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Two stage superchargers and altitude loses


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#1 MatsNorway

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Posted 17 January 2015 - 09:57

Im wondering if there is a good source on the performance at altitude for this configuration. What kind of HP loses where there compared to a turbo engine. Single stage is of interest too ofc.

 

Here is some: http://www.enginehis...alysisR-R.shtml


Edited by MatsNorway, 17 January 2015 - 10:06.


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#2 Wuzak

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Posted 17 January 2015 - 12:50

There is some information about 2 stage Merlins in this article.

 

http://www.wwiiaircr...lin-lovesey.pdf

 

Fig. 11 shows a graph of pressure ratio vs efficiency, clearly showing that the 2 stage was superior for PRs > ~3.4 - 3.6.

 

You can work back from air pressure at altitude and required MAP to get a boost vs altitude.

 

The way turbo engines of WW2 worked was that they maintained sea level air pressure up to the turbo's critical altitude at the carburettor deck. This allows the turbo engine better power in 2 ways - the pressure ratio of the compressor is less, the impeller doesn't have to spin as fast so there is less power required to drive the compressor, and the intake air doesn't have to be throttled at full rated boost.

 

The downside is complexity and weight. And loss of exhaust thrust.

 

Exhaust thrust is quite a big factor in high speed high altitude aircraft. Not as much for low altitude and/or slower aircraft (like bombers). The Mosquito gained 13-18mph (depending on altitude) when ejector exhaust stubs were fitted instead of the saxophone exhausts, which themselves had some measure of exhaust thrust.



#3 MatsNorway

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Posted 17 January 2015 - 18:17

The way turbo engines of WW2 worked was that they maintained sea level air pressure up to the turbo's critical altitude at the carburettor deck. This allows the turbo engine better power in 2 ways - the pressure ratio of the compressor is less, the impeller doesn't have to spin as fast so there is less power required to drive the compressor, and the intake air doesn't have to be throttled at full rated boost.

 

That made no sense to me. carburettor deck? the turbos critical... altitude?  Sea level pressure? Did they not run boost?


Edited by MatsNorway, 19 January 2015 - 22:04.


#4 Wuzak

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Posted 17 January 2015 - 23:19

By the carburettor deck I mean the inlet to the carburettor.

 

Critical altitude for the turbo is the maximum altitude at which the turbo can compress the air to sea level pressure. 

 

Turbo equipped aircraft in WW2 used two stages of supercharging. The turbo compressed the air to 1atm abs and supplied it to the engine, which had an integral single stage single speed supercharger. The integral supercharger would then provide boost to the engine.

 

WW2 engines were rated for certain altitudes. The Merlin 45, for example, was rated at 18,000ft at +12psi boost. Below that altitude boost was maintained by throttling, while above the boost fell away.This altitude was called the Full Throttle Height (FTH) by the British and Critical Altitude by the Americans. Thus the maximum power of an engine at a particular boost level was at its FTH, power being less either side of it.

 

An example:

http://www.wwiiaircr...Power_Chart.jpg

 

In contrast, the engines coupled with turbos had a rated altitude of 0ft. The turbo provided compensation for altitude.

 

The difference can clearly be seen between the Spitfire V (single stage single speed), Spitfir IX (2 stage 2 speed) and the P-38 (turbo).

 

http://www.wwiiaircr...l_Speed_RAE.jpg

http://www.spitfirep.../bf274speed.jpg

http://www.wwiiaircr...t-2338-1400.jpg

 

The Spitfire IX chart also shows the boost - how it is maintained constant to a certain altitude and then falls off.


Edited by Wuzak, 22 January 2015 - 12:23.


#5 Wuzak

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Posted 17 January 2015 - 23:39

At 20,000ft the absolute pressure is 46.6kPa. At sea level it is 101.3kPa.

 

For supercharger to supply boost of +15psi (+101.3kPa(g)) at 20,000ft its pressure ratio would need to be ~4.3.

 

For an engine system with a turbo, the engine stage supercharger would have a pressure ratio of 2 to get the same boost, as the ambient pressure "seen" by the engine is 101.3kPa (abs).



#6 Wuzak

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Posted 24 January 2015 - 02:57

This chart shows the effect of exttra boost in aircraft with 2 speed engines - the V-1650-7 in a P-51B.

 

With the 44-1 (150 grade) fuel the maximum boost was pushed up from 67inHg (+18psi boost) to 75inHg (+22psi boost).

 

http://www.wwiiaircr...grade-level.jpg

 

The extra boost raises power, but the full throttle height/critical altitide of that boost is reduced. The supercharger is the limiting factor after that peak, the boost and power falling as altitude above FTH until it matches the power curve with lower fuel grade (100/130) and maximum boost level. This is reflected in the levelspeed curves in the graph.