Towards a Thermally Robust Automotive Pre-Chamber Spark Plug for Turbocharged Direct Injection Gasoline Engines

This study presents a first step towards a thermally robust passive pre-chamber spark plug for boosted direct injection gasoline engines, where the pre-chamber spark plug is used to precisely control the start of combustion, increase the engine efficiency, while decreasing pollutant emissions and providing a more stable combustion, compared to conventionally initiated combustion with a j-gap spark plug. The here presented passive pre-chamber spark plug can be integrated into an existing cylinder head design without introducing additional components. A new passive pre-chamber spark plug is proposed based on a surface discharge electrode configuration, in order to extend the operating range of a previously developed pre-chamber spark plug using a two pin ground electrode configuration. The new passive pre-chamber spark plug is based on a patented surface discharge concept (GB 2 361 264 A), which allowed us to create a barrier-free pre-chamber working volume, to better control the residual gas distribution, turbulence and flame front propagation inside the pre-chamber. The key parameters of the pre-chamber, such as prechamber volume and hole diameter were estimated in a first step by using a simple gas displacement model and by 3D heat transfer simulations, using thermal boundary conditions under typical full-load operating conditions. Furthermore, in order to better understand the inner physical phenomena of our passive pre-chamber spark plugs, additional 3D CFD simulations of the turbulent mixing and subsequent combustion were carried out by means of large eddy simulations (LES). The LES simulations only considered the pre-chamber volume and were solely applied for the compression stroke. The influence of the engine was modelled as time depended boundary conditions coming from 0D simulations. Combustion was modelled by a flame surface density approach and ignition initiated by an increase of the flame surface density in a spherical volume in the vicinity of the spark plug. The 3D CFD simulation revealed a strong sensitivity of the residual gas distribution and turbulent flow field to the prechamber orifice and inner volume geometry, which influenced the combustion progress inside the pre-chamber significantly. First promising experiments on an engine test bench with the new barrier-free concept were conducted, where the working envelop could have been extended compared to our previously developed two-pin pre-chamber spark plug design.