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Research

  • Knock Indexes Normalization Methodologies

    Year: 2006

    Author: Nicolò Cavina, Enrico Corti, Giorgio Minelli, Davide Moro, Luca Solieri

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    Gasoline engines can be affected, under certain operating conditions, by knocking combustions, which can result in serious engine damage. Specific power and efficiency are influenced by factors such as compression ratio and spark advance regulation, that modify the combustion development over the crank angle: the trade-off between performance and the risk of irreversible damages is still a key factor in the design of both high-performance (racing) and low-consumption engines. New generation detection systems, especially based on ionization current technology, allow aggressive advance mapping and control, and future equipment, such as low-cost in-cylinder pressure transducers, will allow following that trend.

    Also HCCI (Homogeneous Charge Compression Ignition) engines need a sophisticated combustion monitoring methodology, since increasing BMEP levels in HCCI mode force the combustion to approach the knocking operation.

    Many methodologies can be found in the literature to recognize potentially dangerous combustions, usually based on the analysis of accelerometer, in-cylinder pressure or ionization current signals. Signals are sampled with high sample rates, then filtered, for a clear recognition of the phenomenon. Filtered signals can then be used to define damage-related indexes, by means of various types of mathematical operations. The indexes are then compared to pre-defined thresholds, for the diagnosis of dangerous combustion events.

    Thresholds setting is a challenging task, since indexes are not usually intrinsically related to the damages caused by abnormal combustion events. Furthermore, the indexes values usually strongly depend on the engine operating conditions (speed and load), and thresholds must therefore vary with respect to speed and load. It can be said that indexes generally depend on the combustion development over the crank angle, and not necessarily on the knock phenomenon. This means that the index value is also influenced by the spark advance regulation, even without knock.

    This paper shows why commonly used indexes values vary with engine running conditions, and how raw indexes can be modified in order to obtain an operating conditions independent information. In-cylinder pressure data are analyzed both in the time and frequency domains, in order to show how parameters such as window angular position and width, and band-pass filter characteristics influence the filtered in-cylinder pressure signals, and, as a consequence, knock indexes. Such parameters can affect both the signal to noise ratio and the operating condition dependence, thus they must be carefully optimized.

    Once operating conditions effects on knock indexes are known, they can be taken into account for indexes evaluation, letting emerge the plain knock effect. This compensation can be carried out in many ways: in the paper two possible methodologies are considered and compared. Both of them perform an index normalization, the first one in the time domain, the second one in the frequency domain.

    The frequency-domain normalization methodology proves to be efficient in finding operating conditions-independent knock indexes, allowing an a significant reduction of the calibration time. The time-domain methodology is more influenced by the window choice

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  • A Heat Flux Damages-Related Index

    Year: 2006

    Author: Nicolò Cavina, Enrico Corti, Luca Solieri

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    Gasoline engines can be affected, under certain operating conditions, by excessive heat flux through the combustion chamber walls, which can result in serious engine damage. Specific power and efficiency are influenced by factors such as compression ratio and spark advance regulation, that modify the combustion development over the crank angle: the trade-off between performance and the risk of irreversible damages is still a key factor in the design of both high-performance (racing) and low-consumption engines. New generation detection systems, especially based on ionization current technology, allow aggressive advance mapping and control, and future equipment, such as low-cost in-cylinder pressure transducers, will allow following that trend. Also HCCI (Homogeneous Charge Compression Ignition) engines need a sophisticated combustion monitoring methodology, since increasing BMEP levels in HCCI mode force the combustion to approach large heat-flux operation. Many methodologies can be found in the literature to recognize potentially dangerous combustions, usually based on the analysis of accelerometer, in-cylinder pressure or ionization current signals. Signals are sampled with high sample rates, than filtered, for a clear recognition of the phenomenon. Filtered signals can then be used to define damage-related indexes, by means of various types of mathematical operations. The indexes are then compared to pre-defined thresholds, for the diagnosis of dangerous combustion events. Thresholds setting is a challenging task, since most indexes are usually not intrinsically related to the damages caused by abnormal combustion events. Furthermore, the indexes values usually strongly depend on the engine operating conditions (speed and load), and thresholds must therefore vary with respect to speed and load. This paper presents a novel approach to the problem, whose objective is to define a damage-related and operating conditions-independent index. The methodology is based on the in-cylinder pressure signal, that is used for the Rate Of Heat Release evaluation. An onset condition is defined, for the dangerous phenomenon identification, and the mean thermal power released during the over-heating part of the combustion is considered as a damage intensity index. The paper shows that this parameter does not depend on the engine operating conditions, and it reaches similar values for different types of engine, under critical conditions. The index, however, must also take into account the malfunction frequency, since permanent damages are not caused by isolated events. The use of a moving average filter on the raw index is aimed at obtaining a stable output, more representative of the permanent damage risk and less influenced by the single combustion. These considerations lead to the definition of a heat flux index, strictly related to the damages caused by abnormal combustions. The diagnostic threshold value is constant over the entire operating range. Once the index is defined, it can be implemented on a control unit for real time diagnosis, or it can be used as a reference for the off-line calibration of other indexes. Examples are shown of other indexes trends and threshold calibrations over the engine operating range.

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