X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 23 Oct 2005 11:41:32 -0400 Message-ID: X-Original-Return-Path: Received: from centrmmtao06.cox.net ([70.168.83.78] verified) by logan.com (CommuniGate Pro SMTP 5.0c6) with ESMTP id 780713 for lml@lancaironline.net; Sun, 23 Oct 2005 10:02:51 -0400 Received-SPF: none receiver=logan.com; client-ip=70.168.83.78; envelope-from=jfuller@cox.net Received: from Notebook ([68.226.50.3]) by centrmmtao06.cox.net (InterMail vM.6.01.05.02 201-2131-123-102-20050715) with ESMTP id <20051023140128.YDFO24602.centrmmtao06.cox.net@Notebook> for ; Sun, 23 Oct 2005 10:01:28 -0400 X-Original-Message-ID: <002b01c5d7da$54a59fb0$0202a8c0@Notebook> From: "jfuller" X-Original-To: Subject: lml_Web_Archive X-Original-Date: Sun, 23 Oct 2005 09:01:52 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0028_01C5D7B0.68AD4910" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2670 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2670 This is a multi-part message in MIME format. ------=_NextPart_000_0028_01C5D7B0.68AD4910 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable My (former) mechanic had trouble keeping this straight. The spark event = is referenced to top dead center, so when the timing is set to 22 = degrees before top dead center on the compression stroke, the plug fires = while the piston is still traveling upward on the compression stroke. = By the time the fuel starts burning, the piston has passed top dead = center and is moving back down the barrel. If the timing is set to 28 = degrees, that means the plug fires earlier, when the crank has another = 28 degrees to turn before top dead center, and that's why the spark is = said to be more advanced. When you advance the spark that much, the fire starts earlier, possibly = even before the piston gets to top dead center. If the peak cylinder = pressure occurs at top dead center, the expanding gas pushes on a piston = that has nowhere to go. The piston is directly above the connecting rod = which is directly above the crankshaft, and the effective lever on which = the piston pushes has a length of zero inches. Since work equals force = times distance, at that instant in time no work is done, so all the = energy in the burning fuel is imparted to the cylinder, and none to = mechanical work. That's why cylinder head temperatures rise and EGT and = power fall if the spark is advanced too much. =20 If the spark is excessively retarded, the moment of peak cylinder = pressure occurs late, when the piston is well along its path to the = bottom of the cylinder. Although the effective lever arm of the = connecting rod is much longer (it is of course at its longest at 90 = degrees of crank travel), the distance over which the force of the = expanding gas can do work on the piston is less, so less work is done. = This leads to increased EGT (the fire is still burning when the exhaust = valve opens), much lower cylinder head temperature, and less power. Optimal spark advance will thus vary with the speed of flame front = propagation (which depends on lots of secondary factors such as = compression ratio, induction temperature, temperature of the combustion = chamber) and angular velocity of the crank. When George Braly talks about theta PP, he's referring to the point at = which peak cylinder pressure occurs during the combustion event. = Controlling theta PP is how to optimize fuel efficiency and engine = temperatures, and this can be done in our fixed timing aircraft engines = only by adjusting the speed of flame front propagation, which is done = with the mixture control. Non-stoichiometric mixtures burn more slowly = than stoichiometric mixtures. When PRISM is available, theta PP will be = optimized by varying spark advance instead of by changing the speed of = flame-front propagation with the red knob. That's also why these = engines will be able to burn lower octane (ie, faster burning) fuel. = The controller will retard the spark to give the same theta PP with a = faster burning fuel. Jonathan Fuller N1538G TNIO-550B A36 ------=_NextPart_000_0028_01C5D7B0.68AD4910 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
My (former) mechanic had trouble = keeping this=20 straight.  The spark event is referenced to top dead center, so = when the=20 timing is set to 22 degrees before top dead center on the compression = stroke,=20 the plug fires while the piston is still traveling upward on the = compression=20 stroke.  By the time the fuel starts burning, the piston has passed = top=20 dead center and is moving back down the barrel.  If the timing is = set to 28=20 degrees, that means the plug fires earlier, when the crank has another = 28=20 degrees to turn before top dead center, and that's why the spark is said = to be=20 more advanced.
 
When you advance the spark that much, = the fire=20 starts earlier, possibly even before the piston gets to top dead = center. =20 If the peak cylinder pressure occurs at top dead center, the expanding = gas=20 pushes on a piston that has nowhere to go.  The piston is directly = above=20 the connecting rod which is directly above the crankshaft, and the = effective=20 lever on which the piston pushes has a length of zero inches.  = Since work=20 equals force times distance, at that instant in time no work is done, so = all the=20 energy in the burning fuel is imparted to the cylinder, and none to = mechanical=20 work.  That's why cylinder head temperatures rise and EGT = and power=20 fall if the spark is advanced too much. 
 
If the spark is excessively retarded, = the moment of=20 peak cylinder pressure occurs late, when the piston is well along its = path to=20 the bottom of the cylinder.  Although the effective lever arm of = the=20 connecting rod is much longer (it is of course at its longest at 90 = degrees=20 of crank travel), the distance over which the force of the expanding gas = can do=20 work on the piston is less, so less work is done.  This leads to = increased=20 EGT (the fire is still burning when the exhaust valve opens), much lower = cylinder head temperature, and less power.
 
Optimal spark advance will thus vary = with the speed=20 of flame front propagation (which depends on lots of secondary factors = such as=20 compression ratio, induction temperature, temperature of the = combustion=20 chamber) and angular velocity of the crank.
 
When George Braly talks about theta PP, = he's=20 referring to the point at which peak cylinder pressure occurs during the = combustion event.  Controlling theta PP is how to optimize fuel = efficiency=20 and engine temperatures, and this can be done in our fixed timing = aircraft=20 engines only by adjusting the speed of flame front propagation, which is = done=20 with the mixture control.  Non-stoichiometric mixtures burn more = slowly=20 than stoichiometric mixtures.  When PRISM is available, theta PP = will be=20 optimized by varying spark advance instead of by changing the speed of=20 flame-front propagation with the red knob.  That's also why these = engines=20 will be able to burn lower octane (ie, faster burning) fuel.  The=20 controller will retard the spark to give the same theta PP with a faster = burning=20 fuel.
 
Jonathan Fuller
N1538G
TNIO-550B = A36
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