Impact Resistance of Spark Plug’s Ceramic Insulator During Ultra-high-Pressure Combustion Under Deto-Knock Conditions

The ceramic insulators of spark plugs in gasoline engines are especially prone to damage when deto-knock occurs. To understand the damage process and mechanism, the present work investigated the impact resistance of ceramic insulators using detonation waves as impact sources. A test device that generates detonation waves was developed, representing a novel means of evaluating the knock resistance of ceramic insulators. Various impact types and detonation intensities were employed, and detonation initiation and propagation at peak pressures greater than 100 MPa were assessed using synchronous high-speed direct photography and pressure measurements. The test results demonstrate that ceramic insulators tend to break at the base of the breathing chamber when damaged by a single high peak pressure detonation wave impact. In contrast, multiple low pressure impacts eventually break the insulator into multiple fragments. The data also show that the positioning of a ground electrode upstream of the ceramic insulator greatly increases the resistance of the ceramic to the detonation impact. A two-dimensional computational fluid dynamics simulation coupled with a chemical kinetics analysis demonstrated that this improved resistance can be ascribed to a reduced peak pressure that appears after the detonation wave diffracts from the electrode prior to contacting the ceramic insulator.