Effect of Combustion Characteristics on Knocking in a Direct Injection Turbo-Charged Gasoline Engine

Experimental study on knocking characteristics in a direct injection turbo-charged gasoline engine was carried out. The thermodynamic analysis was conducted to investigate effects of the combustion phasing and the burning rate on the knocking behavior. The localization of knock events and the characterization of the early flame kernel propagation were conducted with the fiber optic sensor. The advanced combustion phasing and the slower combustion speed generally increased the knocking probability. However, not only quasi-dimensional thermodynamic combustion characteristics but also the spatial parameter such as the flame propagation direction significantly affected the knocking occurrence. From the fiber optic sensor test results, knocking onset location was found to be closely correlated with the flame propagation direction and mainly observed in the opposite side to the main flame propagation direction. The flame propagation direction leaning to the exhaust side was identified to be favorable for the knocking mitigation because the end gas location on hotter exhaust side could be avoided. Engine tests for various squish designs and tumble port designs were implemented to study the effect of the in-cylinder flow, which significantly affects previously discussed knocking-related parameters. As a result, tumble and squish flow significantly increased combustion speed and advanced combustion phasing. Fuel consumption could be also reduced due to suppressed knocking combustion. In addition, new tumble port design enabled the flame propagation to have favorable leaning direction.