Correlation of Oil Originating Particle Emissions and Knock in a PFI HD SI Engine Fueled with Methanol 2023-24-0036

A viable option to reduce global warming related to internal combustion engines is to use renewable fuels, for example methanol. However, the risk of knocking combustion limits the achievable efficiency of SI engines. Hence, most high load operation is run at sub-optimal conditions to suppress knock. Normally the fuel is a limiting factor, however when running on high octane fuels such as methanol, other factors also become important. For example, oil droplets entering the combustion chamber have the possibility to locally impact both temperature and chemical composition. This may create spots with reduced octane number, hence making the engine more prone to knock. Previous research has confirmed a connection between oil droplets in the combustion chamber and knock. Furthermore, previous research has confirmed a connection between oil droplets in the combustion chamber and exhaust particle emissions. However, the co-variation between oil originating particle emissions and knock has not been investigated. The current study examines the connection between knock and particle number in the exhaust, when running on fuel with low soot production. A single cylinder spark ignited heavy-duty engine was used. It was equipped with port fuel injection and fueled with methanol, which produces very little soot at lambda 1. Consequently, the measured exhaust particle numbers were assumed to origin essentially from engine oil. Three grades of oil, in combination with three piston ring configurations, were used to vary the amount of oil entering the combustion chamber. Results from knock limited operation at both medium and high engine load showed that an increased number of particles in the exhaust was associated with an increased likelihood of knock. The authors find the hypothesis that an increase in particle number correlates with an increase in auto-ignition tendency to be confirmed.