Thread: Mileage
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Steve W Steve W is offline
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Join Date: Nov 2001
Location: PV Estates, CA
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I'm far from an expert in gasoline, but oxygenated fuels do have a significant effect on fuel consumption and it's effect on air fuel ratios. Two common oxyenates are ethanol and MTBE(derived partly from methanol), which can be mixed with gasoline at a 10 and 15 percent rate respectively. Ethanol with an octane of 129 and MTBEs with a rating of 118 are mixed in with low octane gasoline to bring up the octane level of the fuel to the required level. However alchohos have one drawback in that they only have half of the energy output per cc compared to gasoline. This means for alcohol to produce the same amount of energy and maintain the same air fuel ratio of gasoline, twice the volume of alchohol must be injected. If 10% ethanol has displaced gasoline at your pump, the gasoline you are now pumping is only 95% of straight gas, which means your car will now run leaner at full and part throttle and deliver less power, or if your fuel system is adaptive or in the case with closed loop fuel systems, will get poorer fuel efficiency to make up for the reduced density.

When driving under part throttle load conditions, the O2 sensor constantly tries to maintain the stoichiometric ratio of 14.7:1, and sensing the leaner fuel ratio of the oxygenated fuel, will try to constantly richen the fuel mixture, resulting in increased fuel consumption. At full throttle though, the O2 sensor is ignored and a preset/preprogrammed fuel curve is used. But now the fuel system has no idea it is using an oxygenated fuel and not being adaptive here, will actually cause the engine to run leaner at full throttle than it should. For example if your full throttle AFR is tuned/programmed to with an afr of 13.0:1, using an oxygenated fuel with 10% ethanol, it will now lean out to 13.67:1, resulting in reduced power and a higher propensity to predetonate.
Old 01-02-2004, 10:21 AM
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