Supercharger - Question.

[172_driver]

Hi all,

A technical question about a supercharged engine. Normally the power available increases up to full throttle height while mainting sea level boost pressure. But how come the pwr. available actually increases? Is it because of the reduced back pressure on the gas leaving the engine or are there any other reasons why power increases?

TIA

[411A]

Gear driven superchargers boost the intake pressure, and therefore can maintain that manifold pressure up to the critical altitude...that altitude at which full throttle is used.
TURBOsuperchargers do much the same, except that they are exhaust driven.

Several engines in the past have used both...the Boeing Stratocruiser comes to mind, using Pratt&Whitney R4360's.
Others include the B-29 and several models of the B-17.
However, so far as I know, only the Stratocruiser used both a supercharger AND a turbosupercharger, in civvy design.
However, the Boeing Stratoliner may well have been another.
And then we come to the superlative of piston engine design, the CurtisWright R3350 turbocompound engine, using both a two stage (speed)supercharger and power recovery turbines (3), which delivered BHP directly to the crankshaft, thru fluid couplings....not unlike the 'ole Buick Dynaflow idea.

More than you wanted to know, I expect...:rolleyes:

[172_driver]

Haha, thanx for the history lesson.

However, it did not really answer my question why the power output increases with an increase in altitude. If you are able to maintain manifold pressure at MSL you will not loose any power with increasing altitude, right? But according to the graphs the power output even seems to increase with altitude, and that confuses me a bit :)

Turbochargers decrease the 'rate of decrease' of power available up to full throttle height and superchargers even increase the power available. However I can imagine that the supercharger, connected to the crankshaft, has a more powerful source of drive than the turbocharger driven by exhaust.

Brgds

[White Bear]

172 driver,
The first thing you must understand is that as far as the engine is concerned, it does not care how manifold pressure is maintained. Turbo-superchargers and gear driven superchargers, essentially do the same thing.

The big differences between them is turbo’s do not work very well at low rpm and light loads, but do work very well at high rpm, and heavy loads. Disadvantage is, this is the opposite of when you really need boost, but the advantage is they do not use much power to drive them.

Gear driven superchargers do work very well at low rpm, but not very well at high rpm, which generally follows the needs of the engine, and are not ‘load’ dependant which turbo’s are. The big disadvantage is they require a lot of power to drive them, and they can be heavy.

Any time you can supercharge your engine intake above atmospheric (and assuming you match the increase in air flow with a corresponding increase in fuel flow) you will make more power.

Normalizing, which is what you are talking about is using a supercharger (which type is not relevant) to maintain MSL manifold pressure and therefore maintain power output at altitude.

In a way you are supercharging your engine, and in a way you are not.
In short you are only maintaining MSL manifold pressure, and not really ‘supercharging’ it, you have not raised the manifold pressure above MSL. If you did, you would really be “supercharging”, as in using pressure above atmospheric to over (super) charge your cylinder with air.

If either type of supercharger (turbo or gear driven) increases manifold pressure above MSL at any altitude then you have ‘boost’, you are now supercharging your engine, and it will make more power provided you increase the fuel flow to match..

411A,
I have seen cutaways of both the 3350, and the 4360, very impressive, and perhaps the most complicated piston engines ever built. A true marvel of ‘old’ technology.
Regards,
W.B.

[EchoMike]

Well you can set this up to increase the power at altitude - to some extent.

Turbocharged engines have a waste gate, which is a spring-loaded safety valve which prevents overboost - too much boost and you explode the engine. By increasing the spring tension, you increase the allowable pressure in the induction system, but again, it is *very* easy to have too much of a good thing.

This doesn't directly answer the question of why more HP with altitude, but it can - if the waste gate is manually adjustable you can set your HP output to maximum on the ground, and as you climb, increase it somewhat. (You better have a good reason to be in this much of a hurry - mistakes get really expensive, very quickly.)

Turbocharged engines have a "critical altitude" - manifold pressure (and thus HP output) is maintained until the aircraft reaches this critical altitude, which is where the air density is too thin for even the turbocharger to maintain sea level manifold pressure. These can be set up to maintain slightly more (or sometimes considerably more) than sea level manifold pressure, and in cars, often are. We are somewhat more conservative in aircraft, though.

Superchargers, which are direct driven, sometimes have variable gear ratios, known as high blower and low blower - this keeps from overboosting at low altitudes, and allows a change of gear ratio for maximum boost at higher altitudes - there is still a critical altitude, however, even with geared superchargers.

Incidentally, the altitude record for piston engine aircraft is held by NASA - the aircraft is a drone (darn it) powered by a Rotax 912 with FOUR cascaded turbochargers feeding it!

Best Regards,

Echo Mike

[Feather #3]

The part which hasn't been answered is, yes, that reduced back-pressure at altitude gives an increase in power.

This is seen in the reduction in max allowable MAP on the R-3350 at 7,500ft vs MSL. It's a supercharged engine and there may be a different altitude for a turbocharged one, but the effect is there.

It's seen in normally aspirated pistons where the most effective operation is at full-throttle height; reduction in backpressure being part of the equation.

G'day ;)

[172_driver]

The most straight forward answer one could wish Feather#3 :ok:

Of course, engines and other components are not as straight forward as one could hope which basically means I feel a bit lost sometimes. Progress test tomorrow :eek:

[411A]

>>I have seen cutaways of both the 3350, and the 4360, very impressive, and perhaps the most complicated piston engines ever built. A true marvel of ‘old’ technology.<<

Indeed they are, White Bear, and I have flown 'em both, on the 1649 Constellation and the Stratocruiser.

The flight engineer is your best friend with these....:ok:

[100BMEP]

411A...you have a great memory...
as I recall a sudden 20BMEP drop was a pretty good indicator that you had just lost a PRT. (Perhaps a valve wiping out the turbine blades as it found its way through the exhaust manfold!):E
Ring any bells?

(an old Conny and DC7 driver)

[BEagle]

The R-3350 was a very successful engine indeed.

But don't forget the UK's own Napier Nomad, for a truly complex compound engine! http://users.bigpond.net.au/Shackleton/nomad.html . Shame it never entered service though!

My car has a supercharged engine producing around 110 bhp/litre. The supercharger is a Lysholm-type geared blower, belt driven through an electromagnetic clutch and a complex engine management system. It is seamless in operation, running at over 20000 rpm and 1.2 bar of boost, but only runs when needed, so avoids unnecessary losses. If the digital computer had been sufficiently advanced in the 1950s, I wonder what sort of SFC efficiencies would have ultimately been available from high-powered reciprocating engines with FADEC systems and prop management devices?

[411A]

Twenty or so sounds about right, 100BMEP, but what I really remember is the torching involved, when a PRT let loose.
Quite a sight at night.

BEagle,
The 'ole R3350 turbocompound engine had a SFC of 'round about 0.37 pounds fuel/HP/hour, so I suspect this was about all you could get out of an air-cooled radial piston engine, now or then.

[ZQA297/30]

172 Driver.
It's been a while, so I may be a bit hazy, but temperature has a big effect on charge density, so that even with constant MP, as you climb the reduction in OAT means the charge density is higher. Sort of natural intercooler!

411A, remember sitting listening to the drone of big recips, and observing the F/E constantly trolling thru the oscilloscope, and the dreaded "oh sh*t" followed by "double secondary short" or some other catastrophy.
Have to admit I am glad those days have gone. Along with runaway props, and janitrol heaters cycling away next to thousands of gallons of 115/145.