X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Wed, 12 Dec 2007 10:05:02 -0500 Message-ID: X-Original-Return-Path: Received: from mail12.syd.optusnet.com.au ([211.29.132.193] verified) by logan.com (CommuniGate Pro SMTP 5.2c1) with ESMTPS id 2575439 for lml@lancaironline.net; Mon, 10 Dec 2007 20:40:10 -0500 Received-SPF: pass receiver=logan.com; client-ip=211.29.132.193; envelope-from=fredmoreno@optusnet.com.au Received: from fred ([202.139.5.198]) (authenticated sender fredmoreno) by mail12.syd.optusnet.com.au (8.13.1/8.13.1) with ESMTP id lBB1dAq2014657 for ; Tue, 11 Dec 2007 12:39:19 +1100 From: "Fred Moreno" X-Original-To: "Lancair Mail" Subject: GAMI Injectors - question for Walter X-Original-Date: Tue, 11 Dec 2007 10:38:54 +0900 X-Original-Message-ID: <00e001c83b96$9de4a640$c6058bca@fred> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_00E1_01C83BE2.0DCC4E40" X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.6822 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3198 Thread-Index: Acg6sEwq+IEFV3GKQQWh8nQ4W8fI4wA4Twyg Importance: Normal This is a multi-part message in MIME format. ------=_NextPart_000_00E1_01C83BE2.0DCC4E40 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Referring to Scott's set up using a pitot tube in the cowl inlet to pressurize the fuel injector air inlets, Walter wrote:=20 =20 "Very nice set-up. Did you use a turbo-type injector to hook to the ram = air? [Yes]" =20 =20 This topic raises some questions in my mind that just won't go away. = Pesky things. =20 The fuel injectors have holes in them so that air can flow from a high pressure region into the injector to the lower pressure region in the = intake port. Along the way the flow of air helps to atomize the fuel assisting combustion.=20 =20 There are the bits of "data" (I use the term loosely) rumbling around in = my mind that suggest the injector air flow is not important at wide open throttle and thus irrelevant to high power cruise performance. =20 =20 The question is: True or False? =20 Data bit #1: I recall the following when talking to an auto engine = "expert." Modern electronic fuel injection systems control fuel flow by varying = pulse length, but the fuel pressure remains constant. A jet of fuel is = pointed at the back of the intake valve. The intake valve which runs moderately = hot helps to vaporize the fuel. Further vaporization occurs as the intake = air is drawn by the intake valve into the cylinder. The space between valve = and valve seat creates an annular venturi of sorts, and as we know, venturis = are highly effective at atomizing liquid fuel. So in auto engines higher = fuel pressure squirting on the intake valve plus a bit of heat plus the high shear stresses that occur when the flow passes the valve and seat all combine to provide good atomization over a wide range of power = conditions.=20 =20 Data bit #2: I spent some years working in gas turbine combustion to = reduce NOx emissions. This led me to Arthur Lefebvre's book Gas Turbine = Combustion which reports on the huge amounts of work done on fuel atomization in = jet engines. The problem is that between cold day sea level take off power = and flight idle at the start of descent at 36,000 feet, there is a huge difference in fuel flow. This creates major problems for atomization = over the entire range of operation, and has resulted in a lot of fancy fuel injector designs.=20 =20 Our simple constant flow fuel injection systems are more like turbine injectors than modern electronic auto injectors.=20 =20 Data point #3: Aircraft fuel injectors tend to squirt at the wall of the intake port (also warm) and not at the back of the intake valve. So = cold, low power (idle) atomization maybe particularly poor. And low power = which calls for low air flow in the intake port also leads to poor = atomization. Help needed.=20 =20 So methinks the following: =20 In our constant flow fuel injectors, at idle and low power settings, the fuel flow is so low that the injector by itself does not squirt, but dribbles fuel out in a low pressure stream. Poor atomization =3D lousy combustion. So to improve atomization, we add air assist atomization. Fortunately, low power also means low manifold pressure, and so ambient pressure (or upper deck pressure) will force air through the fuel = injector where it can atomize the fuel dribble and - magic! - good (or at least improved) combustion. One can expect at least 5 or 10 inches of mercury delta P between intake port and ambient, good enough to spray gasoline.=20 =20 At high power and in cruise (in particular, wide open throttle at = altitude) the manifold pressure virtually equals the ambient pressure. No delta P means no atomization air through the injector. But not to worry - the cylinder head is hot so the fuel is hitting a hot surface inside the = intake port, and moreover, the MASS FLOW RATE of air through the intake port is high so that the fuel is well sheared and mixed when air and fuel pass through the venturi between intake valve and valve seat. =20 =20 Conclusion (Sorry Scott, if I am right) - The extra effort to gain a few inches of WATER pressure using the pitot ram tube set up to pressurize = the injector air is not worthwhile at higher power settings. At lower power settings, the delta P IS large enough to provide atomization. =20 =20 So for higher power settings or wide open throttle, there is little to = be gained by fancy air distribution systems for air injection flow. More importantly for Scott, the speed contribution may be zip. =20 =20 There are two things that can modify this argument. =20 =20 The first is that the pressure in the intake port is not constant at the "manifold pressure" but surging up and down as the cylinder goes gulp-gulp-gulp, and so there may well be some delta P during the intake stroke when the cylinder is sucking on the intake port. If this = pressure fluctuation is not too fast for the flow in the injector, it may assist = in atomization. But the effect is due to periodic low pressure in the = intake port due to induction, not a tiny bit of additional pressure from the = pitot ram air set up.=20 =20 The second thing that can modify the argument is DATA! One sound data = point is worth 1000 speculations. Has a good before and after test under carefully controlled conditions shown performance improvements in terms = of power, mixture distribution, or beneficial effect on the harmonic convergence? If so, Publish or Perish! =20 What say you, Walter? Teach us some more about fuel atomization at = higher power settings and wide open throttle. What does the data show? If no data, you are invited to speculate as well. One Walter speculation =3D = 1000 Fred speculations. =20 Always Curious Fred =20 =20 ------=_NextPart_000_00E1_01C83BE2.0DCC4E40 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

Referring to Scott’s set up = using a pitot tube in the cowl inlet to pressurize the fuel injector air inlets, = Walter wrote:

 

“Very nice = set-up. Did you use a turbo-type injector to hook to the ram air? [Yes]”

 

 

This topic raises some questions = in my mind that just won’t go away.  Pesky = things.

 

The fuel injectors have holes in = them so that air can flow from a high pressure region into the injector to the = lower pressure region in the intake port.  Along the way the flow of air = helps to atomize the fuel assisting combustion.

 

There are the bits of = “data” (I use the term loosely) rumbling around in my mind that suggest the = injector air flow is not important at wide open throttle and thus irrelevant to = high power cruise performance. 

 

The question is: True or = False?

 

Data bit #1: I recall the = following when talking to an auto engine “expert.”  Modern electronic fuel = injection systems control fuel flow by varying pulse length, but the fuel pressure remains constant.  A jet of fuel is pointed at the back of the = intake valve.  The intake valve which runs moderately hot = helps to vaporize the fuel.  Further vaporization occurs as the intake air = is drawn by the intake valve into the cylinder.  The space between valve and = valve seat creates an annular venturi of sorts, and as we know, venturis are = highly effective at atomizing liquid fuel.  So in auto engines higher fuel = pressure squirting on the intake valve plus a bit of heat plus the high shear = stresses that occur when the flow passes the valve and seat all combine to provide = good atomization over a wide range of power conditions.

 

Data bit #2: I spent some years = working in gas turbine combustion to reduce NOx emissions.  This led me to = Arthur Lefebvre’s book Gas Turbine Combustion which reports on the = huge amounts of work done on fuel atomization in jet engines.   The problem is that between cold day sea level take off power and flight = idle at the start of descent at 36,000 feet, there is a huge difference in fuel flow.  This creates major problems for atomization over the entire = range of operation, and has resulted in a lot of fancy fuel injector designs. =

 

Our simple constant flow fuel = injection systems are more like turbine injectors than modern electronic auto = injectors.

 

Data point #3: Aircraft fuel = injectors tend to squirt at the wall of the intake port (also warm) and not at the = back of the intake valve.   So cold, low power (idle) atomization = maybe particularly poor.  And low power which calls for low air flow in = the intake port also leads to poor atomization.  Help needed. =

 

So methinks the = following:

 

In our constant flow fuel = injectors, at idle and low power settings, the fuel flow is so low that the injector = by itself does not squirt, but dribbles fuel out in a low pressure stream. =  Poor atomization =3D lousy combustion.  So to improve atomization, we = add air assist atomization.  Fortunately, low power also means low manifold pressure, and so ambient pressure (or upper deck pressure) will force = air through the fuel injector where it can atomize the fuel dribble and = – magic! – good (or at least improved) combustion.  One can = expect at least 5 or 10 inches of mercury delta P between intake port and ambient, = good enough to spray gasoline.

 

At high power and in cruise (in = particular, wide open throttle at altitude) the manifold pressure virtually equals = the ambient pressure.  No delta P means no atomization air through the injector.  But not to worry – the cylinder head is hot so the = fuel is hitting a hot surface inside the intake port, and moreover, the MASS = FLOW RATE of air through the intake port is high so that the fuel is well sheared = and mixed when air and fuel pass through the venturi between intake valve = and valve seat. 

 

Conclusion (Sorry Scott, if I am = right) – The extra effort to gain a few inches of WATER pressure using the pitot = ram tube set up to pressurize the injector air is not worthwhile at higher = power settings.  At lower power settings, the delta P IS large enough to = provide atomization. 

 

So for higher power settings or = wide open throttle, there is little to be gained by fancy air distribution systems = for air injection flow.   More importantly for Scott, the speed = contribution may be zip. 

 

There are two things that can = modify this argument. 

 

The first is that the pressure in = the intake port is not constant at the “manifold pressure” but = surging up and down as the cylinder goes gulp-gulp-gulp, and so there may well = be some delta P during the intake stroke when the cylinder is sucking on the = intake port.  If this pressure fluctuation is not too fast for the flow in = the injector, it may assist in atomization.  But the effect is due to periodic = low pressure in the intake port due to induction, not a tiny bit of = additional pressure from the pitot ram air set up.

 

The second thing that can modify = the argument is DATA!  One sound data point is worth 1000 = speculations.  Has a good before and after test under carefully controlled conditions = shown performance improvements in terms of power, mixture distribution, or beneficial = effect on the harmonic convergence?  If so, Publish or = Perish!

 

What say you, Walter?  Teach = us some more about fuel atomization at higher power settings and wide open = throttle.  What does the data show?  If no data, you are invited to speculate = as well.  One Walter speculation =3D 1000 Fred = speculations.

 

Always Curious = Fred

 

 

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