Combustion control of engines has long been used to achieve various purposes. The three most notable purposes are to maximize power output, to maximize fuel economy, and to minimize emissions. In particular, emissions control has become a major factor in the development of engine controls.
Various conventional techniques exist for controlling the emissions of an engine. Most techniques deal in some form of controlling the amount of fuel, the amount of air or other combustible gas, and the timing of the ignition. These techniques vary significantly in the precision with which they are able to control emissions to a desired level. U.S. Pat. No. 4,867,127 to Quirchmayr, et. al. discloses regulating the combustion mixture pressure to achieve a desired NOx level. While this method may present some advantages over past strategies, further improvements are still sought by the industry.