An On-Line Model for Predicting Residual Gas Fraction by Measuring Intake/Exhaust and Cylinder Pressure in CAI Engine

CAI (Controlled Auto Ignition) combustion is already well known to be advantageous over conventional cycles in that it facilitates higher engine efficiency and has low emission characteristics. The CAI combustion process is mainly governed by in-cylinder RGF (Residual Gas Fraction), therefore achieving good control of in-cylinder RGF is essential in the development of CAI combustion engine. Usually, in-cylinder RGF controlled via low lift cam, short valve duration and negative valve overlap. More importantly on the other hand, accurate and instantaneous prediction of RGF must be done as a prerequisite to control. However, on-line prediction of RGF is not always practical due to the requirement of expensive fast response exhaust gas analyzers in the empirical case or otherwise due to theoretical models which are just too slow for application by means of simulation solving. In this paper, a newly enhanced theoretical model for predicting on-line in-cylinder RGF is introduced. This enhanced model predicts an RGF value in less than five seconds for every engine cycle. A Fortran code for predicting RGF in a single cylinder CAI engine was developed using MOC (Method of Characteristics). The code takes in measured dynamic pressures of intake, exhaust and in-cylinder gases to generate a corresponding RGF figure in a few seconds. The code was then inserted into Labview DAQ system as a form of DLL (Dynamic Link Library) to make on-line prediction possible. This model has been verified in various speeds and valve timing conditions of CAI mode by comparing results with those of the commercial code GT-Power.