flyrotary@lancaironline.net Mailing List Archive

Ernest,

There is no need to take any analysis that I made as correct. The data is there for interpretation by anyone interested enough to look at it.

I think you are on the same theory page as I am. Your proposed modification to the theory IS the theory. The changes I've seen to the electrode affecting thermal conductivity are presented in SAG part 1. The pointlessness of cleaning SAGGED spark plugs was also pointed out there.

The reference to carbon deposits in the main combustion chamber was with the intent to indicate that preignition from them would be distinctly different from preignition originating at the spark plug. I'm sorry if that was not clear.

I am not sure that your statement that SAG would decrease the temperature of the electrode tip is correct. My contention is that the opposite may occur and that this is the reason that SAG does not cure itself and a change in operating conditions is needed to recover from it.

It is possible that a colder plug may withstand degredation longer before SAG occurs.

It's my opinion that heat sinks or sources on the insulator on the spark plug wire side would have minimal effect on the temperature of the electrode tip. The proof, of course, would be to try it and see. I would encourage you to do so.

Steve

From: Rotary motors in aircraft [flyrotary@lancaironline.net] on behalf of Ernest Chrisltey [echristley@att.net]
Sent: Monday, October 08, 2012 8:25 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: colder spark plugs

On 10/06/2012 02:12 PM, Steven W. Boese wrote:

David,

Thank you for the compliment.

I have installed a set of the -10 spark plugs and at this point they are long past the time where the stock plugs normally needed to be replaced. How long they will continue to work is unknown, but at least that one data point is consistent with the study and encouraging.

Taking your anaylysis as correct, how does a colder plug help? I read the analysis as saying that carbon deposits build up and glow hot enough to ignite combustion early. If it is carbon deposits, will (should?) the heat range of the plug make a significant difference?

http://www.totallyamaha.com/snowmobiles/aaTECH/sparkplug/Spark Plug Information Overview.htm

This page indicates that the plug tip needs to stay between 500*C-800*C. Any hotter, and you risk me melting the electrode or blistering the ceramic insulator. That would account for the "bubbly" appearance to the electrode in the pictures you posted earlier. An electrode developing bubbles would become an increasingly better insulator, and trap more heat at the tip. Once it traps enough heat to start SAG, the combustion heat drops (due to SAG), and further melting ceases. The negative feedback will stop the phenomena from becoming catastrophic, as long as you have enough runway to clear the trees.

I propose a modification to your theory. The pre-ignition is caused by mechanical changes within the electrode of the sparkplug itself, instead of a carbon deposit. The changes increase the thermal insulation of the plug tip.

The test for the correctness of this modification to the theory would be to insulate or heat a new, normal heat range plug in some way. Verify SAG occurs. If I am correct, wrapping the porcelain of a plug with nichrome wire connected to a variable power supply would allow you to create SAG on demand. The significance of this modification is that cleaning the plugs to remove deposits would be pointless. But it might be possible to use the cheaper plugs with a heat sink to make them last.

Rather than purchasing the expensive thin walled socket, a tool was made to enlarge the spark plug well in the housing so a conventional socket could be used. While not elegant, this was done with the engine in the plane and involved minimal expense.

I've chucked a cheap socket in the lathe and turned it down to make thin-walled sockets.