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  #1  
Old 04-16-2018, 06:28 AM
clarkefarm clarkefarm is offline
 
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Posts: 62
Default Auto Engine ECUs in aircraft

I have been running a Motec M4 ECU in a number of Subaru EJ25 engines for quite a few years. The M4 was an older generation ECU which amongst other things was not generally recommended to be run in closed loop at higher power settings.
I have recently changed to a new generation Motec M130 in which the fuel tables are broken down into a Volumetric Efficiency Table (how much air passes through the engine at a given inlet MAP and RPM) and a Fuel Aim Table which sets the desired fuel air ratio. The new CAN to Lambda exhaust gas sensors are very fast and have a sensor checking ability so closed loop (automatic fuel adjustment based on exhaust value) is accepted.
However, I noticed that there was a considerable variation in the values I was seeing when tuning for Volumetric Efficiency (VE) depending whether I was in flight or on the ground. Possible reasons were lower pressure in the exhaust system due to either airflow or lower ambient pressure due to altitude.
The actual numbers from an initial review are quite startling and would explain why, over the years, I have had trouble with rod/crankshaft bearings seizing.
At 75% power (inlet MAP 75 kPa) on the ground the VE was 96.7 with an ambient pressure of 95 kPa. At an altitude of 2850' above the field and still at an inlet MAP of 75 but an ambient pressure of 88 kPa the VE number had moved out to 104.9 or an extra 7.55% of air being mixed with the fuel charge (before closed loop trim).
With the older ECU without closed loop the lean condition resulting from lower ambient pressures would have led to detonation and excessive pressure on pistons, rods and bearings.
While the new ECU has closed loop and also some ability to program for ambient pressure, the reliance on an exhaust gas sensor which would be adversely affected by the use of leaded fuel, is less than ideal.
Some aftermarket auto ECUs are capable of using MAF (mass air flow) rather than inlet MAP to control engine load and if anyone is planning an auto engine installation these might be preferable.
Rupert Clarke
RV-9A
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  #2  
Old 04-16-2018, 07:14 AM
slngsht slngsht is offline
 
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Location: Purcellville, VA
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why did they not recommend running closed loop at higher loads?

Interesting observation on VE.

Were these measurements at exactly the same RPM? Technically what's the definition of 75% power? on ground you have 75/98 roughly, but in the air you have 75/88.

How does Motec calculate volumetric efficiency? Probably calculated flow divided by static pumping capacity of the engine?

Does the system have a barometric pressure sensor? If that is the case, here is an explanation for why VE might be higher:

Your static mechanical pumping capacity is essentially the displacement x speed x density of air. At altitude, assuming the ECU reads barometric pressure, this value gets lower based on pressure drop in the atmosphere.

But in your case, your measurement point is still at 75 kpa, which is now really 85% power (75/88). So your actual flow has not changed significantly (as you said, maybe better scavenging???). So the increase in VE could be due to decrease in the denominator.

Have you observed if your closed loop system correction has to drift to higher fuel flow to correct at altitude?

Just throwing it out for discussion.
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  #3  
Old 04-16-2018, 07:59 AM
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rv6ejguy rv6ejguy is offline
 
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The reduced exhaust back pressure is the main reason for the improvement in VE at higher altitude although we'd generally expect about a 4.3% increase at 2850 feet compared to sea level, quite a bit less than what you're seeing.

If you have some ram recovery at the throttle body, that may explain some of the difference in reduced pumping losses.

Did your old M4 have barometric compensation?
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Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 419.2 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
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  #4  
Old 04-16-2018, 05:23 PM
clarkefarm clarkefarm is offline
 
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With the older M4 system the lambda sensor was not capable of sending a diagnostic output quickly enough to prevent engine damage should the sensor fail in closed loop at high power. The M4 did not use an ambient pressure sensor.
The M130 relies on calculated values as you have described to calculate efficiency but then offers the option of correcting the resulting cell entries based on the observed exhaust lambda (AFR) value compared to the fuel aim.
The aircraft does have fairly good pressure recovery with WOT inlet pressure equal to or slightly above ambient pressure in flight. Inlet air is fed from an enclosed radiator duct to the throttle body using only a fairly coarse screen to prevent ingestion of larger particles.
The field here is at 1150' so an ambient pressure of around 95 kPa is normal.
I am yet to operate above 4000' and may swap to a Haltech ECU before going much further as I would prefer to use MAF for engine load.
I am also not sure why I experienced oscillations in lambda values with the M130 when it was apparently calling in one of the ancillary fuel tables (it uses separate tables rather than trims for a number of parameters including EGT, knock and coolant temp.) The ECU should apply the richest value from any of these and the main fuel aim table but an unstable result was worrying.
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  #5  
Old 04-16-2018, 07:19 PM
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rv6ejguy rv6ejguy is offline
 
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Their control strategy seems overly complicated. Why not just use Lambda to instantly correct AFR and forget all the other tables? That's what an O2 sensor is for- actual closed loop to target the desired AFR, no matter what the other variables are doing. Can you set different AFR targets at different MAP and RPM ranges? I assume you're using a wideband sensor on the new M130? Were you using a narrow band on the M4?

You don't need MAF or closed loop to do a good job on aircraft, just need to keep the ignition timing in a realistic range and the AFRs in a narrow, safe range as well. The ECU is just doing what you're telling it to do. Make sure you're telling it the right stuff.

We've got Reno racers running way over 75 inches with no issues using MAP. An atmo engine should not be nearly as challenging.
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 419.2 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm



Last edited by rv6ejguy : 04-16-2018 at 07:43 PM.
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  #6  
Old 04-17-2018, 04:03 AM
clarkefarm clarkefarm is offline
 
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Hi Ross
Yes I have always used a wideband sensor and the M130 is a handful.
I have tried to load images of the fuel and ignition maps to Photobucket and the following is a link http://s1212.photobucket.com/user/cl...tml?sort=3&o=0
Hopefully the arrow icons will enable you to also see the fuel table.
If it does not work I can email you directly. I would be grateful for your thoughts.
Rupert Clarke
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  #7  
Old 04-17-2018, 05:58 AM
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rv6ejguy rv6ejguy is offline
 
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Your timing values look reasonable here for premium mogas and stock CR. You might be able to add a few degrees above torque peak rpm (highest cylinder pressure). 3500-4500 rpm is the critical zone on these engines where you need to keep it in the low 20s at high load.

I could not see the fuel tables on Photo Bucket but you could email them to me.
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 419.2 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm


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  #8  
Old 04-17-2018, 06:38 AM
Tomcat RV4 Tomcat RV4 is offline
 
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As a retired life long reg/race mechanic on akl types autos, SDS is the best
Simple reliable method to control auto engine in aircraft
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