Supercharger Confusion

[Mongoose]

  OK, so what is the difference between the supercharger on the Ki-84, and the supercharger on the P-38?  One of the things I like about the Lightning is the fact that the power curve is stable.  It stays pretty much the same all the way up, there aren't any gaps like you mentioned.  What makes the difference?

[JOACH1M]

  OK, so what is the difference between the supercharger on the Ki-84, and the supercharger on the P-38?  One of the things I like about the Lightning is the fact that the power curve is stable.  It stays pretty much the same all the way up, there aren't any gaps like you mentioned.  What makes the difference?

because there is a turbocharger. The turbo would maintain boost due to the maintained load and exhaust on the engine.

[RELIC]

(High blower engages!)


Let's see how many of community understand that reference. 

[Greebo]

  OK, so what is the difference between the supercharger on the Ki-84, and the supercharger on the P-38?  One of the things I like about the Lightning is the fact that the power curve is stable.  It stays pretty much the same all the way up, there aren't any gaps like you mentioned.  What makes the difference?

Most WW2 aircraft had one or two superchargers driven through gears off the engine's crankshaft. If a two speed gearbox was fitted it allows the supercharger to cope better with differing air pressure, but in steps. The P-38's engines had both a single speed crankshaft-driven supercharger and an exhaust-driven turbocharger. The turbo's RPM was controlled by dumping excess exhaust gas using a waste gate. This allowed it to maintain a constant air pressure to the engine up to its critical height, above this altitude the turbo was working flat out and the engine progressively lost power.

This sort of two-stage supercharging had some advantages to the two-stage crank driven set-up used in the later Merlins. Apart from the lack of altitude pressure steps the turbochargers were powered by waste exhaust energy rather than the engine itself. OTOH the turbo-charging set-up used in planes like the P-38 was both bulky and heavy, needed more pilot attention and groundcrew maintenance, was more vulnerable to battle damage and lost the jet effect the Merlin got from its exhaust.

[Greebo]

(High blower engages!)

Let's see how many of community understand that reference. 

Hmmmm.....something to do with drugs and oral sex? 

[nrshida]

The 109's engine had a barometrically controlled fluid coupling which automatically compensated for altitude. Pretty cool feature.

Was the gear selection in two speed superchargers manually controlled then?

[zack1234]

It was automatic control on the 109 

It used a slide and pressure system which extended the slide on a internal worm gear

[Greebo]

Was the gear selection in two speed superchargers manually controlled then?

On the two stage Merlin the gear change was automatic.

[dirtdart]

That's a throwback Del. all. Still in the middle east. Might be back on this summer.

Fear the Ki 43!

[Vinkman]

Most WW2 aircraft had one or two superchargers driven through gears off the engine's crankshaft. If a two speed gearbox was fitted it allows the supercharger to cope better with differing air pressure, but in steps. The P-38's engines had both a single speed crankshaft-driven supercharger and an exhaust-driven turbocharger. The turbo's RPM was controlled by dumping excess exhaust gas using a waste gate. This allowed it to maintain a constant air pressure to the engine up to its critical height, above this altitude the turbo was working flat out and the engine progressively lost power.

This sort of two-stage supercharging had some advantages to the two-stage crank driven set-up used in the later Merlins. Apart from the lack of altitude pressure steps the turbochargers were powered by waste exhaust energy rather than the engine itself. OTOH the turbo-charging set-up used in planes like the P-38 was both bulky and heavy, needed more pilot attention and groundcrew maintenance, was more vulnerable to battle damage and lost the jet effect the Merlin got from its exhaust.

To add to this, super charged engines can also have a waist gate. But a supercharger is geared to the engine so its speed is a function of engine speed. If you size the supercharger to diliver a certain volume of air at 3000rpm at sea level, then as you you went up in altitude and the air got thinner the supercharger can't speed up to compensate unless it has multiple speed gearing.  A turbocharger is a two sided air driven bump. One side is a turbine driven by exhaust gas. The turbine rotates a comon shaft which turns a centifugal compressor. the speed of the compressor/turbine is limited by the pressure balance across both wheels. So as atmospheric pressure drops so does boost and resistance on the compressor side. This lack of resistance creates a pressure unbalance across the turbine/compressor with the tubrine being higher, so the turbocharger accelerates until balance is re-achieved. Hence the turbo is a variable speed constant boost device.

Hope that helps. 

[kappa]

(High blower engages!)


Let's see how many of community understand that reference. 

about 19k in a p51 8)