Dec

21

What is meant by the stoichiometric ratio in relation to gasoline engines?

Filed Under Maintenance & Repairs 

gasoline
mathiusdragoon asked:

As far as I know, gasoline consists of a blend of several different hydrocarbons which are extracted from raw petroleum. In chemistry the stoichiometric ratio would refer to the the number of mols of each reactant required to make the reaction a balanced equation (or else it would violate the law of conservation of matter) That being said different hydrocarbons would have different stochiometric ratios with the atmospheric oxygen when they combust. So my question is this: How can their be a single value which is considered ’stoichiometric’ for gasoline?

RUSS

Comments

2 Responses to “What is meant by the stoichiometric ratio in relation to gasoline engines?”

  1. Mr. KnowItAll on December 22nd, 2009 12:56 am

    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.

  2. Non Anon on December 22nd, 2009 4:17 am

    Short answer:

    The value varies based on things such as temperature of the incoming air, rpm versus load, etc.

    While 14.7:1 is ideal for an ‘ideal’ situation, cars frequently shoot for a different ratio depending on conditions. The ratio itself can and does get lost sometimes in the logic. The more the throttle is held in a certain position, the closer and more likely the car is to run at 14.7:1.