ignition retard
ignition retard
My antique bike has a points plate with fixed timing, no weights or springs like a modern breaker. You set the timing to 35 degrees advance (by viewing a mark on the flywheel) and leave it there when riding, it fires there -- at all rpm. I have already replaced the points with hall effect sensors (one for each cylinder), magnets and shutter disc, with igbts to run the coil, in a setup of my own design. Now I would like to create an advance curve electronically. With the breaker set to full advance as per usual, the circuit has to delay the spark (in practical terms this will likely involve simply extending dwell time) a varying amount depending on engine speed, more delay at low rpm, with the delay dropping out entirely at 2000 rpm so that the ignition goes to full advance. So you could say the circuit will actually have to create a retard curve.<p>I have built astable and monostable multivibrators of discrete components and used 555 timers. I have enough experience as a ham, and knowledge as an engineering school dropout, to build something, but could use a little jump-start with the design, even if only conceptual. Suggested topologies, helpful web site links and such are much appreciated.
Re: ignition retard
Nice project !
I would say a pulse position modulator should work. I have seen that application with a 555 and should be on its application notes.
The voltage to control the pulse position may come from an integrator, varying with rpm pulses taken from a sensor at any rotating shaft in the motorcycle.
I hope it helps somehow.
Miguel
I would say a pulse position modulator should work. I have seen that application with a 555 and should be on its application notes.
The voltage to control the pulse position may come from an integrator, varying with rpm pulses taken from a sensor at any rotating shaft in the motorcycle.
I hope it helps somehow.
Miguel
- Abolish the deciBel ! -
Re: ignition retard
Timing advance is based on the duration of the gasoline explosion. The accepted time is about .003 seconds. The crank rotates through more degrees in .003 sec's @ 5400RPM than at 1200RPM. The problem lies in the ADVANCE, not that it's 'retarded'. So you have to fire the plug .003 SECONDS before TDC, not "X-number of degrees". Actually, if you relate degrees-before-TDC to seconds, real engines advance about .001 second, and the fuel burns during the rod's swing through the TDC arc, when the piston is basically motionless.
I wanted to do your idea to a Briggs engine. The problem works this way; If you actually RETARD spark as referenced to a prior cycle, it retards MORE as RPM goes up (less ADV.). The only solution I arrived at was; Use a TDC sensor. Count 'ticks' of a 1Mhz clock to the next TDC event, & store in a buffer. At steady RPM, if you count the same number of ticks & fire, it'll be at TDC. But if you START counting at 1,000 ticks, you'll arrive at the count from the prior cycle about .001 second sooner (advanced). When the count matches the count from the prior cycle, the plug fires. 1,000 ticks of a 1Mhz clock yields 7-degrees @ 1200RPM, and 32-degrees @ 5400RPM. 2,000 ticks yields 14-degrees @ 1200RPM, and 32-degrees at 2700RPM (and 64-degrees @ 5400 !!). Lo-tech would be clock/counter/comparator with an 8-pos DIP switch to 'dial-in' the desired start-count. Hi-tech , use a PIC. A Basic stamp or 8051 will allow an actual 'curve' based on a more advanced equation.
Pitfalls ? Remember , 4Mhz requires a different clock count for .001 sec. And, it must reset with every cycle of TDC, or it'll be cumulative ( 7,14,21,28,35,42,49-degrees, it'll never start!). Electrical noise and supply variance can cause misfires (or stalls).
If anyone devises a working prototype , alot of go-kart and small-engine modifiers would be interested.
A best-case can use a GM MAP sensor for vacuum/load sensing, and vary the advance count to retard while accelerating.
Any additive/corrective input ????
Mr No_Vice.
I wanted to do your idea to a Briggs engine. The problem works this way; If you actually RETARD spark as referenced to a prior cycle, it retards MORE as RPM goes up (less ADV.). The only solution I arrived at was; Use a TDC sensor. Count 'ticks' of a 1Mhz clock to the next TDC event, & store in a buffer. At steady RPM, if you count the same number of ticks & fire, it'll be at TDC. But if you START counting at 1,000 ticks, you'll arrive at the count from the prior cycle about .001 second sooner (advanced). When the count matches the count from the prior cycle, the plug fires. 1,000 ticks of a 1Mhz clock yields 7-degrees @ 1200RPM, and 32-degrees @ 5400RPM. 2,000 ticks yields 14-degrees @ 1200RPM, and 32-degrees at 2700RPM (and 64-degrees @ 5400 !!). Lo-tech would be clock/counter/comparator with an 8-pos DIP switch to 'dial-in' the desired start-count. Hi-tech , use a PIC. A Basic stamp or 8051 will allow an actual 'curve' based on a more advanced equation.
Pitfalls ? Remember , 4Mhz requires a different clock count for .001 sec. And, it must reset with every cycle of TDC, or it'll be cumulative ( 7,14,21,28,35,42,49-degrees, it'll never start!). Electrical noise and supply variance can cause misfires (or stalls).
If anyone devises a working prototype , alot of go-kart and small-engine modifiers would be interested.
A best-case can use a GM MAP sensor for vacuum/load sensing, and vary the advance count to retard while accelerating.
Any additive/corrective input ????
Mr No_Vice.
Can't we end all posts with a comical quip?
Re: ignition retard
Hi<p>A very interesting read guys. You should chat about this type of jargon more often. I regually read the below link because im a petrol/ motorbike junky. I have taken you to a really good tech page which also goes on about ignition timming etc, and also there is lots of links on this site. If you have no joy, put a post on there message board, im sure somebody will come back with some help. You all seem very clever in america.<p>Hope Colin Edwards kick arse in spain the weekend.<p>cheer guys, and i hope my link helps.<p>http://www.fatbaq.com/mainpage.phtml?topic=tech
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