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Ongoing fuel pressure problems

 
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dan(at)syz.com
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PostPosted: Thu Jun 14, 2018 3:47 pm    Post subject: Ongoing fuel pressure problems Reply with quote

Hi everyone. I've had my RV-10 flying for about a year and a half now. Everything's working fine except one issue - I periodically get low fuel pressure warnings when just on the engine pump and haven't been able to isolate the problem.

My setup is pretty stock - a factory new Lycoming IO-540 D4A5 through Van's with their pre-installed engine fuel pump, electric fuel pump assembly in the tunnel as per Van's instructions, fuel plumbing as per Van's instructions, etc. The only "deviation" from plans was that I had to install wing root gascolators as per Canada's requirements.

My fuel totalizer is in the tunnel also as per instructions, though I realize now that's a bad location in that it tends to over-read by 50% or more when the electric boost pump is on. Not the end of the world since it's only an issue when the electrical boost pump is on (which isn't really that much) and I'd rather it read pessimistically instead of optimistically anyway. Maybe one day I'll relocate it, but I mention it here in case someone thinks it's relevant to my real problem.

And the real problem is this - normally in cruise above 10000', I have fuel pressure readings around 19-21 psi. But periodically (and unpredictably) sometimes the fuel pressure will plummet - I've seen it get as low as 10 psi before climbing back up again. Originally, as soon as it would start dropping I'd turn on the electric boost pump but as it's happened more and more often and I've become more brave, I've held off at least a few seconds and so far it's always recovered before the engine started stumbling.

I've never seen this happen with the boost pump on - just when the engine pump is working on its own. Though as I said, I don't normally fly with the boost pump on, so I obviously have less data with the boost pump on.

What's particularly interesting is that it only seems to happen at higher altitudes. Above 10000' or so, I can count on it happening at least once an hour - at 13000', it sometimes happens every 10 or 15 minutes (though sometimes it doesn't happen at all either for an hour). At 9500', I've seen it happen only rarely, and I don't recall it ever occurring at 8500' or below.

When it drops, it tends to fall over the course of about 5-10 seconds and then climb back up over 5-10 seconds. The whole event is over with within 15-20 seconds, and doesn't seem to "linger" at low PSI. I'm not sure if this is "real" or an artifact of sensor filtering of the EFIS.

It isn't specific to when the engine is particularly hot - I always climb with the boost pump on, so when the engine is running hotter I never see the issue. This happens after I've been flying around at cruise power settings at altitude, often with a below freezing outside temperature and CHTs in the 310-350 range, and usually lean of peak burning in the neighbourhood of 10 gph.

I have a blast tube aimed towards the fuel pump (though not a full shroud). I've even tried removing my gascolator screens in case they were potentially restricting fuel flow.

It's been suggested that perhaps my fuel pressure sensor is having issues. I could certainly change it out, but since I haven't seen an issue with the electric boost pump on, and since it only seems to happen at altitude, I would expect if it was a sensor issue neither of these two factors would have an effect.

Does anyone have any suggestions on what to try? I have tons of engine sensor data logged from the EFIS that I could send if helpful.

I'm a bit apprehensive of changing out the fuel pump (in that I haven't done so before and would want to ensure that a new one is installed correctly so I don't damage anything). Has anyone done that before and could offer any advice? On the other hand, I've lived with this one issue for long enough and as it's the one thing I'm not completely happy with, I would like to try and finally resolve it so I can better trust things.

Thanks for any advice!

Dan
---
Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213


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maca2790



Joined: 26 Jan 2011
Posts: 52
Location: Australia

PostPosted: Thu Jun 14, 2018 5:08 pm    Post subject: Ongoing fuel pressure problems Reply with quote

Hi Dan, From a Video I watched about Fuel Injected engines as long as there is positive pressure above about 5 PSI at high fuel flows to the FI Servo the engine will run fine. The Video was from Tempest Aero parts and it talked about Engine driven Fuel Pumps and Boost Pumps. The Video was Titled, “The Amazing Ubiquitous Diaphragm Fuel Pump.”
Anyway the Video said nothing to worry about down to about 9 psi. What type of boost pump do you have? My Andair Electric boost pump is in the tunnel and so is the red cube. I have no issues with inaccurate flow readings but I do occasionally get fuel pressure fluctuations in very hot weather. That is ambient ground temps above 35 C. I solve this by running the boost pump until at altitude and the OAT has dropped below 30 C.

Cheers John MacCallum
VH-DUU

On 15 Jun 2018, at 09:45, Dan Charrois <dan(at)syz.com (dan(at)syz.com)> wrote:
Quote:
--> RV10-List message posted by: Dan Charrois <dan(at)syz.com (dan(at)syz.com)>

Hi everyone. I've had my RV-10 flying for about a year and a half now. Everything's working fine except one issue - I periodically get low fuel pressure warnings when just on the engine pump and haven't been able to isolate the problem.

My setup is pretty stock - a factory new Lycoming IO-540 D4A5 through Van's with their pre-installed engine fuel pump, electric fuel pump assembly in the tunnel as per Van's instructions, fuel plumbing as per Van's instructions, etc. The only "deviation" from plans was that I had to install wing root gascolators as per Canada's requirements.

My fuel totalizer is in the tunnel also as per instructions, though I realize now that's a bad location in that it tends to over-read by 50% or more when the electric boost pump is on. Not the end of the world since it's only an issue when the electrical boost pump is on (which isn't really that much) and I'd rather it read pessimistically instead of optimistically anyway. Maybe one day I'll relocate it, but I mention it here in case someone thinks it's relevant to my real problem.

And the real problem is this - normally in cruise above 10000', I have fuel pressure readings around 19-21 psi. But periodically (and unpredictably) sometimes the fuel pressure will plummet - I've seen it get as low as 10 psi before climbing back up again. Originally, as soon as it would start dropping I'd turn on the electric boost pump but as it's happened more and more often and I've become more brave, I've held off at least a few seconds and so far it's always recovered before the engine started stumbling.

I've never seen this happen with the boost pump on - just when the engine pump is working on its own. Though as I said, I don't normally fly with the boost pump on, so I obviously have less data with the boost pump on.

What's particularly interesting is that it only seems to happen at higher altitudes. Above 10000' or so, I can count on it happening at least once an hour - at 13000', it sometimes happens every 10 or 15 minutes (though sometimes it doesn't happen at all either for an hour). At 9500', I've seen it happen only rarely, and I don't recall it ever occurring at 8500' or below.

When it drops, it tends to fall over the course of about 5-10 seconds and then climb back up over 5-10 seconds. The whole event is over with within 15-20 seconds, and doesn't seem to "linger" at low PSI. I'm not sure if this is "real" or an artifact of sensor filtering of the EFIS.

It isn't specific to when the engine is particularly hot - I always climb with the boost pump on, so when the engine is running hotter I never see the issue. This happens after I've been flying around at cruise power settings at altitude, often with a below freezing outside temperature and CHTs in the 310-350 range, and usually lean of peak burning in the neighbourhood of 10 gph.

I have a blast tube aimed towards the fuel pump (though not a full shroud). I've even tried removing my gascolator screens in case they were potentially restricting fuel flow.

It's been suggested that perhaps my fuel pressure sensor is having issues. I could certainly change it out, but since I haven't seen an issue with the electric boost pump on, and since it only seems to happen at altitude, I would expect if it was a sensor issue neither of these two factors would have an effect.

Does anyone have any suggestions on what to try? I have tons of engine sensor data logged from the EFIS that I could send if helpful.

I'm a bit apprehensive of changing out the fuel pump (in that I haven't done so before and would want to ensure that a new one is installed correctly so I don't damage anything). Has anyone done that before and could offer any advice? On the other hand, I've lived with this one issue for long enough and as it's the one thing I'm not completely happy with, I would like to try and finally resolve it so I can better trust things.

Thanks for any advice!

Dan
---
Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213
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Kelly McMullen



Joined: 16 Apr 2008
Posts: 1124
Location: Sun Lakes AZ

PostPosted: Thu Jun 14, 2018 6:24 pm    Post subject: Ongoing fuel pressure problems Reply with quote

It is not that simple. What pressure you are talking about depends on where in the system you are looking, what brand system you have, etc.
In general, the Bendix/Precision style injection system needs about 14-15 psi minimum at the input to the fuel servo. Pressures going to the fuel divider will be lower, and the flow divider shuts down flow at around 4-5 psi to prevent run-on at idle cutoff. The amount of pressure at the inlet is important to have the system function correctly, and to get good atomization at the nozzles. While the engine may run at 9 psi, I'd not be at all comfortable to see below 15.

I don't know what pressures the AFS system needs. The Continental system works quite differently and needs very steady and precise pressures at the inlet that have to be checked annually.
Kelly
Quote:
Sent from my IBM-360 main frame


On Thu, Jun 14, 2018 at 6:06 PM, John MacCallum <john.maccallum(at)bigpond.com (john.maccallum(at)bigpond.com)> wrote:
Quote:
Hi Dan, From a Video I watched about Fuel Injected engines as long as there is positive pressure above about 5 PSI at high fuel flows to the FI Servo the engine will run fine. The Video was from Tempest Aero parts and it talked about Engine driven Fuel Pumps and Boost Pumps. The Video was Titled,  “The Amazing Ubiquitous Diaphragm Fuel Pump.”
Anyway the Video said nothing to worry about down to about 9 psi. What type of boost pump do you have? My Andair Electric boost pump is in the tunnel and so is the red cube. I have no issues with inaccurate flow readings but I do occasionally get fuel pressure fluctuations in very hot weather. That is ambient ground temps above 35 C. I solve this by running the boost pump until at altitude and the OAT has dropped below 30 C. 

Cheers John MacCallum 
VH-DUU
 

Quote:




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dan(at)syz.com
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PostPosted: Thu Jun 14, 2018 9:33 pm    Post subject: Ongoing fuel pressure problems Reply with quote

My Lycoming came with an AVStar AVX-5VA1 injector. I'm not sure what "style" of injection that implies (I'm very admittedly not an engine expert). Though in the IO-540 series operator's manual I got with the engine, on page 3-9 they mention a pressure range for the inlet to the fuel injector of 14-45 psi, so I've considered that 14 is a minimum - though I'm much more comfortable when it is closer to 20. When it gets down to less than 14, I get very nervous. Though I've seen it down to 10 psi briefly while the engine kept running, it sure doesn't give me the warm fuzzies when it does that.

I have the ES Airflow pump and red cube Van's supplied installed pretty much exactly as described in section 37, other than the exception that I have an Andair fuel valve installed instead of the Van's supplied one. Considering that I've always figured the transducer would have a simple vane system to measure fuel flow, I have to admit to not really understanding how it could be so easily confused by the boost pump upstream (no matter how "pulse-y" it may be)... but it is.

Dan

Quote:
On 2018-Jun-14, at 8:22 PM, Kelly McMullen <apilot2(at)gmail.com> wrote:

It is not that simple. What pressure you are talking about depends on where in the system you are looking, what brand system you have, etc.
In general, the Bendix/Precision style injection system needs about 14-15 psi minimum at the input to the fuel servo. Pressures going to the fuel divider will be lower, and the flow divider shuts down flow at around 4-5 psi to prevent run-on at idle cutoff. The amount of pressure at the inlet is important to have the system function correctly, and to get good atomization at the nozzles. While the engine may run at 9 psi, I'd not be at all comfortable to see below 15.
I don't know what pressures the AFS system needs. The Continental system works quite differently and needs very steady and precise pressures at the inlet that have to be checked annually.
Kelly
Sent from my IBM-360 main frame

On Thu, Jun 14, 2018 at 6:06 PM, John MacCallum <john.maccallum(at)bigpond.com> wrote:
Hi Dan,
From a Video I watched about Fuel Injected engines as long as there is positive pressure above about 5 PSI at high fuel flows to the FI Servo the engine will run fine. The Video was from Tempest Aero parts and it talked about Engine driven Fuel Pumps and Boost Pumps. The Video was Titled, “The Amazing Ubiquitous Diaphragm Fuel Pump.”

Anyway the Video said nothing to worry about down to about 9 psi. What type of boost pump do you have? My Andair Electric boost pump is in the tunnel and so is the red cube. I have no issues with inaccurate flow readings but I do occasionally get fuel pressure fluctuations in very hot weather. That is ambient ground temps above 35 C. I solve this by running the boost pump until at altitude and the OAT has dropped below 30 C.

Cheers John MacCallum
VH-DUU






---
Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213


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Kellym



Joined: 10 Jan 2006
Posts: 1548
Location: Sun Lakes AZ

PostPosted: Thu Jun 14, 2018 9:44 pm    Post subject: Ongoing fuel pressure problems Reply with quote

The one aspect I don't like about the design of the fuel lines in the
RV-10 is that they go uphill to the fuel selector, then down to the
filter and pump, then back uphill to the firewall and mechanical pump.
I believe this forces the system to develop sufficient negative pressure
at the mechanical pump to overcome the loop in the fuel system.
The Andair with an extension can be mounted fairly low, but is limited
by tubing bend radius and lines coming out the bottom.
I don't think your fuel cube location should affect the total flow.
However the location upstream of the mechanical pump will affect
responsiveness.

On 6/14/2018 10:31 PM, Dan Charrois wrote:
Quote:


My Lycoming came with an AVStar AVX-5VA1 injector. I'm not sure what "style" of injection that implies (I'm very admittedly not an engine expert). Though in the IO-540 series operator's manual I got with the engine, on page 3-9 they mention a pressure range for the inlet to the fuel injector of 14-45 psi, so I've considered that 14 is a minimum - though I'm much more comfortable when it is closer to 20. When it gets down to less than 14, I get very nervous. Though I've seen it down to 10 psi briefly while the engine kept running, it sure doesn't give me the warm fuzzies when it does that.

I have the ES Airflow pump and red cube Van's supplied installed pretty much exactly as described in section 37, other than the exception that I have an Andair fuel valve installed instead of the Van's supplied one. Considering that I've always figured the transducer would have a simple vane system to measure fuel flow, I have to admit to not really understanding how it could be so easily confused by the boost pump upstream (no matter how "pulse-y" it may be)... but it is.

Dan

> On 2018-Jun-14, at 8:22 PM, Kelly McMullen <apilot2(at)gmail.com> wrote:
>
> It is not that simple. What pressure you are talking about depends on where in the system you are looking, what brand system you have, etc.
> In general, the Bendix/Precision style injection system needs about 14-15 psi minimum at the input to the fuel servo. Pressures going to the fuel divider will be lower, and the flow divider shuts down flow at around 4-5 psi to prevent run-on at idle cutoff. The amount of pressure at the inlet is important to have the system function correctly, and to get good atomization at the nozzles. While the engine may run at 9 psi, I'd not be at all comfortable to see below 15.
> I don't know what pressures the AFS system needs. The Continental system works quite differently and needs very steady and precise pressures at the inlet that have to be checked annually.
> Kelly
> Sent from my IBM-360 main frame
>
> On Thu, Jun 14, 2018 at 6:06 PM, John MacCallum <john.maccallum(at)bigpond.com> wrote:
> Hi Dan,
> From a Video I watched about Fuel Injected engines as long as there is positive pressure above about 5 PSI at high fuel flows to the FI Servo the engine will run fine. The Video was from Tempest Aero parts and it talked about Engine driven Fuel Pumps and Boost Pumps. The Video was Titled, “The Amazing Ubiquitous Diaphragm Fuel Pump.”
>
> Anyway the Video said nothing to worry about down to about 9 psi. What type of boost pump do you have? My Andair Electric boost pump is in the tunnel and so is the red cube. I have no issues with inaccurate flow readings but I do occasionally get fuel pressure fluctuations in very hot weather. That is ambient ground temps above 35 C. I solve this by running the boost pump until at altitude and the OAT has dropped below 30 C.
>
> Cheers John MacCallum
> VH-DUU
>
>
>



---
Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213







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Bob Turner



Joined: 03 Jan 2009
Posts: 833
Location: Castro Valley, CA

PostPosted: Thu Jun 14, 2018 9:53 pm    Post subject: Re: Ongoing fuel pressure problems Reply with quote

My only comment has to do with the totalizer showing a 50% error with the boost pump on. I too have a stock setup, but mine shows a 4% error with the electric pump on. I seem to recall the electric pump has a by-pass with a check valve. I’d make sure there are no issues around that pump and its plumbing.

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bill.peyton



Joined: 19 Sep 2010
Posts: 187
Location: St. Louis, MO

PostPosted: Fri Jun 15, 2018 4:27 am    Post subject: Re: Ongoing fuel pressure problems Reply with quote

I would look at the pressure sending unit. You did not mention what you were using for engine data. Both Garmin and Dynon supply a Kavlico fuel pressure sending unit that has the vent internal to the connector housing. I had an issue with fuel pressure dropping in the climb and never returning to normal. It turns out that the silicone gasket inside the connector, was preventing the pressure from equalizing. Removing that gasket eliminated the problem. The final solution was drilling a #50 hole to allow the pressure to equalize.
This does not sound like your issue since the pressure seems to be stable for some period of time and then drop, but it's worth mentioning.


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gengrumpy



Joined: 07 May 2013
Posts: 126
Location: Tullahoma, TN

PostPosted: Fri Jun 15, 2018 7:22 am    Post subject: Ongoing fuel pressure problems Reply with quote

Dan,

I would not worry too much. Same setup as yours, and occasionally during full power climbs and hot temps, I see the same thing. When that happens I just turn on the boost pump for a minute or two.

grumpy
12 yrs flying mine

do not archive

Quote:
On Jun 14, 2018, at 6:45 PM, Dan Charrois <dan(at)syz.com> wrote:



Hi everyone. I've had my RV-10 flying for about a year and a half now. Everything's working fine except one issue - I periodically get low fuel pressure warnings when just on the engine pump and haven't been able to isolate the problem.

My setup is pretty stock - a factory new Lycoming IO-540 D4A5 through Van's with their pre-installed engine fuel pump, electric fuel pump assembly in the tunnel as per Van's instructions, fuel plumbing as per Van's instructions, etc. The only "deviation" from plans was that I had to install wing root gascolators as per Canada's requirements.

My fuel totalizer is in the tunnel also as per instructions, though I realize now that's a bad location in that it tends to over-read by 50% or more when the electric boost pump is on. Not the end of the world since it's only an issue when the electrical boost pump is on (which isn't really that much) and I'd rather it read pessimistically instead of optimistically anyway. Maybe one day I'll relocate it, but I mention it here in case someone thinks it's relevant to my real problem.

And the real problem is this - normally in cruise above 10000', I have fuel pressure readings around 19-21 psi. But periodically (and unpredictably) sometimes the fuel pressure will plummet - I've seen it get as low as 10 psi before climbing back up again. Originally, as soon as it would start dropping I'd turn on the electric boost pump but as it's happened more and more often and I've become more brave, I've held off at least a few seconds and so far it's always recovered before the engine started stumbling.

I've never seen this happen with the boost pump on - just when the engine pump is working on its own. Though as I said, I don't normally fly with the boost pump on, so I obviously have less data with the boost pump on.

What's particularly interesting is that it only seems to happen at higher altitudes. Above 10000' or so, I can count on it happening at least once an hour - at 13000', it sometimes happens every 10 or 15 minutes (though sometimes it doesn't happen at all either for an hour). At 9500', I've seen it happen only rarely, and I don't recall it ever occurring at 8500' or below.

When it drops, it tends to fall over the course of about 5-10 seconds and then climb back up over 5-10 seconds. The whole event is over with within 15-20 seconds, and doesn't seem to "linger" at low PSI. I'm not sure if this is "real" or an artifact of sensor filtering of the EFIS.

It isn't specific to when the engine is particularly hot - I always climb with the boost pump on, so when the engine is running hotter I never see the issue. This happens after I've been flying around at cruise power settings at altitude, often with a below freezing outside temperature and CHTs in the 310-350 range, and usually lean of peak burning in the neighbourhood of 10 gph.

I have a blast tube aimed towards the fuel pump (though not a full shroud). I've even tried removing my gascolator screens in case they were potentially restricting fuel flow.

It's been suggested that perhaps my fuel pressure sensor is having issues. I could certainly change it out, but since I haven't seen an issue with the electric boost pump on, and since it only seems to happen at altitude, I would expect if it was a sensor issue neither of these two factors would have an effect.

Does anyone have any suggestions on what to try? I have tons of engine sensor data logged from the EFIS that I could send if helpful.

I'm a bit apprehensive of changing out the fuel pump (in that I haven't done so before and would want to ensure that a new one is installed correctly so I don't damage anything). Has anyone done that before and could offer any advice? On the other hand, I've lived with this one issue for long enough and as it's the one thing I'm not completely happy with, I would like to try and finally resolve it so I can better trust things.

Thanks for any advice!

Dan
---
Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213







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PostPosted: Fri Jun 15, 2018 11:01 am    Post subject: Ongoing fuel pressure problems Reply with quote

Hi Bill.

I have a Kavlico fuel pressure sending unit as well - I'll be sure to have a good look at it. I'll probably try replacing the sensor first anyway - it's an easier and likely less expensive thing to try before tackling the fuel pump itself.

Dan

Quote:
On 2018-Jun-15, at 6:27 AM, bill.peyton <peyton.b(at)sbcglobal.net> wrote:



I would look at the pressure sending unit. You did not mention what you were using for engine data. Both Garmin and Dynon supply a Kavlico fuel pressure sending unit that has the vent internal to the connector housing. I had an issue with fuel pressure dropping in the climb and never returning to normal. It turns out that the silicone gasket inside the connector, was preventing the pressure from equalizing. Removing that gasket eliminated the problem. The final solution was drilling a #50 hole to allow the pressure to equalize.
This does not sound like your issue since the pressure seems to be stable for some period of time and then drop, but it's worth mentioning.

--------
Bill
WA0SYV
Aviation Partners, LLC




Read this topic online here:

http://forums.matronics.com/viewtopic.php?p=480908#480908










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Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213


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Lenny Iszak



Joined: 23 Mar 2008
Posts: 246

PostPosted: Fri Jun 15, 2018 4:03 pm    Post subject: Re: Ongoing fuel pressure problems Reply with quote

Dan,

Does the pressure drop affect the EGTs, RPM or fuel flow readings at all? If it does, it's definitely not a pressure sensor problem.


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maca2790



Joined: 26 Jan 2011
Posts: 52
Location: Australia

PostPosted: Fri Jun 15, 2018 4:08 pm    Post subject: Ongoing fuel pressure problems Reply with quote

That’s correct there does need to be a differential pressure from the inlet to outlet of the FI Servo. The point that Tim Henderson makes in the Amazing Ubiquitous Diaphragm Fuel Pump Video is that down to about 9 psi being delivered to the Servo at High fuel flows the engine will still run fine. High fuel flows like take off power and around 94 ltrs an hour. At cruise power lean of peak and around 46 ltrs per hour I would expect to see fuel pressure between 95 - 172 kpa (14-25 psi). As for take off power I always run the boost pump so it’s always up around 170 kpa.

Cheers John MacCallum

On 15 Jun 2018, at 12:22, Kelly McMullen <apilot2(at)gmail.com (apilot2(at)gmail.com)> wrote:
Quote:
It is not that simple. What pressure you are talking about depends on where in the system you are looking, what brand system you have, etc.
In general, the Bendix/Precision style injection system needs about 14-15 psi minimum at the input to the fuel servo. Pressures going to the fuel divider will be lower, and the flow divider shuts down flow at around 4-5 psi to prevent run-on at idle cutoff. The amount of pressure at the inlet is important to have the system function correctly, and to get good atomization at the nozzles. While the engine may run at 9 psi, I'd not be at all comfortable to see below 15.

I don't know what pressures the AFS system needs. The Continental system works quite differently and needs very steady and precise pressures at the inlet that have to be checked annually.
Kelly
Quote:
Sent from my IBM-360 main frame


On Thu, Jun 14, 2018 at 6:06 PM, John MacCallum <john.maccallum(at)bigpond.com (john.maccallum(at)bigpond.com)> wrote:
Quote:
Hi Dan, From a Video I watched about Fuel Injected engines as long as there is positive pressure above about 5 PSI at high fuel flows to the FI Servo the engine will run fine. The Video was from Tempest Aero parts and it talked about Engine driven Fuel Pumps and Boost Pumps. The Video was Titled, “The Amazing Ubiquitous Diaphragm Fuel Pump.”
Anyway the Video said nothing to worry about down to about 9 psi. What type of boost pump do you have? My Andair Electric boost pump is in the tunnel and so is the red cube. I have no issues with inaccurate flow readings but I do occasionally get fuel pressure fluctuations in very hot weather. That is ambient ground temps above 35 C. I solve this by running the boost pump until at altitude and the OAT has dropped below 30 C.

Cheers John MacCallum
VH-DUU


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Kelly McMullen



Joined: 16 Apr 2008
Posts: 1124
Location: Sun Lakes AZ

PostPosted: Fri Jun 15, 2018 6:02 pm    Post subject: Ongoing fuel pressure problems Reply with quote

IIRC the minimum fuel spec for the Bendix/Precision RSA 5 system is around 14 psi. I would be very uncomfortable with one showing 9-10, high fuel flow or not. Assuming there isn't a sensor problem giving a false reading.

I see 24-25 psi on takeoff without boost, 26 with it, and have the Dynon\Kavlico sensor.
Quote:
Sent from my IBM-360 main frame


On Fri, Jun 15, 2018 at 5:06 PM, John MacCallum <john.maccallum(at)bigpond.com (john.maccallum(at)bigpond.com)> wrote:
Quote:
That’s correct there does need to be a  differential pressure from the inlet to outlet of the FI Servo. The point that Tim Henderson makes in the Amazing Ubiquitous Diaphragm Fuel Pump Video is that down to about 9 psi being delivered to the Servo at High fuel flows the engine will still run fine. High fuel flows like take off power and around 94 ltrs an hour. At cruise power lean of peak and around 46 ltrs per hour I would expect to see fuel pressure between  95 - 172 kpa (14-25 psi). As for take off power I always run the boost pump so it’s always up around 170 kpa. 

Cheers John MacCallum 

On 15 Jun 2018, at 12:22, Kelly McMullen <apilot2(at)gmail.com (apilot2(at)gmail.com)> wrote:
Quote:
It is not that simple. What pressure you are talking about depends on where in the system you are looking, what brand system you have, etc.
In general, the Bendix/Precision style injection system needs about 14-15 psi minimum at the input to the fuel servo. Pressures going to the fuel divider will be lower, and the flow divider shuts down flow at around 4-5 psi to prevent run-on at idle cutoff. The amount of pressure at the inlet is important to have the system function correctly, and to get good atomization at the nozzles. While the engine may run at 9 psi, I'd not be at all comfortable to see below 15.

I don't know what pressures the AFS system needs. The Continental system works quite differently and needs very steady and precise pressures at the inlet that have to be checked annually.
Kelly
Quote:
Sent from my IBM-360 main frame


On Thu, Jun 14, 2018 at 6:06 PM, John MacCallum <john.maccallum(at)bigpond.com (john.maccallum(at)bigpond.com)> wrote:
Quote:
Hi Dan, From a Video I watched about Fuel Injected engines as long as there is positive pressure above about 5 PSI at high fuel flows to the FI Servo the engine will run fine. The Video was from Tempest Aero parts and it talked about Engine driven Fuel Pumps and Boost Pumps. The Video was Titled,  “The Amazing Ubiquitous Diaphragm Fuel Pump.”
Anyway the Video said nothing to worry about down to about 9 psi. What type of boost pump do you have? My Andair Electric boost pump is in the tunnel and so is the red cube. I have no issues with inaccurate flow readings but I do occasionally get fuel pressure fluctuations in very hot weather. That is ambient ground temps above 35 C. I solve this by running the boost pump until at altitude and the OAT has dropped below 30 C. 

Cheers John MacCallum 
VH-DUU
 

Quote:









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