Insights into abnormal combustion in advanced gasoline engines

Thesis

Abnormal combustion has been a significant problem for engine manufacturers and owners for the last century. This has taken several forms, aggressive normal knock, pre-ignition and mega-knock. It has had severe impacts throughout engine development programmes and in production, impacting timing, cost and warranty. Abnormal combustion continues to be a problem in world markets, with potential risks to future engines due to in-market exposure to 'bad fuels', varying lubricant properties, ambient conditions and driving style. Researchers have made significant steps in the understanding of the causal mechanisms surrounding abnormal combustion. Detection controls have been implemented in the form of dedicated test cycles. There has also been research regarding the role of the lubricant, in particular detergent packs containing alkaline metallic compounds. However there are still significant gaps in the knowledge. One such gap identified was 'why does calcium promote abnormal combustion, whilst magnesium doesn't?'. This thesis presents the findings of investigations relating to the impact of specific fuel and lubricant properties. A cold driven shock tube was used to visualise interactions between metal oxide (calcium and magnesium) powers and (carbon dioxide, nitrogen and argon) gasses. Results suggest a new mechanism for Low Speed Pre-Ignition (LSPI) via a previously unseen reaction between calcium oxide and nitrogen. It was noted that typical end of compression temperature would be insufficient to provoke the observed response, suggesting a contributory factor from internal residual gasses. Fuel and lubricant from the top land crevice volume of a single cylinder gasoline engine were collected and analysed to establish an understanding of the in-service composition of the top land crevice material. This study included the use of advanced diagnostic techniques (Petroleomics) carried out in collaboration researchers at the University of Warwick. The results indicate the presence of new chemical compounds present in the top land crevice not present in the new fuel and oil. Multi-cylinder engine testing of LSPI promoting fuels, with the addition of Methylcyclopentadienyl Manganese Tricarbonyl, with a lubricant containing calcium detergent was also conducted to determine the abnormal combustion characteristics. The results show previously unseen levels of abnormal combustion, so severe that it led to terminal engine damage not previously seen in the public domain.

Authors: McAllister, MJ

• DOI: 10.5287/ora-kobxrxygb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: abnormal combustion; lspi; pre-ignition; magnesium; calcium
• Subjects: Combustion; Internal combustion engine industry; Thermodynamics; Gasoline