Hsien-Hsin Liao

Modeling and Control of an Exhaust Recompression HCCI Engine Using Split Injection

Homogeneous charge compression ignition (HCCI) is currently being pursued as a cleaner and more efficient alternative to conventional engine strategies. Control of the load and phasing of combustion is critical in the effort to ensure reliable operation of an HCCI engine over a wide operating range. This paper presents an approach for modeling the effect of a small pilot injection during the recompression process of an HCCI engine, and a controller that uses the timing of this pilot injection to control the phasing of combustion. The model is a nonlinear physical model that captures the effect of fuel quantity and intake and exhaust valve timings on work output and combustion phasing. It is seen that around the operating points considered, the effect of a pilot injection can be modeled as a change in the Arrhenius threshold, an analytical construct used to model the phasing of combustion as a function of the thermodynamic state of the reactant mixture. The relationship between injection timing and combustion phasing can be separated into a linear, analytical component and a nonlinear, empirical component. Two different control strategies based on this model are presented, both of which enabled steady operation at low load conditions and effectively track desired load-phasing trajectories. These strategies demonstrate the potential of split injection as a practical cycle-by-cycle control knob requiring only minimal valve motion that would be easily achievable on current production engines equipped with cam phasers.

Control of exhaust recompression HCCI using hybrid model predictive control

Homogeneous Charge Compression Ignition (HCCI) holds promise for reduced emissions and increased efficiency compared to conventional internal combustion engines. As HCCI lacks direct actuation over the combustion phasing, much work has been devoted to designing controllers capable of set-point tracking and disturbance rejection. This paper presents results on model predictive control (MPC) of the combustion phasing in an HCCI engine based on a hybrid model formulation composed of several linearizations of a physics-based nonlinear model. The explicit representation of the MPC was implemented experimentally and the performance during set point changes was compared to that of a switched state feedback controller. The hybrid MPC produced smoother transients without overshoot when the set point change traversed several linearizations.

Reducing combustion variation of late-phasing HCCI with cycle-to-cycle exhaust valve timing control

Abstract A framework for modeling late-phasing homogeneous charge, compression ignition (HCCI) combustion shows that combustion instability can be represented as a negative real-axis pole in a discrete-time linearized system. A simple, nonlinear model of the HCCI process presented in previous work effectively captures decreases in heat transfer at late-phasing HCCI points. These decreases in heat transfer result in highly oscillatory responses of combustion phasing at late-phasing HCCI points. Root loci generated by linearizing the nonlinear model at different operating conditions illustrate that a system pole moves from the origin at a nominal case onto the negative real axis for the late-phasing case. Finally, simple proportional control of exhaust valve closing, based on output feedback, demonstrates sufficient reduction in drastic swings in combustion phasing and indicated mean effective pressure.

Modeling and control of exhaust recompression HCCI using split injection

Homogeneous charge compression ignition (HCCI) is currently being pursued as a cleaner and more efficient alternative to conventional engine strategies. This paper presents an approach for modeling the effect of a small pilot injection during recompression on combustion in an HCCI engine with exhaust trapping, and a controller that uses the timing of this pilot injection to control the phasing of combustion. The model is incorporated into a nonlinear physical model presented in previous work that captures the effect of fuel quantity and intake and exhaust valve timings on work output and combustion phasing. It is seen that around the operating points considered, the effect of a pilot injection can be modeled as a change in the Arrhenius threshold, an analytical construct used to model the phasing of combustion as a function of the thermodynamic state of the reactant mixture. The relationship between injection timing and combustion phasing can be separated into a linear, analytical component and a nonlinear, empirical component. A feedback controller based on this model is seen to be effective in tracking a desired load-phasing trajectory and enables steady operation at low load conditions.

Repetitive control of an electro-hydraulic engine valve actuation system

Fully flexible engine valve systems serve as powerful rapid prototyping tools in research laboratories. With the ability to quickly design innovative valve strategies, researchers can explore the possibilities of improving fuel efficiency, power output and emissions through appropriately varying the valve lift, phasing and timing. One means of achieving variable valve motion is through an electro-hydraulic valve system (EHVS). However, with an EHVS, it is difficult to achieve the high acceleration necessary for tracking cam profiles while maintaining 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 position control. The paper first describes an identification method for obtaining a mathematical model of the EHVS. Based on the model, a linear feedback controller is developed. Finally, a repetitive feed-forward controller is added to augment the feedback controller, improving root- mean-square tracking performance to below forty micrometers.

Mid-Ranging Control of a Multi-Cylinder HCCI Engine Using Split Fuel Injection and Valve Timings

Abstract Homogeneous charge compression ignition (HCCI), though a promising piston-engine strategy for the future, presents a significant control challenge due to the presence of cycle-to-cycle dynamics and the absence of a direct combustion trigger. Several actuators can be used for controlling HCCI, but each of them presents unique hurdles to practical implementation. This paper presents an approach for controlling HCCI with exhaust recompression that addresses these challenges using the principle of mid-ranging control. The controller is based on a physical, discrete-time model of HCCI presented in previous work. A split injection strategy is used, with the timing of a small pilot injection of fuel during recompression being used to control the phasing of combustion on a cycle-by-cycle basis. A slower valve motion, easily achievable on an engine equipped with cam phasers, is then used to keep the injection timing in the middle of its range of influence, maintaining the control authority to handle fast transients while respecting actuator constraints. The controller is seen to be effective in tracking desired load and phasing trajectories in simulation, and on a multi-cylinder engine testbed. In particular, the controller enables steady operation at low load conditions on the engine.

Controlling Combustion Phasing of Recompression HCCI with a Switching Controller

Homogeneous charge compression ignition (HCCI) is more efficient and produces significantly less NOx emissions compared to spark ignitions. Using an exhaust recompression strategy to achieve HCCI, however, produces cycle-to-cycle coupling which makes the problem of controlling combustion phasing more difficult. In the past, a linear feedback controller designed with a single linearized model is effective in controlling combustion phasing around an operating point. However, HCCI dynamics can change dramatically around different operating points such that a single linearization is insufficient to approximate the entire operating range. Further investigation shows that the operating range can be roughly divided into three regions where a linear model can capture the qualitative system behavior in each of the regions. As a result, a three zone switching linear model approximates recompression HCCI dynamics far better than a single linearization. This new model structure also suggests that two of the three regions need completely opposite control actions. Therefore, the approach of using a static feedback control based on a single linearziation cannot be appropriate over the entire operating range. We propose a switching controller based on the switching linear model and achieve very good performance in controlling HCCI combustion phasing throughout the entire operating region. Lastly, a semi-definite programming (SDP) formulation of finding a Lyapunov function for the switching linear model is presented in order to guarantee stability of the switching control scheme.

Representing recompression HCCI dynamics with a switching linear model

Homogeneous charge compression ignition (HCCI) promises efficient combustion and less NOx emissions over conventional modes. However, the lack of direct ignition trigger and the cycle-to-cycle coupling in recompression HCCI makes the combustion phasing control problem difficult. To further complicate the matter, we show in this paper that the natural dynamics of HCCI can change drastically from one operating point to another. Our analyses show that the operating range of recompression HCCI can be partitioned into three linear regions such that a single linearized model can reasonably capture the system behavior within each region. As a result, a switching linear model that switches between three linearized systems shows better agreement with the physical testbed compared to using a single linear model. The switching linear model also gives insights on what the appropriate feedback control actions should be in each of the region and reveals that a controller that works well in one region may have a directional error when blindly applied to a different operating region.

Physical Modeling and Control of a Multi-Cylinder HCCI Engine

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 a multi-cylinder 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. Experimental results show good tracking of both the work output and combustion phasing over a wide operating region. In addition, the controller is able to balance out differences between cylinders, and reduce the cycle-to-cycle variability of combustion.Copyright

An analytical method for reducing combustion instability in homogeneous charge compression ignition engines through cycle-to-cycle control

Late-phasing homogeneous charge compression ignition operating conditions have the potential to expand the useful operating range of homogeneous charge compression ignition in internal combustion engines. However, significant combustion instabilities can occur at late-phasing operating conditions. Combustion phasing and work output variations at these conditions are characterized by a pattern in which the combustion phasing alternates between being earlier than the desired timing and later than the desired timing. This characteristic can be traced to a negative eigenvalue in a simplified, discrete-time analytical model of homogeneous charge compression ignition dynamics. The model forms the basis for the design of a simple controller that is shown to reduce combustion timing variations in simulation. Experimental results illustrate the controller successfully reduces the cyclic variations in combustion by altering the exhaust valve timing, resulting in stable combustion and an expanded operating range.