Understanding Timing Principles

ericr

ericr

Well-known member
I did a search for this but could not find an answer.

if the combustion occurs at top dead center with the spark fully retarded, which is the position that gives the most power, why would there ever be a spark adjustment that causes the explosion to occur before the piston hits the top of its travel? doesn't that reduce the amount of power generated?

I suspect someone will comment that this is some relationship between electrical sparking, which is instantaneous, versus mechanical travel, which is slower; but that still doesn't explain it well to me.
 
That's how all most all engines run. The gas exploding/power is not as instantaneous as the spark. As the RPMs increase, you need to have the spark and the start of the combustion of fuel before the piston travels over top dead center so the full expansion/power of the exploding fuel is on the down stroke.
 
ericr said:
if the combustion occurs at top dead center with the spark fully retarded, which is the position that gives the most power,
Click to expand...

Where did you get the info that max power occurs with timing at zero degrees advance? That's where it's easier to start the engine (especially if starting with a hand crank or kick start) but certainly not where max power is developed at higher RPM.

As already mentioned, if you fire the plug at TDC, the piston is already on it's way down the bore when the pressure from combustion is building. Normal combustion is not an explosion, it's not instantaneous, there is a flame front. Instant would be detonation, which is bad.

Normal combustion takes time. Start it at TDC and you miss the party. Also keep in mind that as the piston nears TDC it is slowing down, so it isn't moving as much per degree of crank rotation as it does mid stroke so pressure from combustion isn't fighting mechanical compression as much as you might imagine.
 
Embellishing on the answers above, it takes time for ignition to occur, travel of the flame front across the combustion chamber, combustion and a rise in pressure into the thousands of PSI.

Thus as the engine's crankshaft speed increases from idle, the ignition spark must advance before any piston reaches TDC so the maximum pressure occurs at or just past TDC, and on the downward stroke of the piston.

At idle, there is sufficient time for combustion pressure to reach its maximum a few degrees after TDC because the is piston is relatively slow moving.

By the way, a piston instantaneously stops at TDC, accelerates to a maximum speed at 1/2 of its downward stroke, then decelerates to a stop again at BDC. This is due to the natural reciprocating motion of a crank.
 
see article from #5
Max cylinder pressure has been determined to be needed at 14° ATDC for any motor
the crank pin has to be over center
 
I could be wrong but I think the video is made by a ford Barner Jackson!!!,he is a great guy and a really smart model a mechanic
 
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