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Of note: Advancing the spark timing of an SI engine isn't the actual goal, it's a band-aid for technical design drawbacks. (It wasn't improperly designed, but having been mass produced, its necessary to make a few concessions, in other words). Ultimately, with an SI engine, there should be no pressure build up to and including TDC, and the complete burn should begin, happen, and end through the combustion stroke, with maximum expansion occuring at or near the point where piston acceleration is the fastest, I believe approximately 30* after TDC (probably wrong, can't remember exactly anymore). This is to take advantage of the mechanical advantage which can be gained via the reciprocation/rotation relationship. At some point, the piston begins applying more leveraged force to the crankshaft. At this same point, the amount of energy in the combustion gasses that is being spent is being multiplied by that leverage factor against the crankshaft, thus producing more usable force than is being applied to the face of the piston. For this reason, among others, the maximum expansion of the combustion gasses should occur during the range of peak leverage against the crankshaft to provide maximum usable torque with the least spent fuel.
Advancing the ignition timing makes up for load changes and situational changes as necessary, but does rob some efficiency because for a time, the engine is fighting expansion of spent/burning gasses while trying to further compress the mixture.
The same principles apply to a diesel engine, in part. The gains that were seen by increasing the Cetane rating of the fuel could probably also have been seen by mechanically advancing the fuel racks on the test group to some degree. The added benefit of having done so would be increased power output for a given load/speed without increased fuel use.
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Last edited by Christ; 06-13-2010 at 12:46 AM..
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