X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sat, 24 Dec 2005 21:16:55 -0500 Message-ID: X-Original-Return-Path: Received: from secure5.liveoakhosting.com ([64.49.254.21] verified) by logan.com (CommuniGate Pro SMTP 5.0.5) with ESMTPS id 900190 for lml@lancaironline.net; Sat, 24 Dec 2005 11:07:37 -0500 Received-SPF: none receiver=logan.com; client-ip=64.49.254.21; envelope-from=walter@advancedpilot.com Received: (qmail 20102 invoked from network); 24 Dec 2005 10:06:37 -0600 Received: from ip68-108-237-23.br.no.cox.net (HELO ?10.0.1.4?) (68.108.237.23) by rs5.liveoakhosting.com with SMTP; 24 Dec 2005 10:06:37 -0600 Mime-Version: 1.0 (Apple Message framework v623) In-Reply-To: References: Content-Type: multipart/alternative; boundary=Apple-Mail-33-366890429 X-Original-Message-Id: From: Walter Atkinson Subject: Re: [LML] Re: Where has all the power gone? X-Original-Date: Sat, 24 Dec 2005 10:06:36 -0600 X-Original-To: "Lancair Mailing List" X-Mailer: Apple Mail (2.623) --Apple-Mail-33-366890429 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1; format=flowed Rob: I agree with all of your points but one. **. As rpm and/or combustion duration approach zero the ideal theta PP=20= approaches TDC. As theta PP drifts away from TDC the engine becomes=20 less efficient.** Unless I'm missing something about your point, I don't think that's=20 quite correct. The optimum thetaPP for maximum mechanical advantage is=20= always about 15-16dATDC. By definition and measurement, approximately=20= half of the charge is burned at the peak pressure. If I've=20 misunderstood, please try again! Thanks. And, Merry Christmas and Happy Holidays to you all. Walter On Dec 24, 2005, at 1:30 AM, REHBINC@aol.com wrote: Skip/Walter, =A0 =A0 > **Can you explain why spark advance is a good idea in auto engines and > a bad one in airplanes?** > > It's not necessarily.=A0=A0 It all depends on the power and fuel being=20= > used. > > You can actually gain power in an aircraft engine at takeoff power by > RETARDING the timing from the historically chosen timing.=A0 That has > several advantages. > > OTOH, you can gain power in cruise in most situations (NA) by = advancing > the timing during that power demand. > > If the thetaPP is 10 like it is at takeoff, retarding the timing will > make more HP.=A0 If it is 25, like it can be in cruise, advancing the > timing will increase output.=A0=A0 IOW, if the thetaPP is 16 and you > advance OR retard the timing, the power output goes down. > > The objective is to have a thetaPP of 16dATDC in all conditions.=A0 = That > results in the most mechanical advantage for the fuel charge being > burned.=A0 It improves the BSFC. > > Water-cooled, iron-head engines have a wider detonation margin and > withstand detonation a lot better than our aircraft engines, so we = need > to pay closer attention to these issues.I supect Skip really wants to=20= > know why your car operates with a more or less continuously variable=20= > spark advance verses a static advance in an airplane. The principle=20 > reason is that rpm and cylinder filling are constantly changing in=20 > your car, while these values are almost constant in a typical aircraft=20= > engine. =A0 Engine rpm is important because as the engine turns faster, the crank=20 swings more degrees during the combustion process. This has two effects=20= on the combustion process. First, for a given ignition point=A0the peak=20= pressure occurs occurs later in the power stroke (theta PP). Second,=20 the shorter time at peak chamber pressure reduces the time for the fuel=20= mixture to pyrolize (decompose into its combustible components). With=20 less pyroysis, the detonation margin is increased. This is one=20 reason=A0why your old car may nock a bit at 45 mph steady state and not=20= at 65 mph. =A0 Cylinder filling is also important for two reasons and they work=20 opposite one another. As you open the throttles, manifold pressure=20 increases and the engine is fed denser air fuel mixture. This results=20 in a higher cranking pressure. As the pressure increases, the flame=20 speed decreases, requiring more spark advance to develop best power.=20 (counter intuitive) =A0 The other thing that happens with increasing cranking pressure, and the=20= temperature that comes with it,=A0is that we increase pyrolization and=20= thus get closer to the detonation limit. =A0 In an airplane with a constant prop, the engine barely changes speed=20 the entire time the wheels are off the ground. With a fixed prop, the=20 engine speed still changes much less than in=A0a car. Thus the engine=20 speed effects can be ignored without much loss. =A0 In a typical airplane the most critical engine operation is at takeoff=20= and the critical limitation is detonation. If the timing is adequately=20= retarded to prevent detonation (or at least keep it under control)=A0at=20= full power take off, then it should also be safe when cylinder filling=20= is reduced to 75% power. The reduced cranking pressure also slightly=20 reduces the spark advance for best power/detonation limit=A0because the=20= flame speed is now faster (not a huge increase). Whether the increase=20 in flame speed makes up for the increased detonation margin depends on=20= the engine=A0characteristics, particularly how it heats up. =A0 Keep in mind also that the=A0theta PP for best power is a function is of=20= the combustion duration and=A0of engine speed. As rpm and/or combustion=20= duration approach zero the ideal theta PP approaches TDC. As theta PP=20 drifts away from TDC the engine becomes less efficient. This is why=20 open combustion chambers are so inefficient, the flame has to travel a=20= greater distance and increases the combustion duration. =A0 Ya'll have a merry Christmas, =A0 Rob =A0= --Apple-Mail-33-366890429 Content-Transfer-Encoding: quoted-printable Content-Type: text/enriched; charset=ISO-8859-1 Rob: I agree with all of your points but one. **Arial. As rpm and/or combustion duration approach zero the ideal theta PP approaches TDC. As theta PP drifts away from TDC the engine becomes less efficient.** Unless I'm missing something about your point, I don't think that's quite correct. The optimum thetaPP for maximum mechanical advantage is always about 15-16dATDC. By definition and measurement, approximately half of the charge is burned at the peak pressure. If I've misunderstood, please try again! =20 Thanks. And, Merry Christmas and Happy Holidays to you all. Walter On Dec 24, 2005, at 1:30 AM, REHBINC@aol.com wrote: = ArialSkip/Walter,<= /fontfamily> = Arial=A0 = Arial=A0 Arial**Can you explain why spark advance is a good idea in auto engines = and Ariala bad one in = airplanes?** ArialIt's not necessarily.=A0=A0 It all depends on the power and fuel being = used. ArialYou can actually gain power in an aircraft engine at takeoff power = by ArialRETARDING the timing from the historically chosen timing.=A0 That = has Arialseveral = advantages. ArialOTOH, you can gain power in cruise in most situations (NA) by = advancing Arialthe timing during that power demand. ArialIf the thetaPP is 10 like it is at takeoff, retarding the timing = will Arialmake more HP.=A0 If it is 25, like it can be in cruise, advancing the Arialtiming will increase output.=A0=A0 IOW, if the thetaPP is 16 and = you Arialadvance OR retard the timing, the power output goes down. ArialThe objective is to have a thetaPP of 16dATDC in all conditions.=A0 = That Arialresults in the most mechanical advantage for the fuel charge = being Arialburned.=A0 It improves the BSFC. ArialWater-cooled, iron-head engines have a wider detonation margin and Arialwithstand detonation a lot better than our aircraft engines, so we = need Arialto pay closer attention to these issues.I supect Skip really wants to know why your car operates with a more or less continuously variable spark advance verses a static advance in an airplane. The principle reason is that rpm and cylinder filling are constantly changing in your car, while these values are almost constant in a typical aircraft = engine. = Arial=A0= ArialEngine rpm is important because as the engine turns faster, the crank swings more degrees during the combustion process. This has two effects on the combustion process. First, for a given ignition point=A0the peak pressure occurs occurs later in the power stroke (theta PP). Second, the shorter time at peak chamber pressure reduces the time for the fuel mixture to pyrolize (decompose into its combustible components). With less pyroysis, the detonation margin is increased. This is one reason=A0why your old car may nock a bit at 45 mph steady state and not at 65 = mph. = Arial=A0 ArialCylinder filling is also important for two reasons and they work opposite one another. As you open the throttles, manifold pressure increases and the engine is fed denser air fuel mixture. This results in a higher cranking pressure. As the pressure increases, the flame speed decreases, requiring more spark advance to develop best power. (counter = intuitive) = Arial=A0 ArialThe other thing that happens with increasing cranking pressure, and the temperature that comes with it,=A0is that we increase pyrolization and thus get closer to the detonation limit. = Arial=A0 ArialIn an airplane with a constant prop, the engine barely changes speed the entire time the wheels are off the ground. With a fixed prop, the engine speed still changes much less than in=A0a car. Thus the engine speed effects can be ignored without much loss. = Arial=A0 ArialIn a typical airplane the most critical engine operation is at takeoff and the critical limitation is detonation. If the timing is adequately retarded to prevent detonation (or at least keep it under control)=A0at full power take off, then it should also be safe when cylinder filling is reduced to 75% power. The reduced cranking pressure also slightly reduces the spark advance for best power/detonation limit=A0because the flame speed is now faster (not a huge increase). Whether the increase in flame speed makes up for the increased detonation margin depends on the engine=A0characteristics, particularly how it heats = up. = Arial=A0 ArialKeep in mind also that the=A0theta PP for best power is a function is of the combustion duration and=A0of engine speed. As rpm and/or combustion duration approach zero the ideal theta PP approaches TDC. As theta PP drifts away from TDC the engine becomes less efficient. This is why open combustion chambers are so inefficient, the flame has to travel a greater distance and increases the combustion = duration. = Arial=A0 ArialYa'll have a merry Christmas, = Arial=A0 = ArialRob = Arial=A0= --Apple-Mail-33-366890429--