1. Technical Field
The present invention relates generally to fuel compositions for a motor vehicle, and more particularly to a method for determining the percent alcohol content of fuel used in a motor vehicle capable of burning more than one type of fuel in its internal combustion engine or other power plant.
2. Discussion
Stoichiometric is defined as the chemically correct proportion of fuel and oxygen for the conversion of all of the fuel available for combustion into oxidized products. The value of the stoichiometric fuel to air ratio level depends on the fuel's composition. Basic gasoline fuels comprise hydrocarbons in varying degrees with the accepted value of stoichiometric fuel to air ratio for gasoline being 0.0688:1. When the fuel contains oxygen, such as those with alcohols, the oxygen accompanying the hydrocarbon-based fuel must be compensated for in the fuel management system or a lean-of-stoichiometric combustion will be encountered.
An increase in fuel delivery rates, indicated by an increase of fuel injector pulse width, is required to maintain the desired fuel to air ratios with alcohol or alcohol-gasoline fuel mixtures. Currently M85 fuel, which is 85% methanol and 15% gasoline, and E85 fuel, which is 85% ethanol and 15% gasoline, are in use or are being considered for use in passenger vehicles. The approximate fuel delivery rate increase required for M85 and E85 are +93% and +49% respectively when compared to a gasoline-based system. Dedicated M85 or E85 vehicles have been proposed as well as "Flexible Fuel Vehicles" (FFVs) that will operate on any mixture of gasoline and 0%-85% alcohol. This varied fuel composition needs to be sensed and reflected when selecting the fuel delivery rate.
Sensing the percent alcohol in the fuel has been accomplished with a methanol concentration FFV sensor described in U.S. Pat. No. 5,119,671, which is hereby expressly incorporated by reference in its entirety. However, introduction of ethanol fuel into such a system based upon M85 may not provide an accurate assessment of the alcohol content of the fuel. An FFV sensor utilizes the dielectric properties of the fuel to determine fuel composition. This property is unique for gasoline, methanol and ethanol. As this effect is a function of various alcohols, and the alcohol's temperature, the output of an FFV sensor may be affected. Therefore, introduction of ethanol/gasoline fuel mixtures into a system calibrated for methanol/gasoline mixtures may corrupt the accurate assessment of alcohol content.
Furthermore, a methanol blend with gasoline will carry water in a thoroughly mixed solution rather than as a separated liquid as is the case with water and gasoline. The dielectric property of water is very close to that of methanol and may affect the output of an FFV sensor. It is thus possible that the control system could sense M85 while the actual fuel composition could be a mixture of 75% methanol, 15% gasoline and 10% water. Therefore, pure dielectric sensing cannot be regarded as 100% accurate.
Even with the alcohol content known, the use of exhaust gas feedback information may be a more precise method to maintain stoichiometric fuel to air ratios. A system to determine alcohol percent utilizing a standard oxygen sensor, which only gives a rich or lean fuel to air a ratio signal, has been developed and is described in U.S. Pat. No. 5,400,762, "Method for Determining Fuel Composition" which in its entirety is hereby expressly incorporated by reference. This method checks boundary conditions of the short term oxygen sensor feedback signal and adjusts the percent alcohol content stored in the engine controller unit memory without the use of an FFV sensor or ethanol sensor. Other related inventions are described in U.S. Pat. No. 5,335,637, "Energy Adjust for a Flexible Fuel Compensation System," and U.S. Pat. No. 4,821,701, "Purge Corruption Detection", each in its entirety is hereby expressly incorporated by reference.