Petter Strandh

Cycle-to-cycle Control of a Dual-Fuel HCCI Engine

A known problem of the HCCI engine is its lack of direct control andits requirements of feedback control. Today there exists severaldifferent means to control an HCCI engine, such as dual fuels,variable valve actuation, inlet temperature and compression ratio.Independent of actuation method a sensor is needed. In this paper weperform closed-loop control based on two different sensors, pressureand ion current sensor. Results showing that they give similar controlperformance within their operating range are presented.Also a comparison of two methods of designing HCCI timing controller,manual tuning and model based design is presented. A PIDcontroller is used as an example of a manually tuned controller. ALinear Quadratic Gaussian controller exemplifies model basedcontroller design. The models used in the design were estimated usingsystem identification methods.The system used in this paper performs control on cycle-to-cyclebasis. This leads to fast and robust control. Dual fuels withdifferent octane numbers were used to control the combustion timing.The engine was a 12 liter 6 cylinder heavy-duty diesel engine modifiedwith a port fuel injection system which has dual fuels connected. (Less)

Ion Current Sensing for HCCI Combustion Feedback

Measurement of ion current signal from HCCI combustionwas performed. The aim of the work was to investigateif a measurable ion current signal exists and if it is possible to obtain useful information about the combustion process. Furthermore, influence of mixture quality in termsof air/fuel ratio and EGR on the ion current signal wasstudied. A conventional spark plug was used as ionizationsensor. A DC voltage (85 Volt) was applied acrossthe electrode gap. By measuring the current through thegap the state of the gas can be probed. A comparisonbetween measured pressure and ion current signal wasperformed, and dynamic models were estimated by usingsystem identification methods.The study shows that an ion current signal can be obtainedfrom HCCI combustion and that the signal levelis very sensitive to the fuel/air equivalence ratio. Themost important result from this study is that the ion current signal proved to be an excellent indicator of the actual combustion timing which is crucial piece of information for HCCI control.

Hybrid control of homogeneous charge compression ignition (HCCI) engine dynamics

The homogeneous charge compression ignition (HCCI) combustion engine principle lacks direct ignition timing control, instead the auto-ignition depends on the operating condition. Since auto-ignition of a homogeneous mixture is very sensitive to operating conditions, fast combustion phasing control is necessary for reliable operation. For this paper, a six-cylinder heavy-duty HCCI engine was controlled on a cycle-to-cycle basis in real time. Sensors, actuators and control structures for control of the HCCI combustion were compared. Among several actuators for HCCI engine control suggested, two actuators were compared—i.e., dual-fuel actuation and variable valve actuation (VVA). As for control principles, model predictive control (MPC) has several desirable features and today MPC can be applied to relatively fast systems, such as VVA and dual-fuel actuation. For sensor feedback control of the HCCI engine, cylinder pressure and ion current—i.e., the electronic conductive properties in the reaction zone—were compared. Combustion phasing control based on ion current was compared to control based on cylinder pressure. For the purpose of control synthesis requiring dynamic models, system identification provided models of the HCCI combustion, the models being validated by stochastic model validation. With such models providing a basis for model-based control, MPC control results were compared to PID and LQG control results. While satisfying the constraints on cylinder pressure, both control of the combustion phasing and control of load torque was achieved with simultaneous minimization of the fuel consumption and emissions.

Hybrid modelling of homogeneous charge compression ignition (HCCI) engine dynamics - a survey

The Homogeneous charge compression ignition (HCCI) principle holds promise to increase efficiency and to reduce emissions from internal combustion engines. As HCCI combustion lacks direct ignition timing control and auto-ignition depends on the operating condition, control of auto-ignition is necessary. Since auto-ignition of a homogeneous mixture is very sensitive to operating conditions, a fast combustion phasing control is necessary for reliable operation. To this purpose, HCCI modelling and model-based control with experimental validation were studied. A six-cylinder heavy-duty HCCI engine was controlled on a cycle-to-cycle basis in real time using a variety of sensors, actuators and control structures for control of the HCCI combustion. Combustion phasing control based on ion current was compared to feedback control based on cylinder pressure. With several actuators for controlling HCCI engines suggested, two actuators were compared, dual fuel and variable valve actuation (VVA). Model-based control synthesis requiring dynamic models of low complexity and HCCI combustion models were estimated by system identification and by physical modelling, the physical models aiming at describing the major thermodynamic and chemical interactions in the course of an engine stroke and their influence on combustion phasing. The models identified by system identification were used to design model-predictive control (MPC) with several desirable features and today applicable to relatively fast systems, the MPC control results being compared to PID control results. Both control of the combustion phasing and control of load-torque with simultaneous minimization of the fuel consumption and emissions, while satisfying the constraints on cylinder pressure, were included.

Multi-output control of a heavy duty HCCI engine using Variable Valve Actuation and Model Predictive Control

Autoignition of a homogeneous mixture is very sensitive to operating conditions, therefore fast control is necessary for reliable operation. There exists several means to control the combustion phasing of an Homogeneous Charge Compression Ignition (HCCI) engine, but most of the presented controlled HCCI result has been performed with single-input single-output controllers. In order to fully operate an HCCI engine several output variables need to be controlled simultaneously, for example, load, combustion phasing, cylinder pressure and emissions. As these output variables have an effect on each other, the controller should be of a structure which includes the cross-couplings between the output variables. A Model Predictive Control (MPC) controller is proposed as a solution to the problem of load-torque control with simultaneous minimization of the fuel consumption and emissions, while satisfying the constraints on cylinder pressure. One of the major motivations for using MPC is that it explicitly takes the constraints into account. When operating an HCCI engine there are several contraints present, for example on the cylinder pressure and on the emissions. A drawback of MPC is the potentially large on-line computational effort, which has historically limited its application to relative slow and/or small applications. Today, MPC can be applied in relative fast systems, and we will demonstrate that it can be used for control of HCCI engine dynamics on a cycle-to-cycle basis. As feedback signal of the combustion phasing, the crank angle for 50% burned, based on cylinder pressure, is used. In the control design of the MPC controllers (one for each cylinder), dynamic models obtained by system identification were used. This paper presents cycle-to-cycle cylinder individual control results from a six-cylinder HCCI engine using a Variable Valve Actuation (VVA) system and MPC controllers.

Modeling of HCCI engine combustion for control analysis

Operation of homogeneous charge compression ignition (HCCI) engines are very sensitive to timing variations in the combustion of the air-fuel charge mixture and require precise control of the ignition instant to run properly. It is therefore essential to understand the characteristics of timing variations under various operating conditions in order to find suitable control strategies. This paper presents a first step towards the construction of an HCCI engine model aimed at studies on timing control strategies. The goal is to (qualitatively) reproduce the timing effects that may be observed on a real engine. The proposed model includes a lumped chemical kinetic model for hydrocarbon fuels to predict autoignition. Single-cycle simulations are compared with experimental results from a real engine to validate the model. Comparisons are also made with a model based on the knock-integral.

Multiple-Input Multiple-Output Model Predictive Control of a Diesel Engine

Traditionally, diesel engine control has had to rely on indirect feedback variables and empirical open-loop maps because direct measurements of the variables representing highlevel objectives, such as emissions, have not been available in production engines. With new sensors being developed, the opportunity opens to design the controller directly based on highlevel objectives. In this paper, we propose to use model predictive control as a systematic way to go directly from high-level specifications to a control algorithm. The controller uses four actuator variables and five measured variables and is based on a model obtained through system identification. Experimental results on a six-cylinder heavy-duty engine around a fixed operating point demonstrate the potential of the control scheme. (Less)

Multiple Point Ion Current Diagnostics in an HCCI Engine

Interest in ion current sensing for HCCI combustion arises when a feedback signal from some sort of combustion sensor is needed in order to determine the state of the combustion process. A previous study has revealed that ion current sensors in the form of spark plugs can be used instead of expensive piezoelectric transducers for HCCI combustion sensing. Sufficiently high ion current levels were achieved when using relatively rich mixtures diluted with EGR. The study also shows that it is not the actual dilution per se but the actual air/fuel equivalence ratio which is important for the signal level. Conclusions were made that it is possible to obtain information on combustion timing and oscillating wave phenomena from the measurements. However, the study showed that the ion current is local compared to the pressure which is global in the combustion chamber. This observation triggered the present study where the aim is to investigate the ion current at different locations in the combustion chamber. The ion current was measured simultaneously at seven locations in the combustion chamber. In order to achieve this, 6 spark plugs were fitted circumferentially in a spacer placed between the cylinder block and the head. The seventh spark plug was placed in the cylinder head. Individual DC sources of 85 volts were applied across the spark plug gaps. The present study indicates that the combustion timing seems to be dependent on the wall temperature at the different spark plug locations. The largest difference in timing between different locations in the combustion chamber was 2 CAD. The ion current amplitude varies with different spark plug locations up to 1.5 μA. The signal strength increases with decreasing air/fuel ratio and is also affected by dilution.

Closed-Loop Control of Combustion Phasing in an HCCI Engine Using VVA and Variable EGR

Abstract A homogeneous charge compression ignition (HCCI) engine requires closed-loop control of combustion phasing for reliable operation. Variable valve actuation (VVA) has previously been shown to enable cycle-to-cycle, cylinder-individual control with high precision, but suffers from a narrow operating range. Adding variable exhaust gas recirculation (EGR) to the closed-loop control structure can extend the operating range. A mid-ranging control structure is presented here for combined VVA and EGR actuations in a multi-cylinder engine. The control structure is simple to implement and preserves the fast, cylinder-individual, and precise actuation of the VVA system while extending the operating range. Experimental results verify the performance of the control structure.

Dynamic mapping of diesel engine through system identification

From a control design point of view, modern diesel engines are dynamic, nonlinear, MIMO systems. This paper presents a method to find low-complexity black-box dynamic models suitable for model predictive control (MPC) of NOx and soot emissions based on on-line emissions measurements. A four-input-five-output representation of the engine is considered, with fuel injection timing, fuel injection duration, exhaust gas recirculation (EGR) and variable geometry turbo (VGT) valve positions as inputs, and indicated mean effective pressure, combustion phasing, peak pressure derivative, NOx emissions, and soot emissions as outputs. Experimental data were collected on a six-cylinder heavy-duty engine at 30 operating points. The identification procedure starts by identifying local linear models at each operating point. To reduce the number of dynamic models necessary to describe the engine dynamics, Wiener models are introduced and a clustering algorithm is proposed. A resulting set of two to five dynamic models is shown to be able to predict all outputs at all operating points with good accuracy.