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| Ignition timing for Twin Cylinder eng... |
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laurie_d
Member
Username: laurie_d
Post Number: 18
Registered: 10-2009
| | Posted on Monday, February 22, 2010 - 09:21 pm: | 
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Folks, I have a twin Simplex (1-0/12 hp) and I understand the firing order is not directly opposed Am I confused because the magneto cap appears to distribute the spark ona directly opposed basis? |
gregoryan
Senior Member
Username: gregoryan
Post Number: 59
Registered: 09-2004
| | Posted on Tuesday, February 23, 2010 - 01:17 am: |
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I just had to go and look at mine; it too has 180 deg opposed distributor terminals. The firing order is; 1,2,0,0, 1,2,0,0, 1,2,0,0, 1,2,0,0, So it sparks every 1/2 a turn,,,, the 0,0 sparks just go to waste, at the top of the exhaust strokes. Note that you will have to engage the mag drive in the correct 1/2 turn so that the sparks dont go to the wrong plug. |
laurie_d
Member
Username: laurie_d
Post Number: 19
Registered: 10-2009
| | Posted on Tuesday, February 23, 2010 - 04:42 pm: |
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Thanks for that. This is one reason why I like OME so much.
I checked this explanation by pulling a plug out and looking at when there was a spark and it's true. Each spark plug fires twice in every (4 stroke) cycle. The order is actually 1,2,1,2 so the second spark for no 1 plug is either wasted at the top of the exhaust stroke or just tries to burn whatever is left of the fuel in the exhaust. At a max of 1,000rpm I guess the magneto and plugs are hardly likely to wear out in a hurry with this extra effort. The Simplex actual firing order helps create it's unique sound as against the Hardman twin which, as I understand it have both pistons at TDC at the same time but ignition opposed. |
jb_castagnos
Senior Member
Username: jb_castagnos
Post Number: 511
Registered: 07-2002
| | Posted on Tuesday, February 23, 2010 - 08:38 pm: |
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Usually a wasted spark is used when you have a coil with two high tension leads firing at the same time, one fires the compression stroke, the other fires on the exhhaust stroke and is wasted. Lawn mower engines fire every stroke for simplicity, wasting one of the sparks, also it allows the magnets to spin much faster at cranking speeds, makng a hotter spark for starting. If your mag doesn't have an impulse, it may be spinning at crankshaft speed to improve low speed performance. |
laurie_d
Member
Username: laurie_d
Post Number: 20
Registered: 10-2009
| | Posted on Tuesday, February 23, 2010 - 09:50 pm: |
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Thanks jb, The Simplex twins usualy have a Wico A series maggy which have an impulse fitted but, apart from any problems they may had locating a cap, with the terminals offset to match the offset of the two cylinders, I guess you might be right regarding the need for speed of the magneto. Until I realised this was the case, I had thought the magneto was driven at the same speed as the cam shaft. The motors (at least those with the old style gearbox)have some substantial gears driving the magneto. |
bruce_findlay
Advanced Member
Username: bruce_findlay
Post Number: 49
Registered: 03-2006
| | Posted on Tuesday, February 23, 2010 - 10:00 pm: |
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Thanks JB. The double spark is a mystery that I have been trying to fathom for ages, and you summed it up in just a short post. |
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