An Experimental Characterization of a High Degree of Freedom Spark-Ignition Engine to Achieve Optimized Ignition Timing Control.

Abstract

Pressure to improve fuel economy and emissions allows for the introduction of more complex and expensive spark-ignition engine technologies. As engine complexity increases, traditional ignition timing control methods become restrictive, creating a need for new approaches based on analytical techniques and experimental insight. The addition of variable valve actuation and other intake charge motion altering devices provides unprecedented opportunities for improving engine attributes, but poses significant challenges for developing robust control systems. In particular, internal residual fraction and turbulence level vary over a much broader range than in a traditional engine and have a critical impact on combustion. Hence, the goals of this thesis are two-fold. First, new diagnostic procedures that experimentally characterize key combustion parameters are developed. Then, the new information is used to create a universal physics-based ignition timing prediction model valid over a wide range of residual and in-cylinder turbulence levels. Residual gas fraction is experimentally quantified using several different methods that incorporate fast response emissions analyzers, such as the Fast FID analyzer for unburned hydrocarbons, and a fast NDIR analyzer for CO2. A technique relying on simultaneous measurement of in-cylinder and exhaust CO2 concentration is demonstrated, and proves to be the most accurate and reliable. Turbulence intensity is quantified using a newly developed inverse-model of turbulent flame entrainment in conjunction with experimental combustion diagnostics. Experimental findings are subsequently used to generate semi-empirical models for residual fraction and turbulence intensity capable of running real-time within an engine controller. The newly developed experimental techniques and semi-empirical models enable the development of a physics-based ignition timing control model. The proposed algorithm is loosely based on a well-established turbulent entrainment combustion model, ensuring robust and universal application. The model is divided into two sub-sections; one to predict combustion duration and another for combustion phasing. The duration sub-model predicts the time from ignition to fifty percent mass fraction burned for each operating condition, using an estimated flame entrainment rate, with an RMSE of 2.3 CAD. The combustion phasing sub-model is then used to determine required ignition timing, based on a desired location of fifty percent mass fraction burned.