Not to jump in... but I'm seeing conflicting comments (unless I'm mis-reading this dialogue) wherein AFR value relative RICH/LEAN are not consistently understood.
Stoichiometric air-fuel ratios of common fuels
Fuel By mass By volume [1] Percent fuel by mass
Gasoline 14.7 : 1 — 6.8%
Natural gas 17.2 : 1 9.7 : 1 5.8%
Propane (LP) 15.5 : 1 23.9 : 1 6.45%
Ethanol 9 : 1 — 11.1%
Methanol 6.4 : 1 — 15.6%
Hydrogen 34 : 1 2.39 : 1 2.9%
Diesel 1 4.6 : 1 — 6.8%
Taken from Wiki...
Air-fuel ratio (AFR) is the mass ratio of air to fuel present during combustion. When all the fuel is combined with all the free oxygen, typically within a vehicle's combustion chamber, the mixture is chemically balanced and this AFR is called the stoichiometric mixture (often abbreviated to stoich). AFR is an important measure for anti-pollution and performance tuning reasons. Lambda (λ
is an alternative way to represent AFR.
In industrial fired heaters, power plant steam generators, and large gas-fired turbines, the more common term is percent excess combustion air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used.
A mixture is the working point that modern engine management systems employing fuel injection attempt to achieve in light load cruise situations. For gasoline fuel, the stoichiometric air/fuel mixture is approximately 14.7 times the mass of air to fuel. Any mixture less than 14.7 to 1 is considered to be a rich mixture, any more than 14.7 to 1 is a lean mixture - given perfect (ideal) "test" fuel (gasoline consisting of solely n-heptane and iso-octane). In reality, most fuels consist of a combination of heptane, octane, a handful of other alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol. These compounds all alter the stoichiometric ratio, with most of the additives pushing the ratio downward (oxygenators bring extra oxygen to the combustion event in liquid form that is released at time of combustions; for MTBE-laden fuel, a stoichiometric ratio can be as low as 14.1:1). Vehicles using an oxygen sensor(s) or other feedback-loop to control fuel to air ratios (usually by controlling fuel volume) will usually compensate automatically for this change in the fuel's stoichiometric rate by measuring the exhaust gas composition, while vehicles without such controls (such as most motorcycles until recently , and cars predating the mid-1980s) may have difficulties running certain boutique blends of fuels (esp. winter fuels used in some areas) and may need to be rejetted (or otherwise have the fueling ratios altered) to compensate for special boutique fuel mixes. Vehicles using oxygen sensors enable the air-fuel ratio to be monitored by means of an air fuel ratio meter.
Lean mixtures produce hotter combustion gases than does a stoichiometric mixture, so much so that pistons can melt as a result. Rich mixtures produces cooler combustion gases than does a stoichiometric mixture, primarily due to the excessive amount of carbon which oxidises to form carbon monoxide, rather than carbon dioxide. The chemical reaction oxidizing carbon to form carbon monoxide releases significantly less heat than the similar reaction to form carbon dioxide. (Carbon monoxide retains significant potential chemical energy. It is itself a fuel whereas carbon dioxide is not.) Lean mixtures, when consumed in an internal combustion engine, produce less power than does the stoichiometric mixture. Similarly, rich mixtures return poorer fuel efficiency than the stoichiometric mixture. (The mixture for the best fuel efficiency is slightly different from the stoichiometric mixture.)
In other words:
14.7:1 = Stoichiometric
<14.7:1 = RICH
>14.7:1 = LEAN
NOTE: Given "ideal" conditions
PS: That's my "layperson understanding" coming from a few "Google" hits. ALSO, just because it is "STOICH" doesn't mean that's where YOU want to be under ALL conditions. Understanding the "pros/cons" of varying rations under different conditions, i.e.: BOOST level, load, etc... can help you be successful as you apply the "feedback loop" an CONTROL the variables to maintain a DESIRED ratio accordingly.
-crisp
