Fuel Pressure In Carbs and Injection

[chris]

Hey guys,

I was reading Kersher's book and came across something interesting.

In it, he mentions that for fuel injected engines, fuel psi will vary with throttle and mixture position, as it measures pressure at the fuel distribution manifold.

He also goes on to explain that for carb engines the fuel psi gauge usually measures the pressure somewhere enroute to the carburettor, and so, the fuel psi does NOT vary with power setting.

It seems that fuel psi should still vary with power on the carb engine because the engine driven fuel pump's RPM should depend on the engine's RPM setting (i.e. it is driven by the engine), and with lower throttle settings, the fuel psi should still be lower.

Also, he mentions that if you turn the electric fuel pump on in the injected engine, the engine flood; however, if it is carburetted, you could leave the pump on all day and there will be no flooding. Why is this?

Comments??

Thanks in advance.

 

[ananoman]

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he mentions that for fuel injected engines, fuel psi will vary with throttle and mixture position, as it measures pressure at the fuel distribution manifold

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This depends, many fuel injected engines have a fuel flow gauge that actually is measuring fuel pressure at the flow divider (They do not measure actual fuel flow, they only infer the flow by measuring pressure). Some installations have an additional pressure gauge and some do not. On the engines that have an additional pressure gauge it is usually the pressure delivered to the injection system before the throttle body and does not change much.

[ QUOTE ]
It seems that fuel psi should still vary with power on the carb engine because the engine driven fuel pump's RPM should depend on the engine's RPM setting (i.e. it is driven by the engine), and with lower throttle settings, the fuel psi should still be lower.


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But, even if the fuel pump output is less, the engine uses less fuel, so the pressure does not change much. On alot of the planes that I have flown, it is common to have a fairly low fuel pressure at max power and have the fuel pressure increase in cruise at a lower rpm when not using the electric fuel pump in a carbureted engine.



[ QUOTE ]
he mentions that if you turn the electric fuel pump on in the injected engine, the engine flood; however, if it is carburetted, you could leave the pump on all day and there will be no flooding. Why is this?

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Carburetors rely on having a precise level of fuel in the fuel bowl in order to meter fuel correctly. The fuel bowl contains a float that only lets in a certain amount of fuel to maintain the correct level.

Think of a toilet with a leaky 'flapper' valve. The toilet tank has a float system that maintains the correct amount of water in the tank. If the tank leaks, it only lets in enough water to bring itself up to the correct level. It does not matter how much pressure is in the water line, the tank will not overflow. A carburetor works much the same way.

Some of the fuel injection systems rely on receiving the correct fuel pressure prior to metering the fuel and if too much fuel is received they will flood the engine. Others have a fuel pump that can be used at all times without causing any problems (The Lycomings are like this, you cannot flood the engine with the fuel pumps).