Matthew J. Roelle

Dynamic Modeling of Residual-Affected Homogeneous Charge Compression Ignition Engines with Variable Valve Actuation

One practical method for achieving homogeneous charge compression ignition (HCCI) in internal combustion engines is to modulate the valves to trap or reinduct exhaust gases, increasing the energy of the charge, and enabling autoignition. Controlling combustion phasing with valve modulation can be challenging, however, since any controller must operate through the chemical kinetics of HCCI and account for the cycle-to-cycle dynamics arising from the retained exhaust gas. This paper presents a simple model of the overall HCCI process that captures these fundamental aspects. The model uses an integrated Arrhenius rate expression to capture the importance of species concentrations and temperature on the ignition process and predict the start of combustion. The cycle-to-cycle dynamics, in turn, develop through mass exchange between a control volume representing the cylinder and a control mass modeling the exhaust manifold. Despite its simplicity, the model predicts combustion phasing, pressure evolution and work output for propane combustion experiments at levels of fidelity comparable to more complex representations. Transient responses to valve timing changes are also captured and, with minor modification, the model can, in principle, be extended to handle a variety of fuels.

Model-Based Control of HCCI Engines Using Exhaust Recompression

Homogeneous charge compression ignition (HCCI) is one of the most promising piston-engine concepts for the future, providing significantly improved efficiency and emissions characteristics relative to current technologies. This paper presents a 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. Separability between the effects of the control inputs on the desired outputs provides an opportunity to develop a simple linear control scheme, where the fuel is used to control the work output and the valve timings are used to control the phasing of combustion. The controller is tested on both a single and multi-cylinder HCCI engine, demonstrating the value of a physical model-based control approach that allows an easy porting of the control structure from one engine to another. Experimental results show good tracking of both the work output and combustion phasing over a wide operating region on both engines. In addition, the controller is able to balance out differences between cylinders on the multi-cylinder engine testbed, and reduce the cycle-to-cycle variability of combustion.

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

Implementation and Analysis of a Repetitive Controller for an Electro-Hydraulic Engine Valve System

Variable valve actuation plays an important role in modern engine design. Fuel economy, emissions, and power output can be improved through appropriately varying valve lift and timing. One means of independently and continuously adjusting these valve profile parameters is with an electro-hydraulic valve system (EHVS). However, with an EHVS, it is very difficult to achieve the same level of accurate position control that a mechanical cam provides. In particular, the response time delay and the nonlinear dynamics of the hydraulic system can lead to error in valve position control. The paper first describes the identification method used to obtain a mathematical model of the EHVS. Based on the model, two linear feedback controllers are developed and compared. To further improve the tracking performance, a repetitive feed-forward controller is added to augment the feedback controller and the root mean square tracking error is improved to under 40 μ m. Stability and steady-state tracking error variance analyses complete the mathematical framework of this work.

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.

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

A PHYSICALLY BASED TWO-STATE MODEL FOR CONTROLLING EXHAUST RECOMPRESSION HCCI IN GASOLINE ENGINES

Homogeneous Charge Compression Ignition (HCCI) presents several advantages over conventional IC engines, including improved efficiency and emissions. It is, however, difficult to implement and control due to the lack of an external combustion trigger. One way to achieve HCCI is to trap and recompress a portion of the exhaust in the cylinder to increase the sensible energy of the air-fuel mixture. Such a strategy, however, introduces a cyclic coupling through the exhaust gas retained from cycle to cycle, making dynamic control non-trivial. In order to develop model-based controllers for HCCI, the authors present a physically motivated two-state model of the HCCI process. This model specifically captures the behavior of a direct inject gasoline engine with an exhaust-recompression strategy used to achieve HCCI. As the trapped exhaust is pivotal in setting up the cyclic coupling, its temperature and the amount of oxygen present in it are selected as the states of the system. The systems dynamics are developed through these states to give a discrete-time nonlinear model that can be validated against a more complex continuous-time model. In this form, the model represents a control-oriented description of the HCCI engine as a thermodynamic system, and can therefore be used as a platform to synthesize various control strategies. As a demonstrative example, a linear representation of the system is derived and used to synthesize an LQR controller to track a desired state trajectory in simulation.Copyright

A DYNAMIC MODEL OF RECOMPRESSION HCCI COMBUSTION INCLUDING CYLINDER WALL TEMPERATURE

In a Homogeneous Charge Compression Ignition (HCCI) engine, pre-compression gas temperature and cylinder wall temperature are two of the conditions that determine combustion timing. However, they operate in completely different dynamic realms. The pre-compression gas temperature will stabilize in just a few cycles. Conversely, the cylinder wall temperature requires a minute or more. To predict combustion timing under changing thermal conditions, this paper describes a multi-state continuous-time model of HCCI combustion achieved with exhaust recompression and a discrete model of cylinder wall temperature updated once per cycle. The continuous model is necessary to capture ignition on each cycle. Wall temperature changes so little every cycle, it is only updated between combustion cycles. This scheme is investigated by comparing experimental and simulated fuel step changes, which alter residual gas temperature quickly and cylinder wall temperature slowly. Dynamic comparisons show several similar behaviors and provide insight into the physical processes.

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.