DB-60X manifold pressure handling

roger Yo-Yo

This is from here,

http://www.kurfurst.org/Performance_tests/109G_DB-G6AS_wMW/DB_109G6_ASM.html

it has no accompanying text.

It appears from the illustration (2nd right) that blower output declines with decreasing barometric pressure above VdH? Clearly shows no disadvantage when throttling back to 1.3ata above vdh, speed is only 22kph lower at 8.6km (1.3 vs 1.7 ata same height.)

Me109 GJ+FX was a 109g used as engine test bed, would have had small blower (not db603 type) fitted I think. Suspect your restriction had no effect on performance. Hope this is helpful.

A stylised diagram of the system from Flugmotoren und Strahltriebwerke:

Yo-Yo not sure if the following document is relevant but it's an examination of the DB-601N engine and controls.

http://kurfurst.org/Engine/DB60x/files/Flight_16April1942_DB601N_Engine.pdf

And another page related to the DB60X engines, documents related to increasing engine revs.

http://kurfurst.org/Engine/DB60x/DB601_datasheets_A1.html

While I can confirm that the throttle workings in the DB are somewhat odd in that the pilot controlled throttle does not fully open above full throttle altitude, I found no pilot instructions that there was something to do about this as routine. The basic handling was to go by rpm, not boost, with no rpm increase noted above full throttle altitude, even on a couple of Betriebsdatentafeln that have an optional, separate column for high altitude. The column was left blank. I looked into this just recently.

It seems to me that the DB throttle system is much simpler than on other engines, as it does not need to do much to limit the air pressure delivered by the supercharger. All it does is maintaining the required manifold pressures and prevent too high pressures.

Whereas on simpler supercharger designs like the Merlin the supercharger always develops a fixed maximum pressure (ie. a LOT more than desired) via RPM through a fixed engine-to-supercharger gear ratio, which then needs to be regulated and then throttled to the desired engine setting.

The DBs gear ratio is, in contrast, flexible (at least with "second" supercharger speed, the first one is fixed ratio) and limits supercharger pressure (Geblaesedruck) in the supercharger itself to a very slightly higher pressure than required - in other word, before the air would meet the throttle(s).

So the DB throttles only need to concern themselves to choke and limit the air pressure to engine, they do not really seem to regulate in the traditional way the air pressure from the supercharger. That is already done by the barometrically controlled hydraulic coupling!

The DB 601/605 Lehrtafel notes that there 1+1+2 throttles in the system:

1) Regulator throttle (Reglerklappe), this is directly afterwards the supercharger. It is automatically controlled and is largely independent from the pilots lever position. Its main purpose is to limit the pressures at low altitudes (normally to Kampfleistung, ie. 1,3ata on the 605A (that would be 1,45ata for 605DB/DC)

It seems the only special position is when the pilot lever is advanced to Notleistung lever position is reached, it opens to allow for Notleistung pressures). It prevents too large pressures, and its main use is to limit pressure near sea level, since the first supercharger speed of the DB 60x series is a fixed ratio, just like on other engines, and can and will deliver very high pressures at SL, well above allowable boost pressures. Otherwise its always fully open at higher altitudes like Volldruckhöhe.

This is how it works in detail:

2) Idle air inlet (Leerlaufbohrung), a fixed size inlet (just a drilled hole, really) that is the only thing letting air to the engine at idle, its air let-through capacity is governed by a screw adjustable by the ground crew.

3) Idle throttle (Leerlaufklappe), a varying small throttle that practically opens fully above idle pilot lever position

4) Main/Performance throttle (Leistungsklappe), that opens gradually abve idle pilot lever position and is fully open at Notleistungs lever position

Note that real regulation is not neccesary anymore in the second supercharger speed, since that is done by the barometric hydraulic drive of the DB-supercharger..! These throttles only work to limit the charge pressure, ie. maximumizing the allowed pressure in the engine. To me its seem it would logically follow, that if the air pressure actually coming from the supercharger is less than that (ie. higher altitudes above VDH), then its not a problem since the throttles are already in a position that would let more air to enter into the engine than what actually is supplied...

The DB Lehrtafel note three special stages of throttling:

I. Idle lever position

2) Idle air inlet (Leerlaufbohrung), this is actually just an opening of fixed diamater giving a minimum air into the engine during idle throttle position. The amount of air can be set with a screw.

When at idle, all other throttles (except the first Reglerklappe, whichsee below) are fully closed.

II. Interim lever position

The small sized Idle throttle opens (it does seem that it practically fully opens anywhere above idle lever position), and the larger main/performance throttle also begins to gradually open.

So now the air goes into the engine via the idle air inlet (fixed), idle throttle (practically maximum) and the performance throttle (Leistungsklappe, with the opening increasing as does the lever position).

III. Starleistung position

All throttles, including the Leistungsklappe are fully open, the air goes into the engine at maximum rate.

So I think there is probably no trouble with the manifold pressure position, because there is not just the main throttle opening (which indeed does not open fully in an interim lever position, and would limit boost pressure IF IT WOULD BE THE ONLY THROTTLE CONTROLLING IT - BUT IT IS NOT!), but also the idle throttle plus the idle air inlet that lets in air.... the main/performance throttle is just "extra"...

Ok, but the main question is what is the authority and control curves for the automatic regulator and manual throttle.

As for me, I would design (If I were a German engineer :) of 40') automatic regulator maintaning main significant MP ratings and then manual throttle that maintains idling and low power ratings.

 

Studying the DB manuals I can see that automatic regulator is controlled with the engine lever, so initially I presumed that it works in wide range of MP. But I was told that the MP regulator adjusment is effective only to change Start and Notleisung AS LIMITS.

 

THe diagrams shows that the MP regulator is controlled linear from the lever.... where is the truth? :)

As I understand from the 601/605 Lehratfeln posted just above, the automatic regulator is NOT controlled by the pilot lever at all, normally it sets itself to whatever the Kampleistung rating is (which is 1,3ata onthe 605A, and would be 1,45ata on the 605DB/DC).

And it keeps itself that way, unless the pilot lever is advanced to Notleistung, which works as kind of an override and opens the regulator throttle further to give Notleistung pressure levels.

The only really "moving" throttles are the next ones operated directly by the pilot lever, but these just further throttle down the pressure, if needed, from that 1,3/1,42ata (or whatever the max. Kampf/Noteleistung ratings are) that is allowed through by the (first) automatic boost control throttle.

I think this is where from the the confusion originates.

What I don't understand about the whole thing, sorry YoYo for hijacking a bit here, is that the pressure between the two throttles was for the most part controlled by a single aneroid, the only exception being the manual input for the Notleistung override. Now both the documentation and my technical understanding say that this system limited the boost to a single fixed value (of 1.3 ata), provided the supercharger managed this. Now this works OK for as long as the pilot chose combat power, but at lower power settings with lower rpm the supercharger only manages to deliver 1.3 ata at low altitudes, for instance at 2300 rpm only up to ~1300m. If the automatic regulation now always limits to 1.3 ata, this means the first throttle is fully open and the second throttle is closed a little, which it is since it is controlled directly by the pilot. But this would only results in the 1.15 ata going with the 2300 rpm at altitudes below 1300m, and will be slightly less, and varying, at altitudes above 1300m.

However, the engine chart gives a full throttle altitude of 5500m for 2300 rpm and 1.15 ata, which is exactly what the supercharger can manage - at the exit of the supercharger according to DB documentation, not behind two throttles. Which in turn means that both throttles need to be fully open, at least not restricting anything, in order to deliver the full full throttle altitude. But then I also don't see anything to automatically adjust the pilot linkage, which leaves me in a bit of a dilemma since they way I understand the system - automatic only regulation on the first throttle for 1.3 ata and manual only regulation on the second throttle doesn't result in a constant 1.15 ata up to 5500m, as indicated by the DB documentation.

For the record - a throttle does reduce pressure. It always does, there's no "sufficiently large" or something. So that's not the explanation.

One question I could not answer myself - where does the boost gauge in the cockpit get the boost information from? Would it be possible DB fed the supercharger pressure instead of the pressure directly before the cylinders to the pilot?

Would it be possible DB fed the supercharger pressure instead of the pressure directly before the cylinders to the pilot?

Pretty sure not as this would be nonsensical. Let's not get this thread derailed though.

This is diagram of a DB601N's operation. Below view of throttle connections and components.

These are from http://www.ww2aircraft.net/forum/engines/diamiler-benz-601-603-aircraft-engine-manuals-38202.html Upper link for db601

You may have to register :)

It would seem the DB supercharger was set up so that it would always deliver a bit more pressure than needed, even above FTH, by about 0.05 ata.

So for example on this chart (n.b. that it seems the engine develops a lot less FTH than it should, probably had some fault) at 7000m the supercher develops 1.18 ata pressure (Geblaesedruck curve), which is then throttled down 1.14 ata manifold pressure (Ladedruck curve).

While this is not full performance, the FTH decrease is just about 200-250 m from the theoretical maximum, which is probably not practically achievable at all times anyways because of tolerances, external conditions etc.

But it also means that there always a surplus pressure before the (first) pressure regulator (which is always letting through at least Kampfleistung pressure, i.e. presumably up to 1,45ata on the 605D), so I have doubts if the performance throttle would really not let through the full pressure at all times, since it always receives more pressure than needed! Note that (at least if I read the description of the Laderdruckregler correct) at high altitudes, the first pressure regulator is completely overridden and is fully open at all times!

See Bild III, Reglerstellung in Volldruckhöhe (Regulator setting at full throttle height):

It's also interesting and could be related or a hint to how things were working that DB manuals usually note that the actual manifold pressure may exceed nominal manifold pressure for a given rating by 0.03 ata during climbs and this is normal.

For achieving cruise or econo conditions, the manuals I have seen suggest manual prop pitch control.

Also, increasing the revs above rated altitude to maximum revs was permitted, so I guess in practical conditions the pilot would just advance the throttle a bit, which would drive the supercharger harder and open up the throttle more. See the K-4 performance chart indicating "Drehzahlsteigerung" or rpm increase for the 30 minute rating (which is normally 2600 rpm) at about 8800 meter. The performance matches that of the full rating, so it would seem the full 2800 rpm was permitted at higher altitudes for extended periods.

YoYo, do you have the DB 605 Einbaumappe available from Luftfahrtarchiv Hafner? It contains a few charts that may be helpful - for instance a chart with several boost over altitude curves for various rpm. It states the figure I quoted above. Edit: There's also useful info regarding the relation between throttle opening and power control lever position in the DB 605 Anleitung fuer Kontrolle, page 168, available there. It can for instance be seen there, that the throttle can be considered fully open with Kampfleistung already. Activating Notleistung will only increase rpm and setting for the boost control. If you need help translating, please let me know.

I have for the 605AS if that helps.

Found this description of engine operation here: http://www.pilotfriend.com/aero_engines/aero_db605.htm

Sadly doesn't cite any original docs, but interesting because of reference to start up boost being controlled by 'clockwork mechanism'. First time I've come across that! Useful description of interrelationship between blower and throttles. Best of luck Yoyo :)

"Centrifugal supercharger on port side of engine driven through a fluid coupling by a shaft at right angles to crankshaft. This shaft is driven through bevel gears from the crankshaft, variation in propeller speed secured through variable filling of fluid coupling by two-stage enginedriven pump receiving lubricating oil from the main pressure filter.

First stage delivers oil direct to coupling and second stage delivery is passed in varying proportions between crankcase and coupling by piston valve controlled by a capsule which is sensitive to inlet pressure. Second stage cuts in at approximately 5,000 ft. and full delivery occurs at approximately 11,500 ft.

Butterfly throttle which is capsule controlled regulates supercharger delivery, second throttle which is pilot operated controls air supply to engine and manifold pressure, first throttle subjected to pressure between two throttles, increased boost for take-off controlled by clockwork mechanism, mixture delivered by supercharger to looped manifold by large diameter pipe, dry-sump pressure-feed lubrication, gear type oil pumps, spray of oil directed upon reduction gears, main oil pressure line feeds crankshaft bearings, secondary line feeds supercharger fluid pump. "

Found this description of engine operation here: http://www.pilotfriend.com/aero_engines/aero_db605.htm

 

Sadly doesn't cite any original docs, but interesting because of reference to start up boost being controlled by 'clockwork mechanism'. First time I've come across that! Useful description of interrelationship between blower and throttles. Best of luck Yoyo :)

The clockwork mechanism was only there on the 601A series, later ones did not have it. But the special hydrocoupling of the supercharger remained, hence the characteristic power curve of the DB 600 series.

Some bits from the book Powering the Luftwaffe

Yo-Yo, I think it is pretty clear what happens for Notleistung und Kampfleistung (Regelklappe regulating to 1.42 ata or 1.30 ata (DB605A), Leistungsklappe fully open). I'm still not sure what happens at lower power settings, it still seems to me that the dual throttle will not maintain constant boost over altitude (Regelklappe fully open for the most part, leaving through whatever the supercharger can deliver (at cruise settings less than 1.30 ata at most altitudes for the DB605A), Leistungsklappe at constant opening). Boost at cruise settings not being constant over altitude is new to me, and doesn't really agree with the popular engine charts. These might of course be simplified, but still it seems odd. I'd appreciate if you could share your findings, understanding and conclusions on the subject, if you can spare the time.

Forgive me, I'm hardly and expert on this, but let me see if i get this straight.

So as i understand it, the automatic system for regulating boost and RPM on the 109 only works up to the max continuous setting? And that over that setting you have to manage pitch, rpm, and throttle manually like a old style BoB era plane? That sounds like a terrible way to do things. I am probably not understanding correctly however.

Its automatic all the way (with manual pitch override as an option), these details are about how actually the system works.