Return-Path: Sender: "Marvin Kaye" To: lml@lancaironline.net Date: Wed, 29 Dec 2004 18:37:20 -0500 Message-ID: X-Original-Return-Path: Received: from imo-m21.mx.aol.com ([64.12.137.2] verified) by logan.com (CommuniGate Pro SMTP 4.2.5) with ESMTP id 583347 for lml@lancaironline.net; Wed, 29 Dec 2004 12:42:20 -0500 Received-SPF: pass receiver=logan.com; client-ip=64.12.137.2; envelope-from=Sky2high@aol.com Received: from Sky2high@aol.com by imo-m21.mx.aol.com (mail_out_v37_r3.8.) id q.fc.97293f4 (4410) for ; Wed, 29 Dec 2004 12:41:43 -0500 (EST) From: Sky2high@aol.com X-Original-Message-ID: X-Original-Date: Wed, 29 Dec 2004 12:41:42 EST Subject: RAM, SLAM, BAM - MAP, WHAP, ZAP - HUH? X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1104342102" X-Mailer: 9.0 Security Edition for Windows sub 5000 -------------------------------1104342102 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en =20 Gentleman (I know some who are) and Ladies,=20 There has been a gargantuan struggle to get one=E2=80=99s arms around the r= am air=20 affect on MAP =E2=80=93 Manifold Absolute Pressure =E2=80=93 a struggle acc= ompanied by=20 missteps, pratfalls, traps, blind alleys and just plain bad measuring equip= ment of=20 all sorts.=20 The discussion will focus on Vision Microsystem engine monitors used for=20 normally aspirated piston avgas-sucking engines. I believe the sensors us= ed by=20 VM 1000 and EPI 800 systems are identical. Other systems may operate =20 differently.=20 MAP (displayed in inches of Hg) is not actually =E2=80=9Cabsolute=E2=80=9D=20= since it is=20 ambient pressure (AP) plus gauge sensed pressure (GP). That is indicated= MAP =3D=20 AP + GP. At wide open throttle (WOT), GP can be positive from pressure=20 created via a ram air induction system and limited by KIAS. GP can be nega= tive=20 (vacuum) at reduced throttle (reducing air in the induction system). Ram a= ir=20 may overcome the slight closing of the throttle to where the MAP =3D AP.=20 Ambient Pressure (AP):=20 1) Before engine start: Theoretically, a properly calibrated sensor would=20 be expected to read the barometric setting before engine start. However, t= he=20 AP part of the MAP sensor is not required to match the TSO requirements of=20 even a non-sensitive altimeter. There also may be display constraints whe= n=20 the engine is not running. In my case, the MAP indication on my EPI 800=20 generally reads 29.2=E2=80=9D over a broad range of barometric settings. Si= nce my airport=20 is 710 MSL, this reading (710 ft =3D 29.16) is consistent, all things being= =20 equal. =20 2) The VM MAP sensor is usually mounted thru the firewall with the AP port=20 open to the cabin. This means that the ambient pressure varies with speed,= =20 venting and altitude. That is, the cockpit low pressure is less at lower=20 speeds, less when vents are opened. Generally, in my airplane at cruise s= peeds,=20 the inside pressure is .25=E2=80=9D Hg less that =E2=80=9Cstatic=E2=80=9D a= mbient pressure, depending=20 on altitude. This gives rise to the question: Should I connect the AP port=20 to the same static air used by the altimeter? =20 Side Note: If an electronic ignition is used and one of the controlling=20 parameters is MAP, the ambient pressure is obtained from the position of th= e=20 sensor, generally located in the =E2=80=9Cbrain=E2=80=9D box that is often =20= placed under the=20 cowling and is sensing the cooling air output pressure. This is generally=20 slightly above =E2=80=9Cstatic=E2=80=9D ambient pressure. I misplaced the=20= measurement I once=20 made of this but I remember it was maybe about .25=E2=80=9D Hg greater. At= cruise, this=20 is a true difference of about .5=E2=80=9D in what the MAP indicator is show= ing and=20 what the electronic ignition is using for timing adjustments.=20 3) True ambient (static) pressure at altitude is an elusive number. This =20 pressure is dependent on air density which is sensitive to absolute altitude= , =20 temperature and humidity. Note that our altimeters are built to make=20 assumptions about temperatures at altitude and the MAP sensor is not. Addi= tionally,=20 the true pressure is dependent on what has gone on at lower altitudes with=20 respect to temperature, moisture, etc. The higher the altitude the more =20 difficult it is to even estimate the ambient pressure, let alone back calcul= ate it=20 from the altitude and OAT not adjusted for compressibility due to air speed= .=20 Luckily, most of us use sensitive altimeters built to TSO standards and se= t=20 to the local barometer setting so that we read altitudes the same way even=20 though we may be far off from the true altitude. Maybe this helps to=20 understand one of the reasons why GPS altitude determination and vertical g= uidance=20 is not trivial. Also, I better understand why the Flight Levels above 18000= =20 MSL require the 29.92 Kollsman setting.=20 Remember: Look out below when going from hi to lo or hot to cold.=20 Gauge Pressure (GP):=20 GP is obtained from a tap into the induction system, usually from a primer=20 port on one of the cylinder heads ahead of the intake valve (at least that= =E2=80=99s=20 where it generally is on an injected Lycoming engine). Any reduction in the= =20 throttle denies air to the engine thus creating a vacuum in the induction=20= =20 system. The sensor is protected against pressure pulses (valve operation)=20= in=20 the induction system by a =E2=80=9Csnubber=E2=80=9D valve. =20 I went back to my parts box and checked an older one from a prior VM MAP=20 sensor. As Lorn reported, it is a thin porous metallic disk contained in t= he=20 large brass fitting on the firewall side of the sensor. It seems like it w= on=E2=80=99t=20 pass air when one tries to blow thru it. At least 4 people I have talked t= o=20 have had problems with the snubber valve causing problems in MAP readings,=20 especially after throttle change. Remember that its placement creates a =20 chamber from which the pressure is sensed. There can be at least two proble= ms=20 because of fouling:=20 1) Inability for the chamber to return completely to ambient pressure= =E2=80=93 This might yield a low MAP reading before engine start?=20 2) Fouled in a way that makes it very slow to sense vacuum or =20 incorrectly requires a greater vacuum to reduce the pressure in the chamber.= =20 How can it get fouled? There is no movement of air thru the sensing system= .=20 Is the sensor located lower than the tap into the induction system? If so,= =20 can fuel, oil or water condensations run down the line to the snubber? Thi= s=20 certainly happens in those canard airplanes that park with the nose on the=20 ground. Hmmmmmm=E2=80=A6=E2=80=A6 What about the sensor in the electronic=20= ignition?=20 Ram air effect:=20 Precisely what is the ram air effect and how much does it improve the MAP?=20= =20 The airspeed system is a perfect example of a closed system which measures=20 the differential pressure between ambient (static) and ram in the pitot whi= ch=20 is then converted to the airspeed display. The KIAS takes into account th= e=20 less dense air at altitude where both the engine and wing have to deal with= =20 fewer molecules. The rise in the air pressure difference due to ram in a=20 closed system is thus:=20 KIAS Inches of Hg=20 100 .5=20 150 1.1=20 200 2.0=20 250 3.1 =20 These numbers could be considered a maximum assuming that there is a specia= l=20 inlet and no drop due to a filter. If filtered induction air were taken=20 from a well sealed engine cooling air plenum, the filter and ram effect may= =20 cancel each other out at say, 160 KIAS (a guess from pressure measurements=20= taken=20 from my upper cowl area). Of course, none of this takes into account that=20 the air is just passing thru.=20 Side note: Consider that ram air can raise the induction air pressure=20 higher than that utilized by the fuel injectors (obtained, for example, fro= m the=20 upper cooling plenum) to help atomize the fuel. This may suppress the best= =20 atomization and shrouded injectors utilizing the higher ram air, available =20 before the throttle body, may improve atomization. Such a system is similar= to=20 the way super/turbo charged engines operate.=20 Well, what the heck, with unfiltered ram air I know my induction system is=20 performing better, at a lower altitude than my wing is seeing. Something fo= r=20 nothing (except cash). Now I want to know precisely how much better so tha= t I=20 can have bragging rights over somebody else=E2=80=99s system. However, as c= an be=20 seen from the above discussion, comparable numbers are hard to come by.=20 Wait! There is a way. Join me in the next Air Venture cross country race=20 and we shall see how well your ginned up system works. Yep, the gauntlet ha= s=20 been tossed at your feet.=20 Disclaimer: These data and opinions are provided free and, as such, are=20 worth what you are paying for them.=20 Scott Krueger AKA Grayhawk Lancair N92EX IO320 Aurora, IL (KARR) I am going back to my cell now - Meds are being increased. -------------------------------1104342102 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en

Gentleman (I know some wh= o are)=20 and Ladies,

 

There has been a gargantu= an=20 struggle to get one=E2=80=99s arms around the ram air affect on MAP =E2=80= =93 Manifold Absolute=20 Pressure =E2=80=93 a struggle accompanied by missteps, pratfalls, traps, bli= nd alleys=20 and just plain bad measuring equipment of all sorts.

 

The discussion will focus= on=20 Vision Microsystem engine monitors used for normally aspirated pis= ton=20 avgas-sucking engines.  I beli= eve=20 the sensors used by VM 1000 and EPI 800 systems are identical.  Other systems may operate=20 differently.

 

MAP (displayed in inches=20= of Hg)=20 is not actually =E2=80=9Cabsolute=E2=80=9D since it is ambient pressure (AP)= plus gauge sensed=20 pressure (GP).   That is=20 indicated MAP =3D AP + GP.  At= wide=20 open throttle (WOT), GP can be positive from pressure created via a ram air=20 induction system and limited by KIAS. = ;=20 GP can be negative (vacuum) at reduced throttle (reducing air in the=20 induction system).  Ram air ma= y=20 overcome the slight closing of the throttle to where the MAP =3D AP.

 

Ambient Pressure (AP):

1) Before engine start:  Theoretically, a properly calibrat= ed=20 sensor would be expected to read the barometric setting before engine=20 start.  However, the AP part o= f the=20 MAP sensor is not required to match the TSO requirements of even a non-sensi= tive=20 altimeter.  There also may be=20 display constraints when the engine is not running.  In my case, the MAP indication on=20= my EPI=20 800 generally reads 29.2=E2=80=9D over a broad range of barometric settings.=   Since my airport is 710 MSL, this=20 reading (710 ft =3D 29.16) is consistent, all things being equal. 

 

2) The VM MAP sensor is u= sually=20 mounted thru the firewall with the AP port open to the cabin.  This means that the ambient pressu= re=20 varies with speed, venting and altitude.&n= bsp;=20 That is, the cockpit low pressure is less at lower speeds, less when=20 vents are opened.  Generally,=20= in my=20 airplane at cruise speeds, the inside pressure is .25=E2=80=9D Hg less that=20= =E2=80=9Cstatic=E2=80=9D=20 ambient pressure, depending on altitude.&n= bsp;=20 This gives rise to the question: Should I connect the AP port to the=20= same=20 static air used by the altimeter?

 

Side Note:  If an electronic ignition is used=20= and=20 one of the controlling parameters is MAP, the ambient pressure is obtained f= rom=20 the position of the sensor, generally located in the =E2=80=9Cbrain=E2=80= =9D box that is often=20 placed under the cowling and is sensing the cooling air output pressure.  This is generally slightly above=20= =E2=80=9Cstatic=E2=80=9D=20 ambient pressure.  I misplaced= the=20 measurement I once made of this but I remember it was maybe about .25=E2=80= =9D Hg=20 greater.  At cruise, this is a= true=20 difference of about .5=E2=80=9D in what the MAP indicator is showing and wha= t the=20 electronic ignition is using for timing adjustments.

 

 3) True ambient (static) pressure a= t=20 altitude is an elusive number.  This=20 pressure is dependent on air density which is sensitive to absolute altitude= ,=20 temperature and humidity.  Not= e that=20 our altimeters are built to make assumptions about temperatures at altitude=20= and=20 the MAP sensor is not. =20 Additionally, the true pressure is dependent on what has gone on at l= ower=20 altitudes with respect to temperature, moisture, etc.  The higher the altitude the more=20 difficult it is to even estimate the ambient pressure, let alone back calcul= ate=20 it from the altitude and OAT not adjusted for compressibility due to air=20 speed.  Luckily, most of us us= e=20 sensitive altimeters built to TSO standards and set to the local barometer=20 setting so that we read altitudes the same way even though we may be far off= =20 from the true altitude.  Maybe= this=20 helps to understand one of the reasons why GPS altitude determination and=20 vertical guidance is not trivial. Also, I better understand why the Flight=20 Levels above 18000 MSL require the 29.92 Kollsman setting.

 

Remember:  Look out below when going from hi=20= to lo=20 or hot to cold.

 

Gauge Pressure (GP):

GP is obtained from a tap= into=20 the induction system, usually from a primer port on one of the cylinder head= s=20 ahead of the intake valve (at least that=E2=80=99s where it generally is on=20= an injected=20 Lycoming engine). Any reduction in the throttle denies air to the engine thu= s=20 creating a vacuum  in the indu= ction=20 system.  The sensor is protect= ed=20 against pressure pulses (valve operation) in the induction system by a =E2= =80=9Csnubber=E2=80=9D=20 valve. 

 

I went back to my parts b= ox and=20 checked an older one from a prior VM MAP sensor.  As Lorn reported, it is a thin por= ous=20 metallic disk contained in the large brass fitting on the firewall side of t= he=20 sensor.  It seems like it won= =E2=80=99t pass=20 air when one tries to blow thru it. =20 At least 4 people I have talked to have had problems with the snubber= =20 valve causing problems in MAP readings, especially after throttle change.  Remember that its placement create= s a=20 chamber from which the pressure is sensed.=  =20 There can be at least two problems because of fouling:

 

1)      =20 Inability for the chamber to return completely to ambie= nt=20 pressure =E2=80=93 This might yield a low MAP reading before engine start?

 

2)      =20 Fouled in a way that makes it very slow to sense vacuum= or=20 incorrectly requires a greater vacuum to reduce the pressure in the chamber.=

 

 

How can it get fouled?  There is no movement of air thru t= he=20 sensing system.  Is the sensor= =20 located lower than the tap into the induction system? If so, can fuel, oil o= r=20 water condensations run down the line to the snubber?  This certainly happens in those ca= nard=20 airplanes that park with the nose on the ground.  Hmmmmmm=E2=80=A6=E2=80=A6  What about the sensor in the elect= ronic=20 ignition?

 

Ram air effect:

Precisely what is the ram= air=20 effect and how much does it improve the MAP?  The airspeed system is a perfect e= xample=20 of a closed system which measures the differential pressure between ambient=20 (static) and ram in the pitot which is then converted to the airspeed=20 display.  The KIAS takes into=20 account the less dense air at altitude where both the engine and wing have t= o=20 deal with fewer molecules.  Th= e rise=20 in the air pressure difference due to ram in a closed system is thus:

 

KIAS  Inches of Hg

100      .5

150    1.1

200    2.0

250    3.1

 

These numbers could be co= nsidered=20 a maximum assuming that there is a special inlet and no drop due to a=20 filter.  If filtered induction= air=20 were taken from a well sealed engine cooling air plenum, the filter and ram=20 effect may cancel each other out at say, 160 KIAS (a guess from pressure=20 measurements taken from my upper cowl area).  Of course, none of this takes into= =20 account that the air is just passing thru.

 

Side note:  Consider that ram air can raise th= e=20 induction air pressure higher than that utilized by the fuel injectors=20 (obtained, for example, from the upper cooling plenum) to help atomize the=20 fuel.  This may suppress the b= est=20 atomization and shrouded injectors utilizing the higher ram air, available=20 before the throttle body, may improve atomization.  Such a system is similar to the wa= y=20 super/turbo charged engines operate.

 

Well, what the heck, with= =20 unfiltered ram air I know my induction system is performing better, at a low= er=20 altitude than my wing is seeing. Something for nothing (except cash).  Now I want to know precisely how m= uch=20 better so that I can have bragging rights over somebody else=E2=80=99s syste= m.  However, as can be seen from the a= bove=20 discussion, comparable numbers are hard to come by.

 

Wait!  There is a way.  Join me in the next Air Venture cr= oss=20 country race and we shall see how well your ginned up system works.  Yep, the gauntlet has been tossed=20= at=20 your feet.

 

Disclaimer: These data an= d=20 opinions are provided free and, as such, are worth what you are paying for=20 them.

 

 
Scott Krueger=20 AKA Grayhawk
Lancair N92EX IO320 Aurora, IL (KARR)
I am going=20 back to my cell now - Meds are being=20 increased.

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