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EFI for small industrial engine--aircraft use 
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Greetings. I've just started pawing through the materials in this forum and the DIYEFI.org site, lots of great material, much of it over my head. I'm hoping to get opinions on whether FREE EMS may be right for my application.

I'm hoping to find/build an EFI system suitable for fitting to small (25-35 HP) V-twin industrial engines that are increasing popular in very small experimental aircraft. Most folks flying them are using carburetors, often the inexpensive ones from the OEMs. These can be fussy, and I think we can maybe do better.

The single most important attribute is reliability. Next is reliability. Ease of use and in setting it up will also be important. Fuel efficiency would be very welcome, but is less critical than remaining airborne. Low cost is desirable (the engines themselves cost approx $1000 new)

Megasquirt/Microsquirt could surely be made to work, but my impression is that the code is immense, not entirely transparent, and the cost isn't especially low. The problem (IMO) with huge software, which probably does a lot of great things across a wide variety of users, is that in this application it makes it hard to anticipate failure modes. I (think) I'd do best with the minimal number of sensors and rock-solid defaults that can be put in for any of them so the engine continues to chug along at sufficient output to remain safely airborne.

Attributes of this application:
-- Open loop is desirable, but closed loop might be acceptable if it proves best for other reasons. The most common fuel for general aviation in much of the world ("100LL") still contains tetraethyl lead. Lambda sensors don't like it. Still, users of these engines might be willing to take their own unleaded gasoline to the airport for local flying, if this proves to be the best way.
-- The uneven induction pattern of the V-twin engines requires separate induction and exhaust circuits for each cylinder. So, two injectors.
-- Contrary to automobile use, a smooth idle isn't very important, power settings are fairly constant and generally high. Air temperature and density will vary. Any "limp home" mode must prioritize adequate power output over preservation of the engine.
-- Users are generally willing to make adjustments in flight. They are used to carburetors requiring manual mixture adjustments, etc. CHT and EGT readouts for each cylinder will be available in the cockpit.
-- Power available: These engines generally have 16A alternators for the 12V electrical system, so about 200 watts produced. If we subtract other required power uses and a desire to avoid overtaxing the charging system, it would be best if we could limit the EFI to about 100 watts, if that is feasible.
-- Injection timing: not especially critical, especially if we can do away with an RPM input in the bargain (one less failure point). Squirt the fuel into the runner at the right rate and it'll get into the jug when air flows. Too cavalier?
-- The engines are air cooled, so there's no coolant temperature to monitor (though CHT is available).

Maybe down the road I'd be interested in electronic ignition, but the very simple "Magnetron" system used by these engines is free and quite reliable (negatives: fixed advance). For now, EFI is the priority.

My very naive/uninformed thoughts:
1) Can we run the engine acceptably with just a throttle and a MAF input and run everything from that? No TPS, no temperatures, no RPM, etc. Bias a little bit to the rich side so the engine can increase RPM with increases in throttle, and the new (higher) MAF will drive the increase in injector "on" time? Would this method be self-compensating for altitude? A "mixture" knob in the cockpit for each cylinder could be used for fine-tuning (based on displayed EGT).
2) Similarly, could we get by with just RPM and absolute manifold pressure?
3) Just lambda? (aside from the reservations described above regarding leaded fuel). Sounds primitive, but if the engine is running, we can get the mixture right using lambda alone, and we bias the mixture to allow acceleration if the throttle is opened--would it work?

Or, can Free EMS accommodate some sort of graceful backup to any of the above (or other--better-- degraded operation modes) in the case of sensor failures? I don't think the sensor failures need to be detected automatically, our pilot can flip the switch to alternate modes if the engine starts running poorly.

Sorry for the too-lengthy first post. Thanks for any assistance.


Sun May 17, 2020 12:14 am
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This is very interesting and I definitely have some comments and thoughts for you, but I'm behind on hours at work, and tonight I have to keep busy. Approved and in public so the few other people that still lurk here can see it and respond if they so desire :-) I'll be back, hasta la vista, o hasta pronto :-D

Where are you based, if I may ask? My understanding was that unleaded high octane aircraft fuel was available in most places now? But I could be totally wrong. FWIW I've not even implemented closed loop, though it is desirable for subtle drivability stuff in cars, not so applicable for continuous steady running like in a boat or plane. IE, all of my cars have run open loop for years at a time.

Reliability is a more complex topic and needs to be treated carefully for your application, no matter which device you choose. A forum member here and former sponsor of yours truly was running Link ECUs on a carbon bellied sports plane and had a few dead sticks and carbon damage as a result. No deaths, though. :-o Basically go overkill on power supplies and shielding and noise suppression and protection etc. You'd proabably also want to do some weird stuff like run double FETs on each injector and twin over rate the coils so you can run them at low duty, etc. I burned up a COP in seconds with the wrong dwell here on my most recent car to run this stuff on, but 2+ years later the rest have been fine with the right settings. You need to understand every single bit of hardware you bolt onto this intimately enough to personally guarantee it is reliable yourself from your own testing and experiments (burn up some coils etc, learn the limits).

More later.

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Mon May 18, 2020 7:52 am
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Fred,
Thanks very much for the reply.

Quote:
Where are you based, if I may ask? My understanding was that unleaded high octane aircraft fuel was available in most places now? But I could be totally wrong. FWIW I've not even implemented closed loop, though it is desirable for subtle drivability stuff in cars, not so applicable for continuous steady running like in a boat or plane. IE, all of my cars have run open loop for years at a time.

I'm in southern Ohio, USA. There's no unleaded high octane aviation fuel of any kind at my home airfield, at it still isn't as ubiquitous as the (nasty) 100LL. The Rotax engines that are increasingly popular can't handle leaded fuel, so unleaded fuel is
becoming increasingly available due to their demand, but it isn't nearly as common here as the leaded stuff.


Quote:
Reliability is a more complex topic and needs to be treated carefully for your application, no matter which device you choose. A forum member here and former sponsor of yours truly was running Link ECUs on a carbon bellied sports plane and had a few dead sticks and carbon damage as a result. No deaths, though. :-o Basically go overkill on power supplies and shielding and noise suppression and protection etc. You'd proabably also want to do some weird stuff like run double FETs on each injector and twin over rate the coils so you can run them at low duty, etc. I burned up a COP in seconds with the wrong dwell here on my most recent car to run this stuff on, but 2+ years later the rest have been fine with the right settings. You need to understand every single bit of hardware you bolt onto this intimately enough to personally guarantee it is reliable yourself from your own testing and experiments (burn up some coils etc, learn the limits).


Yes, you've hit on the very important hardware bits. I'm equally as concerned about software. MegaSquirt/MicroSquirt seem very opaque to me and also very complicated (offering every bell and whistle that users have requested, no doubt. Or which the developers thought would be good). "Opaque" and "huge, complex" make it very hard for me to know how it will react to every type of hardware failure.

My (uninformed) opinion at this time is that I'd be best served by a very very simple system. The fewest sensors I can get away with, and cheap enough so I can just install two independent systems for each cylinder. Test them each during the before-takeoff runup check (as we do with the normal dual magneto ignitions), then fly on one FI system unless something goes wrong, toggle to the other in that case. Nobody will want an elegant "failover" system--just a manual toggle switch.

I don't really know which EMS I should be exploring at this point. If it uses very powerful software, I'd probably end up trying to "comment out" most of the features just to avoid unanticipated problems. If there's no stone-simple hardware/software that seems suitable, perhaps I could write an Arduino program? It will probably elicit chuckles, but the software-free old-school discrete component 555-based EFI approach used in this project (https://web.archive.org/web/20190913133244/http://www.rotaryeng.net/simple-cheap-555.html ) is pretty appealing (though it doesn't look like it had been tested at the time the article was written). Ignition is handled elsewhere, I'm just looking for a reliable replacement for the carburetor that will feed fuel reliably (if not optimally) to keep the engine running at all times. I'm willing to tweak a mixture knob (with an eye on the EGT's) to save fuel in cruise.

Other thoughts:
- Batch injection is fine. (That's what is used in the systems sold by Simple Digital Systems (SDS), one of the most respected companies in the aviation EFI biz)
- About 1/2 of aircraft EFI installations have no TPS, and that's fine with me (one less thing to go wrong). Unlike a car, rapid/smooth throttle response just isn't very important in typical aviation use. As long as the engine keeps running when we move the throttle and soon catches up, that's fine. (Most aircraft carburetors don't have an accelerator pump).
- See the OP for my other ideas.


Thanks again!

Mark


Mon May 18, 2020 7:59 pm
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Fred,
BTW, thanks for your comments (below). I think I have another post from a few days ago that is likely stuck in the "newby who might be a spammer" quarantine box.

Fred wrote:
Basically go overkill on power supplies and shielding and noise suppression and protection etc.

Yes, airplanes are a tough EMI environment. And vibration can be a problem. And it can get hot.

Fred wrote:
You'd proabably also want to do some weird stuff like run double FETs on each injector and twin over rate the coils so you can run them at low duty, etc.

No coils on this project it's just EFI. My thoughts on reliability/redundancy below.

Fred wrote:
You need to understand every single bit of hardware you bolt onto this intimately enough to personally guarantee it is reliable yourself from your own testing and experiments


Yes, I agree. And the software. One source of irritation with some OEM systems (including the EFI fitted by Briggs and Stratton) is that the "limp home" modes are clearly characterized. How much power will the engine produce if the O2 sensor dies and the default value is inserted by the software?

On reliability/redundancy--I'll probably want to take a dual-track approach using both "hardening" and redundancy. So, good EMI shielding and clean power, but also redundant power (enough capacity in the battery to run the fuel pump and injectors, plus other essential loads, for 1 hour in case of alternator failure).

After reading a bit more, I think a speed-density EFI system (RPM and MAP) will be more practical in this application than a MAF-based system. I know little about it, but there's not much room in the typical cowling, and the MAF sensors appear (from what I've read) to need to be locatedi n a long, straight run to do their best. Also, with the very choppy airflow in each independent runner (one per cylinder), I'd be concerned about getting accurate readings. On the other hand, some folks who work with these little engines have had trouble getting good MAP data, probably because the flow in each induction runner (one per cyl) is so lumpy. It's probably a matter of choosing the right sensors, the right refresh rate, and appropriate smoothing/averaging in the data processing.

I'd welcome input on what hardware platforms would be good for this project. I don't need bells and whistles, wireless data transmission, etc. Should I be looking at an MCU specifically built to perform EFI functions? Should I do it with a general-purpose processor/hobbiest platform (Arduino? Speeduino?).

Thanks!


Thu May 21, 2020 4:49 am
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You raise some good points, however you're taking a paranoia approach to the code side of things. The things to fear are flash corruption and hardware lockup/latchup leaving the code helpless in the first place. Hence the comments about power supplies and so forth - those things prevent potentially deadly silicone level ECU failures/freeze ups.

What you say about MS bells/whistles/complexity is fair - a single bit in the wrong place can screw any software, but the more you jam in there and the more spaghetti the code is, the worse your chances are.

No coil is a good thing - it's difficult/impossible to run redundancy on one plug, you really need a twin plug cylinder to do that well. An engine that supports that would be a great choice if you were choosing from scratch.

However, what coil is there and what type of leads/delivery? Simple systems designed for use without electronics present can be wholly or partly unusable with EFI nearby. You need resistor plugs, and standard resistance leads etc, or you can totally forget about reliably EFIing the thing. Ignition and starter motors are your biggest enemies in general, and that doesn't change when they're not controlled by the ECU that's doing the fueling.

One software consideration you probably haven't thought about is speed to recovery and frequency of recovery of the RPM/position signal. Some setups do this VERY well with high certainty and frequent resyncs (with my code, not with MS, sometimes), others do not, either once per engine cycle or once per revolution. Why does this matter? In the case where you have resyncs close to the output point, you could run the engine with sync lost 2/3 of the time and still have it run well or okay whereas if the once per engine cycle setup was installed it'd cease running or run at best on one cylinder (assuming 4+ for the example). This could be bespoke with bespoke code for best effect. Honda and Nissan have some good systems standard, though.

About "don't want TPS" etc, don't shy away from sensors, if you're an ECU you need the more the merrier and having them is not the same as *relying* on them. However on that note, if you rely on SD/MAP or you rely on AN/TPS or you rely on measured/MAF then when something fails, you're screwed. If you have ALL three there, and all three tuned, it's trivial to say "which one is the outlier" and then ignore it based on trusting the other two that agree. You'd need this dialled in for altitude though, to work effectively for you. For example, you couldn't do a pure TPS based system unless you had some way of saying "we're this high" to the device be that a MAP sensor reading atmospheric or a knob that you turn or whatever, the system would need to know. Even if you don't use the TPS for running the thing, having it there as a cross check for your data logging (I strongly assume you'd datalog every flight) is valuable as it gives you system-input timings as well as output timings.

Have a physical reset button that breaks power to the device and reapplies it so you can restart the engine if there's a glitch - depending on the inertia in the prop, the back feed from air speed, and the briefness of the press, you could probably fix a software or silicone lock up with a brief press and not interrupt running much longer than it.

About leaded gas and octane - if these engines don't need high octane at sea level, they sure as shit won't need it way up in the sky with way way less cylinder pressure. Ordinary pump gas could be a great choice for you, given that. Then the wideband you will *want* for tuning will not fail as quickly as it otherwise would, and you could run it full time.

About batteries, I'm converting all of my cars to LiFePO4 - it's compatible with lead-acid in terms of Voltage and far exceeds in performance and longevity. 2.8kg of 4S2P Headway 38120HP high current cells/pack will easily start any of my cars up to 3.0 litres or beyond, if the engine is 1.5 or smaller 4 cells would be sufficient at 1.4kg or 2.8kg gives you the redundancy you want. Those are 8AH and 16AH respectively. For longevity without charging you'd need to do the math yourself, but you can't run them flat or they die forever - moot point in a plane though :-D

Final thought, you could have 3 way redundancy with two ECUs and another simpler analogue circuit based on a timer and some frequency in/tps in mapping and a trim knob for you to tweak as you go. Engines will run surprisingly well with a surprisingly wide/imprecise mixture range from black smoking around 9-10:1 AFR through to coughing/bucking lean up around 19-20:1 AFR - you only have to be in the right place for high/sustained loads or on extreme engines. For fuel pumps, choose the most efficient ones you can

Sorry about the spammer filter, a few more posts and it'll go away. :-)

Cheers for the conversation! It's a fun thought experiment.

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DIYEFI.org - where Open Source means Open Source, and Free means Freedom
FreeEMS.org - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!


Sun May 24, 2020 9:09 am
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Fred, thanks for the input.

Fred wrote:
You raise some good points, however you're taking a paranoia approach to the code side of things. The things to fear are flash corruption and hardware lockup/latchup leaving the code helpless in the first place. Hence the comments about power supplies and so forth - those things prevent potentially deadly silicone level ECU failures/freeze ups.

What you say about MS bells/whistles/complexity is fair - a single bit in the wrong place can screw any software, but the more you jam in there and the more spaghetti the code is, the worse your chances are.

This is why I'm reluctant to include software-driven failover modes, etc. If the engine stops running (well), it would seem best and safest for the pilot to select an independent system rather than have the software enter the Gordion's knot of decision trees to figure out which inputs are most reliable/accurate. By "independent systems" I mean really independent--software and all hardware.


Fred wrote:
No coil is a good thing - it's difficult/impossible to run redundancy on one plug, you really need a twin plug cylinder to do that well. An engine that supports that would be a great choice if you were choosing from scratch.

Yes, unfortunately, there's not room in the cylinder head of these small V-twins for another plug. Folks flying air cooled VW engines (as I do) typically do drill for another plug per cylinder.

Fred wrote:
However, what coil is there and what type of leads/delivery? Simple systems designed for use without electronics present can be wholly or partly unusable with EFI nearby. You need resistor plugs, and standard resistance leads etc, or you can totally forget about reliably EFIing the thing. Ignition and starter motors are your biggest enemies in general, and that doesn't change when they're not controlled by the ECU that's doing the fueling.

The OEM ignition uses two "Magnetrons (TM)", one per cylinder. An unfortunate name--not at all related to magnetrons in raar or microwave ovens. Very simple and reliable, more here: https://www.homebuiltairplanes.com/forums/threads/b-s-49-series-810cm3-49ci-tipis-conversion-for-aircraft-use.32368/post-496488. Magnets in the flywheel produce electricity in fixed windings, stored in a condenser, triggered by a secondary coil with transistor. Cheap, one per cylinder.
Cons: Not the most energetic spark (keep the plug clean), fixed timing, need 300 RPM or so to get sufficient juice.
I don't know about the EMI/electronic noise issue with these systems, though I'm pretty sure reducing it wasn't a big concern of the designers. Folks are flying with them and using radios, so maybe it's not terrible. As they aren't wired to the plane's electrical system, noise wouldn't be getting to other components directly via the wiring, except by induction/EMI. (Right?)
It's a wasted spark system (one spark per crank rotation)

Fred wrote:
One software consideration you probably haven't thought about is speed to recovery and frequency of recovery of the RPM/position signal. Some setups do this VERY well with high certainty and frequent resyncs (with my code, not with MS, sometimes), others do not, either once per engine cycle or once per revolution. Why does this matter? In the case where you have resyncs close to the output point, you could run the engine with sync lost 2/3 of the time and still have it run well or okay whereas if the once per engine cycle setup was installed it'd cease running or run at best on one cylinder (assuming 4+ for the example). This could be bespoke with bespoke code for best effect. Honda and Nissan have some good systems standard, though.

The flywheel on these engines does a few things:
- Rotating mass needed for mechanical reasons (though I don't need it in my application, the propeller's moment of inertial is enough).
- Provides a mount for the starter ring gear.
- Provides a home for the alternator magnets (that work with fixed stator coils under the flywheel to make power for the lights, starting battery, etc)
- Provide a location for the (separate) magnets used by the Magnetron for the ignition (described above).
Right now, I'm thinking the magnet used by the Magnetron might be used to trigger the injectors. That gives us batch injection of both cylinders twice per engine cycle. If we wanted more than that, we could mount as many trigger pickups as desired, wherever desired. Would a magnetic pickup like this (Hall effect sensor?) be as reliable as other methods?

Fred wrote:
About "don't want TPS" etc, don't shy away from sensors, if you're an ECU you need the more the merrier and having them is not the same as *relying* on them. However on that note, if you rely on SD/MAP or you rely on AN/TPS or you rely on measured/MAF then when something fails, you're screwed. If you have ALL three there, and all three tuned, it's trivial to say "which one is the outlier" and then ignore it based on trusting the other two that agree. You'd need this dialled in for altitude though, to work effectively for you. For example, you couldn't do a pure TPS based system unless you had some way of saying "we're this high" to the device be that a MAP sensor reading atmospheric or a knob that you turn or whatever, the system would need to know. Even if you don't use the TPS for running the thing, having it there as a cross check for your data logging (I strongly assume you'd datalog every flight) is valuable as it gives you system-input timings as well as output timings.

Datalogging would be nice, as would a simple in-cockpit display (RPM, fuel flow, MAP).
On sensors: Due to the uneven induction/firing cycle of this V-twin, I assume I'll need independent sensor suites for each cylinder.
-- Is there a suitable MAF sensor used in this aftermarket world that can work for this application? I've read that the flow past these sensors needs to be very smooth to be useful (requiring a long, straight run before the sensor) and that the intermittent/turbulent nature of this single-cylinder use will lead to crummy data from the MAF.
-- MAP: The pressure in these two manifolds will be jumping all over the place due to the long period between valve opening, then a big rush, then the rebound off the valve, etc. I'm not sure what this means for sensor selection, desirable refresh rates, desirability of MAP averaging/smoothing when selecting fuel flow, etc. Some folks working to tune/optimize induction with these engines are unable to get reliable MAP information with a stock engine monitor (used for auto engines, etc), I'm guessing it may be because of some of these issues.
-- TPS: A major manufacturer of aftermarket aircraft EFI systems (SDS inc) doesn't use a TPS on most of their installations. They find it's just not needed in typical aircraft use. If it can provide me a benefit (e.g. maybe a backup user-selectable N-Alpha operating mode in the case of MAP sensor failure), that might be okay--but it would be more straightforward and cheaper to just provide another MAP source (or skip all this entirely: rough running = select independent EFI System B and troubleshooot System A on the ground).

Fred wrote:
Have a physical reset button that breaks power to the device and reapplies it so you can restart the engine if there's a glitch - depending on the inertia in the prop, the back feed from air speed, and the briefness of the press, you could probably fix a software or silicone lock up with a brief press and not interrupt running much longer than it.

Good point. I'd imagine a three-position toggle switch: EFI 1 -- off -- EFI 2. "OFF" gives us a chance to reset either system.

Fred wrote:
About leaded gas and octane - if these engines don't need high octane at sea level, they sure as shit won't need it way up in the sky with way way less cylinder pressure. Ordinary pump gas could be a great choice for you, given that. Then the wideband you will *want* for tuning will not fail as quickly as it otherwise would, and you could run it full time.

Yes, and the leadded fuel problem isn't a giant issue, but it is a factor. Many fields here have only Jet A (we can't use that) or 100LL. Lugging 20 gallons of pump gas in cans in the car trunk isn't the most convenient option, but it's do-able (and cheaper). When traveling cross-country, though, being able to use 100LL in a pinch is nice/convenient. It is possible to plan around that, though. And, given the nature of ghe spark we have and the issue of lead fouling, avoiding 100LL when possible would probably be best. But, if using wideband O2 for anything in flight, we'd definitely need to be able to choose the mixture we need. Some OEM systems are hardwired to "best economy" and that's not a good thing if we need a richer mixture for "best power" to avoid terra firma. Also, if not using a TPS, running a bit rich is better, we'll need that to allow acceleration when the throttle is opened.

Fred wrote:
Final thought, you could have 3 way redundancy with two ECUs and another simpler analogue circuit based on a timer and some frequency in/tps in mapping and a trim knob for you to tweak as you go. Engines will run surprisingly well with a surprisingly wide/imprecise mixture range from black smoking around 9-10:1 AFR through to coughing/bucking lean up around 19-20:1 AFR - you only have to be in the right place for high/sustained loads or on extreme engines.

Thanks--that's the spirit! So, maybe a 4 position toggle switch: "EFI 1 - off - EFI 2 - Analogue Hail Mary" :)
I do think a fixed interval (no RPM info required) fixed pulsewidth injection could be selected to keep the engine running, especially if we have a "go to" throttle setting ("pull the throttle knob out to the mark on the shaft"). And a trim knob to fine-tune the PWM. EGT info will be available, so getting the mixture right won't be hard.

Fred wrote:
For fuel pumps, choose the most efficient ones you can
I've seen folks who did homebrew Arduino-based EFI on their lawn tractors using stock GM injectors and 4-7 PSI injectors. Is low pressure like that a bad idea in an EFI system? The pumps are inexpensive and don't draw much current (yes, important when on battery mode, but also at other times. The alternators on these engines only make 7-20 amps (84-240 watts), and we need to recharge the battery, run a radio and external lights, the injectors, etc. And for longevity/reliability of the alternator circuits, its best not to run them close to their capacity.

Fred wrote:
Cheers for the conversation! It's a fun thought experiment.

Thanks very much for sharing your knowledge.


Sun May 24, 2020 5:21 pm
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Been flat out, will get a proper response to the above written up ASAP. Also owe someone a long email. Probably both honoured the same night. Cheers.

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Sun May 31, 2020 3:07 pm
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No rush, I appreciate any ideas/insights you have to offer. Hopefully there's a simple, inexpensive hardware/software approach for this application. I'd like to pare things down to a minimalist approach.


Mon Jun 01, 2020 1:49 pm
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Well, it's been quite the month, and it's still going. Never did write that email, either. Took me 5 minutes to even remember what it was. A few urgent chores still on the plate right now unfortunately. :-( Sorry!

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FreeEMS dev diary and its comments thread and my turbo truck!
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The ever growing list of FreeEMS success stories!


Thu Jul 02, 2020 4:59 am
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