Nikhil Ravi

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.

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

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.

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.

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.

Controller-Observer Implementation for Cycle-by-Cycle Control of an HCCI Engine

This paper presents experimental cycle-by-cycle control of a single cylinder HCCI engine. The controller is developed from a discrete-time nonlinear model presented in previous work. The model captures the behavior of a gasoline direct-injection engine with an exhaust-recompression strategy used to achieve HCCI. This model is linearized about an operating point so as to enable the synthesis of linear controllers. The model states are represented by the temperature and oxygen content of the retained exhaust, and so are not measurable in practice. Therefore, an observer is used to estimate the states based on a measured ignition proxy. The state estimates are then used by a reference-input tracking controller to track a desired system trajectory. Experimental results show tracking of the model outputs that is comparable to tracking achieved in simulation. The controller is also seen to reduce the cycle-to-cycle variability of combustion significantly, particularly at later combustion phasing. This stabilizes combustion, lowers the instances of misfires, and enables steady operation at points that are normally unstable.

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.

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.