VansAirForceForums  
Home > VansAirForceForums

-POSTING RULES
-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.

  #21  
Old 10-10-2021, 08:43 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 10,292
Default

Quote:
Originally Posted by Cumulo View Post
You're keeping us hopping - had to review Paschen's Law. Wondering, though, where your 4 - for 4 atmospheres comes from.
The figure came from the above noted graphs in Heywood, and a 4 bar reference in a Mike Busch article, but that doesn't make it correct for a given Lycoming at full throttle. You're right, it's too low in the context of this discussion.

As previously noted, the Heywood text is at the office, so I'll get back to it tomorrow. My guess? The 4 bar figure is probably typical for a late model automobile operating at part throttle cruise, as emissions and fuel economy research would be foremost at the MIT-Sloan lab.

Cylinder pressure at spark initiation would is not be a simple product of geometric compression ratio. All the model inputs would be in Gordon Blair's "Design and Simulation...", another text on the office shelf.

On the practical side, a Champion plug tester is 125 psi, 8.6 bar.
__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 10-10-2021 at 09:11 AM.
Reply With Quote
  #22  
Old 10-10-2021, 08:55 AM
Pilot8 Pilot8 is offline
 
Join Date: Sep 2018
Location: California
Posts: 107
Default

Back to the original poster's thread, which is very interesting- why three coils have failed. It would be interesting to see a schematic for the Lightspeed Plasma III. From the schematic, it would be easy to calculate the voltage and currents versus time- the coil specs, and knowing temperature, would get us closer to the answer of why the coils are failing. It's a binary search of probable causes to any solution.
Reply With Quote
  #23  
Old 10-10-2021, 04:32 PM
Cumulo Cumulo is offline
 
Join Date: Apr 2015
Location: KHMT
Posts: 86
Default

Quote:
Originally Posted by rv6ejguy View Post
I'm not sure where you are getting 20 atmospheres from. An 8.5 CR engine will have a PR of around 8.5. The density ratio will be less however as a result of the temperature rise from compression and losses from having a VE less than 100%. Density would be the key factor here in required spark energy though the compression temperature rise would have a slight offsetting effect as the spark would have to input less energy into raising local temperature of the mixture between the the plug electrodes to initiate combustion.

A compression tester will illustrate what pressure ratio your engine develops at cranking rpm at least and this will be lower at 2500 rpm BTW, due to VE loss. You certainly don't see 294 psi on a Lycoming with a compression tester.

I totally agree on the last two paragraphs. Introduce a 2-3 inch spark gap somewhere in the system is bad news for a coil. It will arc internally which can damage some coils in short order. Have seen that more than a few times.
Thank you, Ross, for your sage observations. Both you and Dan among others here are artisans in both talk and walk. Very few with that combo. Thx.

Re: the 20 bar pressure for 8.5 CR.
The justification for that figure is just straight thermodynamics.

Verbally, when you trap a volume of air in a cylinder and then reduce the volume, two separate things happen. First, the pressure increases as the volume is reduced. Second, the temperature increases as well, which adds additional pressure.

Getting into a little math, the exponent of the first is 1, the exponent of the second is .4 for air. Combined, it is 1.4

So, The relation of CR to PR is CR^1.4, or in the Lyc case, 8.5^=20.0072 PR

That pressure derived is of course, only a starting point, but is the proper starting point when we then roll in all of the other well known factors that will tend to reduce the number.
------------
As an interesting reverse example, those amazing jet engines that push the tubeliners across the sky have pressure ratios(PR) near 40. In terms of compression ratio(CR) it would be 40^(1/1.4)= 13.9CR

Ron

Last edited by Cumulo : 10-10-2021 at 06:15 PM. Reason: added "which adds additional pressure" to clarify
Reply With Quote
  #24  
Old 10-11-2021, 05:41 PM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 10,292
Default

This is the illustration I was thinking about Saturday, from Internal Combustion Fundamentals, Heywood. Note the line "The 4 bar pressure roughly corresponds to engine conditions at spark onset". The reference calls out a book called Flow and Combustion in Reciprocating Engines...which magically, is available here:

https://r11tc.files.wordpress.com/20...ng-engines.pdf

Ahh, more reading. Looks interesting, and hopefully contains the rest of the story
-
Attached Thumbnails
Click image for larger version

Name:	Spark Power Energy Volts, Current.jpg
Views:	25
Size:	166.4 KB
ID:	17011  
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #25  
Old 10-11-2021, 11:18 PM
rv8ch's Avatar
rv8ch rv8ch is offline
 
Join Date: Feb 2005
Location: LSGY
Posts: 4,162
Default

Quote:
Originally Posted by DanH View Post
This is the illustration I was thinking about Saturday, from Internal Combustion Fundamentals, Heywood. Note the line "The 4 bar pressure roughly corresponds to engine conditions at spark onset". The reference calls out a book called Flow and Combustion in Reciprocating Engines...which magically, is available here:

https://r11tc.files.wordpress.com/20...ng-engines.pdf

Ahh, more reading. Looks interesting, and hopefully contains the rest of the story
-
Hi Dan, could you please double-check the URL? Seems the automatic post URL formatter changed it. Thanks!

Never mind - found it here: https://gctbooks.files.wordpress.com...-b-heywood.pdf
__________________
Mickey Coggins
http://rv8.ch
"Hello, world!"

Last edited by rv8ch : 10-11-2021 at 11:20 PM.
Reply With Quote
  #26  
Old 10-12-2021, 01:55 AM
Cumulo Cumulo is offline
 
Join Date: Apr 2015
Location: KHMT
Posts: 86
Default Its density

My original assumption is right. Just density.
Found a straightforward document about "pressure" as it relates to spark ignition. Dan might like it. NACA 1920 report number 54.

https://core.ac.uk/download/pdf/42795274.pdf

Bottom line: pressure is only relative to spark requirement as it effects DENSITY.

BTW, I worked out the geometric CR for an 8.5 CR Lyc. at ignition time.
Included rod angle's small effect as well in the calc:
20 deg BTDC = 6.54 CR
25 deg BTDC = 5.80 CR
30 deg BTDC = 5.12 CR

my conclusion : don't give up ANY reliability just to get a "super spark", its not needed.

Ron
Reply With Quote
  #27  
Old 10-12-2021, 05:07 AM
Tim Lewis Tim Lewis is offline
 
Join Date: Aug 2006
Location: Bristow, VA
Posts: 167
Default

Quote:
Originally Posted by kaweeka View Post
I currently have a Lightspeed Plasma III ignition system however, after 3 coil failures...
Three things I do to avoid Lighspeed coil failures (gleaned from my conversations with Klaus):
- Replaced the spark plug wires at 500 hours (per the Lightspeed manual). Klaus will make them for you to the lengths you need. Or you can make your own.
- Use the IK27 iridium plug, keep gap below .040"
- Spark plug wire supports - replaced Adel clamps with nylon cable clamps (allegedly reduces chance of spark plug wire shorting to ground).

My Lighspeed coil failures stopped when I took the first two steps.
__________________
Tim Lewis
CFI, A&P
RV-10 N31TD - 1450 hrs
RV-6A N47TD - 1100 hrs (sold)
Reply With Quote
  #28  
Old 10-12-2021, 05:43 AM
rv6ejguy's Avatar
rv6ejguy rv6ejguy is offline
 
Join Date: Mar 2005
Location: Calgary, Canada
Posts: 6,094
Default

Quote:
Originally Posted by Cumulo View Post

Bottom line: pressure is only relative to spark requirement as it effects DENSITY.

BTW, I worked out the geometric CR for an 8.5 CR Lyc. at ignition time.
Included rod angle's small effect as well in the calc:
20 deg BTDC = 6.54 CR
25 deg BTDC = 5.80 CR
30 deg BTDC = 5.12 CR

my conclusion : don't give up ANY reliability just to get a "super spark", its not needed.

Ron
Yup. Agreed.
__________________

Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 449.8 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
https://www.youtube.com/channel/UCiy...g2GvQfelECCGoQ


Reply With Quote
  #29  
Old 10-12-2021, 06:45 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 10,292
Default

Quote:
Originally Posted by Cumulo View Post
My original assumption is right. Just density.
Found a straightforward document about "pressure" as it relates to spark ignition. Dan might like it. NACA 1920 report number 54.
https://core.ac.uk/download/pdf/42795274.pdf

You bet, thanks. I like the old NACA papers.

Quote:
Originally Posted by rv8ch View Post
Looks like an older edition of Heywood.

"Flow and Combustion..." here, add the usual https:// to the URL:

r11tc.files.wordpress.com/2010/05/flow-and-combustion-in-reciprocating-engines.pdf

Here's the 2007 Mike Busch article referencing Paschen's Law and 4 atmospheres. First time I had heard of Paschen and made a note on my hard drive.

https://www.avweb.com/ownership/the-...itude-misfire/
__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 10-12-2021 at 06:59 AM.
Reply With Quote
  #30  
Old 10-12-2021, 08:41 AM
rv8ch's Avatar
rv8ch rv8ch is offline
 
Join Date: Feb 2005
Location: LSGY
Posts: 4,162
Default

Quote:
Originally Posted by DanH View Post
...
"Flow and Combustion..." here, add the usual https:// to the URL:

r11tc.files.wordpress.com/2010/05/flow-and-combustion-in-reciprocating-engines.pdf

Here's the 2007 Mike Busch article referencing Paschen's Law and 4 atmospheres. First time I had heard of Paschen and made a note on my hard drive.

https://www.avweb.com/ownership/the-...itude-misfire/
Thanks Dan - got it.
__________________
Mickey Coggins
http://rv8.ch
"Hello, world!"
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 04:31 PM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.