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Old 01-12-2005, 11:43 AM   #1
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Default Blast from the past : Perfect Ignition at TDC

How do we know that our 2 stroke engines get perfect ignition at TDC ??

I get headache ( too much guessing ) what nitro % and plug # are the best match to blast at TDC, not too early or too late.
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Old 01-12-2005, 11:47 AM   #2
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Default Re: Blast from the past : Perfect Ignition at TDC

Quote:
Originally posted by asw7576
How do we know that our 2 stroke engines engine get perfect ignition at TDC ??

I get headache ( too much guessing , no EE degree ) what nitro % and plug # are the best match to blast at TDC, not too early or too late.

Right know I have : 25% nitro o'donnel fuel, blue novarossi plug # 7, nova & jp engines in my tool box.
You don't want to have ignition at TDC, you want to some degrees before.
Wtith your combination it should be somewhere 0.3 mm shim at sea level and 20-28 C ambient temperature.
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Old 01-13-2005, 11:22 AM   #3
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why not at TDC for ignition ? why some degree before TDC
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Old 01-13-2005, 01:55 PM   #4
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Quote:
Originally posted by asw7576
why not at TDC for ignition ? why some degree before TDC
To make long story short-in couple of words. Combustion process is taking some time and to have all combustion process do the work it should do-it needs some time. If combustion process will happened at TDC it will be not enough time for fuel to burn and create pressure before exhaust is open, so in this case all pressure insted of pushing piston down will go to exhaust. If ignition happened before TDC ( we are assuming that timing was correct) max pressure will be somewhere at TDC and it will do it mechanical work, and then after pressure went down due to the expended volume ( piston went down under burned gases pressure)-it will go to exhaust window. On top of it if we use squish band correctly to get right squish speed and pressure it should start work some degreece before TDC.
I am not sure if I express everything right due to me English.
Edward
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Old 01-13-2005, 06:20 PM   #5
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I believe what Edward is trying to say is.....

The ignition point should be prior to TDC because the "combustion explosion" does not happen instantaneously. It takes mere milliseconds for the "spark" to cause combustion, and by the time the gasses ignite, the piston is going through TDC.

If ignition is too soon (cold plug- nitro % too low), combustion will happen too late. The fresh intake charge will be leaving the exhaust port and there will be less fuel and air in the engine to ignite.

If ignition point is too soon, (plug too hot- nitro % too high)detonation occurs. The rod becomes fatigued because the combustion pressure is working against the momentum of the piston which is moving towards TDC. Hot spots can also occur on the sharp edges of the combustion chamber and piston. You may see aluminum on your glow plug (engine fragments).
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