Lubricant oil influence on the combustion process of conventional and Innovative internal combustion engines

The negative impacts of climate change are mounting very fast and bold actions are required, so that significant efforts are needed to improve the current technologies for mobility because of the competitive marketplace as well as of the ever-tightening regulations concerning emissions and fuel economy. In this scenario, internal combustion engines (ICEs) need radical modifications to be still considered a sustainable option for the future. The motivation for the proposed research stems from evidence reported in the literature, which suggests that lubricant oil droplets are the primary inducer of abnormal combustion modes. Additionally, lubricant oil can represent a major source of very fine soot particles emitted from engines. These adverse effects severely hinder the further development of efficient and cleaner internal combustion engines. However, the influence of lubricant oil on the combustion process is not yet fully understood as it is often neglected in numerical simulations, due to the assumption that the amount of lubricant oil reaching the combustion chamber is negligible. Thus, there is a need to fill the knowledge lack of lubricant-oil-related auto-ignition processes in engines. In this scenario, the accurate chemical modelling of the fuel-lubricant interaction represents an essential aspect for enabling the further development of the internal combustion engine for providing information difficult or impossible to obtain solely through experiments. The present work tries to shed light on the criticalities arising from the interaction between fuel and lubricant oil in modern internal combustion engines, through rigorous numerical investigations. A variety of fuels, including traditional fossil fuels (gasoline) as well as carbon-free alternatives (hydrogen), were considered. As a first step a way for considering the presence of trace amounts of lubricant in the combustion chamber was defined, by selecting proper lubricant oil surrogate species. Consequently, several detailed chemical models were developed and validated to isolate the lubricant oil's contribution to abnormal combustion events. However, it is well known that CFD numerical simulations employing detailed chemical models are computationally expensive. In light of this, the detailed chemical models were reduced for the purpose of conducting Three-Dimensional (3D) CFD numerical simulations, in order to reproduce experimental data involving lubricant oil combustion. With the aim to further reduced computational costs, a practical and simple analytical correlation able to predict variations of gasoline ignition delay induced by the presence of lubricant oil, at different temperatures, is presented. Such a correlation was developed by considering both experimental data available in the literature and numerical simulations. The urgent need to reduce both the environmental impact of mobility and the dependence on fossil fuels has re-ignited the interest toward Hydrogen Internal Combustion Engines (HICEs). However, there are still criticalities that need to be assessed for accelerating the development of this technology. The undesired but unavoidable participation of lubricant oil to the combustion process is considered at the basis of the criticalities that affect the development of a reliable and efficient HICEs. In order to investigate and highlight the role of lubricant oil in the development of HICEs a specific reduced chemical model was developed and validated. The newly developed chemical model was employed in Zero-Dimensional (0D) simulations with the aim to quantify the effects that lubricant oil can have on hydrogen ignition delay time (IDT) in engine-like conditions.