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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