Carburetor

[redblue]

The more I read, the more contradictory items surfaced. For example, a book says a carburetor regulates the ratio by weight, not volume. So why would the POH said lean the mixture above 3000 ft?

Also, the book says "..best power is only 2 to 4 pounds of fuel per 100 pounds of air..". Then it immediately gives various ratios for best power (1:14), chemically correct (1:15), etc.

2:100 = 1:50 and 4:100 = 1:25. Neither is close to 1:14.

Comments?

[FunctionedSatis]

It depends on what type of carb you have.

Float carbs are limited by the fact they meter fuel by volume of air that enters the engine. But as we know the power produced by the engine is dependant upon the mass flow of air through the engine.

An engine will take in the same mass of air at sea level as it will at altitude , but the mass of air will be less at altitude as the air is less dense. If the same amount of fuel is metered into this less dense air we create a rich mixture and therefore lose power. So we grab for the red lever and lean off the mixture, as the mixture is leaned temperature is allso increased, temperature has an effect on air densitiy, warmer air is less dense than cooler air, hence the leaning proccess is a balence between creating the correct mixture for a given mass of air and the cylinder (combustion) temperature.

The 15:1 ratio is the stoichiometric ratio of gasolene, this is a purely theoretical figure. The engine manufacture will state best mixture ratios for bes power based od their analysis taking many factors into consideration.

Hope this is of some help, im not an engine person really;) im avionics:E

Cheers

Stu.

[redblue]

So that'd mean that carburetors are metering by volume, not mass. This actually makes sense because I can not see how the mechanical device can actually meter by mass. If it is a venturi type, metering by flow (volume), is easier.

Is that how most carburetors for the general aviation work?

[FunctionedSatis]

pressure cabs use the air pressure in the venturi and the ram air pressure in the intake to act as a metering force accross a bellows. This is in effect a way of mass flow metering as the carb leans as the intake air pressure drops (and density) when you climb to altitude. they are generally auto lean carbs although they have a mixture control too.

I dont work in GA but i think most light GA engines use float carbs.

You do get mass airflow meters. thats how fuel injection on a car engine is done but aviation engines tend to keep it simple.

Ill try to find some more detailed info on pressure carbs and pressure-injection carbs, its been a while since ive looked into them.

Stu.

[ShyTorque]

A light aircraft carburettor is a dumb object and doesn't really know anything, unlike modern electronic fuel injection systems that have a computer driven feedback loop.

It doesn't actually care whether it measures by mass or volume. The amount of fuel drawn into the manifold depends on the pressure drop across the venturi and is set for sea level ops by fitting an appropriately sized set of metering jets, emulsion tubes or whatever, depending on the manufacturer's whim.

The pilot provides the feedback loop with a carb and can (should) adjust it for altitude in flight by use of the manual mixture control.

[411A]

Pressure carbs, as used on large multi-row radial engines, on the other hand, are fantastically complicated devices, and work as advertised, by providing proper mixture adjustments as altitude is increased/decreased.

Generally considered non-icing as well.

[barit1]

The reason the pressure carb is relatively ice-free is that the fuel is introduced downstream of the venturi. In a float carb it's sucked in right at the venturi, where the ice tends to form.

My father experienced a carb-ice forced landing at age 92. To him it was no big deal - he was on downwind so he just made a shorter pattern, and planted his 65hp T-craft right on the numbers. After all, he learned to fly when forced landings were commonplace.:p

[Volume]

the ammount of fuel flowing through the main jet is dependant on the pressure difference between the float chamber and the venturi. The pressure drop in the venturi depends on the sqared airspeed in the venturi and the density.

Airflow (volume) is product of speed and venturi cross section,
Airflow (mass) is product of speed, venturi cross section and density.
So you can state, pressure drop in the venturi (metering the fuel) is product of mass flow and volume flow, divided by two times the squared venturi cross section (later is constant).

So the carburator actually regulates the fuel by the product of mass and volume.

In a CD carburator (i.e. Rotax) cross section of venturi and fuel main jet is coupled by means of a conical jet needle, so these carburators are metering fuel by air mass.

[FunctionedSatis]

Volume

Have u any more detailed info on the rotax carb you mentioned? Im quite intrested to learn.

Thanks

Stu.

:E

[Volume]

The Rotax carb is produced by BING, the german carburator manufacturer which deliveres the carbs for the older BMW motorbikes. (This is where the Rotax carb is designed for).

Bing Homepage (http://www.bing.de/english/home.html)

Bing Carburaotor Information Sheet (Rotax Carb) (http://www.bing.de/shared/images/datenblaetter/type64e.pdf)

General Carburator Information (German) (http://www.bing.de/shared/images/datenblaetter/vergasertechnik.pdf)