Discussion with Eagle EMS engineer
Hi Dennis,
Thanks for the email. I'm assuming you're referring to my comments posted on the VAF Forum. I'll post your email there, along with my reply to it (below). But I also encourage you to join the forum yourself and participate in the discussion there directly. Your input would be most welcome. And if there are any misconceptions or misunderstandings about your product, then by all means, let's clarify them.
On 1/14/2013 8:51 AM, Dennis Anttila wrote:
Hello,
I am commenting on your statements about the Eagle EMS from Precision Airmotive. You seemed concerned that one had to use a manual mixture control. This is not true. The Eagle does all the computations for air-fuel and there is no need for a mixture control. The Lean Pot you are talking about is used as a economy tool in that when at cruise altitude one can lean out from 12.5 to 1 to 17.5 to 1 or what value give a comfortable EGTs. This can be also set up in tables via a laptop so one does not need to use the Lean Pot for economy. It seems most pilots like the manual mode always setting it to their best number and fly from there. Either way the lean function is disabled when full throttle is requested such as a change in altitude. This system has been proven up to 27,000 feet (with oxygen) and set a record.
The Eagle does not have prop control but could be an option with additional hardware. Please feel free to contact me for any other information as I work for Precision Airmotive and am the Hardware and software designer here. All I want is to clarify any mis-beliefs about the system.
Sincerely, Dennis Anttila
Precision Airmotive LLC
Marysville, WA 98271
Unless I missed a subtlety in your explanation, it is entirely consistent with my understanding of your system (which I obtained from your published documentation, as well as from presentations and discussion with your colleagues at Oshkosh in 2009/2010).
As you said, when talking about manual mixture adjustment with your system, I was referring to the Lean Pot. I understand that your system can function on its mixture base maps alone, without the Lean Pot. I don't doubt this. My criticism pertains to the inclusion and need for the Lean Pot to achieve optimal leaning across different operating conditions associated with different phases of flight.
As I'm sure you're aware, the optimal fuel-air mixture does vary between different operating conditions, spanning at least three possible distinct operating regimes that one might employ: best power, best economy on the rich side of peak EGT ("ROP"), and best economy on the lean side of peak EGT ("LOP"). Best power mixture is called for when full power is commanded, and I understand that you do implement this. But when less than full power is commanded, there is more intelligence required in order to find the optimal best economy fuel-air mixture. A first order cut at it can be just another mixture map, which I think is what you're describing as the alternative to the Lean Pot. This would presumably be only for ROP if there is only one best econ map, because LOP operation must be further restricted to a narrower set of operating conditions. But in any case, further fine-tuning of mixture from the map value does does require closed-loop control, because open loop maps by their nature must be sufficiently conservative to account for all possible worst-case variations in unmonitored operating parameters. And this is where, if my understanding is correct, your system doesn't implement the necessary closed-loop control, but rather leaves it to the pilot to fine-tune mixture via the Lean Pot. Is that correct?
As I'm sure you're aware, finding best economy mixture is not as simple as just adjusting mixture to find a target EGT number. EGT-based leaning is certainly a valid approach, and is the approach commonly used by pilots with legacy fuel metering systems. But there's more to it than just a target EGT number. Absolute EGT numbers are fairly meaningless. They will vary substantially under different operating conditions, from one engine installation to the next, and even between different cylinders on the same engine. Variations in cylinder, intake and exhaust geometry, and tolerances in the injectors themselves will also lead to mixture imbalance between cylinders (which I don't believe you compensate for, but you could!), and subsequent differences in EGT peaking between cylinders. So with all that, EGT-based leaning requires (by the conventional approach) sweeping mixture over a range, detecting peak EGT points per cylinder, and then adjusting mix for a prescribed minimum margin on the desired side of peak EGT, which in itself is also a choice dictated by operating conditions. Optimal leaning also requires careful prevention of operating in detonation regions, as well as monitoring for signs of actual detonation (typically via cylinder head temps for lack of a knock sensor...).
This is the level of engine management required by the pilot to keep the engine healthy and operating at optimal fuel economy. And this is the level of engine management I would expect a FADEC to implement for it to have a an attractive value proposition. It could implement that level of functionality by these methods or others. But in any case, it should implement this level of functionality transparently and autonomously, relieving the pilot of the mixture management task.
P.S. About prop control, are you saying simply that it's possible (of course it is) and that your architecture could hypothetically be built upon to support it? Or are you saying that you actually have this option implemented, tested, and available to customers?
Thanks,
-Roee
