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From: (no name) (no email)
Date: Thu Mar 06 2008 - 10:14:42 EST
This is more about fuel pumps than most people want to know.
Most electrical fuel pumps, like the Walbro, the old Bendix, the old SU, and
the Facet, work in a similar manner. At one end of the pump there is a pumping
chamber usually consisting of a cavity closed by a diaphragm or piston. Input
and output valves are at the top of the cavity to direct the fuel flow. A
solenoid core is attached to the diaphragm. When the diaphragm is at the top of
its stroke, an electrical contact is made which activates the solenoid and
pulls the core down also pulling down the diaphragm. This is the intake stroke.
Fuel is sucked into the cavity. When the solenoid core reaches the bottom of its
stroke, the electrical contact is broken. A spring attached to the solenoid
core pushes it back up, compressing the diaphragm or piston and forcing fuel
out of the cavity. The cycle then repeats. The fuel pressure is determined by
the strength of the return spring. Most standard fuel pumps are set at 3 to 7
psi. Adjustable pressure pumps contain a screw to vary the compression on the
spring and thus increase or decrease fuel pump pressure.
In old carbureted engines, the carburetor float regulated the amount of fuel
that could be admitted. If the float valve was closed, the spring did not have
enough force to push the incompressible fuel past the float valve and the
pump would stop cycling until more fuel was needed. In diesel engines the
injection pump serves the same purpose. The fuel pressure in the primary fuel system
is controlled by the strength of the electrical pump spring.
The valves offer very little resistance to the free flow of fuel in the
correct direction, hence the pump can be connected in series with another
mechanical or electric pump. In fact most boaters simply connect an electric pump
between the primary filter and the engine's mechanical lift pump without additional
bypass valving. There is no harm in bypassing the electric pump. It is just
not necessary in most situations. The only situation where a bypass would be
advisable is if the electric pump were placed before the primary filter.
Most electric pumps contain their own internal filter, usually a fine thimble
sized or smaller screen. This is intended to stop large particles of grit
from passing into the valves and causing them to leak. This filter can be blocked
by a small plaque of algae. Manufacturers void the warranty if the pump is
used without some form of filtering. To be honest, I've known of only a few
cases where unfiltered fuel caused pump failure. It's just that it is hard to
clean the pump's internal filter. To be on the safe side, if you are going to use
a pump before the primary filter, fit a wire mesh screen filter in front of
it. These are available at most small engine shops and cost just a couple of
bucks. In the very unlikely event that particles flake off the fuel tank and clog
the filter, you can replace it with a spare in a few minutes.
Now for the downside of diaphragm pumps. They are very good at pushing fuel
but very bad at sucking it. Most will self prime but not if they are required
to lift fuel more than a couple of feet. This means that they should be
installed at or near the low point of the fuel system. As the pumps age, the valves
tend to leak a slight amount, letting pressurized fuel bleed back into the low
pressure side. When you turn on an electric pump after a period of dormancy,
it will cycle rapidly for 10 or so seconds to build pressure up to its
regulated 3 to 7 lb. pressure. On an engine mounted mechanical diaphragm pump, you
will have to crank the engine for a period of time. Not too good if you have a
weak starting battery. This is another reason for having an electrical fuel pump
in series with the mechanical pump. Turn it on for 15 seconds before you
start the engine and in most cases the engine will start right up with only a
second or two of cranking. At least my Perkins always does.
Modern automobiles finesse the bleed down problem by installing vane or gear
fuel pumps right in the gas tank. Turning on the ignition starts the pump
which immediately pressurizes the 10 feet or so of fuel line between the tank and
the engine. The pump, submerged in the fuel, has no priming problems and is
cooled by the surrounding liquid. Fuel pressure is regulated by a bypass valve
preset at the factory.
Larry Z
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