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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