Fuel pressure variation on FX-3

[Daveembry]

Have you tried turning on the fuel pump and see if the fluctuations stop?

I have a similar issue with my FX-3. Please let us know what you find.

I don't know anything about the orifice you mentioned, but ill see variations in fuel psi of 1-2 psi at lower power settings, 23/2300. At a higher power setting, 25/2500, is when I see larger swings in psi that are rapid and constant. Any where from 3-6 psi while the fuel flow stays consistent down to the tenth.

For example at a power setting of 25/2500 without the electric fuel pump on, fuel psi varies between 19-23 psi but will periodically have more drastic drops or spikes. Same with the electric fuel pump on, except the pressures are bouncing more between 25-30 psi. Again, in both of these scenarios fuel flow stays constant.

[GDFlyer]

Yes. With the electric fuel pump ON the pressure variations are still consistent. Overall fuel pressure will increase by 6-7 psi with the pump ON.

[kiwibob]

Has anyone noted variable fuel pressure on the FX-3 (see attachment). I have the added restrictor orifice in place. The fuel flow is steady, but the pressure fluctuates up to 12 psi.

Hi KCKS, My fuel pressure has always been a bit volatile but yesterday it went nuts (see attachment) with low and high pressure alerts varying from 0 to 53psi. I don't have any restictor orifice as far as I know.

You can see the fuel flow remained stable so I'm inclined to think this is a similar issue to the volatile CHT readings I had a few months ago. This was fixed by reinstalling the connectors to the CHT sensors.

I've booked it into the shop in a couple of weeks for it's first 100 hour so we'll see what the professionals find.

Did you sort yours out?
Screenshot 2020-03-19 at 10.07.03.jpg

[KCKS]

I never did figure out what was going on. I continue to have the problem with 5-10 psi fluctuations, but they come and go. Fuel flow is relatively constant, so I put it on my "conundrums list" which is collecting items bit by bit. I was going to bleed the fitting at the sensor input at my last oil change, but decided against it figuring that it could wait until my first condition inspection when I would have everything torn down. Please let me know what you find out when it gets out of the shop. I would very much like to know what's going on. I suppose it could be a loose pin in the connector as with your CHT.

[andrew.brauer]

All,

I've been asked to chime in here and hopefully add a some clarity. If everything I say is either redundant or obvious please forgive me, but I want to make sure we are all on the same page.

The mechanical pump on the CC363i engine is a normal diaphragm pump common to the majority of Lycoming-type engines. It is driven by the camshaft at 1/2 of engine RPM. It is simply driven by an arm that lifts a spring-loaded diaphragm. There are 2 check valves - one in, and one out. On the intake phase of the cycle (driven by the a cam lobe) the outlet valve close and the intake opens due to the suction of the diaphragm lifting. The lifting arm is then free and the spring forces the diaphragm back down, closing the intake valve and opening the outlet valve. Downstream of the pumps where the unmetered fuel pressure transducer is located, there are spikes at the beginning of each outlet phase from the mechanical pump, and a dip each time the outlet valve is closed for another intake phase. Because the pump can only release what the engine actually takes, the amplitude of those pulses will vary with fuel flow.

The electric boost pump is of an entirely different type, providing much more constant flow and more even pressure, but its output pressure is still subject to engine demand, rising and falling as fuel flow falls and rises. That is why fuel pressure indication will typically even out when turning on the boost pump in a series boost system like the EX3/FX3 has. The peaks and valleys in pressure inherent to a diaphragm pump are smoothed out as the boost pump will simply push straight through the mechanical pump - as long as it can maintain pressure at or above what the mechanical pump maximum output pressure is. At higher flow rates, the diaphragm will still cycle as normal, overcoming the boost pump (closing the inlet valve) and pressurizing downstream with its spring.

All of this is well and good, as the engine really only "takes" fuel in pulses as well, at 2X RPM. For the compression, power, and exhaust phases of each cylinder, the metered fuel nozzle is spraying at the back of a closed intake valve where it mixes with the air already in the intake runner. During the intake stroke, this fuel/air mixture is then drawn with a fresh slug of air and the last bit of fuel from the nozzle into the cylinder.

So. In reality, the fuel pressure IS fluctuating at a frequency that corresponds to engine RPM, and fuel demand will vary the amplitude of those pressure variations. The instrument however, is sampling at a constant rate (I do not know what Garmin's sample rate is vs. EI's etc). The instrument is then only reporting that pressure reading at 1 hz, or once per second. This reporting will match the sampling rate, or it may be an average of 10 samples, or 100 samples etc. At 1 hz, the instrument is reporting 60 times per minute. The engine, however, is cycling at 600-2700 times per minute or 10-45 times per second. At exactly 2400 RPM, the instrument is reporting every 4th revolution of the engine and every other cycle of the mechanical pump and you will see exactly the same point in the pump cycle every single time. Now adjust the RPM to 2450. The reporting rate and pump cycle rate are now out of sync and you will see a what looks like jumpy readings as the instrument is picking different points along the pump cycle. These jumps will still be cyclic, with the pattern getting longer and longer as the engine RPM gets closer and closer to a multiple of instrument sample rate. It is somewhat analogous to the beating sound heard one two instruments are playing the note but slightly out of tune. The beating sounds gets worse as one instrument (frequency) strays further from the other and goes away entirely when they are perfectly in tune (perfect frequency match).

The graph below is taken from data collected on an FX3 using a very fast datalogger that was recording at 100hz. You can clearly see the effect of the mechanical boost pump. You can also see that an example sampling rate of 10hz, there is significant variation. The two graphs are also taken at different engine RPMS, illustrating how sample rate and pump cycle will overlay differently. Note that there are still peaks in the second graph, as the mechanical diaphragm pump leaps into the fray intermittently. This is why a zone of wild pressure fluctuation will change when the engine RPM does.

Fuel Pressure Graphs.jpg


I've gotten a little long-winded here and we haven't even gotten to the point. With an analogue gauge, you see what the engine sees - a relatively constant flow of fuel that is sort of buzzing with pressure pulses. We have implemented a design change that most of you should have installed in the form of a small-orifice slug installed between the mechanical pump and the pressure transducer. This is intended to mechanically break up the pressure waves and even out the over all pressure that the transducer itself actually senses. It does nothing to the function of either pump or the engine, but knocks the pressure peaks and valleys down in order to narrow the indication variation band. What the transducer still "sees" is still subject to the sample rate and RPM overlay, but we have shown to to make an appreciable difference. That said, harmonics are complex and dynamic beasts, and taming them is a game of compromise.

As for readings from zero to 53psi, I would definitely be looking at the electrical side of things: connections or maybe even the transducer itself. The pump maximums are designed to be under 35 psi at zero flow and the engine fuel injection system requires more than 14 psi to run properly. If the engine is running well, the injector inlet itself is seeing 14 psi or above.

I hope this all helps. We understand that there are still some anomalies out there but I just wanted to illustrate the these tight cycles of pressure variation are not necessarily an indication of any malfunction at all.

[Daveembry]

Excellent. Thanks Andrew for jumping in here and clarifying all that.

All,

I've been asked to chime in here and hopefully add a some clarity. If everything I say is either redundant or obvious please forgive me, but I want to make sure we are all on the same page.

The mechanical pump on the CC363i engine is a normal diaphragm pump common to the majority of Lycoming-type engines. It is driven by the camshaft at 1/2 of engine RPM. It is simply driven by an arm that lifts a spring-loaded diaphragm. There are 2 check valves - one in, and one out. On the intake phase of the cycle (driven by the a cam lobe) the outlet valve close and the intake opens due to the suction of the diaphragm lifting. The lifting arm is then free and the spring forces the diaphragm back down, closing the intake valve and opening the outlet valve. Downstream of the pumps where the unmetered fuel pressure transducer is located, there are spikes at the beginning of each outlet phase from the mechanical pump, and a dip each time the outlet valve is closed for another intake phase. Because the pump can only release what the engine actually takes, the amplitude of those pulses will vary with fuel flow.

The electric boost pump is of an entirely different type, providing much more constant flow and more even pressure, but its output pressure is still subject to engine demand, rising and falling as fuel flow falls and rises. That is why fuel pressure indication will typically even out when turning on the boost pump in a series boost system like the EX3/FX3 has. The peaks and valleys in pressure inherent to a diaphragm pump are smoothed out as the boost pump will simply push straight through the mechanical pump - as long as it can maintain pressure at or above what the mechanical pump maximum output pressure is. At higher flow rates, the diaphragm will still cycle as normal, overcoming the boost pump (closing the inlet valve) and pressurizing downstream with its spring.

All of this is well and good, as the engine really only "takes" fuel in pulses as well, at 2X RPM. For the compression, power, and exhaust phases of each cylinder, the metered fuel nozzle is spraying at the back of a closed intake valve where it mixes with the air already in the intake runner. During the intake stroke, this fuel/air mixture is then drawn with a fresh slug of air and the last bit of fuel from the nozzle into the cylinder.

So. In reality, the fuel pressure IS fluctuating at a frequency that corresponds to engine RPM, and fuel demand will vary the amplitude of those pressure variations. The instrument however, is sampling at a constant rate (I do not know what Garmin's sample rate is vs. EI's etc). The instrument is then only reporting that pressure reading at 1 hz, or once per second. This reporting will match the sampling rate, or it may be an average of 10 samples, or 100 samples etc. At 1 hz, the instrument is reporting 60 times per minute. The engine, however, is cycling at 600-2700 times per minute or 10-45 times per second. At exactly 2400 RPM, the instrument is reporting every 4th revolution of the engine and every other cycle of the mechanical pump and you will see exactly the same point in the pump cycle every single time. Now adjust the RPM to 2450. The reporting rate and pump cycle rate are now out of sync and you will see a what looks like jumpy readings as the instrument is picking different points along the pump cycle. These jumps will still be cyclic, with the pattern getting longer and longer as the engine RPM gets closer and closer to a multiple of instrument sample rate. It is somewhat analogous to the beating sound heard one two instruments are playing the note but slightly out of tune. The beating sounds gets worse as one instrument (frequency) strays further from the other and goes away entirely when they are perfectly in tune (perfect frequency match).

The graph below is taken from data collected on an FX3 using a very fast datalogger that was recording at 100hz. You can clearly see the effect of the mechanical boost pump. You can also see that an example sampling rate of 10hz, there is significant variation. The two graphs are also taken at different engine RPMS, illustrating how sample rate and pump cycle will overlay differently. Note that there are still peaks in the second graph, as the mechanical diaphragm pump leaps into the fray intermittently. This is why a zone of wild pressure fluctuation will change when the engine RPM does.

Fuel Pressure Graphs.jpg


I've gotten a little long-winded here and we haven't even gotten to the point. With an analogue gauge, you see what the engine sees - a relatively constant flow of fuel that is sort of buzzing with pressure pulses. We have implemented a design change that most of you should have installed in the form of a small-orifice slug installed between the mechanical pump and the pressure transducer. This is intended to mechanically break up the pressure waves and even out the over all pressure that the transducer itself actually senses. It does nothing to the function of either pump or the engine, but knocks the pressure peaks and valleys down in order to narrow the indication variation band. What the transducer still "sees" is still subject to the sample rate and RPM overlay, but we have shown to to make an appreciable difference. That said, harmonics are complex and dynamic beasts, and taming them is a game of compromise.

As for readings from zero to 53psi, I would definitely be looking at the electrical side of things: connections or maybe even the transducer itself. The pump maximums are designed to be under 35 psi at zero flow and the engine fuel injection system requires more than 14 psi to run properly. If the engine is running well, the injector inlet itself is seeing 14 psi or above.

I hope this all helps. We understand that there are still some anomalies out there but I just wanted to illustrate the these tight cycles of pressure variation are not necessarily an indication of any malfunction at all.

[kiwibob]

All,

I've been asked to chime in here and hopefully add a some clarity.

That's terrific info, many thanks.

Two questions, how can I find out if mine has the orifice fitted? and if not where do I get one?

[Jon Delamarter]

That's terrific info, many thanks.

Two questions, how can I find out if mine has the orifice fitted? and if not where do I get one?

Robert,

If your aircraft has the restrictor slug installed, it will have this elbow coming off the fuel pump tee. The restrictor slug sits inside the elbow. Looking at the records, I suspect you don't have it; it would appear that we introduced this at approximately s/n CCX-2000-0046.


CCX-2000 Fuel Line Restrictor Slug and Elbow.jpg

[mwa3aan]

This is one of the best written replies I have seen in a blog post. Thank you for taking the time to provide this.

I have one question: Are there any downsides to flying with the electric fuel pump always on? One of my instructors taught me that it is safer, helps to avoid risk of clogs caused by low pressure and that in the end these pumps are so robust that they work for thousands of hours. In the end, the cost of eventually replacing the pump is worth the mitigated risk.

Any thoughts or advice on this?

Once again, thank you for the excellent response and the detail you provided. I learned a lot.

AvW

All,

I've been asked to chime in here and hopefully add a some clarity. If everything I say is either redundant or obvious please forgive me, but I want to make sure we are all on the same page.

The mechanical pump on the CC363i engine is a normal diaphragm pump common to the majority of Lycoming-type engines. It is driven by the camshaft at 1/2 of engine RPM. It is simply driven by an arm that lifts a spring-loaded diaphragm. There are 2 check valves - one in, and one out. On the intake phase of the cycle (driven by the a cam lobe) the outlet valve close and the intake opens due to the suction of the diaphragm lifting. The lifting arm is then free and the spring forces the diaphragm back down, closing the intake valve and opening the outlet valve. Downstream of the pumps where the unmetered fuel pressure transducer is located, there are spikes at the beginning of each outlet phase from the mechanical pump, and a dip each time the outlet valve is closed for another intake phase. Because the pump can only release what the engine actually takes, the amplitude of those pulses will vary with fuel flow.

The electric boost pump is of an entirely different type, providing much more constant flow and more even pressure, but its output pressure is still subject to engine demand, rising and falling as fuel flow falls and rises. That is why fuel pressure indication will typically even out when turning on the boost pump in a series boost system like the EX3/FX3 has. The peaks and valleys in pressure inherent to a diaphragm pump are smoothed out as the boost pump will simply push straight through the mechanical pump - as long as it can maintain pressure at or above what the mechanical pump maximum output pressure is. At higher flow rates, the diaphragm will still cycle as normal, overcoming the boost pump (closing the inlet valve) and pressurizing downstream with its spring.

All of this is well and good, as the engine really only "takes" fuel in pulses as well, at 2X RPM. For the compression, power, and exhaust phases of each cylinder, the metered fuel nozzle is spraying at the back of a closed intake valve where it mixes with the air already in the intake runner. During the intake stroke, this fuel/air mixture is then drawn with a fresh slug of air and the last bit of fuel from the nozzle into the cylinder.

So. In reality, the fuel pressure IS fluctuating at a frequency that corresponds to engine RPM, and fuel demand will vary the amplitude of those pressure variations. The instrument however, is sampling at a constant rate (I do not know what Garmin's sample rate is vs. EI's etc). The instrument is then only reporting that pressure reading at 1 hz, or once per second. This reporting will match the sampling rate, or it may be an average of 10 samples, or 100 samples etc. At 1 hz, the instrument is reporting 60 times per minute. The engine, however, is cycling at 600-2700 times per minute or 10-45 times per second. At exactly 2400 RPM, the instrument is reporting every 4th revolution of the engine and every other cycle of the mechanical pump and you will see exactly the same point in the pump cycle every single time. Now adjust the RPM to 2450. The reporting rate and pump cycle rate are now out of sync and you will see a what looks like jumpy readings as the instrument is picking different points along the pump cycle. These jumps will still be cyclic, with the pattern getting longer and longer as the engine RPM gets closer and closer to a multiple of instrument sample rate. It is somewhat analogous to the beating sound heard one two instruments are playing the note but slightly out of tune. The beating sounds gets worse as one instrument (frequency) strays further from the other and goes away entirely when they are perfectly in tune (perfect frequency match).

The graph below is taken from data collected on an FX3 using a very fast datalogger that was recording at 100hz. You can clearly see the effect of the mechanical boost pump. You can also see that an example sampling rate of 10hz, there is significant variation. The two graphs are also taken at different engine RPMS, illustrating how sample rate and pump cycle will overlay differently. Note that there are still peaks in the second graph, as the mechanical diaphragm pump leaps into the fray intermittently. This is why a zone of wild pressure fluctuation will change when the engine RPM does.

Fuel Pressure Graphs.jpg


I've gotten a little long-winded here and we haven't even gotten to the point. With an analogue gauge, you see what the engine sees - a relatively constant flow of fuel that is sort of buzzing with pressure pulses. We have implemented a design change that most of you should have installed in the form of a small-orifice slug installed between the mechanical pump and the pressure transducer. This is intended to mechanically break up the pressure waves and even out the over all pressure that the transducer itself actually senses. It does nothing to the function of either pump or the engine, but knocks the pressure peaks and valleys down in order to narrow the indication variation band. What the transducer still "sees" is still subject to the sample rate and RPM overlay, but we have shown to to make an appreciable difference. That said, harmonics are complex and dynamic beasts, and taming them is a game of compromise.

As for readings from zero to 53psi, I would definitely be looking at the electrical side of things: connections or maybe even the transducer itself. The pump maximums are designed to be under 35 psi at zero flow and the engine fuel injection system requires more than 14 psi to run properly. If the engine is running well, the injector inlet itself is seeing 14 psi or above.

I hope this all helps. We understand that there are still some anomalies out there but I just wanted to illustrate the these tight cycles of pressure variation are not necessarily an indication of any malfunction at all.

[Andy]

What I have learned from this thread is that it appears Garmin logs the raw transducer pressure rather than a smoothed or averaged value. I hope the displayed fuel pressure is not subject to these wild pressure variations.

There may be times during development that it would be useful to log the raw pressure data. However, I would have though that logging the displayed (and hopefully smoothed) pressure would be far more useful in normal service.

If I was able to tailor fuel pressure logging I'd start with a 5 second moving average. All the misleading pressure spikes would go away and there would still be a useful record of the fuel pressure.

Andy