[psalt]

On the CIS engines, I thought the mixture burned too fast and backfired into the intake before the valve closed. ????


Hello Ed,

On a 4 cycle SI engine, the spark plug does not fire until after the intake valve closes at the beginning of the compression stroke. How does that fit with your suggestion ?

If you want to clear up your misunderstanding, I suggest a basic book on combustion science or BOSCH's "Gasoline Engine Management". Like I said, most absolute statements about SI engine are false, because conditions vary so much the answer is usually not a single point or linear relationship, but a peak with contradicting curves on either side. For example, the generalization that "leaner mixtures are hotter" by itself is false. A lean mixture by definition is above an AFR of 14.7:1. EGT actually peaks around stoich and falls off as the mixture gets leaner. One reason pilots are trained to use "rich of peak" and "lean of peak" fueling. "Leaner is hotter" is only true rich of stoich.

If you want to understand fuel burn rates, there is a good chart on page 127 of Jeff Hartman's EMS book. Almost any basic text on fuel and ignition timing will state that a rich mixture burns faster than a lean mixture. Flame speed can vary from 20 to more 100 ft/sec. Burn rate is highest at a rich mixture near 11.1:1 AFR and drops as the mixture leans. It also drops as the mixture gets richer than 11:1, but in a N/A SI engine, you are generally above this outside a subzero cold start. Understanding an ignition timing map can also help. The spark plug is fired to create peak cylinder pressure around 14 degrees ATDC to maximize work done. The burn rate varies with mixture, but is relatively constant for a fixed mixture. Obviously, as rpm increases, you need to fire the plug earlier, because the window of time to accomodate the burn speed gets shorter. Lets say you determine that 30 BTDC give best torque under WOT. Now, at part throttle, low load cruise, the same ignition timing map will give another 10-15 degrees of timing advance. Why ? Because the leaner, less dense mixture supplied under light load takes longer to burn and if the ignition point was not advanced, more of the energy in the fuel would be wasted in the exhaust. If a lean mixture burned quicker than the rich mixture supplied at WOT, the ignition timing would not need to be advanced.