Gregory M. Shaver

Modeling for control of HCCI engines

The goal of this work is to accurately predict the phasing of homogeneous charge compression ignition (HCCI) combustion for a single cylinder research engine using variable valve actuation (VVA) at Stanford University. Three simple single-zone models were developed and compared with experiment. The difference between the three modeling approaches centered around the combustion chemistry mechanism used in each case. The first modeling approach, which utilized a temperature threshold to model the onset of the combustion reaction, did not work well. However, an integrated reaction rate threshold accounting for both the temperature and concentration did correlate well with experiment. Additionally, another model utilizing a simple two-step kinetic mechanism also showed good correlation with experimental combustion phasing.

Physics-Based Modeling and Control of Residual-Affected HCCI Engines

Homogeneous charge compression ignition (HCCI) is a novel combustion strategy for IC engines that exhibits dramatic decreases in fuel consumption and exhaust emissions. Originally conceived in 1979, the HCCI methodology has been revisited several times by industry but has yet to be implemented because the process is difficult to control. To help address these control challenges, the authors here outline the first generalizable, validated, and experimentally implemented physics-based control methodology for residual-affected HCCI engines. Specifically, the paper describes the formulation and validation of a two-input, two-state control-oriented system model of the residual-affected HCCI process occurring in a single engine cylinder. The combustion timing and peak pressure are the model states, while the inducted gas composition and effective compression ratio are the model inputs. The resulting model accurately captures the system dynamics and allows the simultaneous, coordinated control of both in-cylinder pressure and combustion timing. To demonstrate this, an H 2 optimal controller is synthesized from a linearized version of the model and used to dictate step changes in both combustion timing and peak pressure within about four to five engine cycles on an experimental test bed. The application of control also results in reductions in the standard deviation for both combustion timing and peak pressure. The approach therefore provides accurate mean tracking, as well as a reduction in cyclic dispersion. Another benefit of the simultaneous coordination of both control inputs is a reduction in the control effort required to elicit the desired response. Instead of using a peak pressure controller that must compensate for the effects of a combustion timing controller, and vice versa, the coordinated approach optimizes the use of both control inputs to regulate both outputs.

Tackling the Transition: A Multi-Mode Combustion Model of SI and HCCI for Mode Transition Control

Homogeneous charge compression ignition (HCCI) offers a promising way to improve efficiency and emissions. However, when HCCI is induced by reinducting exhaust gases, less power is produced. A possible solution is to couple HCCI with spark ignition (SI) operation at higher loads. This requires a way to smoothly switch between combustion modes. The authors present a multi-cycle, multi-mode combustion model to aid in understanding and controlling the mode transition. The model captures early ignition and low work after a switch from SI to HCCI. Furthermore, the model reveals a need to coordinate intake and exhaust valve timing to correct the HCCI phase and work. To demonstrate the model, the paper concludes with an example trajectory that maintains constant work and ignition phasing after a switch from SI to HCCI.Copyright

Decoupled control of combustion timing and work output in residual-affected HCCI engines

Homogeneous charge compression ignition (HCCI) is a promising low temperature combustion strategy for internal combustion engines. However, when HCCI is achieved with variable valve actuation (VVA) the lack of a direct combustion initiator and cycle-to-cycle dynamics complicate control of the process. This work outlines a strategy for the simultaneous control of both in-cylinder pressure and combustion timing through the use of an approximately decoupled timing/pressure controller. The decoupling is achieved by controlling peak pressure and combustion timing on separate time scales with different VVA-induced control inputs, inducted gas composition and effective compression ratio, respectively. The paper also details how the strategy can be extended to handle decoupled timing/work output control. Experimental results show that in-cylinder pressure or work output can be controlled on a cycle-to-cycle basis, while combustion timing is slowly varied.

Cycle-to-Cycle Control of HCCI Engines

With stated benefits ranging from increased thermal efficiency to significantly reduced NOx emissions, Homogeneous Charge Compression Ignition (HCCI) represents a promising combustion strategy for future engines. When achieved by reinducting exhaust gas with a variable valve actuation (VVA) system, however, HCCI possesses nonlinear cycle-to-cycle coupling through the exhaust gas and lacks an easily identified trigger comparable to spark or fuel injection. This makes closed-loop control decidedly nontrivial. To develop a controller for HCCI, the engine cycle is partitioned into five stages: adiabatic, constant pressure induction of re-inducted product and reactant charge; isentropic compression to the point just prior to combustion initiation; constant volume combustion; isentropic expansion of product gases; isentropic exhaust of product gases. Using this framework, a nonlinear low-order model of HCCI combustion is formulated, where the input is the molar ratio of reinducted products to fresh reactants and the output is the peak in-cylinder pressure. Comparison with experimental in-cylinder pressure data shows that the model, while simple, offers reasonable fidelity. Using the nonlinear model, a linearized model and an accompanying LQR controller are formulated and implemented on a more detailed model presented in previous work. Results from these simulations show that the modeling and control approach is indeed successful at tracking a varying desired work output while maintaining a constant desired combustion phasing.Copyright

A two-input two-output control model of HCCI engines

This paper outlines a 2-input, 2-state control-oriented system model of the residual-affected homogeneous charge compression ignition (HCCI) process. The combustion timing and peak pressure are the model states, while the inducted gas composition and effective compression ratio are the model inputs. In previous work the authors utilized the self-stabilizing characteristic of residual-affected HCCI to neglect the combustion timing dynamics to arrive at a single-input, single-output dynamic model. In this paper the combustion timing dynamics are explicitly included for two reasons: the resulting model is more accurate and allows the simultaneous, coordinated control of both in-cylinder pressure and combustion timing. Following the model development, the experimental results of an H2 controller are given, demonstrating the utility of the control model outlined

Modeling Cycle-to-Cycle Coupling in HCCI Engines Utilizing Variable Valve Actuation

Abstract In order to capture the effect of cycle-to-cycle coupling that is inherent in residual-effected homogeneous charge compression ignition (HCCI) engines, a simple, control-oriented, single-zone model of HCCI combustion is presented. The inclusion of an exhaust manifold model ties the exhausted gas from one cycle to that re-inducted on the next cycle. Multi-cycle simulations are completed and shown to have the same general steady state and transient characteristics as an experimental system. Predicted combustion phasing and in-cylinder pressure values agree very well with experiment.

Contraction and sum of squares analysis of HCCI engines

Abstract By modulating engine valves to reinduct hot exhaust gas together with air and fuel, a clean and efficient form of autoignition can be created. Control of this combustion process, known as homogeneous charge compression ignition (HCCI), requires not only precise valve control but also a combustion control strategy that accounts for the cycle-to-cycle coupling through the exhaust. This paper outlines approaches for proving closed-loop stability of a valve controller and combustion controller using nonlinear analysis tools. Stability of the valve controller is shown using contraction analysis. Stability of the combustion controller is shown using sum of squares decomposition, convex optimization and the Positivstellensatz.