The global automotive industry is undergoing a significant transition as battery electric vehicles enter the market and diesel sales decline. It is widely recognized that internal combustion engines (ICE) are needed for transport for years to come, however, demands on fuel efficiency, emissions, cost, and performance are extremely challenging. Gasoline compression ignition (GCI) is one approach to achieving demanding future efficiency and emissions targets. A key technology enabler for GCI is partially premixed, compression ignition (PPCI) combustion, which involves two high-pressure, late, fuel injections during the compression stroke. Both NOx and smoke emissions are greatly reduced relative to diesel engines, and this reduces aftertreatment (AT) requirements significantly.
Exhaust rebreathing (RB) is used for robust low-load and cold operation. This is enabled by use of 2-Step, mode switching rocker arms to allow switching between rebreathe and normal combustion modes. Exhaust rebreathing involves reinduction of hot exhaust gases into the cylinder during a second exhaust lift event during the intake stroke to help promote autoignition. The amount of exhaust rebreathing is controlled by exhaust backpressure created by the vanes on the variable nozzle turbine (VNT) turbocharger. Due to higher cycle temperatures when rebreathing, exhaust HC and CO may be significantly reduced, while combustion robustness and stability also improve. Increased charge dilution during rebreathing can also lower NOx emissions. Importantly, exhaust rebreathing significantly increases exhaust temperatures to maintain active catalysis in the AT system for ultra-low tailpipe emissions.
A 2-step valvetrain system was designed and developed for exhaust rebreathing on a 2.6l light-duty gasoline compression ignition engine. Tri-roller, switching rocker arms with hydraulically actuated lock pins were built for low friction. The 2-step actuation system was designed for fast response using a pulse-width-modulated oil control valve that regulated the oil pressure feeding the switching rocker arms.
Tests were conducted on the dynamometer demonstrating robust combustion with high exhaust temperatures and active catalysis at low load operation. Calibration mapping tests were also conducted. Overall, the tests demonstrated the simplicity and robustness of the exhaust rebreathing approach while delivering low exhaust emissions.