Engine Confusion

[fw190d11]

I will admit having learned more about engines and their functioning since becoming a serious WW2 fighter buff than I ever had before (not being much of a car buff, all I ever knew was more cylinders/high horsepwr=better [:I]), but there are still certain relationships that confuse me.

Superchargers are described as being "X-Speed/Y-Stage". Is that impeller speed? Multiple speeds produces multiple operating levels (i.e., horsepwr X at A altitude, is the same at A+10K altitude). What do multiple stages (with or without inter/after-cooling) produce? Is that the Boost Pressure (of the air charge or is it air/fuel?) in Inches/Hg? Is there a direct relationship of In.Hg Boost to HP or RPM?

It might be silly, but you dont learn if you dont ask, and I want to nail all these concepts down once and for all.;)

[Wuzak]

Multiple speeds does give different ratings at different altitudes. For example the Merlin 45 in the Spitfire V was a single stage (one impeller) two speed supercharger. The slower speed gives less boost but consumes less power than the faster speed drive. At lower altitudes the faster supercharger speed can produce more boost than the engine can cope with, so the faster speed is usually reserved for higher altitudes.

A two stage supercharger has one impeller feeding a second impeller. The first impeller is the larger of the two, as the second puts through the same mass flow of air, but because the air is already compressed its volume flow rate is less. The second stage then feeds the engine. The idea of a two stage supercharger is that they are more efficient than a single stage supercharger - that is for a given boost they consume less power, and thus allow for greater propellor power.

Turbosupercharged engines are technically two stage engines - most have an engine supercharger too - but operate slightly differently. The turbo maintains sea level atmospheric conditions at the intake of the engine supercharger.

Intercooling is designed to cool the intake charge between supercharger stages. Aftercooling is cooling the intake charge between the supercharger and the engine (what is often referred to as an intercooler in car terms is in fact an aftercooler). The act of compressing air increases is temperature. And increased temperature of the fuel air mix in the combustion chamber is not good for combustion - limiting the amount of boost the engine can take, and therefore power.

[GregP]

Nicely put, Wuzak, as usual.

There are several ways to describe this, but Wuzak did it too well to mess with. The only thing one could add that might help is real-world temperature changes and boost pressures ... and most of those are installation specific.

[Wuzak]

Different manufacturers went about it in different ways too....

Allison's two stage supercharged engines had the engine stage as normal, with the auxiliary stage (the first stage the air sees) being driven with a different gear ratio with a fluid coupling. There was only one gear ratio, but by using a fluid coupling they could mimic the operation of the thurbocharger.

Allison started out with the carburttor feeding the first stage supercharger, but ended up with the carburettor feeding the engine supercharger, so from the carb forward it was the same as the turbo engines. The problem with Allison's system was that the engine was extra long.

Allison didn't use intercoolers on their 2 stage engines, but they did on turbo installations (ie Lightning). The early two stage engines did without aftercooling, but later versions had that installed. It was said to take 50% of the temperature out of the inlet (50% of °F, not of absolute temp).

Rolls Royce coupled their two stage impellers on the same shaft, driven by the same gear ratios - which were usually two for two stage superchargers, except for late Griffons which had 3 speed two stage superchargers. Rolls Royce also cleverly included an intercooler around the first stage supercharger volute (the tract through which the air was expanded and then guided to the second stage supercharger). All RR two stage engines, as far as I know, had a large aftercooler mounted at the back of the main engine. This looks very similar to the Alliosn version of the aftercooler.

Pratt & Whitney had several variations on the two stage theme (not counting turbo installations). They had an engine mounted pair, another with an extension shaft to the auxiliary supercharger (so that it could be mounted remotely), the famous sidewinder setup and they drove them in a variety of different ways, I believe.

[GregP]

Damn Wuzak, you're good. But, then again, we already KNEW that ...


WUZAK
Galatic Engine Explanation Overlord :)

[Wuzak]

quote:Originally posted by GregP

Damn Wuzak, you're good. But, then again, we already KNEW that ...


WUZAK
Galatic Engine Explanation Overlord :)


Haha....

So I'm a GEE-O?

[Red Admiral]

The given boost pressures can be confusing.

The British engines are measured in lb/sq in and gauge pressure. This represents the pressure above atmospheric. 87-oct fuel gives +12lb as the maximum, 100-oct fuel as around +18lb and so on up to tested maximum of around +35lb on 150PN IIRC

The US engines are measured in Inches of Mercury and absolute pressure. This is the boost + atmospheric pressure. Theres a lot of variety in the boosts used, up to around 105" maximum.

[Kutscha]

The Germans used ata in their boost. 1 ata was almost the same as 1 atmosphere.

A nice chart for conversion (left extra large for saving)

http://img.villagephotos.com/p/2005-12/1114844/boostconversion.jpg

[GregP]

You guys seem to uncover these nice technical explanations that I never seem to come across.

Thanks for that graph. Where did you find it? :)Does anyone know what an Ata really is? Wonder how the Germans came up with it as a unit?

[conrad]

quote:Originally posted by GregP

You guys seem to uncover these nice technical explanations that I never seem to come across.

Thanks for that graph. Where did you find it? :)Does anyone know what an Ata really is? Wonder how the Germans came up with it as a unit?


I read somewhere that the Germans were a logical people and Ata's were the logical way to measure perssure.

[Kutscha]

Greg, off the web a long time ago.

ata - atmosphere absolute

The Germans also used 'atm'.

Formula to convert ATA in PSI (relative pressure - British equivalent)
(ATA-1) x 14.7 = PSI (relative)
1.42-1) X 14.7= 6.174 PSI

Formula to convert ATA in PSI (absolute pressure - US equivalent)

ATA x 14.7 = PSI (absolute)
1.42 X 14.7 = 20.874 PSI

[Wuzak]

quote:Originally posted by Red Admiral

The given boost pressures can be confusing.

The British engines are measured in lb/sq in and gauge pressure. This represents the pressure above atmospheric. 87-oct fuel gives +12lb as the maximum, 100-oct fuel as around +18lb and so on up to tested maximum of around +35lb on 150PN IIRC

The US engines are measured in Inches of Mercury and absolute pressure. This is the boost + atmospheric pressure. Theres a lot of variety in the boosts used, up to around 105" maximum.


Good point....

The American method measures the Manifold Absolute Pressure, or MAP.

[GregP]

Thanks Kutscha.

I thought Ata was related to atmosphere, but was wondering about absolute versus gauge (absolute minus 14.7 inches) versus ISA (29.92 inches). The graph above makes it seem like 1 Ata is just about 28.9 inches or so, not 29.92. Generally, 14.7 psi is considered to be 29.92 inches of Mercury or 1 atmosphere, not 28.9 inches of Mercury ... maybe I am reading it wrong?

Once, a few years ago, I purchased an electronic test set for the company I was working for at the time that was made in Italy and the air pressure requirements were specified in units of "NL," which I was informed meant "normalitres" or something to that effect.

I was never able to find an equivalent SI or English unit and had to have the OEM factory convert it into SI units.

[fw190d11]

Man, you guys truly rock when it comes to the special tech stuff.[8)]
Thanks to all, especially to Wuzak!