Altitude Compensating Carburettors, Pt.3: German (continued) A

[LewisS]

I may be mistaken, but wasn't there an inline 8 version of the DIV it was troubled with reliability issues but it was used in the later Albatros two seat bomber. Was it also altitude compensated

[Bletchley]

Sorry, I am not sure about the geared Mercedes D.IV 8 cylinder - I don't have enough information on that one from German sources. I know that it wasn't altitude compensated (as it did not have an altitude compensating carburettor), but I don't know if it was altitude adjusted (and if so, to what extent). The evidence I have presented in this thread indicates that most of the other mid-war but lower compression and larger German engines used on 2-seaters (such as the Benz Bz.IV, Mercedes D.IVa, and Argus As.III) were 'altitude adjusted' by around 4%-6% of their nominal power to about 1300 m, so I think it is possible (likely?) that the D.IV was as well (CR 4.78:1, measured by the British) - but I have no evidence for this. Most of the information that I have for this engine comes from:

Ministry of Munitions. Report on the 240 hp (8 cylinder) Mercedes aero engine. March, 1918 (in, 'Piston Aero Engines of the Great War', ISBN 9781859758014).

'Die deutschen Militarflugzeuge 1910-1918' (from a post by mossie, Gerry Mos, 16 February 2005) has it rated at 220 hp [PS?], and delivering 235 hp [PS?] at 1400 rpm at sea level, but the British tests indicate that it produced around 208 hp on the throttle at 1400 rpm (910 rpm at the airscrew), around 246 hp at 1500 rpm (975 rpm at the airscrew), and a maximum of 268 hp at 1550 rpm. On the power curve, 250 hp at 1400 rpm, 260 hp at 1500 rpm, 275 hp at 1600 rpm, and a maximum of 285 hp at 1750 rpm. There is no indication in either source that this engine was adjusted in any way for altitude, but this does not mean that it wasn't.

The climb performance of an aircraft that used it might give a clue, as most of these 'altitude adjusted' engines show a distinct 'hook' in the climb rate at the threshold altitude - but if this engien, like the others, was adjusted by only 4%-6% to just 1300 m, this would be difficult to spot.

Sorry I can't be more helpful

Bletchley

Second thoughts... looking at the data again, if the German rating of 220 hp/PS at 1400 rpm is correct, then the British reading of around 208 hp on the throttle at 1400 rpm would suggest that this engine, by 1918, might have been 'leaned down' by 4%-6% (so an earlier version would have produced 220 hp/PS on the throttle at 1400 rpm). But the throttle curve meets the power curve at 265 hp at 1550 rpm - this is the point on the throttle opening for maximum permitted speed, I think (and the throttle lever movement appears to have been stopped at this point, so the higher rpm beyond 1550 rpm were probably not available to the pilot), which suggests that although it was officially rated at 220 hp/PS at 1400 rpm the 'normal' running speed may have been around 1450-1500 rpm (225-245 hp), with a maximum of 265 hp at 1550 rpm.

[Kacey]

Originally Posted by Bletchley

Second thoughts... looking at the data again, if the German rating of 220 hp/PS at 1400 rpm is correct, then the British reading of around 208 hp on the throttle at 1400 rpm would suggest that this engine, by 1918, might have been 'leaned down' by 4%-6% (so an earlier version would have produced 220 hp/PS on the throttle at 1400 rpm). But the throttle curve meets the power curve at 265 hp at 1550 rpm - this is the point on the throttle opening for maximum permitted speed, I think (and the throttle lever movement appears to have been stopped at this point, so the higher rpm beyond 1550 rpm were probably not available to the pilot), which suggests that although it was officially rated at 220 hp/PS at 1400 rpm the 'normal' running speed may have been around 1450-1500 rpm (225-245 hp), with a maximum of 265 hp at 1550 rpm.

Hello Hugh,

Actually, I believe there is a little more than meets the eye here.

This original aeroengine was listed by the Aldershof test data as:

Mercedes D.IV
8 cyl, inline, water coooled, 140 x 160mm, 4.73 CR, 235ps@1400rpm@msl

British, RAE test data also reference and report on such an engine. (NACA #15). Airplane Engine Encyclopedia, 1921 & Aerosphere 1939, also repeat this same data (however they misreport the displacement).

Now the first production units of these engines began to break their crankshafts: to long, not supported correctly, inherently unbalanced as most to WWI era standards. Hang a wooden prop in ever changing weather with wartime nicks and scratches, and you have vibration balance problem that started this crankshaft breaking phenomenon.

So, I read it was DERATED/RERATED.

Hence, we see another version reported in other sources. Dechamps reports on page #6 :

Mercedes D.IV
8 cyl, inline, water coooled, 140 x 160mm, 4.60:1 CR, 220ps@1400rpm@msl

This later version (derated) I believe, is the one most people are referring to as the 220ps Mercedes D.IV aeroengine.

Anyway, trying to use the 220ps and the 235ps numbers for the same engine I believe is a crossover between two different versions.

Respectfully Submitted,

KC

[Bletchley]

Thanks KC, I missed those references - I will look at it again. although I expect you are right

Second thoughts again...

Could the higher compression version have been the later one? The one that was tested by the British in May 1918 appears to have been the higher compression version (4.78:1 measured, which is close to the german 4.73:1), and appears to have been capable of producing 250 hp at 1400 rpm, but was then 'held back' to around 208 hp, as shown on the throttle and power curves.

Could they have taken the original 220 hp rated version (CR 4.6:1), maximum power 235 hp at 1550-1600 rpm, increased CR to 4.73:1 and 250 hp at 1400 rpm, and then 'leaned' it back by 18% power (just under an AFR of 20:1 at ground level) to 'de-rate' it to 208 hp at 1400 rpm for full continuous running? It would then maintain this power to around 2500 m, although by moving the throttle to increase engine speed from 1400 rpm to 1550 rpm at lower altitudes this would open the throttle further but also 'enrich' the mixture to provide a maximum of up to 265 hp for just a few minutes only (sea level), but less at higher altitudes, until around 2500 m is reached. Opening up the throttle above 1400 rpm would therefore increase power but also reduce the 'threshold' height at which the mixture became full-rich.

The higher-compression version would not have been regarded as a 'high-altitude' engine as, by German standards, a CR of 4.73:1 is still low and it clearly had no altitude compensating carburettor - but was merely 'altitude adjusted' and would have lost power rapidly at higher altitudes above 2500 m.

Does this make sense? Would an increase in CR of 0.13 have ben enough to raise the output by 30 hp at 1400 rpm?

It is possible that the lower compression engine was also 'altitude adjusted' down from 220 hp to run at around 208 hp at 1400 rpm, as this would have meant it was 'leaned down' by around 5% power to give it a threshold height of around 1300 m. The original engine may have produced 220 hp at 1400 rpm with no 'leaning', hence the 220 hp rating.

Bletchley

[Kacey]

Bletchley,

Could be, might be , however the Aldershof test data on the original 4.73 CR version was in 1914 - 1916 test data.

The British RAE test data I have, from a downed two-seater Albatros on May 12, 1917, also states 4.73:1 CR (Flight mag 5/23/1918).

And the lower compression 4.60:1 data is in Dechamps from 1921.

So I believe it occurred in that order.

KC

[Bletchley]

Sorry, yes, the RAE report is dated March 1918 but the engine was taken from an Albatros brought down in May 1917. You are right about the compression ratio, also, as I mis-read '3' for an '8', so it is indeed 4.73 not 4.78

The data on p.6 of Dechamps & Kutzbach isn't dated to 1921, but I would agree that the two authors appear to have provided data for the latest version of the engines available at that time (so an Argus As.III is listed, but not I or II, the 160 hp Beardmore not the 120 hp, and so on), so I guess you are probably right about the sequence. I wonder when they 'de-rated' it in this way? They were clearly still using the earlier, higher compression version in the spring/summer of 1917.

Interestingly, the March 1918 report states that "this engine, according to reports, is now obsolete, the design having been abandoned in favour of the German standard six-cylinder vertical type", which does suggest that it may have been withdrawn by the spring of 1918, but then goes on to comment that "details and illustrations of the engine will no doubt prove of considerable interest, more particularly to those interested in the design of the special engines used for airship work", so I would guess that it is possible that this engine, or the later and lower compression version, may have been used to power airships instead?

It is also a bit odd that it took the RAE engineers 9 months to assess an older engine, from the 1914-16 period, that was already 'obsolete', and regarded as relatively uninteresting from a technical point of view - unless this is because it had found a new operational use?

Bletchley

[Wad]

Many thanks for your excellent research of WWI engines, I read everything with great interest. I have performed some calculations, you may be interested to look at this in the ROF forum.

[Bletchley]

Hello Wad, and wellcome to the Aerodrome

Thank you for your post at the RoF Forum - I found it very interesting, and I appreciate the time that you have taken to do this. I have replied there - but could you re-post your calculations on the Mercedes D.IIIa carburettor here? I know that there are several people who hang around here who would be very interested in it.

Bletchley

[Wad]

Hello Bletchley, glad to see you.

Perhaps it would be enough if you put a link to my post? I can't do it now because of the the forum's rules - I do not have 10 posts in this forum. If anyone have any questions I can answer here.

[Bletchley]

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Bletchley