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Aircraft engines: Turbochargers vs. superchargers


Doc72
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Recently I came across this American WWII training movie explaining turbochargers: www.youtube.com/watch?v=PKTSCXRjcmQ. The didactic qualities of these movies (including the cartoon elements like the mountain goat at 3:44 to make things more entertaining) are always remarkable.

AFAIK the USAAF was the only air force during WWII to use turbocharged as opposed to crankshaft-driven superchargers on a large scale. The B-17 and B-24 had them and the unusual size of the P-38 and P-47 is explained by the volume and weight of the available turbochargers.

Now I wonder, why did the USAAF make this choice and did it pay off?

I assume that the availability of heat-resistant alloys needed for an exhaust gas-driven turbocharger played a role. The Americans usually had a much better access to raw materials than other states. At least in the case of the failed German FW-190 C the lack of raw materials is said to have been decisive. But what about Britain? I think in the case of the Spitfire, the small airframe spoke against a bulky turbocharger and in favor of the two-stage supercharger.

In the long run turbochargers seem to have beaten superchargers (for example in car engines), but I assume turbochargers were not superior during WWII. At least the 8th Air Force ultimately replaced the turbocharger-equipped P-47 and P-38 in the high-level escort role to a large extent by the Merlin-driven P-51 which had a two-stage supercharger. Even though the turbocharger was thought to be the key to superior high altitude performance, the P-47 and P-38 were pushed into the low-level fighter-bomber role. The situation might have been different with bombers. Maybe the turbocharger was essential for the B-17 and B-24.

What do you think?

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Well, a not terribly scientific answer:

 

Turbochargers have a theoretical efficiency advantage because they are harvesting "waste energy" from the exhaust gases.  Probably most significant for sustained high-altitude operation.  The flip-side is that, at least during WWII, they tended to be relatively bulky systems, especially on something like a fighter, so you're carrying some extra weight from both the system and the extra structure, and you're also sacrificing volume that could, theoretically, go to extra fuel instead. EDIT: and yes, the metallurgy was challenging because of the high temps and corrosive atmosphere, as with jets.

 

Superchargers, which most if not all of the combat aircraft engines had, were driven by the engine, so took some "useful" energy to drive.  It was far easier to adapt an existing airframe to take a new engine with a different supercharger (or different drive) than to add a turbocharger.  Numerous types had experimental versions with turbos, and they generally didn't look very elegant (Corsair, Hellcat, Fw family, Curtiss Hawk family, etc).

 

 

Edited by gingerbob
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In addition to the recovery of wasted energy mentioned by Bob, turbochargers have one great advantage: they automatically adapt their output at any altitude while superchargers have to be designed for a certain altitude at which they'll give the best power output. I'm not going into the engineering details as the matter is long and being part of my past I'm sure to make some mistake, in any case it's clear how a charger capable of giving the same engine power at any altitude is way better than a system that has to be tuned for certain altitudes while giving lower power at others.

At the same time the turbochargers have one big problem: turbo lag ! The throttle response of a turbocharged engine is not the same as that of a supercharged one and this can be a serious problem in some aircrafts, particularly in fighters. The same problem affected car engines in the past, today it's totally sorted but in the '40s it wasn't. Fighters in particular require a very good throttle response, so the use of a supercharger actually may make better sense than a turbo. Bombers and transports on the other hand spend most of their time flying at constant altitude and speed therefore a turbo makes more sense.

Using turbochargers IMHO paid off for the USAAF as both the Lightning and the Thunderbolt were succesful aircrafts, the Lightning in particular was crucial to air operations over the Pacific. The Mustang may have ended up becoming the best American fighter of the war but the P-47 was still widely used to the end and beyond.

Superchargers rapidly disappeared after the war apart from those engines based on WW2 designs, but then piston engines had become for transports only, with combat types quickly adopting the jet engine...

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It's worth remembering that turbochargers will give an advantage at low altitudes too, which superchargers don't.  (Not including the benefit on the P-47 to that additional belly armour provided by the stainless steel tubing.)   On the other hand that waste energy can be partially regained by shaped exhausts, which benefit is lost on the turbocharged installation.

 

At least two Japanese fighters (J2M Raiden and Ki100-II) had examples with a very neat turbocharger installation: presumably these were in some way less ambitious?  That on the experimental Ki87 appears more cumbersome.  Another externally neat installation is that of the BMW801 on the Ju388, but there's certainly fairly ambitious and complicated engineering under the aircraft skin.

 

However I think that the basic answer is that turbocharging is a complicated technology that takes some study and trials to master, that the US was simply the first to take it seriously, and the timescale fitted nicely (for the US) with that of WW2.

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I've never driven a turbocharged car engine I didn't absolutely hate (including the most modern ones), and they are the best of the technology

 

For a fighter, the wartime turbochargers needed constant control by the pilot. What Giorgio says is correct in that the turbocharger automatically adjusts itself with altitude, but that is theoretical. In practise, the turbocharger is spooled up to maximum RPM (around 17000rpm in the P-47 from memory) and as you climb it speeds up, so with altitude and throttle setting changes the pilot has to also make continuous adjustment to the manually operated waste gate controls to prevent the turbocharger from overspeeding and grenading itself. The theoretical advantages of the turbocharger are very much offset by the additional workload to use them and their fragility - particularly in fighters.

 

The reason superchargers were replaced by turbochargers in the post-war transports like the Connie, DC-6 etc was because they had a Flight Engineer to look after all aspects of these capricious engines and waste gate control was just an incremental addition. Once established in a long high altitude cruise it's more important to get that small improvement on fuel consumption.

 

Coming back to cars, modern supercharged engines are superior in all respects to turbocharged engines from a drivers' point of view. The aging 5.0 litre supercharged Jag V8 is just a better engine to drive enthusiastically than BMW's twin turbo engine - it has better torque curves and much better throttle response - essential for control. Turbochargers give a small improvement on emissions and that's what defines VED or equivalent taxation, so designers are actively making cars worse to make them more sellable on paper. Turbochargers have had a lot of engineering attention over the past 20 years due to the increase in popularity of diesels which have high waste energy levels in exhaust gases due to high compression ratios, but extremely limited RPM ranges making turbos suitable and superchargers unsuitable. This has migrated to petrol engines too, but if you look at the common failures in modern forced induction engines, it's mostly either high pressure fuel injectors or turbochargers failing rather than old fashioned failures like mechanical ignition.

 

Back to aeroplanes, even before technology in aircraft shifted to jets, the almost comical unreliability of ever more complicated piston engines was helping to remove any appetite to operate these big turbocharged radials in transports. There's a reason why many DC-3s are still flying but the big beasts are not!

 

Can you tell I've never liked turbochargers? (and yes, I have one now, and it's a bad engine - but it's also a diesel so inherently so but there's not much choice in commercial vehicles!)

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Now that you mention it, I drive a boat that has a Cat turbo-diesel engine.  The high-pitched whine- even after the "hush-kit"- at certain rpm is annoying enough that it helped me find my standard operating speeds- I need enough rpm to get me to the next dock in time, but minimizing the whine, finding the smoothest feel, and the most bearable overall noise-level drives the precise throttle setting!  None of which is terribly relevant to this discussion, but c'est la vie.

 

 

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Couple of clarifications: the piston engines used on large airliners in the postwar years were not troublesome because of the turbocharger, it was the introduction of the turbo-compound systems that made them very complicated and introduced a lot of problems. Turbo-compound engines recover energy from the exhausts and convert this energy in additional power on the shaft. Fortunately turboprops and jet engines soon spelled the end of these engines...

 

Slightly OT, it's true that supercharged engines give a great driving experience thanks to the huge torque at low RPM but they are not immune from problems and limitations themselves. They are less efficient than turbos, suck energy from the engine (meaning worse fuel economy and more strain on the engine), have limits in terms of maximum pressure (not a problem with petrol engines but can be with others), result in heavier powerplants... there's a reson if many tried to produce supercharged engines for everyday cars (Lancia Volumex, VWs G-lader, Mini, DB7...) but in the end most attempts were short lived with the exception of the Mercedes Kompressor series and VWs twin-charged engines. A supercharger today may make sense for top end Jaguars or Mercedes where the maker wants to give a certain driving experience to their customers, but they are generally not cost effective for smaller cars and are not the best for proper sport cars

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I for one am quite happy with my turbo-charged car. It doesn't go on racetracks, so it gets driven normally, which it does absolutely fine and it's quite good at overtaking things if I feel like doing so. I think for normal diesel and petrol cars the turbo is a good idea as it allows you to have a smaller more economical engine with the power and performance of something bigger. I'm thinking of swapping my 2.0 diesel Octavia for a 1.4 petrol when the current lease ends. The 1.4 Octavia petrol has around 150bhp and is faster and more economical than a Mazda 6 2.0 petrol, for example.

 

Just a point about the P-47, it had a turbo-supercharged engine, which certainly gave plenty of power but as we know resulted in a huge for the time fighter aircraft. Most fighter designers seemed to think that fighters should be relatively light and streamlined, hence things like the Spitfire and Mustang and clearly the available turbo charger technology at the time was too big and complex, so supercharging must have been the better option for most.

 

thanks

Mike

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Or indeed a Firebrand or Tempest.  Or Ki87.  If you want a large fighter with a turbocharger try the BV155C.

 

The Bearcat was designed as a short-endurance fighter for the escort carriers, so perhaps is something of a special case.

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I would think that comparing the 2 technologies in the WW 2 timeframe is a little ahead of things.

The two items ( turbocharging , supercharging ) were not at all any where near the top of their technological learning curves.

Metallurgy , air movement , engine air requirements were all burgeoning  technologies and understanding was in its infancy in the '40's.

Comparing the technology to todays knowledge of air movement compression etc , is literally like comparing apples and apple sauce , the same thing but vastly different in todays world.

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An often overlooked advantage of a mechanically-driven supercharger for a radial is that it really gives the fuel-air mixture a good shake-up and can be a big help in improving mixture distribution. Unsupercharged radials with central carbs often have terrible mixture distribution, i.e. different cylinders running at different fuel-air ratios. This makes them inefficient as you end up having to run the engine too rich overall to compensate for the weak cylinder(s), because they are air-cooled and can't tolerate hot spots very well.

 

I don't think any radials for proper mainstream combat aircraft during WW2 were unsupercharged, even those which had turbochargers as well. 

 

Supercharging in no way died out after ww2 for aviation. It continues to be used in general aviation for radial engines: the highly successful Ivchenko AI-14 and its evolved Vedeneyev M14P family being the dominant case. In fact those are the most widely manufactured post-war piston aero engines of any make, and generally fabulous things to fly around behind. They're used in lots of different kinds of Yaks, in helicopters and loads of other applications including gas pipeline compression. There have also been supercharged in-line post-war engines such as the dH Gipsy Queen, and Walter / Avia 337 types.

 

krow113 is absolutely right about the advances in metallurgy during WW2. Turbocharging makes things very hot indeed, and in the early post-war aviation world those materials were geerally more effective used in gas turbines than turbochargers for piston-enginer airliners.

 

And as Giorgio says above, it is important not to confuse turbo-compounding (complicated, expensive, unreliable, a step too far) with turbocharging. Both systems put windmills in the exhaust, but that's about all they have in common.

 

Edited by Work In Progress
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Harleys are basically the same radial engine technology , superchargers work pretty well , the uneven exhaust gas pulses at low rpm tend to 'baffle' the turbocharger vanes , variable vane technology solved that to some degree but the extra 'gizmos' needed to make it work really uglified the bikes.

Again ; applying todays knowledge to yesterdays knowledge is a disingenuous comparison.

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I flew a turbocharged helicopter in the Army, the TH-13T. Turbocharging was added to compensate for the increased weight of the avionics required for instrument training. Since it was powered by a piston engine, rotor rpm was maintained manually with a twist-grip throttle on the collective. The turbo lag was very obvious in a hover, since at first you rolled on too much throttle and then in compensating for that, rolled off a little too much. Somehow, you quickly learned to roll on the amount of throttle that following the lag, was just right. The other problem was that when the turbocharger failed, the amount of power available was drastically reduced (e.g. not enough to hover).

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Modern turbo cars are also ECU controlled - they get an electric pre-spool to reduce lag, an advantage not available until recent times. Even that isnt perfect; the Veyron at 1000 - 1200 bhp is one of the few road cars even approaching a piston fighters bhp, and that has 4 turbochargers rather than one big one. As others have said, chalk and cheese.

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  • 1 month later...

My Dad's VW Golf is a 1.4 petrol with a TSI engine which I believe means it has a turbo charger, a super charger and an intercooler. Seems a lot to go wrong to me but it doesn't lack power- but as someone else said, the CPU on the car is doing a lot of the management.

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:hmmm: nobody mentioned (?) the difficulty in collecting the exhaust gases in a non-radial engine, like a Merlin in a Spitfire or a DB 60x in a Bf 109, and then ducting them to that lumpy turbocharger... Compared to having a series of short exhaust stubs exiting the airframe? The concept for turbocharging just wasn´t feasible in my opinion. V-P

P.S. The greatest car ever? The Saab 900 turbo 16 Aero, of course :offtopic:

 

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Hello, vppelt68

Early turbochargers had been tested during WWI on SPAD XIII C.1 fighter, and in early twenties small series of Rateau turbocharged Breguets XIV A.2 and B.2 had been in service with l'Armee de l'air, as had been P-5 Superhawks with USAAC. All these types had been equipped with V-engines and IIRC turbocharged radial engines did not appear until some time later. Why would collecting exhaust gasses from inline or V-engines be so difficult? Cheers

Jure

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1 hour ago, Jure Miljevic said:

Hello, vppelt68

Early turbochargers had been tested during WWI on SPAD XIII C.1 fighter, and in early twenties small series of Rateau turbocharged Breguets XIV A.2 and B.2 had been in service with l'Armee de l'air, as had been P-5 Superhawks with USAAC. All these types had been equipped with V-engines and IIRC turbocharged radial engines did not appear until some time later. Why would collecting exhaust gasses from inline or V-engines be so difficult? Cheers

Jure

 

It's true that the U.S.A.A.C. employed early turbo-chargers in the thirties. The original Curtiss P-6 (the tubby version) fetched up as the P-6D with a turbo-charged motor --- about a dozen were in service. The Consolidated PB-2/P-30 monoplane also had a turbo-charger, and several dozen were operated.

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For what its worth, 2 books that are a worthwhile read are Graham White's books on the R-2800 and R-4360. In these books he goes into some detail on the practical aspects of supercharging vs. turbocharging.

 

From a first hand standpoint, listening to an F4U-5 with the R-2800-32W "Double Twister" supercharger is something you will never forget. P-38s equipped with GE turbos are surprisingly quiet.

 

I can't remember if its in Graham White's book on the R-4360 or Geoff Hays' book on the B-50, but one of 'em goes into detail about the Boeing XB-44 which was essentially a B-50 with supercharging rather than a turbo. By his accounts it turned out to be a more reliable powerplant installation. The USAAF in the postwar years wanted to develop a turbocharger for the B-50 on the cheap, using off- the- shelf GE turbo components. The end result was a bad matching between turbo and engine, that led to a protracted, time and money-wasting series of incremental retrofits and improvements. I don't remember if the B-36 had the same problems or not, but White expands quite a bit on the B-36 powerplant.

 

Regarding the "Comical" reliability of these late engines, as far as the -4360 is concerned it seems the problems had more to do with the add-ons like turbo- or supercharging systems, rather than the "core" engine itself, at least in the case of the "Corncob". Engineers at P&W could have taken both the R-2800 and R-4360 a lot further, but a strategic decision was made by management at P&W to limit funding for recip development , and concentrate on development of what was then the "Turbo Wasp" (J-57/ JT-3). As history has proven, it was the right bet.

 

Curtiss-Wright persisted with the R-3350 and it did become a fairly reliable engine that stayed around a long time, in fact even putting out more brake horsepower than the R-4360. But Curtiss-Wright won the battle and lost the war.

 

The R-3350 has an interesting story that still needs to be told.

 

-d-

 

 

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On ‎4‎/‎7‎/‎2017 at 5:26 AM, Giorgio N said:

Superchargers rapidly disappeared after the war apart from those engines based on WW2 designs, but then piston engines had become for transports only, with combat types quickly adopting the jet engine...

 

The F4U Corsair with the supercharged Pratt & Whitney R-2800 and the AD Skyraider with a supercharged Wright R-3350 both served long after WWII ended as did the supercharged P-51D/K/H and the P-82. The PW R-4360 Wasp Major was also supercharged and found significant post-war use in a slew of aircraft; B-36, B-50, C-97, C-74, C-124, Martin JRM Mars, and C-119.

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My old post has experienced quite a revival.

Still I wonder why turbochargers were (almost) exclusively limited to US designs. Can this be explained by raw-material supply/metallurgy and simple path dependency (nations stuck with whatever technology they had chosen in the 1930s)?

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Just one other thought, that might prompt some more comment on this great post. From personal experience (in cars) when a supercharger breaks, you are left with just a low compression engine, short on power, but running ok. When a turbocharger fails, you are usually faced with an destroyed/siezed engine within seconds as the turbo's bits are ingested by the engine through the intake and the oil feed.
I know which I'd prefer at 30,000 ft over the mid-Pacific.

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3 hours ago, alancmlaird said:

Just one other thought, that might prompt some more comment on this great post. From personal experience (in cars) when a supercharger breaks, you are left with just a low compression engine, short on power, but running ok. When a turbocharger fails, you are usually faced with an destroyed/siezed engine within seconds as the turbo's bits are ingested by the engine through the intake and the oil feed.
I know which I'd prefer at 30,000 ft over the mid-Pacific.

Either that or the busted turbo sucks all your oil out of the sump, same end result. :( Happened to me once on a Volvo coach. Inconvenient to say the least, pleased I wasn't at 20,000 feet. It'd have been a lousy glider. :)

Steve.

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