Mercedes Diiiau Engine Description and Test Data

[No.23_Starling]

All, whilst scanning through Jane's Fighting Aircraft of WW1 I found a decent chapter on the various Mercedes engines, including the Diii, Diiia, and Diiiau. It offers the description and data from tests performed by the R.A.E. on engine no.775 taken from Fokker DVII G2 B/14, captured on 6th June 1918, alongside photos of both the 180hp diiia and 200hp diiiau. 

 

I've included the full text and images below. Should 1C look to add this engine it might help explain its operation and offer some contemporary test data. It highlights the main differences, being the new high compression pistons (similar to the Hispano-Suiza concept for the HS8b 220hp) and the carburetter with altitude control, which the book mentions was "so arranged that it is impossible to run the engine all out on the ground."  It mentions that the throttle control itself was "marked for opening out to its maximum ground level position." The main operation of the throttle past this point was to "uncover extra air holes, thus weakening the mixture to such an extent that the engine stops if the throttle is fully opened on the ground."

 

My understanding of the 'over' compression from the 5.73:1 ratio is that it was too high to allow full power at lower altitudes as it might risk knocking and predetonation; the carb + throttle design helped avoid this; the throttle notch is visible in the DVII cockpit photo included in Anders' book. Curiously though the British performed a test with the normal diiia carb and managed to produce 204hp at 1,600rpm, but experienced issues with overheating.

 

My question for @Holtzauge and the rest of you engineers is whether the diiiau produced more HP at sea level than the diiia using the special carb, or if the over compression meant that the extra HP would only kick it at higher altitudes where air was thinner? Would the power curve have looked the same as the diiia up to 2km which I understand was the altitude at which the throttle could be fully opened?

 

 

 

 

Edited April 27, 2024 by US103_Rummell

[Holtzauge]

Sorry for the delay in answering @US103_Rummell, but you know why already. Anyway, just sent you a mail with my thoughts on this.

 

 

[No.23_Starling]

On 5/1/2024 at 12:17 PM, Holtzauge said:

Sorry for the delay in answering @US103_Rummell, but you know why already. Anyway, just sent you a mail with my thoughts on this.

 

 

 

Fantastic as ever, @Holtzauge and interesting to see the wealth of research already available and done in the last year. I think Jane's took their article from Flight magazine; this is still available online with more data from the British tests and nice technical diagrams: https://archive.org/details/Flight_International_Magazine_1919-02-20-pdf/page/n7/mode/2up 

 

It includes sketches of the carb with features / holes for leaning the mixture in the 'H' position or altitude, and the duplex airpump which can be seen on the front of the engine in the photo above - probably the only GFX model change we might need for the variant to be added. NB the visible difference with the Diiia is its little bell shaped protrusion that we have in game:

   

 

It's fascinating to note that the Flight Magazine figures are in agreement (within 2%) with the performance data from Heinrich Dechamps and Karl Kutzbach (1921), as well as with the surviving test report from Friedrichshafen. At engine speeds below 1350 rpm D.IIIa actually produces more power than D.IIIaü, however at 5km the difference is substantial in orders of magnitude. Interesting also that the original instructions are to open the throttle to the H position from 2km gradually to raise RPM to max till around 2.5-3km when it can be opened fully without RPM drop from over-leaning. Excellent to see real photos of the carb too provided in the different throttle positions (hohengas open and then closed, showing the teeth aligning to lean the mixture):

 

 

 

Summary chart below taken from YavorD from the Aerodrome post - the sources used are in the thread.

Probably the best summary of power vs alt performance I've seen and what the devs would need:

 

 

As per your note to me, it appears the Diiiau produces slightly more power at sea level than the standard diiia, but due to the leaning from the carb to prevent overheating (as seen by the British in their tests) the additional power starts to be felt at 2km as the throttle is pushed into the H position (gradually!). This also rules out pilots opening the Hohengas down low without first changing the carb, as the engine simply cuts - even if they did have the older carb fitted they would have seen engine damage and potential loss of power up high due to the new mixture conditions. I've not seen any sources suggesting pilots did this.

 

I believe that between these sources and your collective minds there is sufficient data to build models for these engine variants. @LukeFF I know the developers only try to make changes when strong data is available - @Holtzauge on this forum you, Bletchley, and a few others provide plenty of primary and secondary sources.

 

There's at least x4 airframes that need this engine for 1918 scenarios to be closer to reality for purposes of simulation: Dvii, Alb Dva, Pflz Diiia, Pflz Dxii with the Fok Dvii being the most urgently needed as the most commonly fitted engine, far more so than the BMW which also borks MP balancing.

 

Next we could use the 220hp HS8B engine for the SPAD XIII, Dolphin, and SE5a (before the Viper), as well as the Alb DV with the older Mercedes engine for 1917 scenarios!

Edited May 5, 2024 by US103_Rummell

[US103_Baer]

Ha! Was just reading that Flight mag (Feb 1919) a couple of weeks ago after going thru the DXII thread on theaerodrome. Essential reading for anyone interested in the DIIIa and DIIIau engines.
The Flight article has a bunch of original graphs including power curve comparison with the 160hp engine too.

 

It also has fascinating Performance data for aircraft used by the Italian Air Service and those built and used by the US. 

 

Something that really caught my eye was actually the start of the next article.

Edited May 6, 2024 by US103_Baer