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  #11  
Old 05-30-2022, 07:54 AM
Toobuilder's Avatar
Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by JBrenneise View Post
By ignition timing, I mean advancing or retarding the spark to compensate for lower octane fuel(s).
Yes, SDS pioneered this capability years ago and it works. My 330+ HP (dyno numbers) 540 runs just fine on regular car gas from the corner gas station thanks to the adjustability of the SDS system.

Heres a thread with some real world data: https://vansairforce.net/community/s...d.php?t=186833
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
1940 Taylorcraft BL-65
1984 L39C - SOLD
RV-8 - SDS CPI - SOLD

Last edited by Toobuilder : 05-30-2022 at 08:40 AM.
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  #12  
Old 06-02-2022, 09:11 PM
agent4573 agent4573 is online now
 
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I'd like to do some testing with various fuels in the 7, but there was another post mentioning it's almost impossible to find a knock sensor that works in lycomings. Anyone have any info on why knock sensors don't work, or suggestions on how to make them work?
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  #13  
Old 06-02-2022, 10:10 PM
gasman gasman is offline
 
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They don't work for the same reason you can't hear the engine knock.
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  #14  
Old 06-03-2022, 07:09 AM
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airguy airguy is offline
 
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Too much mechanical noise in an air-cooled engine.
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Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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  #15  
Old 06-03-2022, 07:36 AM
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Knock frequency varies between engines so a sensor optimized for one engine may not be ideal on a different engine.

Std Lyc heads don't have good places to mount knock sensors as they were never designed for them. Placement is important for useful knock detection.

You have to make the engine knock first to detect it. How many times do you want to subject your engine to this? Light knock for short periods probably isn't damaging but continuous medium knock is.

Lycs have individual cylinders so you may need individual sensors to properly detect knock.

Lycs have a lot of mechanical noise which must be filtered out so it's not falsely interpreted as knock. You need a way to filter out the unwanted false frequencies and just display when actual knock is occurring. Can and has been done on air cooled Corvair and Porsche engines with integrated heads at least. These are also mechanically noisy engines.

Lycoming's IE2 engine has different cylinders and multiple knock sensors so it can be done with a bunch of money and engineering thrown at it. Standard cylinders may work with enough experimentation and I read an account online from someone who'd done this on a big Conti or Lyc a few years back. They believed they were able to detect knock in flight.

You then need a programmable EI to effectively map around knock and probably full EFI/EI to optimize as mixture also makes a big difference in knock propensity.

If you decide to experiment, please let us know what you discover. It's an interesting area of study.
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Last edited by rv6ejguy : 06-03-2022 at 08:00 AM.
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  #16  
Old 06-03-2022, 07:40 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by agent4573 View Post
I'd like to do some testing with various fuels in the 7, but there was another post mentioning it's almost impossible to find a knock sensor that works in lycomings. Anyone have any info on why knock sensors don't work, or suggestions on how to make them work?
I am not convinced that a lyc is any noisier than any other engine and the volume of noise has no bearing on electronically observing knocks. In most auto engines, they use a piezo electric sensor to listen for a specific frequency and that sensor is mounted in a strategic position on the engine block. Each sensor is built to only listen for that specific frequency, though it can be done with broader sensors if you know the frequency that is present during detonation. In a lycoming the cylinders and heads are much thinner with very little contact with the block, meaning that it is likely challenging to get a good resonance in the block, possibly requiring a sensor for each cylinder, with few good places to mount them. Auto egnines have everything cast into a block and heads. Therefore all noises can be observed resonating in that block. Not so much with a boxer engine. Also, engineers spend weeks on the dyno with forced detonation, figuring out which frequency to listen for and which spot on the block will best resonate that frequency during detonation. To my knowledge, no one outside of Ada, OK has done that for a Lyc and I don't think they are sharing. I am sure there are others who do this, but don't know. Clearly it is done, as Dan has posted several dyno reports that show detonation as part of the plotted data. These are likely engine lab type settings with instrumentation well outside the reach of guys like us.

That said, it can be done. I installed a custom EFII system on an older, 80's vintage porsche 911, which is an air cooled boxer engine very similar to the Lyc. While I never was able to rig the knock detection into the system (never attempted due to the complexity), I was able to rig a system to hear the knock frequency through headphones and used that when tuning.

Larry
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Last edited by lr172 : 06-03-2022 at 08:10 AM.
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  #17  
Old 06-03-2022, 08:29 PM
N427EF N427EF is offline
 
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According to Santa Clara County:
Quote:
The unleaded fuel available at Reid-Hillview and San Martin is 94 octane and is FAA-certified as safe for use in approximately 68% of the piston-powered aircraft of the type that use the Reid-Hillview and San Martin airports. Larger jet aircraft, such as those operating out of Mineta San Josť International Airport, use a Kerosene-based fuel that does not contain lead.
With 94UL available at your home field, why don't you just determine if you fall within the 68% of (FAA certified as safe) and use it before redesigning your ignition system?
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  #18  
Old 06-03-2022, 11:08 PM
agent4573 agent4573 is online now
 
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Most of you guys are focussing on microphone type knock sensors, and I agree those come with challenges. Not knowing which frequency to listen for can be overcome by using a large range sensor and real-time FFT data analysis. While it varies a little bit based on combustion chamber design, I remember my internal combustion textbook saying its focussed around 3,500Hz for automotive engines. If I run the motor at 2100 rpm, that should give me a mostly noise free zone between 2100 and 4200hz, until the 2/rev vibrations come back into play. As far as mounting the sensor I see 3 options, the primer port which is unused with an SDS system, the CHT port, or one of the 1/4-20 baffle bolts that go through the fins in the head. I would start with a leak-down test to see which cylinder has the best compression, and lean it out a few percent over the other 3 to make it knock first. That would mean I would only have to focus on that 1 cylinder instead of all 4. Using the LOP button with ignition retard instead of advance would allow me to pull massive amounts of timing almost instantly when I do encounter knock.

I think the easiest way to do this would be to remove a spark plug and mount a pressure transducer in its place. We work with this company for a few of our ground test stands, so I might be able to work a deal on a high-temp, 5000psi pressure transducer and amplifier. (https://www.kistler.com/en/solutions...e-measurement/) No noise to worry about and I could plot the trace on a battery powered o-scope in the cockpit. The downsides to this approach would be if I did end up losing the other ignition, I would only have 3 cylinders to fall back on. I would be ok with this as I would do all these tests in gliding distance to the runway, so my only massive risk would be losing the other ignition between 1 and 1000 ft. The biggest issues would be 1 vs 2 plug detonation variables.

This would be a ton easier on a dyno stand, but I think I can make it work on the airplane. I'll think about it more after Oshkosk. If the pressure transducers are out of reach price-wise, I can always hook up a good old stethoscope style system and listen to the cylinder through headphones.
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Last edited by agent4573 : 06-03-2022 at 11:40 PM.
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  #19  
Old 06-04-2022, 07:22 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by agent4573 View Post
Most of you guys are focussing on microphone type knock sensors, and I agree those come with challenges. Not knowing which frequency to listen for can be overcome by using a large range sensor and real-time FFT data analysis. While it varies a little bit based on combustion chamber design, I remember my internal combustion textbook saying its focussed around 3,500Hz for automotive engines. If I run the motor at 2100 rpm, that should give me a mostly noise free zone between 2100 and 4200hz, until the 2/rev vibrations come back into play. As far as mounting the sensor I see 3 options, the primer port which is unused with an SDS system, the CHT port, or one of the 1/4-20 baffle bolts that go through the fins in the head. I would start with a leak-down test to see which cylinder has the best compression, and lean it out a few percent over the other 3 to make it knock first. That would mean I would only have to focus on that 1 cylinder instead of all 4. Using the LOP button with ignition retard instead of advance would allow me to pull massive amounts of timing almost instantly when I do encounter knock.

I think the easiest way to do this would be to remove a spark plug and mount a pressure transducer in its place. We work with this company for a few of our ground test stands, so I might be able to work a deal on a high-temp, 5000psi pressure transducer and amplifier. (https://www.kistler.com/en/solutions...e-measurement/) No noise to worry about and I could plot the trace on a battery powered o-scope in the cockpit. The downsides to this approach would be if I did end up losing the other ignition, I would only have 3 cylinders to fall back on. I would be ok with this as I would do all these tests in gliding distance to the runway, so my only massive risk would be losing the other ignition between 1 and 1000 ft. The biggest issues would be 1 vs 2 plug detonation variables.

This would be a ton easier on a dyno stand, but I think I can make it work on the airplane. I'll think about it more after Oshkosk. If the pressure transducers are out of reach price-wise, I can always hook up a good old stethoscope style system and listen to the cylinder through headphones.
The problem with home brew testing of this sort is getting all the parameters in place. With a permanent knock sensor tied to EFII all is good. If you are testing to determine an advance threshhold that is safe, that is hard. Detonation risk increases with heat in several areas. Getting OT, IAT and CHT into the high ranges will be problematic on the ramp, even orbitting the field.

Larry
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  #20  
Old 06-04-2022, 08:39 AM
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Toobuilder Toobuilder is offline
 
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We also need to understand that knock sensors in cars are required because they are trying to optimize timing and mixture to a very, very tight standard to meet fleet emissions standards. Our standard is much easier - do not knock. I have shown that the typical Lycoming running typical take off power mixture is very insensitive to timing retard. Put another way, you can back timing off far enough that you will never have to worry about detonation and retain essentially all of your take off performance - and you can do this open loop with a simple lookup table. This capability is available right now, today. Just give SDS a call.

OTOH, if you are really looking for that absolute corner case where you want to ride right on the edge of detonation reliably with a closed loop system, then please let us know what you find out. We will all benefit.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
1940 Taylorcraft BL-65
1984 L39C - SOLD
RV-8 - SDS CPI - SOLD
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