A New Methodology for Comparing Knock Mitigation Strategies and Their Stability Margin 2023-01-0248

The automotive sector is rapidly transitioning to decarbonized, electric vehicles solutions. However, due to challenges with such rapid adoption, Internal combustion engines (ICE) are expected to be used for decades to come. In this transition period it is important to continue to improve ICE efficiency. A key design parameter to increase ICE efficiency is the compression ratio. For gasoline engines, the compression ratio is limited so as to avoid knock. Engine designers can employ several strategies to mitigate knock and enable higher compression ratios. In this study, a new methodology has been developed to compare various knock mitigation strategies. By comparing the knock limited load at a given combustion phasing the expected compression ratio increase can be inferred. Knock mitigation techniques examined in the paper include coolant temperature, manifold temperature, start of injection, split fuel injection, exhaust gas recirculation (EGR), and water injection, both port and direct. Critically, a knock mitigation technology should not excessively compromise the combustion stability. This methodology includes a test for the combustion phasing at a 5% coefficient of variation (CoV) of the net indicated mean effective pressure (nIMEP). The difference between the CA50 at the knock limited load and the stability limited is noted as the stability margin. It is critical for designers and engineers to insure that an engine operates within both the knock and stability boundaries. Different strategies for knock mitigation affect knock and stability differently. The proposed methodology provides an efficient yet accurate way to quantify the impact of technologies. For example, EGR provides a significant increase in knock limited load but also decreases the stability margin by increasing ignition requirements. Water injection, on a per volume basis, provided a smaller increase in knock limited load but has a smaller impact on combustion stability.