Paul,

I pulled this quote off a discussion group.
The last line is very pertinent.

Also here is a site with lots of in tank fuel pumps to offer.

http://www.car-stuff.com/store/?Ntt=lexus+gs400+fuel+pump

You're probably familiar with "return-type" fuel injection systems,
but have you worked on a "returnless" system?
Returnless systems are found on many late model cars and trucks. The
first ones appeared back in 1993 on certain Chrysler V6 and V8 truck
engines. By 1998, all Chrysler cars and light trucks had them. In 1996,
Toyota introduced its first returnless system, followed by General
Motors and Ford in 1999. Honda went "returnless" in 2001, and today
you'll find returnless fuel injection systems on almost all new
vehicles.
In the older return-type systems, the fuel pump delivers more fuel to the
engine than it actually needs. The excess fuel is then routed back to
the fuel tank through a pressure regulator and return line. But in a
returnless system, there is no pressure regulator on the fuel supply
rail and no return line back to the tank. The regulator is mounted in
the fuel tank and is usually part of the fuel pump module.
A returnless fuel injection system, by comparison, manages fuel pressure
a little differently. Instead of using a spring-loaded vacuum diaphragm
in the regulator to change fuel delivery when throttle opening and
intake vacuum change, the regulator in a returnless system operates at a
constant pressure. The older return-type systems need to vary fuel
pressure to maintain the same pressure differential across the injectors
when intake vacuum drops. When vacuum drops, the regulator increases
pressure to compensate. But in a returnless system, this isn't
necessary because the line pressure is always the same.
So how does the system compensate for changes in engine load and vacuum?
A returnless system uses the Powertrain Control Module (PCM) to regulate
fuel delivery. A fuel pressure sensor mounted on the supply rail allows
the PCM to monitor fuel pressure. When pressure in the supply rail drops
as engine load or speed increase, the PCM compensates by increasing
injector duration (on time) and/or the operating speed of the fuel pump.
Some systems (Ford, for example), vary the fuel pump's output by
changing the voltage supply to the fuel pump module. When more fuel is
needed, pump speed is increased by increasing the pulse-width (on-time)
of the pump's voltage signal (pulse-width modulation).


Doug in Japan

How does it purge air out of the system in a hot soak situation?
How does it deal with water in the fuel?

Paul Lamar

Paul

Not sure on the first question.  Assuming the use of injectors how does
air get in the system unless the tanks are run dry.  In that case there
is an greater problem to worry about:)

Second question.  It doesn't. That is pilot responsibility to sump the
tanks during preflight. Minor amounts of water mixed with hydrocarbon
based fuel is common, if not unavoidable in any motorized equipment.

From another site informing over the counter sales staff:

"In a Returnless fuel system, a fuel rail pressure sensor replaces the
fuel pressure regulator, and the ECM controls the fuel pump either
directly or indirectly through a fuel pump driver module. Fuel pressure
is varied by the amount of voltage applied to the fuel pump as commanded
by the ECM. This has the benefits of active fuel pressure control, full
compatibility with On-Board Diagnostics, and no heat transfer to the
fuel tank as the pumped fuel has a one-way ticket to the combustion
chamber. Since the system is actively monitored by the OBD system,
faults with returnless systems rarely go undetected. One thing to watch
for is warranty fuel pump driver modules. In many performance
applications, the vehicle owner may have fitted an aftermarket high-flow
fuel pump to the vehicle in place of the OE pump that isnB!GBt compatible
with the fuel pump driver module. If a fuel pump driver module comes
back under warranty, double check with the customer to see if a non-OE
style pump was installed or have the installer check the fuel pump
current draw to prevent further problems or failures."

Doug in Japan

Air gets in a fuel injection system during heat soak expanding the
desolved air in the fuel I think. Starting an AC fuel injection
systems when hot has been a problem for years. When you park your
airplane it is not always level.

Paul Lamar


I suspect that the driving force in the auto industry to remove the return
line from the EFI system
is strictly cost savings.  I understand that some manufacturers are
unitizing the
filter-pump-regulator system into a single unit.  What this means is if your
$15 filter needs to be
replaced, you must buy the whole unit for $400+ and still add the
installation cost.

Having started my driving and auto mechanic adventures in the South western
US just after WW-2, I am
very aware of the fraialities of engine driven fuel pumps and vapor locks.
When pumps, located in
the fuel tank, and with recirculating systems were introduced with the
advent of fuel injection, I
thought it was the next best thing since (?).  My mind is slipping, can't
remenber what  IT

Joel

I'm not advocating one system over the other. But after considering the many
points raised in the discussion, and some very good alternative fuel system
architectures, my preference is leaning to stick with a return system.
How-ever, some of that may be based on false perceptions.
Does the pressure regulator of an EFI with return maintain constant pressure
at all altitudes? If yes, score a point for the return system, fuel flow
calculation is simple.
EFI with return, does not require an ECM, but can use very simple Lamar-555
fuel controller and slab throttle with P-port. Score another point.
EFI with return can be implemented without in tank fuel pumps. This greatly
simplifies fuel tank construction, as special flanges requiring a custom
forming die likely required. Score point three.
Service of fuel pumps, external to tank seems easier to implement as well.
Score point four.
External pumps (new pumps, I will not put anything from a junk yard on my
airplane, just my thing, and yes my engine is salvage, but I will complete
overhaul, you can't overhaul late automotive pump units) are less expensive.
Score point five.
IF I have misstated something, I'm open for correction. I want
misconceptions cleared up.
C. Smith


Reason for switcing from a return to a returnless system system was to
reduce evaporative emissions and cost reduction.

Tom Walter


Colin,

Good points all, but really either system is reliable and can be designed
to maintain fuel pressure at any altitude.   The difference is that with
a returnless system, a mechanical fuel pressure bypass valve on the fuel
rail, is replaced with an electronic fuel pressure sensor and controller
affecting the pump speed. Either system can malfunction and needs the
'what if' design rational thought through.  As for the physical
location, the pump  can be in the tank, in a small accessible sump tank,
or outside any tank, in line.  Airplane design and accessibility issues
dictate the choice. If an externally mounted pump is desired, than get
one with an internal adjustable bypass valve.  In case of failure in the
PWM controller, the pump  returns to 100% duty cycle, or said another
way modulation of its rpms ceases. It is on all of the time.  In case
the fuel rail pressure, for instance, is set for 75 psi, then
accounting for line losses and by use of a simple calibration screw, the
pump by-pass valve  could be set to crack open at 80 psi. Some fuel by
passes from the high pressure side of the pump back into the low
pressure side. The distance is less than 6 mm. If the PWM is effectively
controlling the duty cycle of the pump then the by pass valve will never
open. Think of it as a safety feature.  This is a common feature built
into any high pressure pumps, including in tank designs.

The down side of return systems (at least for me) are the  ten foot long
plumbing runs, extra weight, venting issues, and the inefficiency of
having over rated pumps pumping fuel in a circle.
Added to that is the abundance of fuel line connections and plumbing each
that adds up to pumping losses and potential for leaks.  KISS principles
have to be weighted looking at the total picture. Adding complexity in
one area sometimes reduces complexity in the whole system

With today's sensors, fuel flow calibration is relatively easy either
way.  With the returnless system, the fuel flow only has to measured
once. PWM controllers are fairly simple to make and as Paul states, the
555 chip is reliable. Redundancy of pumps and controllers is a good
method of handling the what ifs.  Pumps can Y into one manifold. A home
made unit interfaced to a fuel rail pressure signal is within a home
builder realm of possibilities.  Pumps and plumbing can be simplified
and any leaks stay in the fuel tank. One, (1) high pressure fitting goes
from the tank (if a sump tank is used) to join one fitting on the fuel
rail.  Wing tanks right or left, gravity feed the sump tank if  a high
or mid wing airplane is used.

 Again there is no correct choice for everyone.  It depends on  the
aircraft, the aviator's familiarity with possible technologies, and
his/her comfort level in maintaining them.

Doug in Japan