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Date: Sun, 07 Aug 2005 15:03:20 -0400
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From: Walter Atkinson <walter@advancedpilot.com>
Subject: Re: [LML] Re: Lower Cowl Air Temperature - Induction Air - Heater Air Temp


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Gary:

Thank you for the clarification and detailed explanation.

Walter


On Aug 7, 2005, at 10:22 AM, Gary Casey wrote:

The theoretical temperature drop of the mixture due to fuel evaporation=20=

is about 70F with a normal mixture, correlating closely to Walter's=20
measurement.=A0 But he suggested there is a temperature drop due to the=20=

carburetor venturi - not quite true as the temperature drops in the=20
venturi throat, but during the diverging portion is rises again - very=20=

little net drop. =A0The temperature drop is due to fuel evaporation and=20=

the result is a positive. =A0In other words a given volume of air/fuel=20=

mixture contains more oxygen if the fuel is evaporated than if it is in=20=

liquid form as long as the latent heat comes from the air. =A0A=20
fuel-injected engine gains very little from fuel evaporation as most of=20=

the latent heat comes from the cylinder head and intake valve.=A0 The=20
fuel is mostly splattered on the metal surfaces, which being very hot,=20=

evaporate the fuel very quickly but not at any significant air=20
temperature drop.=A0 One can increase power by moving the injectors=20
upstream from the intake port so that the latent heat is taken from the=20=

air, but that introduces other problems.

Someone else discussed placement of the throttle body -=A0 where in the=20=

intake system the throttle body is placed has a different effect=20
depending on whether you are using a "Bendix" (air-flow) system or=20
Continental (throttle angle) system.=A0 The Bendix system, made by=20
Precision Airmotive, probably gives more consistent air/fuel ratios if=20=

placed downstream of the intercooler.

Gary Casey

> As an example, in my Twin Beech, when the OAT is 92dF, the temperature=20=

> AFTER the venturi in the carburetor is 35dF. That temperature drop=20
> across the venturi is not there in an injected engine. Now add another=20=

> 100dF so that the IAT is 192 and the carb temp is 135. That is not a=20=

> good thing. At high powers, with higher CHTs, the detonation margin is=20=

> quite a bit narrower. This isn't too much of a problem in the lower=20
> compression engines but can quickly cross the threshold in the higher=20=

> compression engines.
>
>
>
> Walter
>

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Gary:


Thank you for the clarification and detailed explanation.


Walter


On Aug 7, 2005, at 10:22 AM, Gary Casey wrote:


The theoretical temperature drop of the mixture due to fuel
evaporation is about 70F with a normal mixture, correlating closely to
Walter's measurement.=A0 But he suggested there is a temperature drop
due to the carburetor venturi - not quite true as the temperature
drops in the venturi throat, but during the diverging portion is rises
again - very little net drop. =A0The temperature drop is due to fuel
evaporation and the result is a positive. =A0In other words a given
volume of air/fuel mixture contains more oxygen if the fuel is
evaporated than if it is in liquid form as long as the latent heat
comes from the air. =A0A fuel-injected engine gains very little from
fuel evaporation as most of the latent heat comes from the cylinder
head and intake valve.=A0 The fuel is mostly splattered on the metal
surfaces, which being very hot, evaporate the fuel very quickly but
not at any significant air temperature drop.=A0 One can increase power
by moving the injectors upstream from the intake port so that the
latent heat is taken from the air, but that introduces other problems.


Someone else discussed placement of the throttle body -=A0 where in the
intake system the throttle body is placed has a different effect
depending on whether you are using a "Bendix" (air-flow) system or
Continental (throttle angle) system.=A0 The Bendix system, made by
Precision Airmotive, probably gives more consistent air/fuel ratios if
placed downstream of the intercooler.


Gary Casey


<excerpt><bigger>As an example, in my Twin Beech, when the OAT is
92dF, the temperature AFTER the venturi in the carburetor is 35dF.
That temperature drop across the venturi is not there in an injected
engine. Now add another 100dF so that the IAT is 192 and the carb temp
is 135. That is not a good thing. At high powers, with higher CHTs,
the detonation margin is quite a bit narrower. This isn't too much of
a problem in the lower compression engines but can quickly cross the
threshold in the higher compression engines.</bigger>


<bigger>Walter</bigger>


</excerpt>=

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