So High pressure by itself will increase risk of vapor lock?
or do you need the additional heat from the fuel pump to make it vaporize. I
know heat will vaporize fuel without igniting it - so I guess higher pressure
exacerbates that condition?
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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>>
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