Timing

Hi Chris,

Actually ignition timing generally is a function of engine RPM, where spark advance increases as RPM increases. This is because it takes a finite amount of time for the spark to ignite the fuel/air mixture. As the pistons move up & down faster, the spark must ignite sooner so that an explosion happens as the piston reaches top-dead-center. Prius basically operates the same way.

Knocking happens when the explosion happens before the piston reaches TDC and shows timing is too advanced. If timing is retarded then power output is reduced since the explosion happens as the piston already is descending from TDC.

 

I wonder how variable valve timing plays into that?

 

Please see the first document which is an excerpt from the 2001 New Car Features Manual. The intake valve side is variable, and the last two pages of the excerpt show the conditions under which timing is retarded and advanced.

2G uses a similar concept although the amount of variability was reduced (see the second document). This reduction means less exhaust gas residue that will accumulate in the throttle body interior and on the throttle plate.

 

Hi Chris,

You are right about the vacuum advance portion of the distributor mechanism; but recall that the distributor also has a centrifugal advance which will advance timing based upon the speed at which the distributor shaft is rotating. And the distributor shaft speed is a direct function of engine RPM.

This mechanical/vacuum system did not work very precisely. Hence the invention of computerized engine controls to specify correct ignition timing based upon engine RPM and load.