Lanotte, Alfredo (2023) Evaluating RCCI and HCCI combustion for reducing emissions in marine engines: a computational analysis. [Tesi di dottorato]

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Abstract

The history and evolution of the internal combustion engine (ICE) have shaped the modern world, revolutionizing transportation and power generation. While ICEs continue to dominate various sectors, the growing focus on environmental sustainability and stringent emissions regulations has driven the need for cleaner and more efficient engine technologies. To address the challenges posed by stringent emissions regulations and environmental concerns, researchers and automakers have implemented various technologies to reduce the environmental impact of ICEs, including low-temperature combustion (LTC) modes, such as Homogeneous Charge Compression Ignition (HCCI) and Reactivity Controlled Compression Ignition (RCCI). LTC modes, particularly HCCI and RCCI, offer improved fuel efficiency and reduced emissions, contributing significantly to environmental sustainability. The ability to operate under lean conditions enhances thermal efficiency, resulting in lower fuel consumption and reduced nitrogen oxide (NOx). Computational fluid dynamics (CFD) and zero-dimensional (0D) modelling techniques have become indispensable tools for optimizing engine performance, controlling emissions, and facilitating virtual prototyping. In this scenario, this doctoral thesis explores the development and optimization of low temperature combustion (LTC) models, specifically focusing on Homogeneous Charge Compression Ignition (HCCI) and Reactivity Controlled Compression Ignition (RCCI) combustion modes. The research primarily concentrates on the development and evaluation of low-temperature combustion models, incorporating advanced approaches such as tabulated kinetics of ignition (TKI) and multi-zone combustion modelling. The TKI approach, utilizing tabulated data to simulate autoignition behaviour, offers a more efficient alternative to conventional chemical kinetics simulations. The efficacy of the proposed method has been validated by comparing it to a detailed chemical kinetics simulation in terms of pressure and temperature evolutions. This comparison was carried out on both an adiabatic homogeneous reactor and an engine, considering various operating conditions. The multi-zone combustion model coupled with the TKI approach enables a detailed analysis of the key characteristics and behaviours of HCCI and RCCI combustion, providing a comprehensive representation of the complex combustion processes involved. Specifically, the validation of the HCCI model involves three engines, each operating with distinct fuels such as hydrogen, methane, n-heptane, and an n-heptane/toluene/ethanol blend. The validation process considers varying boundary conditions including intake temperature, air/fuel ratio, and level of exhaust gas recirculation (EGR). The model's ability to accurately predict engine behaviour is demonstrated by evaluating its performance against measured data, specifically in terms of pressure traces, rate of heat release, and emissions levels. The numerical results exhibit a strong agreement with the experimental counterparts, without the need for case-dependent tuning. The model effectively captures the characteristic thermal stratification within the engine being studied. Through this validation procedure, the reliability and accuracy of the HCCI model are assessed and confirmed. The RCCI model has been validated against the experimental data of large bore single cylinder research engine (SCE) under various operating conditions, which considers a range of engine load, air/fuel ratio, light fuel oil (LFO) quantity, valve timing, and intake air temperature variations. The engine is fuelled by natural gas supplied through the intake port, while LFO is directly injected into the cylinder. The proposed numerical approach successfully simulates the experimental data with high accuracy, utilizing a fixed tuning constant set. The model demonstrates excellent predictive capability, with an average error of less than 5% for global performance and combustion parameters. It effectively captures the engine's behaviour under diverse operating conditions and provides insights into the physics of these advanced combustion concepts. Moreover, a comprehensive evaluation of the emissions characteristics exhibited by the investigated engine has been carried out. A key aspect of this evaluation involved the development of emission models for nitrogen oxides (NOx) and unburned hydrocarbons (uHCs). The simulation of NOx emissions was achieved through the implementation of a widely recognized reduced chemical kinetic approach (Zeldovich approach). Additionally, a refined methodology for modelling uHC emissions was integrated into the code as a submodel. This innovative model incorporates various sources of uHC production, including crevices, and flame wall quenching, also considering post-oxidation process. By considering these distinct contributing factors, the model significantly enhances the accuracy of uHC emission predictions. Furthermore, the models' potential for accommodating alternative fuels, specifically ammonia, is investigated. A comprehensive analysis was conducted under several operating conditions, considering variations in load, equivalence ratio, and valve phasing. The primary objective of this evaluation was to assess the model's capability to simulate and predict the engine's behaviour when utilizing alternative fuels. Moreover, a sensitivity analysis was performed to demonstrate the model's responsiveness to changes in key parameters. This examination aimed to evaluate the model's effectiveness in capturing variations in the engine's main operating parameters and maintaining reasonable responses. The findings confirm the model's credibility as a reliable tool for assessing and analysing the performance of engines that utilize novel fuels and employ low-temperature combustion (LTC) modes. This refined and validated model holds great promise for the development of new engine architectures that employ alternative fuels, facilitating the design of more efficient and environmentally friendly powertrains.

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