Modeling and Control of Exhaust Recompression HCCI Using Variable Valve Actuation and Fuel Injection

Abstract

With the growing focus on energy and environmental issues in the world today, significant efforts are being made in the automotive industry towards the development of sustainable and clean technologies that can power automobiles. One of the most promising engine technologies along these lines is homogeneous charge compression ignition (HCCI). By using variable valve actuation (VVA) to trap a portion of the exhaust gases and using this to increase the sensible energy of the reactant mixture on the next engine cycle, HCCI allows fast compression ignition of a homogeneous and diluted fuel-air mixture, leading to significantly better efficiency and emissions characteristics in comparison to current technologies. However, due to the lack of a direct combustion trigger, as well as the presence of cycle-to-cycle dynamics where the trapped exhaust from one engine cycle influences combustion on the next, closed-loop control is necessary for the operation of HCCI over a wide operating range.

This thesis presents a physical model-based control framework for controlling an HCCI engine with exhaust recompression and direct injection of fuel into the cylinder. A physical model is used to describe the HCCI process, with the model states being closely linked to the thermodynamic state of the cylinder constituents. Simple linear controllers based on this model are used to control the work output and the phasing of combustion on a cycle-by-cycle basis with the use of variable valve timings as well as variable fuel injection quantity. Experimental results from both single and multi-cylinder engine testbeds are presented, demonstrating the value of a physical model-based approach that allows an easy porting of the control structure from one engine to another. The controllers are also seen to be useful in reducing the cyclic variability of combustion at operating points with late combustion phasing, indicating the value of this framework in potentially expanding the operating range of HCCI.

Having validated the basic model structure with simple controllers, this thesis then describes the expansion of the modeling framework to include a simple model for the effects of fuel injection during recompression. This represents the first such model of its kind, and it forms the basis for control strategies that use a split fuel injection, with a variable pilot injection timing, to control the phasing of combustion. These include a mid-ranging control scheme where constraints on a realistic implementation of valve actuation such as cam phasers are taken into account. Finally a more comprehensive control framework is developed based on the principle of model predictive control (MPC). The predictive controller is designed for fast tracking of desired load and phasing trajectories while respecting practical constraints on the different actuators as well as other system variables such as air-fuel ratio. Experimental implementation of the MPC scheme demonstrates the promise of this model-based control framework as a practical tool for HCCI control.