Engine management of the Mercedes DIIIaü

[PreußensGlanz]

On several threads I found these infos about the management of the DIIIaü:
***
DIIIaü
At low altitudes the throttle was advanced to its maximum for low altitude working, to a point on the throttle quadrant marked by an new instruction to pilots to go no further until higher altitude was reached. At 1400 rpm the engine had an output of around 160 Ps (measured at 164 hp adjusted to sea level by the British, using a petroleum fuel) and was 'leaned down' to approximately 90% 'nominal' power - this 'leaning' is indicated by the British attempt to get more power from the engine, replacing the D.IIIaü altitude compensating carburettor with its small 250 cc capacity main jet with the 'standard' D.IIIa carburettor with its main jet 'beefed up' from 350 cc to 450 cc. With this 'standard' carburettor the engine produced its otherwise 'nominal' 180 hp at 1400 rpm (an increase of approx. 10%). Using the D.IIIaü carburettor and maintaining a 1400 rpm, pilots could maintain 160 Ps up to 1800 m (the full throttle height) without any further movement of the throttle - just as with the D.III or D.IIIa, the carburettor 'adjusted' for altitude by letting the mixture go from a 'lean' mixture of just under 18:1 AFR at ground level to a full-rich mixture of around 12:1 or 13:1 AFR at 1800 m. At this point the pilot could advance the throttle into the final 'altitude' section of the throttle quadrant, and this would then uncover additional air holes in the throttle barrel that weakened the mixture even further (up to an AFR of just under 20:1 ground level equivalence, when fully open) but added no (or very little) extra fuel as the small 250 cc jet was by now supplying fuel at full capacity (the D.III/D.IIIa carburettor, by contrast, had a larger 350 cc capacity main jet, and this would support engine speeds of up to 1700 rpm). This 'leaning' in the D.IIIaü carburettor can be seen clearly on the relative fuel consumption graph in the British test report which, instead of having a standard 'dish' shape (increasing from a lean-ish setting to full rich as the throttle is increased from half-throttle) it is down-hill all the way. From altitudes of around 1800 - 2000 m the pilot could continue to 'lean' the mixture by advancing the throttle, manually maintaining a more-or-less constant AFR up to greater altitudes - power would still decline, as normal altitude effects started to have an impact from 1800 m, but the 'double whammy' of both altitude and mixture enrichment effects would be delayed up to much higher altitudes of 4000 m or more. Although the D.IIIaü was rated at 160 Ps at 1400 rpm, British tests indicate that it ran well at up to 1600 rpm. At 1500 rpm the British measured an output of 171 hp adjusted to sea level, and it is therefore likely that the engine could comfortably achieve an output of 180 Ps at ground level, and it is often seen rated at this higher value.


The throttle/mixture control was similar to that in the Mercedes D.IIIa carburettor, but with two important differences - the size of the main fuel jet (ejector) was smaller, at a 250 cc capacity as compared to the larger 350 cc capacity of the D.IIIa, and the throttle barrel had an extension to it that was meant to be engaged only at altitudes above a threshold or barrier height of 1800 m. At ground level and at lower altitudes, the pilot was instructed to use only the first movement of the throttle lever: a maximum 160 Ps at 1400 rpm. The engine was 'leaned' in such a way (by the small jet/injector) that this 160 Ps at 1400 rpm was maintained, without any further movement of the throttle lever or any other mixture control lever, to around 1800 m (going from lean at ground level to fully rich at 1800 m). At 1800 m, or when the pilot noticed engine rpm starting to fall, he could engage the altitude control by pushing the throttle lever forward into the final 'altitude' section of the throttle quadrant - this would then open up additional air ports in the throttle barrel, and the first movement would draw more fuel from the jet/injector to maintain power as the aircraft continued to ascend. By pushing the lever forward every time rpm dropped, to regain 1400 rpm, power could be maintained at 160 Ps until the aircraft reached an altitude of around 2500 m (we cannot be too exact about these threshold heights, as they were influenced by seasonal and atmospheric conditions). At this point, the small 250 cc fuel jet/injector would be delivering fuel at full capacity and further movement of the throttle lever forwards would only add more air - to maintain an optimum mixture as the aircraft continued to ascend (but no longer compensating fully for altitude effects, so power would start to fall). The small fuel jet/injector was necessary not only to provide this lean mixture at ground level, but also as a 'limiter' to prevent the overcompressed Mercedes engine from producing too much power at low altitude (damaging the engine), as the larger jet of the D.IIIa carburettor would have done.

That was the officially 'correct' way to use the throttle, maintaining full power at 160 Ps to around 1800-2500 m, and then maintaining optimum mixture (but no longer at full power) to the higher altitudes above that. But it appears that combat pilots found an 'incorrect' way to use the throttle to gain an extra 10-20 Ps at low altitude (just as they appear to have done with the later BMW IIIa engine's altitude controls in the Fokker D.VIIF). Moving the throttle lever forwards into the 'altitude' part of the throttle quadrant at ground level or low altitude, if moved only part way, would give the pilot an 'emergency power' of about 175-180 Ps at 1500-1600 rpm - but moving it any further would then exhaust the capacity of the jet/ejector to supply any more fuel (the 'limiter' effect), and would progressively weaken the mixture from an AFR of around 17:1 or 18:1 to around 20:1 (the weaker mixture bringing power and rpm back down to 160 Ps at 1400 rpm). Any further movement of the throttle lever much beyond this point and the fuel mixture would then become too weak to support combustion, and the engine would cut out (so, as the British engineers stated, the throttle could not be fully opened at ground level). But if the pilot stopped advancing the throttle lever at the point where maximum power was obtained, he could maintain this 175-180 Ps to around 1800 m - but at this point any further movement of the throttle lever could only be used to maintain the optimum mixture ratio, so the power would then drop gradually from around 1800 m upwards.

You should be able to see from this that there are two possible climb rates to 1800 m, one at a constant 160 Ps, and another at a constant 175-180 Ps. Although this second option (and all the possible variations between the two) was an 'abuse' of the engine, and was no doubt officially discouraged, the British engineers found that the D.IIIau actually ran comfortably at a speed of around 1500-1600 rpm (two one hour endurance runs), so it is unlikely that a few minutes of 'maximum power' at 175-180 Ps in the initial climb-out or in combat would actually have done much serious damage to the engine. I think that this 'abuse' might have been at least partly possible due to the benzol additive to the German fuel (probably in general use for aviation fuel, not just for high altitude engines, from around the Spring of 1917 onwards), as British engineers (using a high grade "Shell A" 100% petroleum) seem to have experienced some problems getting much more than 170 hp from this engine at 1500/1600 rpm - a slight reduction in power, compared to German figures, caused, possibly, by symptoms of pre-ignition or very mild (possibly unnoticed) detonation. Without benzol (a by-product of the German coking industry) the variable quality lower-octane German petroleum from Galician oil fields (after higher quality Romanian fuel had been denied to them) would most likely have had much greater problems with detonation at these engine speeds above 1400 rpm and at low altitude.
***

Can this be confirmed by other forum members?
Any errors in it?