Stoich (short for Stoichiometric) combustion condition is defined as the ratio of air-to-fuel (by weight) which will provide enough oxygen for the complete combustion of all fuel. Different fuels require different amounts of air for this condition. Also, automobile computers control the fuel flow through the fuel injectors so that Stoich conditions are met in an engine’s combustion chambers. Stoich air-to-fuel ratio for real gasoline = 14.7 lbs of air to l lb of gasoline 1 / 14.7 = 0.0680 lbs of fuel for each lb of air through an engine Stoich air-to-fuel ratio for ethanol = 9 lbs of air to l lb of ethanol 1 / 9 = 0.1111 lbs of fuel for each lb of air through an engine Stoich air-to-fuel ratio for 10% ethanol blended gasoline: (.9 x 14.7) + (.1 x 9) = 13.23 + .9 = 14.13 1 / 14.13 = 0.0708 lbs of fuel for each lb of air through and engine The ecm receives information from the O2 sensor and adjusts the injectors for Stoich conditions no matter which fuel is used. More 10% ethanol blended fuel is consumed: 0.0708 - 0.0680 = 0.0028 lbs / lb of air Therefore, compared to real gasoline, 10% blended fuel is used at a higher percentage through an engine: 0.0028 / 0.0680 = 4.1% more of the blended fuel will be consumed by an engine operating at Stoich conditions. Finally, if real gasoline is $3.50 / gallon, the 10% blended fuel must be: .041 x $3.50 = 14.35 cents / gallon less expensive in order to overcome the expected decrease in fuel mileage from the 10 % blended fuel
Chips programmed for real gasoline (Stoich 14.7) will cause the ecm to add fuel when using 10% bended gasoline (Stoich 14.13). Therefore, BLMs will shift to higher values because the O2 sensor signals the ecm of a leaner fuel mixture. For instance, a 1 BLM point change represents a 0.833% change in fueling commanded by the ecm. Therefore, a BLM cell which was set at 128 with real gasoline will indicate 133 when using 10% blended fuel (i.e. 5 BLM points x 0.833% = 4.1%) and 4.1% is the quantity of extra 10% blended fuel required for Stoich combustion. The BLMs can be biased to return to 128 by increasing fuel pressure at the regulator or by making a BLM offset adjustment if the chip (i.e. Extender) allows.
By Leon Boelte