Homogeneous Charge Compression Ignition Control Research Articles

Ion Current Based Data Driven Control of HCCI - the Key to Improve Pressure Based Control Strategies?

Homogeneous charge compression ignition (HCCI) is a low-temperature combustion process that can both increase efficiency and reduce pollutant raw emissions. The process is highly dependent on low-temperature kinetics of the chemical reactions that lead to auto-ignition. Due to strong coupling of consecutive combustion cycles stable control of HCCI is a major challenge. Ion current sensors that detect ionization in the combustion chamber can provide information about chemical reactions taking place inside the cylinder. In contrast, model-based HCCI control strategies, which require accurate models that describe the current cylinder state, are often based only on the cylinder pressure. However, low-temperature chemical kinetics are not sufficiently taken into account by only using the cylinder pressure signal. Therefore, the main hypothesis of this paper is that HCCI strongly depends on the chemical state in the cylinder, which cannot be adequately described by the pressure sensor in the cylinder alone. Hence, a conventional spark plug is used to apply an ion current sensor, which is used alongside the pressure sensor. The aim is to use further information about the state of the chemical mixture for the control of the engine. The features derived from the ion current are integrated into an explicit artificial neural network-based controller. The developed control strategies are then compared against a purely pressure-based controller. Using the ion current features, the standard deviation of the combustion phasing CA50 is reduced by up to 42% compared to the pressure-based approach. The experimental results of this work show that the inclusion of the ion current in the controller provides a significant advantage for low temperature combustion control.

Dynamic measurement with in-cycle process excitation of HCCI combustion: The key to handle complexity of data-driven control?

Homogeneous Charge Compression Ignition (HCCI) is a low temperature combustion technique with a high potential for reducing emissions while simultaneously improving fuel consumption. However, the high sensitivity to changing boundary conditions and low combustion stability at the edges of the operating range has lead to implementation challenges. Additionally, cyclic coupling through internal exhaust gas recirculation causes cyclic variations of the process, resulting in incomplete combustion, or even misfiring. Thus, consecutive cycles must be decoupled to increase the process stability. To achieve an accurate description of the coupling effects on a cycle-to-cycle and an inner-cyclic timescale, a novel measurement methodology is presented to generate data with a high variance. For this purpose, an active process excitation is performed to capture all relevant interactions between operating and feedback variables to enable modeling of the coupling effects on both timescales. To demonstrate the potential of the methodology, the generated data is used to design multiple input, multiple output (MIMO) models for both cyclic and inner-cyclic timescales. Artificial neural networks are then utilized to address the highly nonlinear process by taking advantage of the large amount of training data. Inverse process models are then used to implement a pure cycle-to-cycle and a multiscale MIMO closed-loop controller. Compared to state-of-the-art rule-based control approaches, the process stability and its thermodynamic efficiency are significantly improved. For the multiscale MIMO controller, a reduction of the standard deviation of the indicated mean effective pressure and the combustion phasing of more than 65% is achieved. In particular, the additional inner-cyclic feedback loop achieves a remarkable reduction of the standard deviation of approximately 35% and a 1.2% higher indicated efficiency compared to the cycle-to-cycle MIMO controller. The dynamic measurement with active in-cycle process excitation has proven to be an enabler for data-driven MIMO control of HCCI on multiple timescales.

Detection of transient low-temperature combustion characteristics by ion current – The missing link for homogeneous charge compression ignition control?

Homogeneous charge compression ignition is a promising low-temperature combustion mode because of its high efficiency and low emissions; however, its strong cycle-to-cycle coupling effect, which caused by the recirculation of exhaust gases, may entail problems with low combustion stability. In this study, a new concept that extracts more comprehensive combustion information in homogeneous charge compression ignition is proposed through the integration of ion current and in-cylinder pressure sensing. To analyze the correlations of combustion parameters and their relationships with the ion current parameters, steady-state measurements were conducted. Dynamic measurements were implemented to form a comprehensive database for artificial neural network training. To investigate the hypothesis that the ion current gives additional information beyond the pressure trace, black-box models based on experimental data are trained. The results show that the baseline model trained purely with the manipulated variables has the worst performance, while the model including both in-cylinder pressure and ion current derived parameters has the best predictability, with the overall root-mean-square error reduced by 2.5% in predicting combustion phasing, compared with in-cylinder pressure based model. It demonstrates that a significant improvement in model quality can be achieved by the combination of ion current and in-cylinder pressure sensing, which indicates that the ion current signal contains information that goes beyond a sole analysis of the pressure trace. By complementing the in-cylinder pressure, the use of the ion current as a “chemical sensor” for low-temperature combustion thus appears very promising for the stable control of homogeneous charge compression ignition combustion.

Simulation and experiments of advanced gasoline engine combustion modes from spark ignition to compression ignition

This paper reviews some advanced gasoline engine combustion modes, including direct injection spark ignition (DISI), homogeneous charge compression ignition (HCCI), stratified charge compression ignition (SCCI), and multiple premixed compression ignition (MPCI). To reveal the engine knock mechanism of DISI combustion, the flame propagation and pressure oscillation in a boosted gasoline engine were numerically analysed. To further improve thermal efficiency and reduce emissions, the effects of injection strategies on fuel economy, emissions, and heat release in HCCI combustion were studied. To solve the challenges of ignition control and rapid combustion of HCCI, SCCI and MPCI combustion modes were adopted to achieve high efficiency, low emissions, and low acoustic noise simultaneously.

Effects of refinery stream gasoline property variation on the auto-ignition quality of a fuel and homogeneous charge compression ignition combustion

The combination of in-cylinder thermal environment and fuel ignition properties plays a critical role in the homogeneous charge compression ignition engine combustion process. The properties of fuels available in the automotive market vary considerably and display different auto-ignition behaviors for the same intake charge conditions. Thus, in order for homogeneous charge compression ignition (HCCI) technology to become practically viable, it is necessary to characterize the impact of differences in fuel properties as a source of ignition/combustion variability. To quantify the differences, 15 gasolines composed of blends made from refinery streams were investigated in a single-cylinder homogeneous charge compression ignition engine. The properties of the refinery stream blends were varied according to research octane number, sensitivity (S = research octane number − motor octane number) and volumetric contents of aromatics and olefins. Nine fuels contained 10% ethanol by volume, and six more were blended with 20% ethanol. Pure ethanol (E100) and an un-oxygenated baseline fuel (RD3-87) were included too. For each fuel, a sweep of intake temperature at a consistent load and engine speed was conducted, and the combustion phasing given by the crank angle of 50% mass fraction burned was tracked to assess the sensitivity of auto-ignition to fuel chemical kinetics. The experimental results provided a wealth of information for predicting the HCCI combustion phasing from the given properties of a fuel. In this study, the original octane index correlation proposed by Kalghatgi based solely on fuel research octane number and motor octane number was found to be insufficient for characterizing homogeneous charge compression ignition combustion of refinery stream fuels. A new correlation was developed for estimation of auto-ignition properties of practical fuels in the typical HCCI engine. Fuel composition, captured by terms indicating the fraction of aromatics, olefins, saturates and ethanol, was added to generate the following formula: [Formula: see text]. The results indicate a significantly improved estimation of combustion phasing for gasoline fuels of varying chemical composition under low-temperature combustion conditions. Quantitative findings of this investigation and the new octane index correlation can be used for designing robust HCCI control strategies, capable of handling the wide spectrum of fuel chemical compositions found in pump gasoline.

An HCCI Control Oriented Model that Includes Combustion Efficiency

A control oriented model that includes combustion timing, engine load and combustion efficiency for Homogeneous Charge Compression Ignition (HCCI) engines is developed. In HCCI engines, a lean homogeneous air-fuel mixture auto-ignites due to compression and combustion occurs at lower temperatures compared to spark ignition and diesel engines. The low HCCI combustion temperature results in low NOx level, however unburnt HC and CO levels are high. Higher thermal efficiencies are realized for higher combustion efficiencies when combustion timings is appropriate. First, the effects of valve timing and fueling rate on combustion efficiency are investigated experimentally. Then, the influence of combustion efficiency on HC and CO emissions is studied. A physics based control oriented model of HCCI engine combustion efficiency and emissions for future control design is developed. This model includes the effect of trapped residual gas and fueling rate on combustion timing and output power. The developed model has acceptable accuracy for combustion timing, load and combustion efficiency prediction when compared to experimental data. This model is useful for combustion timing and load control in HCCI engines while simultaneously considering the constraints of combustion efficiency and emission.

An ELM based predictive control method for HCCI engines

We formulate and develop a control method for homogeneous charge compression ignition (HCCI) engines using model predictive control (MPC) and models learned from operational data. An HCCI engine is a highly efficient but complex combustion system that operates with a high fuel efficiency and reduced emissions compared to the present technology. HCCI control is a nonlinear, multi-input multi-output problem with state and actuator constraints which makes controller design a challenging task. In this paper, we propose an MPC approach where the constraints are elegantly included in the control problem along with optimality in control. We develop the engine models using experimental data so that the complexity and time involved in the modeling process can be reduced. An Extreme Learning Machine (ELM) is used to capture the engine dynamic behavior and is used by the MPC controller to evaluate control actions. We also used a simplified quadratic programming making use of the convexity of the MPC problem so that the algorithm can be implemented on the engine control unit that is limited in memory. The working and effectiveness of the proposed MPC methodology has been analyzed in simulation using a nonlinear HCCI engine model. The controller tracks several reference signals taking into account the constraints defined by HCCI states, actuators and operational limits.

Gray-Box Modeling for Performance Control of an HCCI Engine With Blended Fuels

High fidelity models that balance accuracy and computation load are essential for real-time model-based control of homogeneous charge compression ignition (HCCI) engines. Gray-box modeling offers an effective technique to obtain desirable HCCI control models. In this paper, a physical HCCI engine model is combined with two feed-forward artificial neural network models to form a serial architecture gray-box model. The resulting model can predict three major HCCI engine control outputs, including combustion phasing, indicated mean effective pressure (IMEP), and exhaust gas temperature (Texh). The gray-box model is trained and validated with the steady-state and transient experimental data for a large range of HCCI operating conditions. The results indicate that the gray-box model significantly improves the predictions from the physical model. For 234 HCCI conditions tested, the gray-box model predicts combustion phasing, IMEP, and Texh with an average error of less than 1 crank angle degree, 0.2 bar, and 6 °C, respectively. The gray-box model is computationally efficient and it can be used for real-time control application of HCCI engines.

Control of recompression HCCI with a three region switching controller

Homogeneous charge compression ignition (HCCI) presents new challenges in combustion phasing control due to the lack of direct ignition trigger. In addition, recompression HCCI possesses cycle-to-cycle coupling through trapping a large portion of the exhaust gas in the cylinder. This paper examines the change in the cycle-to-cycle coupling around different operating points in recompression HCCI and show that there are three qualitative types of temperature dynamics: smooth decaying when combustion phasing is early, oscillatory when phasing is late, and strongly converging with moderate combustion phasing. As a result, a three region switching linear model that combines three linearized models can capture the qualitative change in system characteristics. Based on this model, a three region switching controller is designed and shown in experiments that it can track a wide range of desired combustion phasing and improve the variance of combustion phasing over open-loop operations. In the last part of this paper, two semi-definite programming (SDP) formulations are presented to prove the stability of the switching control/model framework.

Model predictive control of HCCI using variable valve actuation and fuel injection

Control of work output and combustion phasing on a Homogeneous Charge Compression Ignition (HCCI) engine is essential to realize the benefits of superior efficiency and emissions. This paper presents a model predictive control approach for cycle-by-cycle control of HCCI while respecting constraints on actuators that might exist on a production implementation. The strategy is based on a physical model developed in previous work and uses valve actuation and split fuel injection to achieve the control objectives. In addition, it considers constraints on air–fuel ratio, ensuring that the system stays away from very lean or rich regions. Simulation and experimental results show that the controller works well over a range of conditions, and demonstrate the potential of this approach as a practical cycle-by-cycle control strategy for HCCI.

Ignition control of homogeneous-charge compression ignition (HCCI) combustion through adaptation of the fuel molecular structure by reaction with ozone

The present paper describes a method of controlling the time of ignition in homogeneous-charge compression ignition (HCCI) combustion. In the described experiments some control of ignition timing in HCCI combustion is achieved through alteration of the fuel molecular structure using a chemical reaction of the fuel with ozone, prior to introduction of the fuel into the combustion chamber. Controlling ignition timing is essential, in achieving high thermal efficiency and low pollutant emission in HCCI engine operation. To this end, ignition should occur in the vicinity of piston top-dead-centre (TDC), the point of maximum compression of the fuel–air charge. The present paper proposes a method of controlling the time of ignition of the fuel–air charge by adapting the ignitability of the fuel through prior chemical reaction of the fuel with ozone. Ozone can be readily produced using air in conjunction with a corona discharge ozoniser and may be brought into contact with the fuel in a reaction chamber before its injection into the engine. It was shown through experiments that an acetal fuel which has undergone treatment with ozone, ignites earlier during the engine cycle in HCCI combustion, than fuel which has not undergone treatment with ozone, as a result of changes in its molecular structure prior to combustion. The observed changes in molecular structure consisted primarily in the formation of peroxides within the fuel. This method can be used to operate an engine in HCCI combustion mode with some control over the point of ignition of the fuel–air charge by varying the proportions of fuel previously treated with ozone and fuel not treated with ozone. The experiments showed that the time of ignition could be controlled, whilst keeping other parameters such as the load and speed of the engine, and pressure and temperature of the intake air and fuel, constant.

Effect of injection timing on gasoline homogeneous charge compression ignition particulate emissions

Particulate emissions from homogeneous charge compression ignition (HCCI) engines are often considered as negligible and the measurement of particulate matter (PM) with HCCI combustion systems has been rare. An earlier publication in the literature and the authors' own recently published work suggest that PM emissions from gasoline direct injection (DI) HCCI engines should not be neglected. It has been shown that PM emissions from HCCI engines, although generally lower, can be similar to spark ignition (SI) levels for certain engine conditions, especially in the accumulation mode. The present work shows that although injection timing is effective for control of HCCI, it can have a significant impact on PM emissions. Engine speed also appears to have an impact on PM emissions for the HCCI combustion mode. Typically for HCCI operating at 1500 r/min, the total mass and total number of the PM emissions could be up to six times higher when the fuel is injected during the recompression period compared with the case for injection during the inlet valve open period. Similarly, when the engine was operated in SI mode, an attempt to inject fuel close to induction top dead centre (TDC) made the PM emission noticeably higher, reaching unacceptable levels. The split injection strategy for HCCI mode has been investigated for its effect on PM emissions. It has been found that the PM number concentration for split injection lies between the maximum concentration measured for very early injections (near TDC at recompression) and the minimum concentration measured for the latest injections used (at crank angles near to the inlet valve maximum opening time), with the magnitude of number density varying approximately from 0.2 to 1.2 million/cm3. It is also observed that for split injection the number of particulates in nucleation mode varies less, while the number in accumulation mode level varies within a limit of one order of magnitude; both are dependent on the injection strategy. The shape of the PM distribution is such that the nucleation mode exceeds the number concentration of 1×107 (d N d log Dp) and the accumulation mode falls parabolically to approximately 1×106. For comparison, it can be said that the PM number concentration measured in the authors' engine is approximately one order of magnitude lower in accumulation mode than the level for a typical modern diesel engine.

A physics-based approach to the control of homogeneous charge compression ignition engines with variable valve actuation

Homogeneous charge compression ignition (HCCI) is a promising low-temperature combustion strategy for reducing NOx emissions and increasing efficiency in internal combustion engines. However, HCCI has no direct combustion initiator and, when achieved by reinducting or trapping residual exhaust gas with a variable valve actuation (VVA) system, becomes a dynamic process as the temperature of the residual gas couples one cycle to the next. These characteristics of residual-affected HCCI present a challenge for control engineers and a barrier to implementing HCCI in a production engine. In order to address these challenges, this paper outlines physics-based control strategies for both the VVA system and the HCCI combustion process. The results show that VVA system control can provide arbitrary valve timings on a cycle-to-cycle basis, enabling tight control of HCCI. By abstracting these valve timings further into an inducted gas composition and an effective compression ratio, model-based controllers can be developed to control simultaneously load and combustion timing in an HCCI engine.

Two-stage ignition in HCCI combustion and HCCI control by fuels and additives

A Rapid Compression Machine (RCM) has been used to study the effects of fuel structure and additives on the Homogeneous Charge Compression Ignition (HCCI) of pure hydrocarbon fuels and mixtures under well-determined conditions. Such information is needed for understanding ignition delays and burning rates in HCCI engines, and “knock” in spark-ignition engines. It is also valuable for validating basic chemical kinetic models of hydrocarbon oxidation. The pure fuels used in the study include: paraffins (n-heptane, iso-octane), cyclic paraffins (cyclohexane, methylcyclohexane), olefins (1-heptene, 2-heptene, 3-heptene), cyclic olefins (cyclohexene, 1,3-cyclohexadiene), and an aromatic hydrocarbon (toluene). The additives were 2-ethyl-hexyl-nitrate and di-tertiary-butyl-peroxide. It was found that fuels which contained the structure -CH 2-CH 2-CH 2- showed two-stage ignition with relatively short ignition delays and that the ignition delay depended strongly on the energy released during the first-stage. For primary reference fuel mixtures (n-heptane + iso-octane), the ignition delay depended only on the molar ratio of n-heptane to oxygen and was independent of the octane number (percent iso-octane). On the other hand, the burn rate depended on both these parameters, which uniquely determine the equivalence ratio. When additives were included in the air/fuel mixtures, the ignition delay was reduced but the burn rate was not affected. These results indicate that for HCCI combustion, the ignition delay and the burn rate can be independently controlled using various fuel mixtures and additives.