Dan,
<snip>
You had made comment that you wanted to know more about the mixture control
valve design. To completely understand the reasoning behind this design you
have to know a little history of how this all got started. When we started
our company in 1984 I had already 10 years experience with aircraft fuel
injection systems at the Bendix Corporation. Being the under study of the
inventor of the RS and RSA fuel injection system and later being the project
engineer on that product line gave me insight into the manufacturing
problems and cost associated the RSA design. In Airflow's infancy, we knew
that we would have to design a system to satisfy a large range of horsepower
requirements with a minimum of part and tooling changes. Knowing that we
wanted to be able to run engines from around 80 HP to 1000 HP we designed
the present fuel regulator concept.
Studying the needs in the aviation field we constantly heard of the big draw
back to fuel injection was 1) initial cost, 2) hot starts, 3) high cost of
overhaul. In this design we determined that eliminating part count without
sacrificing performance would help with manufacturing costs, and overhaul
cost.
Studying various manufacturing techniques, we knew that plate valves were
expensive to make (high part count) were susceptible to scoring unless you
used some expensive materials and there's always the issue of making the
parts flat (specialized equipment). Rotary valves on the other hand were
easy to control in manufacture (OD grinding) and round bores were easy to
control with honing. This would allow parts that would not have to be hand
lapped or fitted. The round parts could be made with tight enough
tolerances that matched parts were not necessary. Having a through bore
that both idle and mixture valves ran in gave the bonus of getting cost out
of manufacturing as through bore honing would hold the bore straight and we
could easily hold + .0005" on the entire bore. Brass was chosen as the
material to run in an anodized honed bore. Designing the L/D of the valve
gave excellent bearing surface and I have to admit, we really haven't had
any problems with wear or scoring of these parts in 20+ years of service.
The only down side is continued actuation of the parts when dry can cause
galling of the valve. This is solved by oil flushing the parts after test,
and in service the parts are always in fuel. Of course with a rotary valve
there has to be clearance for the valve to rotate, therefore ICO cannot be 0
leak. We also only shut off the metered side of the circuit in the
regulator. This removed the additional parts required to mechanize an
additional valve to shut off this side of the circuit and since the decision
was made to use the purge valve as standard equipment, a zero leak mixture
valve was not required.
Hot starts were a common problem with low-pressure non-returning fuel
injection systems, and even some early mechanical automotive fuel injection
like the Bosch K Jetronic suffered from this problem. We determined that
the hot start problem was due to heat soak on the fuel system components on
the engine. Since fuel boiled at around 130 degrees F at sea level
pressure, after the engine shut down the fuel on the engine side of the fire
wall in the hoses, engine driven fuel pump, fuel control, flow divider, and
nozzle lines would be partially boiled away. Since the fuel metering system
was non-returning, there was no way to get rid of the hot fuel and vapor.
You had to start the engine flooded or when the engine started you had to
run it up excessively to pass the vapor through the metering system to keep
the engine running. Some people didn't have problems with this technique,
many did. The components that held the most volume of fuel were the
culprits. The #6 fuel hoses, the engine driven fuel pump and the fuel
control. Since our metering system metered fuel to the engine based on
engine airflow consumption there was a limit on how fast fuel would transfer
through the system when the engine was not running. On a typical 4 cylinder
Lycoming the normal calibration set up allowed about 1 cup of fuel to
transfer through the system in 45 seconds of purging with the throttle wide
open. This would pretty much exchange the fuel in the engine driven fuel
pump and the fuel control and hoses. At idle the fuel transfer would be
.038 cup of fuel in one minute. This is why idling the engine will never
get the air out of the system, well at least not for 26 minutes. This is
another reason we want to minimize the volume of fuel on the engine side of
the firewall.
The purge valve was designed on the premise that cleaning out the hot fuel
and vapor from the engine driven pump, fuel control and hoses would cure the
hot start problem. The first system was installed on an IGSO 480 in an
aerobatic airplane, which was pretty much unstartable when hot. The system
worked quite well with pretty much the same start routine hot or cold. Also
the benefit with the purge valve was that it would dump the fuel pressure
when the engine was shut off to keep fuel from bleeding into the engine
after shut down. This was a problem with engines using diaphragm fuel
pumps. We always had complaints of fuel dripping into the air box after
shut down on Bendix servos which basically dead head the fuel pump pressure
against a plate valve. When the plate valve scored a little leakage started
and the engine would not shut down clean. People whine and moan about this
now, but 30 years ago when I was working at Bendix we heard the same thing.
Thus, another reason for the design of the purge valve.
The purge valve design was not something we designed from scratch with a
fresh sheet of paper. The basic valve design was studied as to what design
in the field gave the most trouble free service. Looking at helicopter
service, we found that that seemed to get the most abuse. From both a
vibration and wear stand point this installation typically had fuel tanks
above the engine so the valve had to be near zero leak as possible, yet be
robust enough to withstand the harsh environment it was in. So the valve
bushing was used from a RSA-7 fuel regulator. This same design had been
used on all Hughes 300 and Beechcraft Baron 58P installations. With a few
million flight hours accumulated, there had been not one incident of
malfunction of the valve, let alone the screw backing out because it was not
lockwired. The idle valve bushing on these fuel servos had the same design,
that is, being held in by one screw. Thus the Airflow purge valve was
designed to mimic the Bendix design, with some minor changes in the venting
of the ports in the bushing, and of course a housing was designed to hold
the valve.
So there you have it. A history and reasoning behind the mixture control
and purge valve design. This design was done to satisfy requirements that
we determined customers wanted in the field. After all, if the status quo
was accepted, why build anything? It would not address any of the issues
that existed, and you would end up with a clone of the same 40-year-old
design. Kind of like a Silver Hawk. All of these parts were designed for a
reason and with lot of forethought. Are there other ways to do it? You
bet. Is there a better cost effective way to address the problems
associated with low-pressure non-returning fuel injection systems? Probably
not, with the market as it is today.
