Etude de la variation cyclique de combustion d'un moteur a allumage commande a partir des temperatures d'echappement

This thesis consists of a study of cyclic variability of exhaust gas temperature of a spark ignited engine. For different engine working conditions, the cyclic variability of exhaust gas temperature is compared to cyclic variability of combustion. Exhaust gas temperature is measured at the exhaust port exit, at about 10 cm from the exhaust valve and cyclic variation of combustion is determined from cyclic variation of the indicated mean effective pressure obtained from in cylinder pressure measurements. This thesis tends to establish the link between the cyclic variability of exhaust gas temperature and combustion cyclic variability. A temperature sensor placed at the exhaust port exit could be used to obtain additional information on the engine working conditions and its stability. The sensor could be used to infer on the engine working parameters, from an input given to the electronic control unit, to keep the engine at optimum working condition. Other studies have been done on the subject but always using thermocouple compensation techniques from the reading of two thermocouples of different diameters. The principal part of the work done in this study uses the temperature reading directly obtained from two thermocouples of very small diameter (25.4 and 50.8 mum) without any temperature compensation techniques. Two articles have been written on the subject during the study. The first part of the work was done using a sheathed exhaust temperature sensor (usually used in gas turbines) and a signal processing technique developed by the company Nexum Research Corporation, located in Kingston Ontario. The second part of the work studies the exhaust gas temperature cyclic variability from the reading of small diameter thermocouples inserted at the exhaust port exit. During this part of the study, a temperature compensation technique from the reading of two thermocouples of different diameter was also explored. The principal hypothesis for the two parts of this thesis was that there will be a positive correlation between cyclic variability of combustion and cyclic variability of exhaust gas temperature. The results obtained from the first part of the work done with a sheathed exhaust gas temperature thermocouple was that there is a positive correlation between the variation of the signal of the temperature sensor and the cyclic variability of combustion when groups of successive cycles are used to evaluate the respective coefficients of variation. Also, the signal obtained from the sensor is sensible to the engine working conditions. The results obtained at different speed and load showed that the signal obtained from the sheathed temperature sensor could vary for a given value of combustion cyclic variability. The work done during the second part of the study shows firstly, the limits associated with a temperature reconstruction technique used to determined cyclic variation of combustion. Secondly, the results obtained from the temperature measurements show that a positive correlation exists between the cyclic variation of mean effective pressure and the cyclic variation of maximum exhaust gas temperature when groups of successive cycles are used to calculate the different coefficients of variation. Also, a link exists when all values of mean effective pressure and maximum temperature from a particular test (250 consecutive cycles) are drawn together. For both cases, the correlation is stronger when cyclic variation of combustion is higher. In conclusion, cyclic variability of combustion is reflected in cyclic variability of exhaust temperature for tests done at different air fuel ratios, different spark timing and different injection timing. The intensity of the correlation depends on the test done. When the engine operates in a stable regime, when the engine load is varied at constant speed or when the engine speed is varied at constant load, the results obtained are less significant.Keywords: Spark ignited engine, cyclic variability of combustion, exhaust temperature.