Method and apparatus for determining optimum skip fire firing profile

A method of selecting an operational skip fire firing fraction suitable for use in operating an internal combustion engine in a skip fire manner to produce a desired engine output, the method comprising, during operation of the internal combustion engine:

• determining the desired engine output;
• calculating a candidate cylinder load for each of a plurality of candidate firing fractions that are each capable of delivering the desired engine output, wherein each candidate cylinder load represents a cylinder torque fraction at which an associated cylinder would need to operate at an associated candidate firing fraction of the plurality of candidate firing fractions in order to deliver the desired engine output;
• for each of the candidate firing fractions, determining whether the calculated candidate cylinder load exceeds a maximum allowable cylinder load associated with such candidate firing fraction under selected current engine operating conditions, wherein the maximum allowable cylinder load indicates a maximum allowed cylinder torque fraction when the internal combustion engine is operating at the associated candidate firing fraction under specified operating conditions, and wherein the maximum allowable cylinder load for at least some of the candidate firing fractions at some specified operating conditions is a cylinder torque fraction that is less than one;
• eliminating one or more of the candidate firing fractions for which the associated candidate cylinder load exceeds the maximum allowable cylinder load under the selected current engine operating conditions, and
• after the eliminating step, selecting one of the candidate firing fractions that has not been eliminated as the operational skip fire firing fraction;
• and
• operating the internal combustion engine in the skip fire manner using the selected operational skip fire firing fraction, wherein at least some of the time, the selected operational skip fire firing fraction has an associated maximum allowable cylinder load that corresponds to a cylinder torque fraction that is less than one when operated to deliver the desired engine output under the selected current engine operating conditions.