Fuel PSI In Carbs/ Injected Engines

[uwochris]

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.

 

[uwochris]

bumparoo

 

[Fuzzy_is_Hungry]

Fuel return lines maybe? Usually there is some kind of line that returns unused fuel to the tank so that there is always a constant supply at the pump. Automobile engines have this kind of system, not sure if airplanes do too, but that would account for the constant pressure indication if the measurement is taken somewhere upstream like that.

 

[avbug]

Chris,

The generalizations made in Kershner's book aren't true in all cases. In fact, turning on the fuel pump in most aircraft in flight will not flood the engine. Fuel pumps are used for several different purposes in aircraft, and one needs to consider the individual system.

A piston engine is an air pump. It draws air in the carburetor side, and pushes it out the exhaust side. In the carburetor, decreased pressure through the internal venturi is used to draw or "suck" fuel into the engine. The faster the engine, the more air flowing through the carburetor, the faster the airflow, and the greater the pressure drop. The greater the pressure drop, the greater the force drawing fuel out of the carburetor, and therefore the greater the amount of fuel flowing.

In a typical light airplane carburetor, the fuel is delivered to the carburetor either by gravity, or by pressure from either an engine driven fuel pump or an electrical pump...often a combination of each. Not much pressure is required to deliver this fuel...it just needs to get to the carburetor and the engine does the rest. When the fuel arrives at the carburetor, it's dumped into the float chamber. The float chamber works a lot like the float chamber on the back of your toilet. When the chamber is full, a float rises and blocks off any further fuel flow into the chamber. That fuel pressure is more or less to that point isn't relelvant, and the float prevents it from going any further.

All that the float cares about is that enough pressure is available somewhere to deliver the fuel to the carburetor...the carburetor will take it from there. After that, the amount of fuel consumed depends on the speed of the engine. The pilot controls the speed of the engine by opening and closing the throttle. This doesn't control fuel flow directly, but instead controls only the amount of air the engine can "suck." Allow more air past the throttle (which should be thought of as an air valve), and more air is available to pass through the carburetor, which picks up more fuel, and the engine accelerates.

A fuel injected engine works a little differently. Several different systems are in use, and your car works a little differently yet. In your aircraft, fuel is delivered under pressure to a fuel flow divider (the little "spider" on top of your engine with the little silver lines going to each cylinder). From there, it's delivered to the cylinders individually. The throttle still acts as an air valve, but also includes either linkage to a valve conrolling the fuel directly, or in most cases, a fuel controller. The fuel controller in turn know show to work by sampling the air pressure going to the cylinder, and them metering the fuel according to a preset mechanical ratio. This differs from your car, which usually uses a computer to do the same thing.

The difference between your car and your airplane, of course, is that you need a mechanical failure to interrupt the fuel metering in the airplane, and there's no computer to go bad.

Computers are coming into vogue more and more in aircraft fuel systems...you'll see this increasingly in the future.

The fuel pump you turn on in the aircraft is an electrical pump, not an engine driven pump. The electrical pump is a constant duty pump in some aircraft, meaning that it runs all the time the aircraft is running. Many turbine engines utilize pumps for this purpose in different forms, mostly forms that aren't found in piston airplanes (such as jet pumps). The purpose is generally to deliver fuel to the engine driven pump. The pump doing the delivering can't always keep the engine running on turbine equipment (but almost always can on piston equipment, due to lower fuel flow and pressure demands), but the engine driven pump needs the fuel supply. The engine driven pump then does the rest.

Fuel systems usually use a metering orfice or fuel pressure regulator to limit the amount of pressure delivered to critical points in the fuel system. If the fuel presure values increase beyond this, bypass valves return the excess fuel, the fuel that isn't being used, back to holding tanks, the main fuel tank, or often the inlet side of the pump. The bypass valves open and close automatically to prevent an unwanted pressure buildup and to relieve the dangerous fuel pressure that might damage other valves or components downstream of the pump.

The fuel controller may also deliver excess fuel back to the pump, fuel tank, or other places in the fuel system, as a way of regulating the fuel flow to the engine. The engine driven pump or electrical auxilliary fuel pump delivers the maximum amount of fuel to the controller that it "thinks" the controller might need, and the controller throws back everything it doesn't need by way of a bypass system. A lot of turbine engines work the same way. This accounts for variances in engine speed...the engine is always getting the fuel it needs, but throws back what it doesn't want.

A good example of this is the 200 series Cessnas, that use a header tank, or kidney tank, between the main fuel tanks and the engine. This tank, about two and a half gallons, receives fuel from the main wing tanks, and also from the engine fuel system as bypass fuel. The 200 series Cessnas (excepting the 208) also have a proceedure for "fuel flow fluctuation" because of this design. Fresh, cold fuel is arriving from above, from the wing tanks. Hot, vapor-laden fuel is arriving from the engine fuel system. That fuel tends to want to rise, and may be more prone to the formation of gas or vapor due to it's heat...and it tries to return to the wing tank right up the same line that's feeding the engine. This results in some cases in fuel stoppage, which causes a fuel flow fluctuation in the cockpit, and a rough engine or a failed engine.

In those airplanes, your only choice is to switch tanks, which almost always restores power right away. The book proceedure also has you turning on the boost pumps, and then adjusting the mixture...as the increased pressure from the boost may enrichen the mixture, and *could* flood the engine. Here again, however, turning on the boost and pushing more fuel under greater pressure to the fuel controller doesn't mean that you're shooting more fuel straight into the engine...you're only seeing more fuel returned back to the header tank. More aircraft fuel is being delivered to the engine fuel system, and more is being returned again as the engine only "wants" so much.

Continental fuel injected engines use a different injection system and controlling system than lycoming engines. Different components in fuel systems act differently or perform different functions, depending on what aircraft and fuel system it is to which you refer. Each system has it's own strengths and weaknesses, as the system in the 200 series cessnas illustrates.

In many aircraft fuel pumps don't serve the engine at all but move fuel around from one part of the aircraft to another. These are often referred to as transfer pumps. Some aircraft such as the ercoupe, use transfer pumps to move fuel to a place where gravity can take over and feed the carburetor.

Chris, getting to know your specific airplane and it's system is always important, especially with critical things such as fuel systems. If you are flying a fuel injected aircraft, when you get up and away from terra firma sometime, turn on the boost pump and see what happens. Try different combinations of throttle, boost, mixture, etc. Know the aircraft before you go, follow the proceedures outlined in your aircraft flight manual with special attention paid to any cautions. Errors with fuel systems can cause fires or engine failures. Doing low G pushovers in float carburetted aircraft, for example, can cause the carb float to stick in certain aircraft, and the result can be a flooded engine or a fire...Cessna had a problem with this years ago and released warnings not to do that maneuver in some of it's aircraft. In others, such as the 200 series Cessna identified above, specialized proceedures are given to deal with the fuel system peculiarities...know your aircraft.

As an aside the proceedure provided by Cessna for the 200 series aircraft as explained above can actually lead to engine failure or make the situation worse. Cessna suggests applying boost (turning on the fuel pump) first, then switching fuel tanks, and finally adjusting your mixture. However, applying boosted pressure only returns more hot fuel to the header tank, and can quickly result in an engine failure. Swapping tanks first is a much better way to go, but if you envision the system, you can also see why Cessna made the recommendations they do. If you turn on the boost pump before switching tanks you're ensuring a positive fuel pressure and helping purge the return lines of contaminated (hot aerated fuel) fuel before going to your clean source with the other tank. Again, look at your specific aircraft and fuel system.

If you're wanting to better understand any system on your aircraft, I strongly suggest going to the aircraft maintenance publications for more information. I also always encourage pilots to visit the shop floor to see it first hand with a mechanic. They say a picture is worth a thousand words, but seeing the actual system and how it works in the shop or hanger is even better, and it will help you better visualize what's going on when you're flying the airplane. It expands on the "bigger picture" and gives you more "situational awareness" when it comes to dealing with the system in flight. The more you know about the aircraft, the better you are equipped to deal with both normal routine operations, and abnormal proceedures or situations as they may arise. Good luck!