X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Thu, 21 Feb 2008 11:51:06 -0500 Message-ID: X-Original-Return-Path: Received: from mta16.adelphia.net ([68.168.78.211] verified) by logan.com (CommuniGate Pro SMTP 5.2.0) with ESMTP id 2751174 for lml@lancaironline.net; Thu, 21 Feb 2008 09:16:20 -0500 Received-SPF: pass receiver=logan.com; client-ip=68.168.78.211; envelope-from=glcasey@adelphia.net Received: from [75.82.216.175] by mta16.adelphia.net (InterMail vM.6.01.05.04 201-2131-123-105-20051025) with ESMTP id <20080221141539.NQWO17537.mta16.adelphia.net@[75.82.216.175]> for ; Thu, 21 Feb 2008 09:15:39 -0500 Mime-Version: 1.0 (Apple Message framework v753) In-Reply-To: References: Content-Type: multipart/alternative; boundary=Apple-Mail-43--189644947 X-Original-Message-Id: <592CE7E3-3AE2-4487-A10A-5F9E2D34362A@adelphia.net> From: Gary Casey Subject: Re: Exhaust thrust X-Original-Date: Thu, 21 Feb 2008 06:15:37 -0800 X-Original-To: "Lancair Mailing List" X-Mailer: Apple Mail (2.753) --Apple-Mail-43--189644947 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed Some thoughts on exhaust energy recovery (jet thrust): Most of the energy, and all of the "free" energy in the exhaust, is released during the blow-down event just after the exhaust valve opens. This is when most of the exhaust leaves the cylinder and it results in the highest velocities. If the exhaust pipe is short and has minimal volume this pulse arrives at the tip with little loss and can be used for thrust. The rest of the flow, which occurs during the exhaust stroke, is at a much lower rate and is therefore not very useful. And during the rest of the engine cycle there is no flow, so the pipe is nothing more than drag and weight. Since the pulse is so short multiple cylinders can be combined to use one exhaust pipe - however, the total volume of the pipe then increases and much of the pulse energy is lost by expanding back up toward the other cylinders. It is logical, but probably not much more beneficial, to combine cylinders so that the necessary minimum volume is combined with evenly-spaced pulses. This works well with a 6-cylinder engine since each bank is even-firing, but on a 4 cylinder HO engine two cylinders from opposite banks must be combined. The pipe must be as small as practical to produce the highest velocity at the exit and a surprisingly small pipe can be used to advantage. This is because the backpressure that is important to engine power is that during the exhaust stroke, not during the blowdown, at which time the piston is still going "down" and increased pressure actually helps power (don't bother trying to take advantage of this, though, since the piston is barely moving and some of the backpressure will certainly remain when the piston starts back "up"). Excessively small pipes are probably not good because of the backpressure caused by the discontinuity at the exhaust port - might as well keep the pipe about the same size as the port or maybe just a little smaller. So individual stacks are best, combining cylinders on each bank next best, combining two cylinders on opposite banks third best and combining all cylinders is the least desirable. Remember, combining cylinders thinking that the net velocity will be higher is incorrect - it's just the free pulse energy that can be used effectively. The angle of the exhaust to the slipstream is important for two reasons: The exhaust "plume", if directed across the free air stream, creates drag just as though a pipe were there. The thrust is proportional to the cosine of the angle to the direction of flight, so the angle should be kept below maybe 30 degrees - being exactly zero isn't important. A 30-degree angle will recover 86% and a 15-degree angle is 97% effective. Is an effort to recover this energy worth it? Since you have to have an exhaust system anyway, configuring it to take advantage of this energy makes sense. To throw away an otherwise good system to build another optimized for thrust doesn't as my calculations showed a thrust horsepower in the single digit range. And I would bet that the more thrust there is the more noise will be produced. I did everything like described above, but then added mufflers - probably negating all the efforts to increase thrust. For what it's worth.. Gary Casey > --Apple-Mail-43--189644947 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1 Some thoughts on exhaust energy recovery (jet thrust): =A0Most of the = energy, and all of the "free" energy in the exhaust, is released during = the blow-down event just after the exhaust valve opens. =A0This is when = most of the exhaust leaves the cylinder and it results in the highest = velocities. =A0If the exhaust pipe is short and has minimal volume this = pulse arrives at the tip with little loss and can be used for thrust. = =A0The rest of the flow, which occurs during the exhaust stroke, is at a = much lower rate and is therefore not very useful. =A0And during the rest = of the engine cycle there is no flow, so the pipe is nothing more than = drag and weight. =A0Since the pulse is so short multiple cylinders can = be combined to use one exhaust pipe - however, the total volume of the = pipe then increases and much of the pulse energy is lost by expanding = back up toward the other cylinders. =A0It is logical, but probably not = much more beneficial, to combine cylinders so that =A0the necessary = minimum volume is combined with evenly-spaced pulses. =A0This works well = with a 6-cylinder engine since each bank is even-firing, but on a 4 = cylinder HO engine two cylinders from opposite banks must be combined. = =A0The pipe must be as small as practical to produce the highest = velocity at the exit and a surprisingly small pipe can be used to = advantage. =A0This is because the backpressure that is important to = engine power is that during the exhaust stroke, not during the blowdown, = at which time the piston is still going "down" and increased pressure = actually helps power (don't bother trying to take advantage of this, = though, since the piston is barely moving and some of the backpressure = will certainly remain when the piston starts back "up"). =A0Excessively = small pipes are probably not good because of the backpressure caused by = the discontinuity at the exhaust port - might as well keep the pipe = about the same size as the port or maybe just a little smaller. =A0So = individual stacks are best, combining cylinders on each bank next best, = combining two cylinders on opposite banks third best and combining all = cylinders is the least desirable. =A0Remember, combining cylinders = thinking that the net velocity will be higher is incorrect - it's just = the free pulse energy that can be used effectively. =A0The angle of the = exhaust to the slipstream is important for two reasons: =A0The exhaust = "plume", if directed across the free air stream, creates drag just as = though a pipe were there. =A0The thrust is proportional to the cosine of = the angle to the direction of flight, so the angle should be kept below = maybe 30 degrees - being exactly zero isn't important. =A0A 30-degree = angle will recover 86% and a 15-degree angle is 97% effective.

Is an effort to recover = this energy worth it? =A0Since you have to have an exhaust system = anyway, configuring it to take advantage of this energy makes sense. =A0To= throw away an otherwise good system to build another optimized for = thrust doesn't as my calculations showed a thrust horsepower in the = single digit range. =A0And I would bet that the more thrust there is the = more noise will be produced. =A0I did everything like described above, = but then added mufflers - probably negating all the efforts to increase = thrust.

For = what it's worth..
Gary Casey
=A0=A0

= --Apple-Mail-43--189644947--