Two stage Superchargers

Snowygrouch said:

I would add (respectfully), that the last three points above can be google-searched quite easily,....so I hope you might try to do so from now on - and keep the questions to things you need the specific knowlege of the forum to answer..Its not very fair to ask me to spend time writing all this out saving you from typing "adiabatic", "choke" or "air pressure ata" into google..

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Good luck with that part

Snowygrouch said:

I suspect I`m overcomplicating what you are trying to say here but for your information German engines didnt have carburettors - and hence had NO chokes, so comparing any ram effects between Allied and German aircraft engines is impossible without correcting for pressure drop across the choke, which is worth a jump from 3:1 to 3.3:1 in compressor pressure ratio on a Merlin 45 for example (October 1942 Royal Aircraft Establishment Report#3958). This sort of thing makes it very difficult to compare performance of the actual compressors....

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But they did have throttle plates, which is what causes the pressure drop.

wuzak said:

But they did have throttle plates, which is what causes the pressure drop.

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You need to define what you are referring to as "THE" - before I can answer that in a really useful way.

Pending that happening - I`d add only that very many German engines certainly do NOT have throttle plates....like virtually the entire Jumo series for example. I`m assuming there that you are referring by "plates", to a throttle butterfly used to regulate engine speed so it can cruise and idle and so forth. Most of which doesnt really apply to the entire DB series of engines either.

Snowygrouch said:

Without a choke (another case of changing nomenclature, its known now more often as the carburettor venturi, and choke is now typically known as the cold start assist enrichment valve) - a carburettor doesnt work, as you need a pressure difference to suck fuel out the float-bowl. Hence a venturi (which chokes the flow a bit) is inserted

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So you would have better ram compression without it changing the air pressure and velocity?

You need to be a bit careful with the term adiabatic, its a "closed" process in as far as we assume that the actual physical compressor housing does not transfer its heat to the outside air, BUT - we are certainly saying with it that its non-reversible. In that to change the volume (eg compress the air) it DOES take energy away to do so, which IS removed from the process (and hence needs to be put IN for it to be done), as mechanical work.

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So, it's "close to closed"...

One technical atmosphere (ata) = 0.98 Bar.

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And a bar is atmospheric pressure, so basically I could just do 0.98 * n(14.7) = manifold pressure?

Snowygrouch said:

You need to define what you are referring to as "THE" - before I can answer that in a really useful way.

Pending that happening - I`d add only that very many German engines certainly do NOT have throttle plates....like virtually the entire Jumo series for example. I`m assuming there that you are referring by "plates", to a throttle butterfly used to regulate engine speed so it can cruise and idle and so forth. Most of which doesnt really apply to the entire DB series of engines either.

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I could understand Daimler engines not having a throttle to control mass flow, as they had the variable speed supercharger - but they did.



The small section of pipe just after the supercharger outlet is the throttle.

And you can see it in this cutaway picture from Flight magazine
http://www.aviation-history.com/engines/db601n.jpg

Jumos had fixed speeds, not variable drives. So they definitely need a throttle body.
View: https://twitter.com/wingsjumo/status/384387950936879105


That looks like the throttle in there.

Just because an engine is direct injection doesn't mean you can get away without controlling the air flow. The DB series engines have the variable speed supercharger drive, but that would be insufficient to give control over the wide range mass flows required.

1) The Jumo has a throttle barell, NOT a "throttle plate" - this is zero restriction once fully open - and hence plays no part whatsoever in limiting maximum engine performance, unlike a carburettor choke, which is FIXED in all conditions. (although the two components have nothing to do with eachother functionally anyway).

2) The DB601/605 has two butterfly throttles - which I`m well aware of as I`m sitting here translating the memiors of the Chief designer of DB. Because it has a variable speed drive, neither of these throttles play much of a role after take-off, except to modulate between cruise or all-out, because most of the mass flow control is taken care of by the hydraulic coupling changing speed. This means the DB engines are in any situation except for the slowest cruise or idling on the ground, very little affected by the fact there is a throttle anyway. A fact which is almost entirely responsible for the low level power advantage of the whole variable speed drive system in the first place.

3) The throttle (unlike a choke) can be sized anything you like in terms of its diameter, too large of course and you`ll get full throttle after only a very small change in angle, which is undesirable for fine-control, hence people tend to make them slightly OVERSIZE, such that when they are open - they do not pose any significant flow restriction at all. A choke, to work - must have a diameter LOWER than the mean intake pipe size as it requires to bring about a permanent and significant low pressure region where the main jet emerges into the airflow. So its a fudamental, fixed and immovable flow restriction in all conditions.

For these reasons - it is I`m afriad totally incorrect to bring into a comparison of "is it bad to have a choke" the fact that all engines have "a throttle" - this applies equally to a caburettor as well. Hence, any pressure drop that the throttle "plate" gives at any given throttle angle will hurt flow in all engines equally, so is a moot-point in any comparison argument about chokes - or injection, as its a common denominator and so is irrelevant to the comparison.

As I`ve explained, once the engine is operating near to max power, the throttle is almost invisible to the airflow anyway (as long as its designed well), and so in any actual combat scenario - the throttle can actually be totally ignored as a flow influence anyway - totally irrespectively of which fueling system you are running.

So - once again, in broke-record fashion - I`m afraid that a choke will typically limit the pressure ratio of your average WW2 supercharger by 10%. Which unlike throttles, is a permanent - immovable flow limitation which applies at ALL engine powers and conditions from idle all the way up to war-emergency rating.

So- returning to your quote.

"but they did have throttle plates which is what causes the pressure drop"

I would propose a correction.

"Throttle systems cause a pressure drop below fully open in all engines, and virtually zero at fully open - but chokes cause the main pressure drop in an intake system with a carburettor which causes them to be permanently around 10% down in compressor pressure ratio in a typical ww2 supercharged V12 compared to an engine without a carburettor - regardless of its throttle system design"

If you go and take a look at a Merlin 100 series, you`ll see for youself where much of its power came from. No chokes !...because - no carburettor.

I suspect we`re all wandering somewhat off course of this original threads purpose...possibly a "throttle systems" thread is required if anyone wants to talk more.

Keep going, people, there is so much to learn here.

Snowygrouch said:

1) The Jumo has a throttle barell, NOT a "throttle plate" - this is zero restriction once fully open - and hence plays no part whatsoever in limiting maximum engine performance, unlike a carburettor choke, which is FIXED in all conditions. (although the two components have nothing to do with eachother functionally anyway).

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Thanks for the clarification.

The throttle valve would not be fully open most of the time in a Jumo in the same way that the Merlin's.

As you noted, the DB series is different, as the variable speed drive reduces the need for throttling.

And 10%? I would have thought that much for the carburetor.

Yes 10% is a high number indeed, it will have been different for all engines/carbs. The Royal Aircraft Establishment in Farnborough, reckons about 7.5% in a Wright Radial. Although they had hand calculated that the removal of the chokes ought to be 5% benefit; which they were a bit surprised about as you can read below.

Keep in mind this doesnt mean a peak power drop everywhere - it means less boost, which you can correct for by changing the boost controls - but means you`ll run out of puff at a lower altitude, so once you correct the boost control for having a choke - the real effect is the loss in aircraft ceiling of several thousand feet.

(i.e. you`ll have to open the throttle MORE at any given altitude then to obtain the same power)

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Snowygrouch said:

You need to define what you are referring to as "THE" - before I can answer that in a really useful way.

Pending that happening - I`d add only that very many German engines certainly do NOT have throttle plates....like virtually the entire Jumo series for example. I`m assuming there that you are referring by "plates", to a throttle butterfly used to regulate engine speed so it can cruise and idle and so forth. Most of which doesnt really apply to the entire DB series of engines either.

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Spark ignition engies are regulated by controlling air flowing because there is only a narrow range of air:fuel ratios where stable combustion is possible; they are all throttled. Unthrottled spark ignition engines require more than fuel injection.

Nobody, anywhere on this entire thread has suggested that HCCI was being done in ww2 in service. (HCCI being the current fancy acronym for a petrol engine geninely without any air throttles at all).

If you read the posts - you`ll see that the discussion you`ve picked a paragraph from was about throttle and carburettor choke losses, I stated that the Jumo has no plate throttle. Which it doesnt - it has a barell throttle - which when fully open forms a hole...literally - so has zero flow restriction at 100% open position. Its important not to try to call every single throttle system a "plate" - which was what that paragraph was about.

Just as an aside: German engines were not unique in not having throttle plates. See, for example Aircraft Carburetors and Fuel Systems: A Brief History - 09

The stuff on AEHS, which you`ve linked to is always extremely good. But I am unable to find the words "throttle" or "plate" in that particular link... did you mean to post a different report ?

GregP said:

The correct terminology depends on when you talking, and seems to change whenever marketing comes up with a new catch-name to charge more for it since it now seems "new."

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..and you only have to look at the dozens of different names for cockpits and flight manuals as couple of prime examples of terminology changes

cockpit = crew compartment = flight deck = crew centre = = = = =

Flight manual = pilot notes = pilot operating instructions = pilots flight operating instructions = handbook of operating and flight instructions = flight crew operating manual = = = = =

gumbyk said:

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.
View attachment 408321

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This is coming kinda late, but I don't visit here very often. Is this from a UK manual? I ask because it the case of a Merlin Mark 3 or Mark 7 (the diagram would apply in either case. The part that's labeled "intercooler" in these drawings, is called the "aftercooler" in the US. The detail on the right side of the drawing that says "Delivery from intercooler radiator" shows the "plumbing" that runs into a fitting on the supercharger case in between the two stages. The fitting leads to a cast-in liquid passage that more-or-less encircles the front (discharge) end of the first stage - near the point where the two stages are bolted together. This "liquid passage" is actually part of the case of the first stage (the rear stage) and absorbs some of the heat of compression from the first stage before it enters the "eye" of the second stage supercharger, and because it's in between the 2 stages of the supercharger system, it's called the "INTERcooler."

The large aftercooler (as noted above, says "intercooler" in the drawing), has that name "AFTERcooler") because it cools the charge after it's discharged from the last (2nd) stage of thje supercharger.

This naming convention, at least in the US, is based on what I've tried to explain above.

Thanks!