Thanks Wilhelm, I had a handle on the basic concept of how that engine worked, but never really heard the details of how the two throttles interacted.
Also it's good to hear the Mercedes D.IIIau engine mentioned. IMHO the self-damage aspect of those two engines has been under-modeled; at one time they were far more vulnerable to improper operation in RoF. However, there was a huge outcry from the community as to their fragility, and the response was to make them much more resistant to over throttling. I've even heard pilots say that they thought the Halb CL.II 200-HP "just sounds that way" normally when the throttle is too far forward at low altitudes, and had no idea it was because the engine was knocking and theoretically damaging itself. It's become standard procedure for those Halb pilots to run with the engine knocking all the time, and never incur any noticeable damage effects to the engine in the game. I will admit that in the early days of RoF when all the engines were far more vulnerable to self damage, there were times that the engine would "oil" for reasons impossible to fathom, and the game did need improvement, but not maybe to the extent that things are now. Even the S.E.5a was considered to have a "glass" engine, although in my opinion it could easily be operated without killing it if proper attention was paid to the RPM limit.
There are still planes in RoF that need "fixing" to make them reasonably tolerant, and one of these is the Fokker D.VIII. Simply flying this plane at high altitude for a short time (above 4 Km or so) will destroy the engine even in level flight, due to the effects of cold temperature on the rotary engine. To see this effect it is only necessary to enable full gauges, and then watch the redline of the tachometer creep slowly down until it actually goes below the current RPM and the engine oils itself. This is something that should not happen, and it's indicative of the types of DM problems that existed for many engines when RoF was newer.
There are may pilots in RoF that are very acclimated to flying planes in which it is almost impossible to cause the engine to over-rev, a lot of which is caused by it's prop pitch and the resistance of a large rotary engine to changing speeds quickly due to the flywheel effect. They then complain loudly when they get into different planes which don't act the same, and accidentally destroy the engine.
I don't know how much of this will change in the future with FC, but over all in RoF things ended up being far more realistic than in other, previous sims. Hopefully, they'll provide a Mercedes D.IIIau version of the the Fokker D.VII, the lack of which in RoF is something that causes a lot of degradation of that plane's reputation in that sim. Right now, we are stuck with an "either-or" situation for the D.VII, and it's seen either as a crappy plane which is not worth flying, or an uber plane that needs to be severely limited because of it's huge perceived advantages, and pilots of which are not respected because they're seen as too easy to win in. Neither perception is accurate, but it illustrates how people think.
BTW, at one point GregHF states that the Mercedes D.IIIau is also used in the Albatros D.Va. I do not think that was ever the case in RL; in RoF it is not the case, otherwise it would operate exactly the way the Halb CL.II 200-HP does. In RoF the Alb D.Va has the normally aspirated Mercedes D.IIIa, which they show as having 180 HP at sea level, but which loses power in a conventional manner at higher altitudes. There are some pilots in RoF that think, or at one time thought, that the Alb D.Va has a more powerful engine than the Alb D.III, which was never the case. In the early days of RoF, the Alb D.III OAW was a slightly better plane than the D.Va, which was an opinion that many German WWI pilots actually held. Since RoF "fixed" the top speeds of many planes in it's final update, it is no longer the case.