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The Anatomy of a AA/Supercharged Fuel System
The Anatomy of a AA/Supercharged
Fuel System: The Final Chapter
The
AA/Supercharged fuel system settings are key to the vehicle performance. The fuel volume must be set to match the capability
of the motor to burn the fuel for a given atmospheric condition. At 8000 RPMs, a typical AA/Supercharged fuel system
is delivering 7.5 to 9.5 gallons per minute at around 100 PSI. The fuel volume could be compared to your garden hose
running wide open.
The science behind the fuel system is known as the fuel curve. The fuel curve is
the change in the fuel volume to fuel pressure relationship through the entire RPM range. The fuel curve is designed
to match the need of the engine at a given RPM.
The fuel volume is trimmed by returning fuel back to the tank.
This is accomplished with the main jet, high speed or lean out jets. The fuel pressure is set by adjusting the
combined area of the delivery nozzles, located in the injector hat and intake ports. The AA/Supercharged rules dictate
that any changes effecting the fuel system must be controlled by the fuel pressure, or be actuated by the driver, typically
with a gear change.
For a given fuel curve, the relative amount of fuel is typically adjusted with the main jet.
If the main jet is too large, you will burn pistons. If the main jet is too small, the engine will be too rich
and not make optimum horsepower. The team that matches the fuel curve to the needs of the engine and balances the main
jet to be not too rich and not too lean, will have a horsepower advantage on the competition.
Part
3: Fuel
Delivery System
The AA/Supercharged fuel delivery system has three modes of operation:
Idle, Stage and Wide Open Throttle
The barrel valve controls the fuel delivery in each mode of operation.
Barrel Valve
The barrel valve is the heart and soul of the fuel delivery system and has two main
features to control the fuel delivered to the engine. At idle and stage, the amount of fuel delivered to the engine
is restricted by a small V-shaped slot in the face of the barrel valve. At full throttle, the barrel valve rotates
90 degrees and the entire U-shaped center allows all of the fuel from the pump to pass without restriction. The
rotational position of the barrel valve is directly controlled by the throttle peddle. I would normally include a photo
of the barrel valve for a better description, but in this case that information is proprietary, based on modifications that
can be made.
At idle, the tip of the V-shaped slot allows only a small amount of fuel to pass
to the engine.
At stage, the driver uses the throttle peddle to set the stage RPM, typically above 4000 RPMs. Only
a small movement on the throttle peddle is required. In this case, more of the V-shaped slot is exposed to the fuel
coming from the pump and additional fuel is delivered to the engine. The relationship between the exposed V-shaped slot,
for a given throttle position determines the air-fuel mixture at stage and thus the horsepower available when the car leaves
the starting line. This is a critical adjustment and is controlled by linkage between the throttle blades and the
barrel valve.
At full throttle, the V-slot rotates out of the fuel path and the machined out center section of the barrel
valve allows all of the fuel from the pump to pass without restriction.
Hat Nozzles
The
hat nozzles are located between the injector hat and the top supercharger opening. A typical AA/Supercharged car will
have eight hat nozzles. By injecting fuel above the supercharger, the fuel will be automatically mixed with air for
a more efficient burn and it also serves to cool and seal the rotors in the supercharger. Fuel passes through the hat
nozzles at idle, stage and full throttle.
Dribblers
Dribblers are small nozzles located in the
intake ports of the cylinder heads. They serve to cool and regulate the cylinder temperatures at idle and stage.
The fuel delivered from the supercharger feeds the rear cylinders much more than the front cylinders and the dribblers are
used to compensate for the variations in distribution at idle and stage RPMs. The number of dribblers varies from team
to team, from 4 to 6 or 8, with the relative size a function of the amount of fuel passing through the blower. Dribblers
pass fuel at idle, stage and full throttle.
Port Check
The port check is a pressure
valve that prevents fuel from being delivered to the port nozzles at idle and stage. With the low fuel pressures involved
at idle and stage, the port check pressure is typically set below 15 PSI. At a full throttle condition, the port check
opens immediately and allows fuel to be delivered to the port nozzles.
Port Nozzles
The
port nozzles are located at the cylinder head intake ports, immediately adjacent to the dribblers. The port nozzles
are used to regulate the temperatures of individual cylinders at full throttle. The size of individual port nozzles
are determined by reading the spark plugs, rod bearings and computer readings.
Part
2: Fuel Bypass System
The intention of the bypass system is simple. Control the amount of fuel delivered
to the engine, by returning unneeded fuel back to the tank. Fuel returns can be dumped directly into the tank or returned
to the inlet side of the fuel pump.
Idle Check
The Idle check is used to maintain pressure
in the fuel system at idle and can be used to fine tune the idle mixture. The idle check consists only of a spring loaded
valve with no jet involved. Fuel passing the barrel valve at idle is seen by the idle check spring. When the idle
fuel pressure overcomes the idle check spring pressure, the valve opens and returns fuel to the tank and reduces the amount
of fuel delivered directly to the engine. A higher spring pressure will create a richer fuel mixture at idle, as all
of the fuel will go directly to the engine, before the valve opens at a higher pressure.
Main Jet
The main jet is the most critical tuning component used for a AA/Supercharged engine. The main jet
is a bypass jet and determines the overall amount of fuel delivered to the engine. The main jet must always be adjusted
to compensate for atmospheric conditions. If the temperature, humidity or relative atmospheric elevation increases,
the size of the main jet must also increase to reduce the amount of fuel delivered to the engine. In this condition,
the relative air density is decreased and the amount of fuel required by the engine must also be decreased accordingly.
The main jet is also a tool to increase horsepower, by creating a leaner fuel mixture. Leaning the overall fuel mixture
is a fine line to walk, because if you step over the line, burned pistons will result.
High Speed
The High
Speed consists of a jet and a pressure activated valve like the idle check. The high speed is used to trim the
fuel curve to match the efficiency of the blower at higher RPMs. The roots-style blowers used for AA/Supercharged become
less efficient at high RPMs and the High Speed is used to reduce the fuel volume accordingly. The size of the jet is
selected to determine the amount of fuel to return to the tank, thus creating a leaner fuel mixture. The pressure of
the valve is set to determine the RPM range to begin leaning the engine. The pressure is typically set well above 50
PSI to operate the lean out process at higher RPMs only.
Lean Out
The Lean Out jet can
be used to richen or lean the fuel mixture, based on the type of valve connected. The AA/Supercharged rules state this
valve must be driver actuated and is typically connected to the air shifting line, to allow for actuation as a result of a
gear change. Once again a jet is selected to determine the amount of fuel to return to the tank. To richen the
engine, a normally open valve is used. In this configuration, fuel is returned to the tank, until the valve is closed.
When the valve closes, the returned fuel is delivered to the engine instead of the tank and thus richening the fuel system.
A normally closed valve is used to lean the fuel system. When this valve opens fuel is returned to the tank and the
fuel mixture becomes leaner.
Pump Saver
The pump saver is used to relieve stress to
the fuel system, when the driver steps off the throttle at the end of a run. At the finish line, maximum fuel is delivered
to the engine and when the throttle is closed, the fuel pressure goes through the roof. The Pump Saver is a high pressure
valve that remains closed during the run and opens only when the driver steps off the throttle and returns fuel to the thank,
thus relieving pressure from the system.
Part
1: Fuel Tank, Inlet and Fuel Pump
Fuel Tank
A AA/Supercharged fuel tank
requires baffling, to ensure a consistent source of fuelduring acceleration, deceleration and tire shake conditions.
The fuel tank must include a vent to provide sufficient air ventelation during high fuel flow conditions. Insufficient
ventilation can create a vacuum, within the tank, resulting in restricted fuel flow and burned pistons.
Fuel
Inlet
The fuel inlet is a large line between the bottom of the fuel tank and the bottom of the fuel pump.
A tight bend in this line could restrict the high level of flow required during a run. If a rubber line is used, it
must have a stiff wall, to ensure it will not collapse as the fuel pump provides a huge amount of suction during a full-throttle
condition.
Fuel Pump
The standard AA/Supercharged fuel pump is a gear type, that will provide
a direct relationship between engine RPM and fuel flow. A gear pump will continue to provide a constant fuel flow, regardless
of size of the delivery orifice. The smaller the delivery nozzles, the higher the pressure, with fuel flow remaining
constant.
Pump Loop
A pump loop is also known as a pump sizer. If a large fuel pump
is used, a bypass jet in the pump loop can be adjusted to set the forward flow to the desired level. It is not recommended
to used the pump loop as a tool for tuning the fuel curve from run-to-run.
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