Ethanol content learning based on engine roughness

摘要

A method according to the present invention controls engine roughness by varying the ethanol content in a fuel mixture of a flexible fuel vehicle. Initially, a trigger threshold and fix threshold are established based on engine speed and manifold pressure. The fix threshold is less than the trigger threshold. A first ethanol amount is delivered to the fuel mixture if a first measured roughness is greater than the trigger threshold. A second measured engine roughness is determined after delivering the first ethanol amount. The delivery of the first ethanol amount is stopped if the second measured roughness is greater than the trigger threshold. A second ethanol amount is delivered if the second measured roughness is less than the trigger threshold but greater than the fix threshold. A third measured roughness is then determined. Delivery of the second ethanol amount is stopped if the third measured roughness is greater than the trigger threshold. The roughness is measured and ethanol delivery modified accordingly so long as the third measured roughness is less than the trigger threshold and greater than the fix threshold.