What is the effect of changing the air-fuel ratio?
Traditionally, the greatest tendency to knock was near 13.5:1
air-fuel ratio, but was very
engine specific. Modern engines,
with engine management systems,
now have their maximum octane requirement near to 14.5:1. For a given
engine using gasoline, the relationship between thermal efficiency, air-fuel
ratio, and power is
complex. Stoichiometric combustion ( air-fuel
ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither
maximum power - which occurs
around air-fuel 12-13:1 (Rich), nor maximum thermal efficiency
- which occurs around air-fuel 16-18:1 (Lean). The air-fuel ratio is
controlled at part throttle by a closed loop system using the oxygen
in the exhaust. Conventionally, enrichment for maximum power air-fuel
ratio is used during full
throttle operation to reduce knocking while providing better
drivability. An average increase of 2 (R+M) /2 ON is required for
each 1.0 increase (leaning) of the air-fuel ratio. If the mixture is
weakened, the flame speed is reduced, consequently less heat is
converted to mechanical energy,
leaving heat in the cylinder walls and head, potentially
inducing knock. It is possible to weaken the mixture sufficiently
that the flame is still present when the inlet valve opens again,
resulting in backfiring.
Engine Management Systems are designed
to keep Oxygen from increasing power.
A modern system will automatically compensate for all of the
currently-permitted oxygenate levels, thus your
consumption will increase.
This article is from the Gasoline
by Bruce Hamilton with numerous contributions by others.