Challenges regarding greenhouse gas emissions in general, and especially emissions of carbon dioxide, highlight the need to reduce the use of fossilfuels, which requires more efficient combustion engines and a transition to renewable fuels, such as e-methanol. As knocking combustion limits the efficiency of a spark-ignited engine, thereby increasing fuel consumption and the emissions, it is a very relevant research topic of today. The research literature has proposed several explanations for knocking combustion. A generally accepted hypothesis is that knock is predominantly initiated from so-called hot spots, i.e. exothermic centers with a deviation intemperature. Nevertheless, the scientific literature suggests that hot spots may not be present in all engine-fuel configurations. Moreover, some studies indicate that other reactivity spots within the engine, such as fuel-rich spotsand oil spots, can contribute to knock as well.The standard approach to mitigating knock is to retard spark timing when knock is detected in earlier cycles. Therefore, this approach penalizes thecycles that would have experienced normal combustion at optimal spark timing, thereby reducing overall combustion efficiency. Hence, a preferred solution for controlling knock is to predict in-cycle if knocking will occur and adjust spark timing accordingly. However, the research literature presents conflicting results regarding the possibility of predicting knock before spark timing. This thesis evaluates the potential for predicting the conditions that lead to incycle knocking combustion in a heavy-duty spark-ignition engine running onmethanol, as well as assessing strategies for mitigating knock and enhancing engine efficiency.The thesis also investigates other potential root causes of auto-ignition in theengine-fuel configuration, including whether lubricant oil entering the combustion chamber can be a contributing factor. The results indicate that it is not possible to accurately predict prior to spark timing whether a cycle will knock. Knock control after spark timing is unlikely to be effective due to the significant overlap in combustion characteristics between normal and knocking cycles. Lubricant oil, rather than hot spots or fuel-rich spots, was demonstrated to be the most likely cause of knock in the current engine-fuel configuration.

Utmaningar kopplade till utsläpp av växthusgaser i allmänhet och koldioxid isynnerhet understryker behovet av att minska användningen av fossila bränslen. Detta kräver mer effektiva förbränningsmotorer och en övergång till förnybara bränslen, såsom e-metanol. Eftersom knackande förbränning begränsar verkningsgraden hos en gnisttänd motor och därmed ökar både bränsleförbrukningen och utsläppen är det ett högaktuellt forskningsområde. Den vetenskapliga litteraturen har föreslagit flera förklaringar till knack. En allmänt accepterad hypotes är att knack främst initieras av så kallade "hotspots", det vill säga exotermiska centra med temperaturavvikelser. Samtidigt visar litteraturen att hot spots inte alltid förekommer i alla motorbränslekombinationer. Dessutom finns studier som pekar på andra reaktiva områden, såsom bränslerika zoner och oljefläckar, som möjliga orsaker till knack. Standardmetoden för att motverka knack är att senarelägga tändtidpunkten när knack upptäckts i tidigare cykler. Denna metod bestraffar dock även decykler som skulle ha haft en normal förbränning vid optimal tändning, vilket därmed minskar den totala förbränningseffektiviteten. En önskvärd lösning för att reglera knack är därför att kunna förutsäga inom en förbränningscykel om knack kommer att inträffa och justera tändtidpunkten utifrån denna information. Tidigare forskning har dock visat motstridiga resultat vad gäller möjligheten att förutsäga knack före tändning. Denna avhandling syftar specifikt till att klargöra, för en tung gnisttändmotor som drivs med metanol, potentialen att inom en förbränningscykel förutsäga om knackande förbränning kommer att inträffa, samt om den kan mildras inom samma cykel. Avhandlingen har även undersökt andra möjliga grundorsaker till självantändning i motorbränslesystemet, däribland huruvida smörjolja som tränger in i förbränningskammaren kan vara en bidragande faktor. Resultaten visar att det inte är möjligt att med hög noggrannhet förutsäga före tändtidpunkten om en cykel kommer att knacka eller inte. Reglering av knack efter tändning bedöms inte som fördelaktigt, på grund av stort överlapp i förbränningskaraktäristik mellan normala och knackande cykler. Smörjolja, snarare än hot spots eller bränslerika zoner, visade sig vara denmest sannolika orsaken till knack i den aktuella motorbränsleuppsättningen.

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 arelimited 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 theengine can emit large spikes in particle numbers (PNs) under stationary operating conditions. Thesespikes were several orders of magnitude higher than for the base particle emissions level and occurredseemingly at random. The source of the spikes was believed to be the cylinder-piston-ring systemand as 50–99% of the particles stemmed from these spikes, the influence on the particle emissionsmade it an interesting investigation to find the root cause of it. The experiments were performedfor different piston ring loads, locked ring positions, and different oil compositions. The resultsindicate a possibility to control the PN emissions through the experiment alterations, with lockedpiston rings having the greatest influence at a higher load. There was no clear relation between ringrotation and flutter with the spikes observed. The locked piston ring configurations did indicate thering gap not to be the main contributor to the spiking as fully aligned gaps did not result in thehighest levels of particle emissions. Variations to the oil composition indicate that a high-volatilityoil will emit higher levels of small, sub-10 nm particles compared to the standard baseline oil. Ahigh-viscosity oil instead lowers the particle emissions, possibly due to the higher inner friction athigh temperatures reducing the oil ingress into the combustion chamber. The link between the PNspiking phenomenon and the oil pathway past the piston ring was not established through theexperiments reported in this publication.

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.