Study on the Efffect of Cylinder Wall Temperatures on Knock Characteristics in Spark-Ignited Engine

Abstract

To cope with stringent regulations of fuel economy and emission, development of high efficiency gasoline engine is getting urgent than ever before. Due to its intrinsic characteristics, the increment of compression ratio has a direct relationship to efficiency enhancement. However, increasing compression ratio provokes knock-prone in-cylinder condition. The temperature and pressure of the end gas area increase during the combustion period, and this may cause spontaneous ignition of air-fuel mixture which is known as knocking phenomena.<br/> Knock phenomena has to be avoided because of the engine failure and its unfavorable noise during engine operation. Numbers of attempts have been made to reduce the temperature of end-gas to mitigate knock. Exhaust gas recirculation and water injection studies are now actively being introduced, and there have been experimental suggestions on cooling optimization, dual-loop cooling and introduction of insulated intake port. It has been shown by a simulation study that the intake port has a great effect on gas temperature reduction, however, quantification and in-depth study of effects of other wall components has not been investigated. Furthermore, reflection of temperature variation on control is needed for future utilization of knock or auto-ignition for higher efficiency.<br/> In this study, a systematic investigation was conducted to observe how wall temperatures affect the overall knock behavior. Therefore, study for experimental methodology of knocking combustion was conducted according to the necessity of exquisite test and analysis. It was found that decreasing coolant temperature shows a significant effect on knock mitigation.<br/> With the segregation of the head and liner coolants and the introduction of the piston oil cooling gallery, the wall temperature of each component was controlled. Temperature measurement including piston surface facilitated by designing an optimized linkage system, showed that independent temperature control was achieved. Individual impact of each wall component on knocking behavior was assessed using refined experimental techniques. As a result, the effect of head temperature reduction was found to be greater than that of the liner temperature reduction under various conditions such as different engine geometry. Also piston cooling showed a remarkable effect by reducing its surface temperature.<br/> In addition, 3D simulation analysis was conducted for deeper understanding. As a result, it was confirmed that the effect of temperature decrease on heat transfer from the combustion gas to the wall was restrictive due to originally large temperature difference. Reducing wall temperature showed a large knock mitigation effect due to reduction the gas temperature during the intake and compression process. The intake port has the greatest effect during the intake process, and after insulation of intake port, temperature reduction by the liner wall was found to be more effective in reducing temperature than that of head wall. The liner cooling effect was higher in long-stroke engine, and intensified tumble flow showed a potential of gas temperature decrease by increasing the heat transfer from gas to wall during compression stroke.<br/> To establish a cooling strategy of liner wall cooling depending on knock position, knock localization was demonstrated by PCB ion-probe gasket. No significant change in knock location was observed while chilling the liner wall. Knock was mainly occurred in intake and exhaust side under weak knock condition and occurred simultaneously around the cylinder bore with multiple spots regardless of wall temperature variation. This confirms that the knock location was not heavily affected by wall temperature, but rather critically affected by flame propagation.<br/> Lastly, for a future control of auto-ignition and knock phenomena under transient engine operation, a fast 0D knock prediction model was established based on ignition delay correlation and individual cycle analysis including improved knock onset determination. The validity under variation of coolant temperature was identified, and the robustness was thoroughly secured.