Hi
Al,
Didn’t think I was
glossing over your point (a good one) – as I mentioned in my previous e mail
about returnless systems –
“…
However, these are systems with the pumps generally embed in the fuel tank –
thereby minimizing (if not eliminating) your
concern
about the return being “T” back into the line close to the high pressure pump
intake…..”
well, its pretty
clear, that statement didn’t lessen your concern.
You certainly made a
valid point about how important it is to know how close you are operating to a
possible failure point. However, its not quite as easy as implied.
In the past I have used the Clausius_Clapeyron phase change equation to try
and get a handle on crucial temperatures – however, it was simply frustrating
without any useful results - the principal problem I found was the very wide
range of boiling point of gasoline.
Mogas has a range of
around 100-400F due to the various additives put in during different seasons
and by different refineries. Even 100LL aviation gasoline is shown to
have a boiling point range of from 95-330 F
So this makes it
difficult to really get a handle on doing that type of analytical assessment
and produce useful results – and that is for a static system not a dynamic one
like we have in real life where you have heat being added and removed
over time.
http://www.engineeringtoolbox.com/fuels-boiling-point-d_936.html
http://www.globalsecurity.org/military/systems/aircraft/systems/engines-fuel.htm
Fuel delivery
interruption is not something you want to encounter under any
circumstance. However, to blankly state (or at least imply) that there
is no alternative to a returning fuel to the fuel tank system , I believe does
not provide a complete picture.
Your used the
example of millions of autos in the past having used the “Return to tank”
method is not any different than me countering with the fact that millions of
autos today do NOT use the return to tank system – same level of validity (or
invalidity {:>))
I agree, that
in the past that was certainly the way millions of auto fuel injected systems
were designed. However, millions of autos are now using the “returnless”
fuel system – what would once have been complex and difficult (meaning costly)
to do with a mechanical control system is very amenable to electronic
control. Progress does continue – or else we would all be flying with
the same 1932 air cooled designs. . The returnless system has been
proven to work in automobiles so why would it not work in aircraft
systems?
A control system
design to accommodate a returnless system similar to that used in many autos
today does not appear that complicated, it would need two control
parameters. The pressure in the fuel line and the temperature of the
fuel. So that would entail adding two transducers to the fuel
plumbing. Both fuel pressure and temperature data then processed by
microchip such as I use in my projects could easily vary the PWM duty cycle to
change the pump speed and therefore the fuel pressure in the line. Not
necessarily something your typical builder would want to take on – not
something the majority might want to use - but certain not beyond the
capability of some.
I personally do not think a returnless system is out of the question for aircraft usage – like any fuel delivery system safe requirements would need to be thoroughly understood and carefully designed.
Again, I was not attempting to gloss over your points – I did not find anything in any of your points to challenge – I simply believe your implied assertion that “returning to the tank” was the only way to go because million of autos previously used that system needed to be balanced by recognizing the fact that today - million of newer autos do not use that method any longer.
That said, the returnless approach may not turn out to be suitable for our use for some reason, but I don’t see any reason - at this point- why it should be discounted out of hand.
In fact, this discussion has intrigued me to the point, I am going to look into what it would take to add such a pressure management system to my current system. My small header tank (1 pint) already has fittings welded in that would make it easy to install a pressure and Temperature sensors. I am not concerned about vapor lock in the high pressure part of my system – but the low pressure part is obviously where elevator temperatures would have an easier time of causing fuel vapors to manifest themselves.
Everyone on this list ultimately makes up their own mind about what information to use and how to evaluate it, all view points are welcome on this list – that is why it exists. There is no moderator to make those judgment calls about what should or should not be discussed or mentioned – it’s up to everyone to take the input provided and make their own decision about it. That’s why this list is great!
Onward indeed!! {:>)
Ed
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Wick
Sent: Tuesday, June 08, 2010 10:59
AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: high/low
pressure pumps question
Hi Ed. You often make significant
contributions that help improve flight safety. If you share your fuel design,
at least do the calculations and let users know how close this is to vapor
lock. We're talking about life risk here. You are glossing over my point and
changing the topic. Returning fuel to pump inlet is very risky and should not
be done unless you desire to operate on the threshold of vapor
lock.
I agree, there is one exception.
That's if you have one of the new fuel pump controllers that measures
fuel parameters and alters pump speed. I looked into that one years ago. Talk
about added complexity!
----- Original Message -----
Sent: Monday,
June 07, 2010 8:00 AM
Subject:
[FlyRotary] Re: high/low pressure pumps
question
Points noted,
Al.
However, would like
to point out that today many automotive fuel systems DO NOT return fuel to
the tanks – they use the “No-Return” fuel systems whereby pump
pressure is regulated to prevent vapor lock. However, these are systems with
the pumps generally embed in the fuel tank – thereby minimizing (if not
eliminating) your concern about the return being “T” back into the line
close to the high pressure pump intake. With millions of autos now
using this system – it would appear to be a viable approach if done
correctly (always the caveat {:>))
That is one point
that Charlie made - was the possibility of using a simple PWM circuit to
control pump pressure similar to the no-return Auto
systems.
For those
interested, here is a fairly good non-technical description of a no-return
system used in automobiles.
Ed
http://www.picoauto.com/tutorials/fuel-injection.html
Returnless
Fuel Systems
Have been adopted
by several motor manufacturers and differ from the conventional by having a
delivery
pipe only to the fuel rail with no return flow back to the
tank.
The returnless
systems, both the mechanical and the electronic versions, were necessitated
by emissions laws. The absence of heated petrol returning to the fuel tank
reduces the amount of evaporative emissions, while the fuel lines are kept
short, thus reducing build costs.
Mechanical
Returnless Fuel Systems
The ‘returnless’
system differs from the norm by having the pressure regulator inside the
fuel tank. When the fuel pump is activated, fuel flows into the system until
the required pressure is obtained; at this point ‘excess’ fuel is bled past
the pressure regulator and back into the tank.
The ‘flow and
return’ system has a vacuum supply to the pressure regulator: this enables
the fuel pressure to be increased whenever the manifold vacuum drops,
providing fuel enrichment under acceleration.
The ‘returnless’
system has no mechanical compensation affecting the fuel pressure, which
remains at a higher than usual 44 to 50 psi. By increasing the delivery
pressure, the ECM (Electronic Control Module) can alter the injection pulse
width to give the precise delivery, regardless of the engine load and
without fuel pressure compensation.
Electronic
Returnless Fuel Systems
This version has
all the required components fitted within the one unit of the submersible
fuel pump. It contains a small particle filter (in addition to the
strainer), pump, electronic pressure regulator, fuel level sensor and a
sound isolation system. The electronic pressure regulator allows the
pressure to be increased under acceleration conditions, and the pump’s
output can be adjusted to suit the engine's fuel demand. This prolongs the
pump’s life as it is no longer providing a larger than required output
delivery.
The Electronic
Control Module (ECM) supplies the required pressure information, while the
fuel pump’s output signal is supplied in the form of a
digital
squarewave. Altering the squarewave’s duty cycle affects the pump’s delivery
output.
To compensate for
the changing viscosity of the fuel with changing fuel temperature, a fuel
rail temperature sensor is installed. A pulsation damper may also be fitted
ahead of or inside the fuel rail.
Here
is one rendition of such a system in an auto
application
Ed
Anderson
Rv-6A N494BW Rotary
Powered
Matthews,
NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://www.dmack.net/mazda/index.html
http://www.flyrotary.com/
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.rotaryaviation.com/Rotorhead%20Truth.htm
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Wick
Sent: Monday, June 07, 2010 9:25
AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: high/low
pressure pumps question
I'm really concerned for your
life risk. This is a very high risk fuel design. Puts your fuel pressure at
pump inlet very very close to the vapor pressure of fuel. This suddenly
makes the design sensitive to a whole bunch of environmental factors.
Ask this question: Millions of
automobiles. When they designed automotive fuel systems, why did every
single engineer return fuel to tank instead of pump inlet? So much more
expensive to send to tank.
You can bench test your design
and prove how close it is to failure. Just need to measure fuel pressure at
pump inlet, measure pump temp. Use hair dryer to force pump to higher temp.
Put all the numbers in spreadsheet, then calculate distance between vapor
pressure and your readings. Adjust for worst case, which would be high
altitude airport, hot day sitting on tarmac for 1 hour, heat soaked engine
compartment, car fuel containing ethanol.
Marginal designs can fly for
years without failure. This is the nature of failure. Use care, I'm
concerned.
----- Original Message -----
Sent:
Sunday, June 06, 2010 6:47 AM
Subject:
[FlyRotary] Re: high/low pressure pumps
question
Hope this
helps...
On 6/6/2010 2:21 AM, Todd Bartrim wrote:
>
>
Hi Charlie;
> Hmmmnn, I gotta say a picture (or drawing) is worth a
thousand words.
> Todd (sent on my new-fangled google phone with a
really damn small
> keyboard)
>
>> On 2010-06-05
8:46 PM, "Charlie England" <ceengland@bellsouth.net
>> <mailto:ceengland@bellsouth.net>>
wrote:
>>
>> Hi Todd,
>>
>> My thought
is to set up the fuel path thus: fuel source through a T
>> into
injection pumps, through filters, through combining T, through
>> the regulator(in cockpit), through firewall to engine. The
>> regulator's bypass port (in the cockpit) would be plumbed
just like
>> yours, except no heat exchanger since the bypass
fuel would never see
>> the heat of the engine compartment. The
bypass would still T into the
>> supply to the injection pumps,
like your system. Obviously, a
>> manifold pressure line would
be required through the firewall into
>> the cockpit to the
pressure regulator.
>>
>> My original plan (different
injection that didn't require a return
>> line) was similar to
your selector setup: main tanks feeding stock
>> van's selector,
with the 3rd port on it being fed by a 2nd vans
>> selector to
select either of the 2 aux tanks. No transfer pump would
>> have
been required, & no crossover valve. Failure of the primary
>> valve could have been a 'show stopper', but the newer valves
seem to
>> be rock solid reliable. Going to this system using
the gear type
>> pumps requiring a return line forced
re-thinking. Using your idea to
>> return the bypassed fuel at
the pump inlet effectively eliminates the
>> 'return' issue,
& tempts me to return to the original fuel selector
>>
layout, with the addition of a Facet boost pump. The fact that the
>> optical sensors will work looking into the side of a fuel
line (the
>> T), instead of needing them in the tank & that
they still give almost
>> a full minute's warning, is very
encouraging. I'll start looking for
>> a convenient place to
mount them.
>>
>> I hope that Tracy will chime in
on how he plumbed the regulator on
>> his
-8.
>>
>> Many thanks for the extra
details.
>>
>>
Charlie
>>
>>
>>
>> On 6/5/2010 12:32
PM, Todd Bartrim wrote:
>>
>>
>
>> > Hi
Charlie
>> > Not
sure I understand correctly what you mean? Can
you
>> sketch it out quickly?
I'...
>>
>> > *From*: Charlie
England <ceengland@bellsouth.net
>>
<mailto:ceengland@bellsouth.net>
>>
<mailto:Charlie%20England%20%3cceengland@bellsout.
>>
<mailto:Charlie%2520England%2520%253cceengland@bellsout.>..
>>
>>
> *Subject*: [FlyRotary] Re: high/low pressure pumps
question
>> > *Date*: Sat, 05 Jun 2010
08:38:09 -0...
>>
>>
>>
>>
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