What is the effect of changing the air-fuel ratio?

The tendency to knock increases as spark advance is increased. For an engine
with recommended 6 degrees BTDC ( Before Top Dead Centre ) timing and 93
octane fuel, retarding the spark 4 degrees lowers the octane requirement to
91, whereas advancing it 8 degrees requires 96 octane fuel. It should
be noted this requirement depends on engine design. If you advance the spark,
the flame front starts earlier, and the end gases start forming earlier in
the cycle, providing more time for the auto-igniting species to form before
the piston reaches the optimum position for power delivery, as determined by
the normal flame front propagation. It becomes a race between the flame front
and decomposition of the increasingly-squashed end gases. High octane fuels
produce end gases that take longer to auto-ignite, so the good flame front
reaches and consumes them properly.

The ignition advance map is partly determined by the fuel the engine is
intended to use. The timing of the spark is advanced sufficiently to ensure
that the fuel-air mixture burns in such a way that maximum pressure of the
burning charge is about 15-20 degree after TDC. Knock will occur before
this point, usually in the late compression - early power stroke period.
The engine management system uses ignition timing as one of the major
variables that is adjusted if knock is detected. If very low octane fuels
are used ( several octane numbers below the vehicle's requirement at optimal
settings ), both performance and fuel economy will decrease.

The actual Octane Number Requirement depends on the engine design, but for
some 1978 vehicles using standard fuels, the following (R+M)/2 Octane
Requirements were measured. "Standard" is the recommended ignition timing
for the engine, probably a few degrees BTDC.

Basic Ignition Timing

Vehicle      Retarded 5 degrees      Standard      Advanced 5 degrees

    A                       88                       91                        93
    B                       86                       90.5                      94.5
    C                       85.5                     88                        90
    D                       84                        87.5                     91
    E                       82.5                     87                        90

The actual ignition timing to achieve the maximum pressure from normal
combustion of gasoline will depend mainly on the speed of the engine and the
flame propagation rates in the engine. Knock increases the rate of the
pressure rise, thus superimposing additional pressure on the normal
combustion pressure rise. The knock actually rapidly resonates around the
chamber, creating a series of abnormal sharp spikes on the pressure diagram.
The normal flame speed is fairly consistent for most gasoline HCs, regardless
of octane rating, but the flame speed is affected by stoichiometry. Note that
the flame speeds in this FAQ are not the actual engine flame speeds. A 12:1
CR gasoline engine at 1500 rpm would have a flame speed of about 16.5 m/s,
and a similar hydrogen engine yields 48.3 m/s, but such engine flame speeds
are also very dependent on stoichiometry.

This article is from theGasoline FAQ, by Bruce Hamilton with numerous contributions by others

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