B&S 49-series (810cm3/49ci) for aircraft use - TiPi's Q&A thread

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Chilton

Well-Known Member
Joined
Jun 28, 2014
Messages
103
Location
Jersey, channel islands
Maybe the carb heat should be half on normally all the time. The pilot could "override" this carb heat momentarily for takeoff with a spring loaded gate valve that snaps back to normal.
That sounds exactly like the DH Gipsy engine system, the warm air (not hot as it comes from around the cylinders not the exhaust) is the normal feed and the selector flap is spring loaded to this, the last half inch or so of throttle lever travel over rides the spring and selects cold air for full power.

This is one of the reasons why DH teaching was to pul the power back slightly at around 500 feet on the climbout to give the engine warm air. It is not necessarily a good habit to carry forward to the Lycoming and Continental engines.
 

rtfm

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Joined
Jan 3, 2008
Messages
3,233
Location
Brisbane, Australia
Hi,
Not sure if this has been mentioned before, but the Valley Engineering Big Twin has a small outlet drilled in the exhaust, and a narrow pipe tapped into this feeds directly into the air intake just before the carby. Instant and constant carb heat. Simplle and fail-proof. What power loss? Have no idea.

Duncan
 

TiPi

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Log Member
Joined
Aug 25, 2019
Messages
292
Location
Mackay (AUS)
Carb icing and the evaporation are 2 separate problems linked by the temperature of the intake air:
- carb icing can happen at the butterfly even with a warm manifold if the carburetor is isolated from the manifold
- fuel evaporation is helped by air temperature from the time the fuel enters the airstream
- it is better to have the carburetor isolated from the manifold to reduce heat-soaking after shutdown to prevent hot-start issues

Carb icing and fuel evaporation benefit from an increase in air temperature entering the carburetor. The downside is the powerloss due to decreased air density and the added mixture change (richer) as the temperature increases.

Ideally, at WOT the air temp is as cold as possible and then increases to about 25-30deg C with closing trottle, and 50-60deg at idle to help with the fuel evaporation at low air volume and velocity. Carburetted cars used to have a thermo vacuum valve that controlled the intake air temp regardless of throttle position.
1594852440102.png
 

Hephaestus

Well-Known Member
Joined
Jun 25, 2014
Messages
1,752
Location
YMM
Got some excess battery power?

Diesels have intake air heaters...fuel-systems-dih1-64_1000.jpg

Warm air no recirculation, just takes amps. Would be fairly easy to build a fancy driver to have it come on as needed without input.
 
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