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Fuel is a critical component for the operation of most vehicle engines, especially internal combustion engines widely used in India and around the world. Simply put, fuel is a substance that stores energy in chemical form and releases it when burned (combustion) inside an engine, producing the power needed to move the vehicle.
The efficiency, performance, and environmental impact of a vehicle depend heavily on the type and quality of fuel used. Understanding fuel types, their properties, and how fuel interacts with the engine's combustion process is essential for aspiring mechanical engineers preparing for competitive exams.
This section covers the main fuel types used in vehicles (including petrol, diesel, and alternative fuels such as CNG and biofuels), key fuel properties, how fuel combustion occurs, and how fuel consumption and efficiency are calculated using metric units. Real-world examples and Indian contexts are referenced wherever appropriate.
Fuel types can be broadly classified into conventional fuels like petrol and diesel, and alternative fuels such as compressed natural gas (CNG), liquified petroleum gas (LPG), and biofuels. Each fuel type has its chemical composition, physical properties, availability, and typical applications in vehicle systems.
| Fuel Type | Calorific Value (kJ/kg) | Density (kg/m³) | Flash Point (°C) | Common Applications |
|---|---|---|---|---|
| Petrol (Gasoline) | 44,000 | 740 | -43 | Petrol engines in cars, motorcycles |
| Diesel | 45,500 | 830 | 52 | Diesel engines in trucks, buses, tractors |
| CNG (Compressed Natural Gas) | 50,000 | ~0.8 (gas at standard conditions) | Not applicable (gaseous fuel) | CNG vehicles, lower emissions in public transport |
| LPG (Liquified Petroleum Gas) | 46,000 | 520 | -104 | Two-wheelers, small vehicles, cooking fuel |
| Biofuels (Ethanol, Biodiesel) | 26,000 (ethanol) | 790 (ethanol) | 13 (ethanol) | Blended fuels for petrol/diesel engines |
Note: The calorific values and densities can vary depending on fuel quality and formulation. Flash point is the lowest temperature at which the fuel emits enough vapor to ignite in air, indicating safety considerations.
The performance and safety of vehicle fuels depend on several intrinsic properties. Key among these are calorific value, viscosity, and flash point.
Calorific value is the amount of energy released when a unit mass of fuel is completely burned. It is expressed in kilojoules per kilogram (kJ/kg). Higher CV indicates more energy content, leading to better engine performance and fuel economy.
Viscosity measures a fluid's resistance to flow. It affects how well fuel can be pumped and atomized in the engine. Diesel has higher viscosity than petrol, requiring different fuel injection systems.
Flash point is the lowest temperature at which the fuel vapor ignites in air. A low flash point indicates high volatility, which increases fire hazards but also better vaporization for quick ignition, especially in petrol.
The combustion process in an engine refers to the chemical reaction where fuel reacts with oxygen (from air) to release energy in the form of heat and expanding gases. Understanding this process aids in optimizing engine efficiency and controlling harmful emissions.
graph TD A[Air and Fuel Intake] --> B[Mixing in Combustion Chamber] B --> C{Air-Fuel Ratio?} C -- Stoichiometric --> D[Complete Combustion] C -- Rich or Lean Mixture --> E[Incomplete Combustion] D --> F[Energy Released] E --> G[Unburnt Fuel & Pollutants] F --> H[Engine Power Output] G --> I[Emissions & Efficiency Loss]The stoichiometric air-fuel ratio (AFR) is the ideal ratio where just enough air is present to burn all fuel completely, without excess oxygen or fuel remaining. For petrol, this is approximately 14.7:1 by mass.
Complete combustion produces maximum energy and mainly carbon dioxide (CO₂) and water (H₂O). Incomplete combustion occurs when oxygen is insufficient, leading to unburnt hydrocarbons (HC), carbon monoxide (CO), soot, and lower efficiency.
The combustion chamber is designed to ensure proper mixing of air and fuel, facilitate ignition, and contain pressure from expanding gases. Its shape and materials impact combustion efficiency and emission control.
Step 1: Identify given data: Mileage = 18 km/L
Step 2: Use the fuel consumption formula:
\(\text{Fuel consumption} = \frac{100}{\text{Mileage}}\)
Step 3: Substitute values:
\(\text{Fuel consumption} = \frac{100}{18} = 5.56\) litres/100 km
Answer: The car consumes approximately 5.56 litres of petrol per 100 km.
Step 1: Given mass \(m = 2\, \mathrm{kg}\), \(CV = 45500\, \mathrm{kJ/kg}\)
Step 2: Use the energy formula:
\[E = m \times CV\]
Step 3: Substitute values:
\[E = 2 \times 45500 = 91,000\, \mathrm{kJ}\]
Answer: Burning 2 kg of diesel releases 91,000 kJ of energy.
Step 1: Write the balanced combustion reaction:
\[ C_8H_{18} + a (O_2 + 3.76N_2) \to b CO_2 + c H_2O + d N_2 \]
Step 2: Balance carbon and hydrogen first:
Carbon: 8 C atoms -> \(8 CO_2\)
Hydrogen: 18 H atoms -> \(9 H_2O\)
Step 3: Balance oxygen atoms on right side:
Oxygen needed = \(8 \times 2 + 9 \times 1 = 16 + 9 = 25\) atoms
Since oxygen is diatomic (\(O_2\)), number of \(O_2\) molecules: \(a = \frac{25}{2} = 12.5\)
Step 4: Calculate mass of air required.
Mass of fuel (octane) per mole:
\(C: 12 \times 8 = 96\, \mathrm{g}\)
\(H: 1 \times 18 = 18\, \mathrm{g}\)
Total = 114 g = 0.114 kg
Mass of oxygen:
\(12.5 \text{ mol} \times 32\, \mathrm{g/mol} = 400\, \mathrm{g} = 0.4\, \mathrm{kg}\)
Mass of nitrogen (in air, N₂/O₂ ratio is 3.76 by mole):
\(12.5 \times 3.76 = 47\) mol of \(N_2\)
Mass of nitrogen:
\(47 \times 28 = 1316\, \mathrm{g} = 1.316\, \mathrm{kg}\)
Step 5: Total air mass = oxygen + nitrogen
\[ 0.4 + 1.316 = 1.716\, \text{kg} \]
Step 6: Calculate air-fuel ratio (AFR):
\[ \text{AFR} = \frac{1.716}{0.114} \approx 15.05 \]
Answer: The stoichiometric air-fuel ratio for petrol (octane) is approximately 15:1 by mass.
Step 1: Calculate cost per km for petrol:
Cost per litre = Rs.110
Mileage = 15 km/litre
Cost per km (petrol):
\[ \frac{110}{15} = Rs.7.33 \text{ per km} \]
Step 2: Calculate cost per km for CNG:
Cost per kg = Rs.60
Mileage = 20 km/kg
Cost per km (CNG):
\[ \frac{60}{20} = Rs.3 \text{ per km} \]
Answer: CNG is more cost-effective at Rs.3/km compared to petrol at Rs.7.33/km.
Step 1: Calculate the mass of unburnt hydrocarbons (HC):
\[ \text{HC} = 10\, \mathrm{kg} \times 0.05 = 0.5\, \mathrm{kg} \]
Step 2: Calculate the mass of carbon monoxide (CO):
\[ \text{CO} = 10\, \mathrm{kg} \times 0.03 = 0.3\, \mathrm{kg} \]
Answer: The engine emits 0.5 kg of hydrocarbons and 0.3 kg of carbon monoxide due to incomplete combustion.
When to use: Quick combustion and efficiency problems.
When to use: Problems involving mixed units; avoids calculation errors.
When to use: Conceptual questions linking theory with practical observations.
When to use: Efficiency and financial calculation problems in exams.
When to use: Complex chemical reaction problems and emission calculations.
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