posted for Gary Casey <glcasey@adelphia.net>   -Rob

I've been fairly intimate with the Lycoming (Bendix/Precision
Airmotive) fuel system, but I'm not an expert regarding the
Continental system.  However, maybe a description of the basic
control methodology would clear the air - or at least remove a
vapor lock or two..

The Continental system is what some describe as a "speed-theta"
system as opposed to a "speed-density" (Honda and Chrysler) or
"air-flow" system (used in most cars and by Lycoming).  The
concept relies on a positive-displacement pump turned by the
crank to pump (actually "meter") a constant volume of fuel per
revolution.  Then a valve connected to the throttle restricts
(or allows a bypass in the case of the Hilborn racing system)
the flow in proportion to throttle angle (the "theta" of
"speed-theta").  A very simple system in concept, but not
so straightforward in execution.  Remember that the fuel,
under atmospheric pressure in the fuel tank, must snake its
way through the cabin into the fuel selector valve and from
there through the electric pump, pushing open the check valve
that allows it to bypass the pump.  Then to the gascolator
and through its screen and finally it gets to the engine driven
pump.  The pump is a gear-type pump sized to deliver somewhat
more fuel than the engine could ever use - which means it is
fairly small as these pumps go - much smaller than an oil pump.
A gear pump works by "sucking" fuel into the gaps between the
gear teeth as the separate, carrying it around the pump and
then squishing out from between the gear teeth as they mesh
on the other side.  Not too elegant.  Since there must be
clearance around the teeth there is always leakage present
with fuel flowing backwards around the teeth (remember, the
pump discharge pressure is higher than the inlet pressure).
Above the pump is an orifice that bypasses some fuel directly
to the tank through the fuel selector valve.  I believe this
is a fixed orifice, but I'm not absolutely sure.  From the pump
the fuel goes through the fuel flow transducer and then to the
metering valve.  This is a carefully-designed valve that restricts
the flow more at low throttle openings and less at high openings,
with the shape of the transition carefully matched to the flow
capacity of the throttle at each angle.  From there it goes to
the "distributor" which has a relief valve that requires a
certain fuel pressure before fuel is allowed to go to the
injectors.  This valve actually has a separate flow area
(in the form of a slot) that goes to each injector.  These
slots gradually open as the flow is increased until at some
flow substantially more than idle they are wide open, connecting
all injectors to a common source.  Finally the fuel makes it to
the injector, which is another fixed orifice that has the downstream
side vented to atmosphere (or compressor discharge pressure in the
case of a turbocharged engine).

So what does all this mean?  First, the pump delivers a fixed
VOLUME of fuel, so if there are any vapor bubbles in the inlet
the MASS flow of fuel is instantly reduced.  Deliver too much
vapor to the pump and the leakage around the gear teeth will
increase (more volume of vapor will flow through the leakage
than liquid) to the point that the pump won't deliver any fuel
at all - a "vapor lock."  The pressure at the pump inlet is
below atmosphere because of the restrictions of all the stuff
in the line from the tank.  How to turn fuel into vapor?
Lower the pressure and increase the temperature.  The pressure
can be low because of a nose-high attitude during climb,
restrictions in the filter, or a low fuel level in the tank.
The temperature goes up as the fuel is heated in the engine
compartment - the fewer components exposed to that heat the
better.  How to fix it?  Turn on the boost pump as that will
pressurize (supercharge) the engine pump inlet, preventing vapor
formation.  But the engine fuel "pump" is actually a metering pump,
delivering a presumably precise volume of fuel and the resulting
flow into the engine is a result of fuel flow through a combination
of restrictions and bypass orifices - in essence a delicate balance
of forces.  When the metering pump is supercharged, the former
pressure rise across the pump becomes a pressure drop - the pump
is actually "holding back" against the pressure of the electric
pump.  The flow due to internal leakage now reversed in direction
and the pump delivers more fuel than before.  How much more?
Depends on the magnitude of the leakage and all the other operating
conditions, like altitude, engine speed, boost pressure and throttle
position).  If the engine is already almost too rich it might flame
out.  If it is lean it will just get richer.

If the flow is predictable, then why is there so much effort
required to "set up" the system for each engine?  I'm not sure,
but the internal leakage of the metering pump surely varies
considerably from unit to unit.  The large throttle valve changes
air flow dramatically with very little opening when at idle.  Also,
since the fuel delivery is volume-based there can be significant
flow changes based on fuel temperature.  Fuel density easily
changes by 10% from cold to hot.

I hope this description helps - I hope it wasn't so simplistic
to be patronizing or too obscure to be useful and if there is
anything incorrect I would appreciate hearing about it so I can
be corrected.  The principle of operation of this fuel system
is the main reason I installed a Lycoming engine in my ES.

Gary Casey
ES157, N224SG