Fuel-Air Ratio Determination From Cylinder Pressure Time Histories

Abstract

Determining fuel-air ratio quickly over a wide range of engine operating conditions is desirable for better transient engine control. This paper describes a method based on cylinder pressure time history pattern recognition which has potential for providing such a high bandwidth measurement. The fact that fuel-air ratio has an effect on the shape of the cylinder pressure trace is well-known. It should therefore be possible to obtain the fuel-air ratio of an engine by examining the pressure trace if the engine speed, load, and EGR are known. The difficulty lies in separating the effects of unknown engine load, speed, and EGR from the fuel-air ratio effects. An algorithm was developed using a wide range of steady state experimental data from a single cylinder engine. Application of the algorithm requires the calculation of first, second and third moments of the cylinder pressure time history. Verification of the algorithm showed that the root mean square error in estimates were about 5 percent for fuel-air ratio and 3 percent for a combination of fuel-air and EGR. These results were obtained using a single pressure trace which yields a response time of 1.5 engine revolutions. The algorithm was also found to be relatively insensitive to the use of different fuels, errors in spark advance, and variations in relative humidity. Research is continuing to verify the accuracy under transient engine conditions. An operational count shows that this algorithm should be well within the limits of present microprocessor technology.