Two stage Superchargers

[Wavelength]

Please excuse me if this overly elementary, but whats the diiference between a two speed supercharger and two stage supercharger system?

[gumbyk]

There's nothing overly elementary at all.

A two-speed supercharger has a single impeller (that does the compressing) that you can change gears on. there is a lever in the cockpit that has high/low that when you get to altitude, you can change the speed of the impeller to high to give you more compression.

A two-stage super-charger has two impellers, usually one smaller than the other (hence the two stages) set up so that one feeds into the other, causing a greater increase in pressure than a single stage.

Hope this helps.

[Wavelength]

Thanks. I was thinking along those lines but was almost thinking it was two seperate superchargers in series. I take it its two impellers in one case then.

[gumbyk]

Here is a diagram of the Merlin 2-stage supercharger set-up. They are both in the same case. Its a bit hard to see, but they are set up in series.

[Wavelength]

Thanks for going to trouble of posting the diagram. It makes perfect sense. What's the main advantage of using two impellers in series? Better efficiency of the compresser blades per rpm?

[gumbyk]

Yeah, better efficiency. If you have one impeller with a compression ratio of 5:1, and then run that through another one with a compression ratio of 5:1 you end up with a final ratio of 25:1. That would require either a large, heavy impeller, or one spinning at extremely high speeds.

So, better efficiency, and sometimes the only practical method of getting the high compressions required for high altitude flight.

[Shortround6]

Thanks. I was thinking along those lines but was almost thinking it was two seperate superchargers in series. I take it its two impellers in one case then.

The 2 stages can be different cases. The Allison 2 stage supercharges used separate cases as did all US engines that used turbo chargers. Bristol engine that set altitude record in late 30s used separate cases.

all that is needed is one impeller feeding another in series.

[hrandy]

I believe the compression ratios are added not multiplied, i.e. two 5:1 stages would result in a 10:1 overall compression. In a slide prepared for his paper "Gas Turbines for Aircraft Propulsion" Sir Stanley Hooker shows the Merlin 61 having a maximum compression ratio of about 6.5:1 compared to a maximum of about 4.5:1 for the Merlin 46 and about 3.5:1 for the Merlin III.

The concept of two stages in one case with air to liquid cooling, which seems so logical now, was a stroke of genius by Sir Stanley and his colleagues at Rolls-Royce in the early 1940s. This let the Merlin stay ahead of the much larger displacement DB 600 series engines in both total power output and altitude performance.

[gumbyk]

I believe the compression ratios are added not multiplied, i.e. two 5:1 stages would result in a 10:1 overall compression. In a slide prepared for his paper "Gas Turbines for Aircraft Propulsion" Sir Stanley Hooker shows the Merlin 61 having a maximum compression ratio of about 6.5:1 compared to a maximum of about 4.5:1 for the Merlin 46 and about 3.5:1 for the Merlin III.

The stages are multiplied:
From wikipedia article on Gas Compressor (staged compression)

If each stage has a compression ratio of 7 to 1, the compressor can output 343 times atmospheric pressure (7 x 7 x 7 = 343 atmospheres).

This is talking specifically about a compressor, as opposed to a supercharger, but the principle is the same.

[Shortround6]

I believe the compression ratios are added not multiplied, i.e. two 5:1 stages would result in a 10:1 overall compression. In a slide prepared for his paper "Gas Turbines for Aircraft Propulsion" Sir Stanley Hooker shows the Merlin 61 having a maximum compression ratio of about 6.5:1 compared to a maximum of about 4.5:1 for the Merlin 46 and about 3.5:1 for the Merlin III.

The concept of two stages in one case with air to liquid cooling, which seems so logical now, was a stroke of genius by Sir Stanley and his colleagues at Rolls-Royce in the early 1940s. This let the Merlin stay ahead of the much larger displacement DB 600 series engines in both total power output and altitude performance.

As has been noted by "gumbyk" the results ARE multiplied.

However there are very definite limits to what can be used by an engine in a given circumstance. The higher the pressure ratio that is used the higher the power needed to drive the supercharger. The more power used by the supercharger the less that is available for the propeller. It is also a fact that two impellers, both operating at a 2 to 1 ratio, will require less power to drive and heat the intake air less than a single impeller operating at a 4 to pressure ratio.
The amount of charge heating an in-efficient supercharger causes limits the amount of boost that can be used with a give performance number fuel.
Another big factor (often over looked) is the compression ratio an engine uses.
For pure power at the prop what matters is the amount of fuel and air that can be moved through the engine in a given amount of time. This is also affected by the thermal efficiency of the engine. How much of the potential power of the fuel is turned into crankshaft HP.
the higher the compression ratio an engine uses the more efficient it is. The more power per pound of fuel burned. However the higher the compression ratio in the cylinders the less boost that can be used and the less total amount of fuel and air put through the engine in that given amount of time. Less peak power.
These facts were known in the late thirties to some car racing teams and some other supercharger experts. The questions at the time (and the inspired guess? of Hooker and crew) were what level of expense and complication were they willing to pay for what level of improvement/performance.
Rolls-Royce (and Hooker?) had made a deliberate decision that their two stage engine should fit as well as possible into the space/volume of the singer stage engine. From this decision comes the single case lay out. inspite of some other drawbacks. The liquid intercooler also follows from this as an air to air inter cooler simply won't fit in some of the aircraft, like the Spitfire. This fact was also known at the start of work.

[vintage radials]

OK guys, enough already. The combos are one stage with single, two or three speed drives. Two stages single or two speed drives some with intercooler. The correct answer on the boost or compression is additive with these aircraft engines. The merlin two stage will generate 5 ata of boost before the drag of the unit drops the shaft HP. see reno engines

[hrandy]

I was wrong Gumbyk and Shortround6 are correct in stating that compression ratios in staged superchargers are multiplied. The compression ratios are so low in these engines that is easy to make that mistake (i.e. 2x2= 4, 2+2=4).

[Trilisser]

Just a short note: compression ratio is not correct term for superchargers, it is pressure ratio. In jet engines the compressor's performnace is also expressed as pressure ratio...

[fibus]

Yes but the pressure ratio can be used to compare compression ratio. The it has more meaning to piston engine people.

[Trilisser]

Better to stick to proper nomenclature and not dumb down everything.