Correlating Particle Number Emissions to the Rotation of the Piston Ring

Reaching the particle emissions regulatory limits for the combustion engine is a challenge for developers. Particle filters have been the standard solution to reduce particle emissions, but filters are limited in storage capacity and need to be regenerated, a process emitting more carbon dioxide (CO₂) as more fuel is consumed to regenerate the filter. In previous research, it was found that the engine can emit large spikes in particle numbers (PNs) under stationary operating conditions. These spikes were several orders of magnitude higher than for the base particle emissions level and occurred seemingly at random. The source of the spikes was believed to be the cylinder-piston-ring system and as 50–99% of the particles stemmed from these spikes, the influence on the particle emissions made it an interesting investigation to find the root cause of it. The experiments were performed for different piston ring loads, locked ring positions, and different oil compositions. The results indicate a possibility to control the PN emissions through the experiment alterations, with locked piston rings having the greatest influence at a higher load. There was no clear relation between ring rotation and flutter with the spikes observed. The locked piston ring configurations did indicate the ring gap not to be the main contributor to the spiking as fully aligned gaps did not result in the highest levels of particle emissions. Variations to the oil composition indicate that a high-volatility oil will emit higher levels of small, sub-10 nm particles compared to the standard baseline oil. A high-viscosity oil instead lowers the particle emissions, possibly due to the higher inner friction at high temperatures reducing the oil ingress into the combustion chamber. The link between the PN spiking phenomenon and the oil pathway past the piston ring was not established through the experiments reported in this publication.