In automotive terms, STOICHOIMETRIC (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.
- Stoichiometric Combustion. Taftan Data, 1998. (http://www.taftan.com/thermodynamics/COMBUST.HTM)