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.  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.  The temperature drop is due to fuel evaporation and the result is a positive.  In 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.  A fuel-injected engine gains very little from fuel evaporation as most of the latent heat comes from the cylinder head and intake valve.  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.  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 -  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.  The Bendix system, made by Precision Airmotive, probably gives more consistent air/fuel ratios if placed downstream of the intercooler.

Gary Casey

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.



Walter