Advance and Idle

[mleder]

I would like to know why when I have my A running and it is at the slowest idle and the spark lever is at full retard (all the way to the top) there is little shake or vibration in the car. As I go down the quadrant to full advance the shake and vibration increases. The shake is not terrible but it does become more noticeable. Like most anyone I like it as smooth as possible and hate to see that I need to trade a little shake for performance. Do most of you find this same trait in your A? Is there something I can do to get it more steady? The increased shake is again comparing the spark lever position at the same idle speed but at both ends of the quadrants travel. Would like to get an explanation as to why this happens. Thanks all

[40 Deluxe]

How deep do you want to get into combustion theory of a gasoline engine? And, into the effects of ignition timing on combustion (flame speed, cylinder pressure, rate of pressure rise, etc.)? To me, it seems that over advancing the timing at idle causes erratic combustion thus the uneven firing pulses, and retarding the timing causes a loss in efficiency, thus the drop in RPM and the sound of 'lugging' or the engine struggling just to keep running. Will see what I can find on the web.

[MikeK]

Although you think of the engine idling at a constant speed, it is actually accelerating and decelerating with each cylinder firing. The engine wants to torque in the opposite direction it is trying to push the flywheel, and jump at a point directly above the particular cylinder that is firing. If you graph those acceleration pulses, you find that ignition advance make the line peaky like this ^v^v. Retarded timing gives smoother, weaker acceleration pulses like this ~~~~. If you reduce the mass of all the accelerated parts to zero (impossible) with no load you would get no vibration pulses like this - - - -, but adding load (driving) they would be severe | | | |.

To alleviate the problem in an I-4 engine you need a design with a purposely double-imbalanced counter shaft. It still will not be perfect.

[skip]

"I would like to know why when I have my A running and it is at the slowest idle and the spark lever is at full retard (all the way to the top) there is little shake or vibration in the car."

Well in the first scenario, slow idle no advance, smooth as glass. Your Not making any power when firing the mixture as the pistons come up to top dead center. Your firing the mixture late. The piston is already on the way down. "Your blowing the tree over after it has already fallen down." Barley getting the piston up over the top-O-center-stroke, and the job done. No shake.

Secondly with a closed throttle and advancing the timing one will detonate this 'no power condition' too soon, a shaky condition at best. Pushing the piston back down the bore while it's on it's way up the bore.

And last... in-line fours are inherently shaky, and with no power this kind of hides this shake.

Some [Jap] modern day cars have chain driven counter rotating shafts to quell this inherent shake. More cylinders can defeat this shake. My 12 cylinder Jaguar never shook. Could stand a coin on edge and not have it fall over when running.

[mleder]

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Thank you guys for the time spent explaining the answer to my question. I especially want to thank Skip for the simple way of getting the theory across.
Mark