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Estudo da combustão num motor de combustão interna
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Abstract(s)
O mundo hodierno enfrenta desafios urgentes, entre os quais se destaca a dependência energética associada a elevadas emissões poluentes. O setor dos transportes, particularmente resistente à transição energética, é um dos mais críticos. Neste contexto, este trabalho investiga o uso de misturas de combustível com diferentes proporções, em volume, de metano e hidrogénio em motores que operam segundo os ciclos teóricos de Otto e Atkinson. Foram analisadas as emissões de monóxido de carbono (CO), dióxido de carbono (CO2) e monóxido de azoto (NO) em função de diferentes razões de compressão (Otto) e de expansão (Atkinson), considerando variações na composição do combustível e na quantidade de ar admitida. A modelação teórica baseou-se no cálculo do equilíbrio químico via folha de cálculo em Excel, para compreensão do processo de combustão, complementada por simulações de cinética química realizadas através de uma rotina de cálculo escrita em linguagem Python, permitindo acompanhar a formação e destruição de poluentes durante a expansão. A formação de NO foi estimada pelo método de Zeldovich. Os resultados indicam que temperaturas e pressões mais baixas reduzem as emissões. Combustíveis com menor poder calorífico produzem menos poluentes. Maior teor de metano aumenta as concentrações de CO, CO2 e NO, devido ao incremento energético e disponibilidade do carbono para reação. O aumento de ar admitido reduz a temperatura e a formação de CO em detrimento do CO2. Com a diminuição da quantidade de ar verifica-se o inverso. Elevadas quantidades de ar promovem menor temperatura e conseguinte menor formação de NO após a combustão, mas no fim da expansão,
devido à maior presença de oxigénio, são as misturas com maior excesso de ar que apresentam maior concentração de NO. As razões de compressão e expansão influenciam apenas as concentrações na medida em que afetam o volume da câmara de combustão, sem alterar as quantidades molares totais.
The contemporary world faces urgent challenges, among which energy dependence associated with high pollutant emissions stands out. The transport sector, particularly resistant to energy transition, is one of the most critical. In this context, the present work investigates the use of fuel mixtures with different volumetric proportions of methane and hydrogen in engines operating according to the theoretical Otto and Atkinson cycles. Emissions of carbon monoxide (CO), carbon dioxide (CO2), and nitric oxide (NO) were analysed as a function of different compression ratios (Otto) and expansion ratios (Atkinson), considering variations in fuel composition and the amount of intake air. The theoretical modelling was based on chemical equilibrium calculations performed in an Excel spreadsheet to understand the combustion process, complemented by chemical kinetics simulations carried out using a computational routine written in Python, allowing the monitoring of pollutant formation and destruction during expansion. The formation of NO was estimated using the Zeldovich mechanism. The results indicate that lower temperatures and pressures reduce emissions. Fuels with lower calorific value produce fewer pollutants. A higher methane content increases CO, CO2, and NO concentrations due to the energy increment and greater availability of carbon for reaction. An increase in intake air reduces temperature and CO formation in favour of CO2. Conversely, a reduction in intake air leads to the opposite effect. High amounts of intake air result in lower post-combustion temperatures and consequently lower NO formation, however, at the end of the expansion, due to the greater availability of oxygen, mixtures with higher excess air present higher NO concentrations. Compression and expansion ratios influence concentrations only insofar as they affect the combustion chamber volume, without altering the total molar quantities.
The contemporary world faces urgent challenges, among which energy dependence associated with high pollutant emissions stands out. The transport sector, particularly resistant to energy transition, is one of the most critical. In this context, the present work investigates the use of fuel mixtures with different volumetric proportions of methane and hydrogen in engines operating according to the theoretical Otto and Atkinson cycles. Emissions of carbon monoxide (CO), carbon dioxide (CO2), and nitric oxide (NO) were analysed as a function of different compression ratios (Otto) and expansion ratios (Atkinson), considering variations in fuel composition and the amount of intake air. The theoretical modelling was based on chemical equilibrium calculations performed in an Excel spreadsheet to understand the combustion process, complemented by chemical kinetics simulations carried out using a computational routine written in Python, allowing the monitoring of pollutant formation and destruction during expansion. The formation of NO was estimated using the Zeldovich mechanism. The results indicate that lower temperatures and pressures reduce emissions. Fuels with lower calorific value produce fewer pollutants. A higher methane content increases CO, CO2, and NO concentrations due to the energy increment and greater availability of carbon for reaction. An increase in intake air reduces temperature and CO formation in favour of CO2. Conversely, a reduction in intake air leads to the opposite effect. High amounts of intake air result in lower post-combustion temperatures and consequently lower NO formation, however, at the end of the expansion, due to the greater availability of oxygen, mixtures with higher excess air present higher NO concentrations. Compression and expansion ratios influence concentrations only insofar as they affect the combustion chamber volume, without altering the total molar quantities.
Description
Keywords
Hydrogen Methane Chemical Equilibrium Chemical Kinetics Zeldovich Runge-Kutta Hidrogénio Metano Equilíbrio químico Cinética química