pressure carburetors

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h 1998Pressure Carburetors

• The basic engine needs for pressurecarburetors are the same as float.

• The primary differences between float andpressure carburetors are:– the fuel distribution point is at a different

location from the fuel meter, usually after thethrottle.

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• The fuel is pressure fed into the airstream,maximizing atomization.

• Fuel is fed into the initial staging area by apressure pump

• Typical unmetered fuel pressure is 9 - 14 psismaller, and 14 - 25 psi on the large multi-bore carburetors

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• Air metering force is created with a pressuredifferential:– Air inlet impact pressure (high)

– Venturi pressure (low)

• Fuel metering force is that pressure differencefelt across the metering jet.

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• This is a dynamic pressure drop created by fuelflowing through a restrictor at varying rates.

• Metered fuel pressure is lower than unmeteredfuel pressure when fuel flows through themetering jet.

• Regulating inlet fuel pressure creates accuratefuel metering.

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• Fuel is regulated by a poppet valve that isdriven by two pressure differentials:– Impact air to venturi air.

– Unmetered fuel to metered fuel.

• Springs are used to alter these forces.

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• These forces move the poppet valvediaphragms and separate pressure chambers.

• Impact pressure is known A chamber.

• Venturi pressure is B chamber.

• Metered fuel is C chamber.

• Unmetered fuel is D chamber.

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• Fuel inlet pressure is E chamber.

• If A pressure increases and or B pressuredecreases the poppet will open moreallowing more fuel into D increasingpressure.

• This causes more flow across the meteringjet and closes the poppet valve some.

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• As fuel flows across the metering jet, Cchamber will unbalance the poppet valveopen more.

• Pressure differential across the metering jetand the C/D diaphragm directly regulatefuel quantity delivered to the dischargenozzle, by controlling the poppet valve.

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• Pressure differential across the A/Bdiaphragm directly monitors air flow intoengine and modifies poppet valve actionappropriate to the metering jet.

• This allows fuel regulation that isappropriate to the airflow into the engine atdiffering rates.

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• The nozzle diaphragm valve prevents Cchamber pressure from getting low enough tostart closing the poppet valve.

• Smaller carburetors use C pressure for meteringbut not for poppet valve regulation.– D balances against A and B

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• The nozzle discharge diaphram and valve actvery similar to a pressure relief valve.

• They will try to maintain C chamber at aconstant pressure.

• This will vary by 1/4 to 1/2 PSI depending onengine load.

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• The D chamber poppet valve will attempt tokeep a 1/2 PSI pressure differential across thefuel metering jet(s).

• This attempt is modified by differentialpressures across A and B chambers.

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Pressure Carburetors

E D C B A

E = inlet, D = unmetered, C = metered,

B = venturi air, A = impact air

Mid throttle

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E D C B A

Mid throttle

Changes in fuel inlet pressure cause D and Cchambers to regulate D pressure.

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E D C B A

Mid throttle

Chamber D low pressure opens poppet

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E D C B A

Mid throttle

Chamber D high pressure closes poppet

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E D C B A

Mid throttle

Returns to equilibrium.

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E D C B A

Chamber C lower pressure closes poppet until Dchamber drops and reopens poppet

Full throttle

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E D C B A

Mid throttle


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E D C B A

Full throttle

Chamber B low pressure, chamber A highpressure opens poppet

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E D C B A

Mid throttle


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E D C B A

Idle throttle

Chamber B increased pressure chamber Adecreased pressure closes poppet.

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E D C B A

Idle throttle

D and C chamber then find a new balance.

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E D C B A

Mid throttle


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• Inlet impact pressure provides a betterthrottle response then inlet ambientpressure.

• Air bleed comes from impact air and entersfuel stream in the discharge nozzle.

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• Auxiliary components can include:

• Accelerator Pump

• Enrichment / idle control circuits

• Mixture control circuits

• Automatic mixture control circuits

• Separate throttle body/fuel control

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• Accelerator Pump– Operates by venturi pressure reacting against a

spring.

– Low MAP draws diaphragm back fillingchamber with fuel.

– High MAP releases this into discharge fuelnozzle.

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• Enrichment / idle control– Commonly done by throttle linkage.

– May operate in conjunction with nozzlediaphragm needle valve.

– It will react to changes in metered fuel pressureand throttle position.

– Needle will be stepped and tapered.

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– Main metered fuel will be regulated by idleneedle valve and discharge needle valve.

– When throttle opens, idle needle valve isopened and removed from main meteringaction.

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• Mixture control circuits– will be manual

– may have automatic function

– bypasses A pressure to B pressure or:

– flow valve regulating metered fuel

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• Mixture control circuits– idle cut-off can be:

– A-B bypass type has throttle linkage that closesD chamber poppet valve.

– fuel flow regulating type the mixture controlvalve will simply shut off metered fuel flow.

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• Automatic mixture control will use aneroidbellows modulating a needle valve.

• Will be a density controller (pressure andtemperature).

• May be oil dampened.

• Does not replace manual control, just allows forless operational input.

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• Separate fuel control and throttle body– precursor to fuel injection

– one model includes a float vent assembly

– throttle body has all I/O devices

– fuel control has all regulating devices exceptthrottle plate

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– Poppet valve action occurs between C chambermetered pressure, and discharge pressure.

– D chamber is inlet fuel pressure.

– A and B pressures felt between C and D pressures.

– Unit has boost venturi (B) and main venturi (automix con) pressures

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– Power enrichment by valve opened when excessivedifferential exists between C and D during auto-richmixture control setting.

– Idle metered by throttle operated plunger valve, andC - D poppet valve leaf spring.

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– D chamber is high flow fuel staging area.

– Vapor vent float valve will vent vapor back tofuel supply tank.

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• The main advantage of a pressure carburetoris the reduction of icing potential.

• Secondarily throttle transitions and invertedoperations are much better.

• Disadvantage = $$$$$$$$$$$$$$$$

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SECTION END

pressure carburetors

Documents