Tema 2 CS

Based on the concepts of biofuel incorporation discussed in class, what is the primary impact of increasing the ethanol fraction X in a gasoline-ethanol blend E(X) on a vehicle's fuel consumption and CO2 emissions ?. As the ethanol fraction increases, the volumetric fuel consumption (L/100km) decreases because ethanol has a higher energy density than gasoline. Increasing the ethanol fraction leads to higher volumetric fuel consumption (L/100km) because ethanol's energy density (MJ/L) is significantly lower than that of gasoline. The fuel consumption (L/100km) remains constant regardless of the ethanol percentage, as internal combustion engine efficiency improves proportionally to the blend ratio. Higher ethanol content increases the amount of fossil CO2 emitted per kilometer traveled, making the blend less sustainable than pure gasoline.

What is the fundamental difference in the operating principle between Otto and Diesel engines?. In an Otto engine, ignition occurs by auto-ignition of the air–fuel mixture, whereas a Diesel engine uses a spark plug. In an Otto engine, the air–fuel mixture is ignited by a spark plug, whereas in a Diesel engine, fuel is injected into compressed hot air, where it auto-ignite. In an Otto engine, only air is compressed, whereas in a Diesel engine, a mixture of air and fuel is compressed. Both engines use identical ignition mechanisms and differ only in the type of fuel used. Fuel for gasoline engines should have a low octane index.

Within the family of linear alkanes CnHm, how do the properties of these fuels evolve when moving from short molecules like methane (CH4) to long chain molecules like hexadecane (C16H34) ?. The melting point (°C), density (g/cm 3), and viscosity (mPa.s) increase, while the heat of combustion (MJ/kg) decreases. All these properties increase proportionally to the size of the carbon chain. The melting point (°C) and density (g/cm 3) increase, but viscosity (mPa.s) and heat of combustion (MJ/kg) decrease due to molecular complexity. The viscosity (mPa.s) and density (g/cm 3) increase, but the melting point (°C) and the heat of combustion (MJ/kg) remain constant for all linear alkanes.

Which of the following molecular structures is most indicative of a fuel with a high cetane number?. Highly branched iso-alkanes, which strongly resist radical formation during the compression stroke. Aromatic hydrocarbon rings with high bond energies, requiring a spark to overcome the activation energy. Short-chain alcohols, which have high latent heats of vaporization that cool the combustion chamber. Long, unbranched straight-chain hydrocarbons, which easily fracture and auto-ignite under pressure.

Which of the following correctly orders the fuels from the lowest to the highest energy density per mass (MJ/kg)? (Use: Gasoline = G, Diesel = D, Methanol = M, Ethanol = E). M – E – D – G. E – M – D – G. M – D – E – G. E – M – G – D.

Which statement best explains why alcohol fuels generally have lower energy density than gasoline/diesel?. They contain oxygen in their molecular structure. They contain more sulfur. They are denser than hydrocarbons. They release no heat during combustion.

Which type of sustainable fuel can directly replace conventional fuels without modification?. Biomass-based fuels. Drop-in fuels. Hydrogen fuels. Ammonia fuels.

Analyze the following statements about demand management in the transport sector: Statement I: The adoption of remote work (teleworking) is an example of the "Avoid" strategy. Statement II: The transition from gasoline-powered vehicles to battery electric vehicles is an example of the "Shift" strategy. Statement I is correct, while Statement II is incorrect. Both statements are correct. Statement II is correct, while Statement I is incorrect. Both statements are incorrect.

A vehicle with an internal combustion engine is modified to run on different fuels without changing the engine efficiency. Consider the following fuels: • Gasoline • Diesel • Ethanol • Hydrogen Which of the following statements is correct regarding their performance and emissions?. Hydrogen provides the highest energy density per unit volume, leading to the lowest fuel consumption (L/100 km), and produces no CO₂ emissions. Diesel fuel leads to lower fuel consumption (L/100 km) than gasoline due to its higher volumetric energy density, and diesel engines typically have higher thermal efficiency. Ethanol has a higher energy density than gasoline, resulting in lower fuel consumption and lower CO₂ emissions per km. Gasoline produces less CO₂ per unit of energy than hydrogen because hydrogen combustion releases less energy.

An ICE (internal combustion engine) vehicle currently runs on pure gasoline (E0). A driver is considering switching to E85 fuel (85% ethanol, 15% gasoline by volume). Assuming that the energy required per kilometre driven remains constant, and using the energy densities from the course (gasoline: 34.2 MJ/L; ethanol: 23.4 MJ/L), which of the following statements best describes the effect of this switch on fuel consumption and CO₂ emissions from combustion?. Fuel consumption (L/100 km) increases by approximately 44%, and total CO₂ emitted per kilometre from combustion remains roughly the same as with pure gasoline. Fuel consumption (L/100 km) decreases because ethanol has a higher energy density than gasoline, and CO₂ emissions per kilometre also decrease proportionally. Fuel consumption (L/100 km) stays the same because both fuels are liquids, but CO₂ emissions per kilometre drop significantly. Fuel consumption (L/100 km) increases slightly, and CO₂ emissions per kilometre from combustion also increase because ethanol combustion produces more CO₂ per MJ than gasoline.

Why do diesel engines generally provide better mileage than gasoline engines?. Diesel fuel is more volatile than gasoline. Diesel engines have higher thermal efficiency. Diesel engines produce less torque than gasoline engines. Diesel fuel has lower energy density than gasoline.

A country wants to reduce its reliance on fossil fuels while avoiding competition with food production and making use of existing resources such as agricultural waste. Which biofuel generation is the most appropriate choice?. I generation. II generation. III generation. IV generation.

When comparing first-generation bioethanol (derived from corn or sugarcane) to "e-fuels" (synthetic fuels produced via Power-to-Liquid processes using captured CO2 and renewable hydrogen), which of the following statements best describes their relative sustainability and energy logic?. First-generation biofuels are inherently more sustainable because they use biological photosynthesis, which is always more efficient at capturing solar energy than photovoltaic-driven electrolysis. E-fuels offer a potentially "closed" carbon cycle without competing for arable land, whereas first-generation biofuels are limited by the "food vs. fuel" dilemma and indirect land-use change (ILUC). Both fuel types are considered carbon-neutral because they do not emit CO2 at the tailpipe during combustion in an internal combustion engine. E-fuels have a significantly lower volumetric energy density than traditional biofuels, making them unsuitable for long-haul aviation or maritime transport.

In the context of the energy transition and the decarbonization of the transportation sector, sustainable fuels are often classified as drop-in or non-drop-in fuels. Based on this classification, which of the following statements correctly describes a characteristic of a non-drop-in sustainable fuel?. It can be blended directly with conventional fossil fuels without requiring modifications to current engines. It includes substances such as hydrogen and ammonia, which require new distribution infrastructure and propulsion systems. Typical examples include HVO (Hydrotreated Vegetable Oil) and synthetic fuels. They are produced exclusively from conventional biomass to ensure full compatibility with existing technology.

Which of the following thermal engines is classified simultaneously as an internal combustion engine and a rotary engine?. Turbofan Engine. Wankel Engine. Stirling Engine. Turbojet Engine.

Num plano de transição energética, uma pessoa pretende quer reduzir emissões, mas sem substituir num imediato toda a frota e a infraestrutura de abastecimento. Qual opção tem a lógica mais consistente?. substituir diretamente gasolina/diesel por drop-in fuels ou blends sustentáveis. abandonar todos os combustíveis líquidos imediatamente.

Mantendo-se a energia necessária por distância constante, ao aumentar a fração de etanol numa mistura com gasolina (E10 para E85), o consumo de um veículo de combustão interna (L/100Km) aumenta. Verdadeiro, pois o etanol tem menor densidade energética por volume (MJ/L). Falso. O consumo diminui, pois, o etanol tem maior densidade energética por massa.

In the comparison between gaseous hydrogen and traditional liquid fuels (such as gasoline) at atmospheric pressure, hydrogen exhibits a ________ gravimetric energy density and a ________ volumetric energy density. higher; lower. lower; higher. higher; higher. lower; lower.

Which statement best explains why diesel engines typically have higher thermal efficiency than gasoline engines?. Diesel fuel has a higher octane number than gasoline. Diesel engines operate with higher compression ratios. Diesel engines operate at lower temperatures. Diesel fuel contains oxygen in its molecular structure.

Based on the energy density and thermal effeciency of diesel and gasoline, which of the following statements better explains why the diesel model vehicle is typically chosen for long distance transport?. Diesel engines are more efficienty because the fuel contains a higher gravimetric energy density. Diesel's lower viscosity and higher volatility compare to gasoline alow for a more rapid combustion, leading to a better RPM and responsive power. Diesel engines provide more torque because the fuel is more flammable than gasoline, requiring less compression to ignite. Diesel contains more energy per unit of volume and operates on a cycle with higher thermal efficiency.

Why is pure biodiesel (B100) rarely used directly in diesel engines and instead blended with petroleum diesel?. Because biodiesel has lower energy content and cannot power diesel engines. Because biodiesel can be more acidic and prone to oxidation, which may cause corrosion and material compatibility issues. Because biodiesel cannot combust in diesel engines. Because biodiesel produces no lubricating properties.

Internal Combustion Engine (ICE) vehicles can use mixtures of different fuels to be more sustainable, for instance a mixture of gasoline and ethanol. Let’s define a function to describe this mixture E(X) with X corresponding to the fraction of ethanol. If the energy needed per distance remains the same, what is the direct impact in the volumetric consumption of fuel (L/100km) of increasing the fraction of ethanol (for instance E10 to E27) and why?. The volumetric consumption (L/100km) decreases because ethanol burns more efficiently than gasoline, meaning the engine needs less fuel volume. The volumetric consumption (L/100km) increases because the volumetric energy density (MJ/L) of the mixture decreases, since ethanol has a lower energy density than gasoline. The volumetric consumption (L/100km) remains the same because ethanol generates less CO2 emissions per kg, which compensates for its lower energy density. The volumetric consumption (L/100km) increases because ethanol has a lower mass density (kg/L) than gasoline, making the vehicle heavier and requiring more fuel.

In diesel engines, which type of fuel molecular structure is preferable to reduce knocking, and why?. Fuel with highly branched hydrocarbons, because they always increase ignition delay and thus reduce knocking. Fuel with aromatic hydrocarbons, because their slower ignition completely eliminates diesel knock at high pressure. Fuel with cyclic hydrocarbons, because knocking in diesel engines is independent of fuel ignition properties. Fuel with linear hydrocarbons, because they ignite more easily under high temperature and pressure, reducing knocking.

When blending ethanol into gasoline for use in a standard internal combustion engine, it is observed that the volumetric fuel consumption (measured in L/100 km) increases as the fraction of ethanol in the blend increases. Assuming the vehicle's required energy per distance remains constant, which of the following best explains this phenomenon?. Ethanol has a higher mass density than gasoline, meaning each unit volume contains more fuel mass and therefore more energy, increasing fuel consumption. Ethanol has a significantly lower volumetric energy density than gasoline because, although its mass density is slightly higher, its energy per unit mass is lower, meaning more liters must be consumed to provide the same amount of mechanical work. Ethanol has a lower energy per unit mass than gasoline, so less total energy is delivered per unit volume, even if density is similar. The slightly higher mass density of ethanol leads to a proportional increase in energy per unit volume, increasing the required fuel consumption.

The figure below shows three thermodynamic cycles for internal combustion engines. The legend of the figure indicates three curves labeled A, B, and C. Based on the characteristics of the heat addition process and the pressure evolution during expansion, identify which curve corresponds to the Otto cycle, the Diesel cycle, and the Hybrid High Efficiency Cycle (HEHC). Curve A – Otto cycle; Curve B – Diesel cycle; Curve C – HEHC. Curve A – Diesel cycle; Curve B – Otto cycle; Curve C – HEHC. Curve A – HEHC cycle; Curve B – Otto cycle; Curve C – Diesel cycle. Curve A – HEHC cycle; Curve B – Diesel cycle; Curve C – Otto cycle.

Diesel engines are generally more fuel-efficient than gasoline engines in many vehicles. Considering the properties of the fuels and the operation of the engines, which of the following statements best explains this difference?. Diesel fuel has a lower energy density than gasoline, which allows engines to burn it more efficiently. Diesel engines operate at higher compression ratios and diesel fuel has higher energy density per unit volume. Gasoline engines operate at higher compression ratios, increasing their thermal efficiency compared with diesel engines. Diesel fuel vaporizes more easily than gasoline, improving combustion efficiency.

The use of biomass as an energy source presents several logistical and economic challenges. Which of the following options correctly describes one of these challenges and its direct consequence?. The energy density of biomass (comparable to that of crude oil) makes long-distance road transport economically viable, which is the main reason for its widespread global use. The high density of biomass facilitates its storage at conversion plants, compensating for its seasonality and ensuring a continuous and stable supply throughout the year. The low energy density and high variability in biomass composition (moisture content, contaminants) are challenges that can complicate industrial processes and increase transport costs. On-field storage of biomass is the ideal solution because, despite its high cost, it allows for rigorous control of moisture content and completely eliminates the risks of biological degradation.