The Mercedes D.IIIav

[Bletchley]

This post is a continuation of ‘The Mercedes D.IIIaü Carburettor’ post just below this one.

It is a correction to my earlier, very speculative 2009 post on the Mercedes high compression and over-dimensioned D.IIIav post in the original thread on German engines referenced in the post above.

Having read the analysis by ‘Wad’ of the diagram on p.135 of ‘Prüfung, Wertung und Weiterentwicklung von Flugmotoren’ (Dechamps & Kutzbach, 1921), an analysis from 2012 in the 'Rise of Flight' forum that I had not seen until very recently, I am inclined to think that he is right and I was wrong.

I really still don’t know much about the Mercedes D.IIIav, and I have never seen a description of how the carburettor worked - but if management of power with altitude was essentially the same as that for the D.IIIaü then there was just one throttle, with the end of the first part of the movement marked ‘V’ and the part beyond that marked ‘H’ (the latter part of the movement weakening the mixture without adding any further fuel). Back in 2009 I made a rather speculative comment on the D.IIIav at the end of that old Aerodrome thread on German altitude compensating carburettors, arguing that ‘Bodenstellung’ might be the theoretical or ‘nominal’ power developed if the carburettor restrictions were removed (e.g. if a small fuel jet was replaced with a larger one), so that the engine could develop 200 PS at 1400 rpm at ground level. The ‘Höhenstellung’ would then be the ‘carburettor-restricted’ power of 180 PS (reduced down from 200 PS by leaning the mixture) that could be held up to a full throttle height of around 1500 m, at which point the mixture would be rich and the throttle would be advanced into the ‘H’ part to stop the mixture becoming over-rich to an altitude of around 5000 m.

I think now that this is incorrect. I now agree with Wad’s analysis in the ‘Rise of Flight’ forum that the "Bodenstellung” in this context simply refers to the throttle being fully open to the end of the ‘V’ section of the throttle movement (full throttle, 200 PS, for ground level at 1400 rpm) and the "Höhenstellung” is where the throttle lever has been pushed fully into the ‘H’ section (the power dropped to around 180 PS at 1350 rpm at ground level due to the leaning of the air fuel mixture).

Note, however, that the "Bodenstellung” power is achieved with a rich mixture (in the diagram, power falls very quickly from 200 PS at ground level to around 160 PS at 2000 m, and on the two curves for air fuel mixture you can see that the mixture becomes richer immediately from ground level, although by contrast in the “Höhenstellung” position the fuel air ratio remains constant up to 5000 m). This indicates to me that Daimler engineers took a further step back from the extreme leaning of the early D.IIIaü Carburettor, and the partial leaning of the later D.IIIaü Carburettor. The carburettor is now adjusted to run full-rich rather than extremely lean (early D.IIIaü) or economy-lean (later D.IIIaü). This means that there is no longer any sacrifice of power at low altitude for a gain in power and performance at high altitude – the carburettor is now just being used to maintain a constant air fuel mixture from the ground up to 5000 m. This engine would have run very rich at low altitude when delivering the full 200 PS (perhaps to help aid cooling and suppress detonation), so fuel economy would have been very poor and service ceiling and performance at high altitude would have been significantly worse than if the air fuel mixture had been leaned down at ground level to produce 180 PS at 1350 rpm, and then been allowed to recover back up to a full rich air fuel mixture and a nominal 200 PS (actually 160 PS) at 2000 m.

Why would the Daimler engineers do this? The only explanation that I can give is that given by YavorD at the end of that 2009 thread, where he says “Development of such a system was an obvious requirement … D.IIIav equipped Fok. D.VII using "Höhenstellung" was not able to meet 1918 k.u.k. requirement for 0 to 1000 m climb within 2.5 minutes (by memory, I am away from books right now). Something had to be done in order to improve performance below 2000 m! In mid-1918 Daimler was under tremendous pressure to improve its mainstream product, D.III. And German engineers were quite ingenious in designing hydro-mechanical control systems”. I think Yavor was right!

As always, this is work in progress... and comments, criticism, discussion welcome!