> Don't we need to keep R, the universal gas constant, to make the equation > work? > > v ~ sqrt(TR/M) ?
Not when you write it like this, because it's just a constant, so you can pull it out and the result is still proportional to the rest if you drop it, like v ~ sqrt(R) * sqrt(T/M) ~ sqrt(T/M) (you have to put it back in when you want v_E = xxx instead of v_E ~ xxx in the end - but you can gather all numerical constants into a single number in the end, and I guess that's precisely what you'd want to do anyway, see below). > It was my thinking that an increase in RPM would cause a pressure > increase > because: > - The pressure from the exhaust valve falls off over time and having the > valves open more often would increase the average pressure. (untested > hypothosis) > - The piston ejecting the burnt air from the cylinder moves faster as RPM > increases, causing the air to be ejected faster and leading to an > increase > in total pressure. (another untested hypothosis) Air pressure adjusts to changing conditions with the speed of sound (any deviation from equilibrium propagates with the speed of sound). Unless the piston moves supersonically, its movement cannot cause the air to be ejected faster than it would naturally do given its thermodynamics. If it moves supersonically, things become complicated, because then you're into shockwave propagation and then the precise geometry of the engine matters - thermodynamics is then insufficient to provide an answer, you'd need to run fluid dynamics. > Would p_E be the ambient air pressure? And p would not be the intake > manifold > pressure, which we have, it is the exhaust manifold pressure which we do > not > have. [1] I also assumed that a falloff of ambient pressure would be > accompanied by a fall-off in mass flow. *shrugs* Your proposal was to replace the term by a constant. Mine was to use manifold pressure and ambient pressure as proxies for the real conditions (which we do not have). Of course that's not exact - but in all likelihood captures qualitatively what is going on and is better than a constant. In the end, there are hundreds of things you don't know - friction in the exhaust tube for example... its geometrical shape (exhaust velocity isn't actually a constant - there's some spatial profile to the velocity field)... turbulence when the exhaust airstream enters ambient air... and so on. So, you want to write down an equation which gives you the right qualitative behaviour, and have an adjustable constant in front of it which gives you the number right which is supposed to take care of all the things you don't know or have neglected- with the hope that engine-specific constant and qualitative equations are a good enough model for the physics that goes into the problem. If the air mass flow drops with ambient pressure or not is a question of having a turbocharger for the engine or not I guess. Plus, there is of course the dynamic ram pressure due to the aircraft motion which increases air density in the engine. I would guess as long as manifold pressure does not change, the air mass flow does not change, because that measures at the intake. Cheers, * Thorsten ------------------------------------------------------------------------------ This SF.net Dev2Dev email is sponsored by: Show off your parallel programming skills. Enter the Intel(R) Threading Challenge 2010. http://p.sf.net/sfu/intel-thread-sfd _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
