Engine Start Simulation General Comments

[Guest Ian_Riddell]

I've noticed that a lot of desktop simulators fall short when it comes to modelling engine start ever since jet engines with EEC control were introduced (in basically all price categories) .Common flaws:1. With no fuel applied, the engine starter motor keeps on accelerating the N2 almost up to engine idle speed (OK in PMDG by the way). Real life Starter motors are lucky if they can drive the high pressure rotor rpms to 30%, let alone 55% (55% is when the starter motors disengage from the engine on the 737NG). BTW, driving the engine as fast as it can with only the starter motor (i.e. no fuel or ignition) is known as "max motoring" the engine.2. The (normal and alternate) Engine Electronic Controller ("EEC") power sources are not modelled properly, so you get things like engine indication displays not appearing when they should (Some engine sensors feed analog data to the EEC's where the EEC's digitize the information and send this info on to the instrument display computers ("DEU's" on the 737NG)3. EGT's rising too rapidly (unless purposely simulating "hot starts", of course)On PDGM starter ops strangeness...If I "max motor" the engine with the starter, I get a value of around 24% N2 (sounds about right to me). If I then lift the fuel levers to add fuel and ignition, the N2 plummets back to 16% ! If you are adding fuel and fire to an engine, you are not going to slow a spinning engine in any way (unless the fuel is part superglue ;-)). I don't know if everyone else is getting this? Of course, PMDG is brilliant in the fact that you can see all this happening at once (if you push the right buttons) :-)Starter motors can be exceedingly noisy at times because of the rpms they are running at. You should hear a steady rising tone as the starter cranks the engine. I'm a little worried about PMDG's looping sound, however :-) On EEC power sources....The 737 NG Left Engine EEC, for example, is powered by a number of sources.1. AC from the #1 115V AC Transfer Bus (and it's associated dc bus... for power relays).... OR....2. The 2 "dedicated" alternators on the engine (which run completely independently of the airplane electrical system.... That, incidentally, is why the engines can still run in PMDG's 737NG, even when you have a dead ship ;-) However, proper aircraft dc power is required to turn them off ;-)Re the Xfer Bus AC power...If either the respective Fuel Lever is selected to IDLE, OR the respective Start Switch is in GND (or CONT in the air), this will energise the power relays allowing the EEC's to use Xfer Bus Power 115AC power. For example, if your engine wasn't running on the ground and you put the Fuel Lever to Idle (not recommended unless you know what you are doing), things like Oil Pressure, Oil Temperature, Fuel Flow and EGT's should appear on the Engine Displays after a few seconds.http://members.ozemail.com.au/~b744er/737/EECPower.gifRe Dedicated Alternators.Whenever the engine N2 is spinning, the gearbox attached to the N2 rotor should be spinning the dedicated alternators. At only 15% N2, the alternators can provide all the power the EEC needs to operate properly. Indeed, these alternators are the normal power sources for the EEC. When you start an engine using the Start Switch and the engine starter motor, EEC power will first come from the 115Vac Xfer Bus, then at 15% N2, the two Alternators will take over. Why two alternators? The EEC, for safety, has two identical "channels" with one alternator per channel. When the engine is switched off after a flight, the n2 spools down past 15% and all power is removed from the EEC's (There may be some lag, however: I did read that EGT's, for example, disappear from the cockpit displays at about 10%N2. Presumably, the EEC has a few large electrical capacitors in the power circuit, allowing this to happen(???)). So, the result of this will be that the displays will be available for some time after the fuel levers have been selected to OFF.Anyway, time's up...More gibberish tomorrow, I'd imagine :-)Cheers.Ian.