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Air / Fuel Ratio


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 sensor 
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 fuel consumption will increase.

This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.

Read more:http://stason.org/TULARC/vehicles/gasoline-faq/7-3-What-is-the-effect-of-changing-the-air-fuel-ratio.html#ixzz1hhblRLVw




Page Last Edited - 04/03/2022

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