Vivian Meazza wrote: > Back from a holiday in France. The modified software tests OK > here. One small snag: the MP is not ambient when the engine is not > running. Strictly, this should be turning, I suppose, because a > wind-milling supercharger will produce some pressure. Near enough > though.
Yeah, good point. But it's more complicated than running vs. not running I think. If a normally aspirated engine is turning with the throttle closed (whether or not it is burning fuel), it will try to suck a vacuum in the manifold. Superchargers will produce the same RPM-to-boost curve as they do when the engine is running, because their impeller is being driven at the same speed. But turbochargers don't, because their impeller is driven by exhaust pressure; in a windmilling situation, the exhaust pressure is going to be *less* than ambient and the turbo will want to turn backwards! (No idea if this actually happens or not -- is there a relief system to prevent this?) Anyway, the bug here is obvious: playing with the throttle on the ground with the engine shut down cause the manifold pressure gauge to move up and down, which clearly doesn't happen in reality. But a fix is going to be subtle and hard, so I'm willing to live with this until we can come up with a realistic set of requirements. While we're listing obvious shortcomings of the current model, note that it also doesn't handle intercooling, which I'd argue is more important than getting idle/windmilling manifold pressure correct. :) > I've wasted a couple of days coming to the conclusion that the > published graphs for the Merlin show the so-called "boost pressure" > in psi absolute, with zero at 1 atmosphere. You mean "gauge", right? Normally "absolute" pressure connotes interpreting zero as vacuum. > Moving on, the Boost Control/Boost Control Cutout has been written > in Nasal. This works well, but YASim needs to be told to ignore the > wastegate. I tried setting an improbable value for the wastegate, > but of course, YASim uses the wastegate value when solving, so this > doesn't work. I'm a little confused. The boost cutout works by mapping a property name to the wastegate setting. You can set that property to any value you like (say, 1000000) during solution. Note that the "improbably high wastegate" mechanism is actually the way that YASim implements this internally. For a normally aspirated engine, the _maxMP value is set to 1.0e06, so I know that this will work. The supercharger boolean isn't a bad idea in principle, as it will be needed eventually to handle the difference between turbocharging and supercharging. But I'd rather wait until we have an actual implementation in mind before adding it. I'm pretty sure your current requirements can be met with existing code. Andy _______________________________________________ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
