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In automotive terms, Stoichiometric (or Stoichiometric Combustion) refers to a [condition] you will never truly see - The condition of having exactly the right quantities of [fuel] and [air] for both to be consumed completely. This is the point at which your [engine] will reach its [peak] [efficiency]. Having less fuel than this [ratio] is known as running [lean]; Having more fuel is running [rich]. In this state, all the [carbon] (C) is converted to [carbon dioxide] (CO2), all [hydrogen] (H) to [water] (H2O), and all [sulfur] (S) to (SO2).

However, the process is complicated by the [dispersion] of fuel in the [combustion chamber]. The fuel is not distributed or [atomized] evenly. Generally speaking, it is necessary to run [rich] to avoid [premature] [detonation], though there are technologies to avoid this necessity in the name of greater fuel [economy]. The best known (and perhaps earliest) example is [Honda]'s [CVCC], or [Controlled Vortex Combustion Chamber], in which the fuel density is highest near the spark plug. This allows the richer part of the mixture to touch off the leaner part of the mixture. Another well-known design for efficiency is the so-called "[Hemi]" (or [hemispherical]) cylinder head design, which positions the spark in the best location to get an even burn of the fuel-air mixture.

Your car's engine control unit ([ECU], also called the [PCM] or powertrain control module) watches the [oxygen sensor] output voltage (in the case of most sensors) to see if your air-fuel mixture is [rich] or [lean] and then adjust fuel delivery accordingly. The most common oxygen sensor, the zirconium type, will put out a voltage near 450mV when gasoline is burnt at the stoichiometric ratio of 14.2:1 and complete combustion occurs.

References:

1. Stoichiometric Combustion. [Taftan Data], 1998. (http://www.taftan.com/thermodynamics/COMBUST.HTM)