your airline's product and vice versa this is a concept that seems obvious to me probably obvious to you but for some reason I'm having a very hard time getting it through to certain people in my Airline life Delta understood this in 1953 and they put out this manual to their employees talking about it they had high hopes for the dc7 and in some sense the plane did deliver it was fast comfortable safe and it could cross the United States in either direction without a stop in most cases it could also cross the Atlantic or get to Hawaii with previously unseen efficiency from an airplane however it had two problems and Lockheed Starliner had them as well the piston engine technology was being pushed to the Limit here the amount of maintenance required to keep these planes in the air and Performing was very high thus the operating expense was very high the power recovery turbines were really in their infancy in this type of application in fact the mechanics would call them Parts recovery turbin this technology just didn't have the chance to fully develop in these planes before the Jets came along and that's the second issue within a couple of years after the first dc7 went into service the Boeing 707 was in the air by this point the problems with the early Jets had been worked out and in terms of speed Comfort reliability really about anything you can measure the Jets were far superior the prop-driven airplanes still had a place they could be competitive in Airline service on short Hall flights or longer flights that were of less frequency but even in those applications the plan's days were numbered the dc7 and the Starliner were not intended as short Hall airplanes so they had very little time before they became obsolete a few years at best they were so complex that they didn't have much of a Second Life in smaller Airlines or cargo operations they were just too expensive to operate thus the older DC 6s tended to fill that role in fact of all the bigger piston powered four engine airliners the dc6 seems to be the one that hits the Sweet Spot in terms of cost versus capability the DC 7s and starliners faded away very quickly by comparison as for the power recovery turbine well it's sort of faded away as well it's not totally gone there are some Swedish trucks out there with true turbo compound engines there may be other trucks as well with this Tech under the hood I'm not too familiar with big rigs but from what I know it seems that turbo compounding would be a really good solution for them in regards to fuel economy you can also use a blowdown turbine to do things other than put power back into the crankshaft within some limits you can drive a hydraulic pump an electrical generator or something else in this picture from NAA it's driving a dynamometer for the purposes of measuring power some Formula 1 cars have used one to drive a generator which recharged a battery that was used to power an electric motor which spun a centrifugal supercharger a lot of complexity there but it's a very efficient way to raise manifold pressure if you can keep the packaging issues under control space and weight this idea of using it in this way might have some Merit being able to control the speed of a centrifugal compressor independently from engine RPM and without a significant increase in exhaust back pressure could lead to more powerful engines in the future and more efficient engines of course you might ask why not just drive the turbo with a blow down turbine instead of a pressure turbine well you can't do that because you won't be able to get a Turbocharger to spin fast enough and make boost without an increase in exhaust back pressure and at that point you need a pressure turbine not a blowdown turbine there is a weight penalty with turbo compounding of course because you're adding stuff but it's not bad according to Curtis right their turbo compounding system had less weight than a turbo supercharging system it will be interesting to see what the future brings I'm not so sure that turbo compounding is dead it might just be taking a really long nap thanks for watching that's all for now goodbye and have a great day

Channel: Greg's Airplanes and Automobiles

Views: 735,860

Keywords: Turbocharging, Compound Turbo, Turbo Compound, Turbo Compounding, Turbo, DC7, Connie, Lockheed Consellation, Douglas, WW2 aircraft, Engine tech

Id: gbMgwDIdScY

Length: 34min 30sec (2070 seconds)

Published: Sat Nov 11 2023