[스크랩] 점화시기에 영향을 주는 요소들

Lets look at a bunch of factors that affect timing requirements
Factor	Cylinder filling	Flame speed	Burn time	Timing effect
bore/stroke ratio	-	You wouldn't have thought so, but this has an effect on rate of compression and piston dwell time at TDC	-	a long stroke/small bore requires as much as 10° more advance than does an equal displacement engine with short stroke/big bore
Camshaft with more duration	improved at higher speed, worse at low speed	-	-	less advance at high speed, more at low speed
combustion chamber shape	-	-	compact designs take less time to burn to farthest reaches	Minimized for compact designs, maximized for in-piston chambers
fuel atomization	-	liquid fuel does not burn	fuel globules have to be atomized by combustion in other areas before they will burn	Minimized when atomization is complete - atomization also improves with speed
improved exhaust efficiency or lowered backpressure	-	the presence of residual exhaust gas in the cylinder retards the flame front	-	less advance required when exhaust extraction effect is working, ie, higher up in the rev range
improved induction efficiency	The engine has an easier time getting a 'lung' full	faster with improved VE	-	less advance as VE improves
increased bore size	may increase valve shrouding and lower VE	-	more distance from plug to far side of cylinder	more advance required
increased compression ratio	-	a higher CR results in faster burning	-	less advance required for increased CR
mixture swirl in combustion chamber	-	-	a well mixed cylinder charge will burn uniformly	minimized with good swirl. Westlake type heads have heart shaped chambers to improve swirl
piston shape	-	-	pistons which force the charge into a confined space limit the distance the flame front has to travel	minimized for squish pistons, maximum for dished pistons
spark plug position in head	-	-	varies with distance from plug to farthest point in the cylinder, ideally centered in cylinder	minimum when centered, more advance as it moves to the farthest corner of the cylinder
air/fuel ratio	-	anything weaker or richer than the ideal 14.7:1 will burn slower	-	maximized at stoicheometric
coolant temperature	-	increased engine temperature affects final charge temperature	-	less advance required as engine temperature increases
fuel octane rating	-	octane slows burn rate	-	more advance with increased octane, or, more importantly, less risk of over advancing without changing advance
heat transfer rate	-	the higher rate at which the cylinder head can get rid of the combustion heat, the lower the final charge temperature will be	-	less advance needed for poor cooling iron heads, slightly more for efficient turbo heads, even more for aluminum
induction air temperature	Colder charge means denser air and higher VE, warmer charge, lower VE	higher temperature charges burn faster	-	less timing with increased temperature
engine speed	VE increases with speed	-	-	as VE increases, advance decreases, but because flame speed is finite, it must be initiated much earlier with increasing speed
Ambient moisture conditions	-	moisture in the cylinder charge will slow the flame speed	-	more advance required under humid conditions

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