9.5 Engine Fuels

9.5 Engine Fuels

1. Petroleum Chemistry

1. Petroleum Composition:

   • Complex mixture of hydrocarbons (compounds of hydrogen and carbon).

   • Main Hydrocarbon Groups:

     • Paraffins (Alkanes): $C_nH_{2n+2}$, saturated, straight/branched chains.

     • Naphthenes (Cycloalkanes): $C_nH_{2n}$, saturated, ring structures.

     • Aromatics: Benzene ring structures ($C_6H_6$ derivatives), unsaturated.

     • Olefins (Alkenes): $C_nH_{2n}$, unsaturated with double bonds (rare in crude).

   • Other Components: Sulfur, nitrogen, oxygen compounds, metals (V, Ni).

2. Refining Process:

   • Distillation: Separates crude into fractions by boiling point.

     • Light ends: LPG, naphtha (25-150°C)

     • Gasoline: (40-200°C)

     • Kerosene: (150-300°C)

     • Diesel: (250-350°C)

     • Heavy gas oil: (350-500°C)

     • Residue: Lubricants, asphalt, coke.

   • Conversion Processes:

     • Cracking: Breaking large molecules → smaller ones.

     • Reforming: Rearranging molecules to improve octane.

     • Alkylation: Combining small molecules → larger branched molecules.

3. Fuel Properties from Chemistry:

   • Carbon Chain Length: Shorter chains → higher volatility, lower boiling point.

   • Branching: Branched isomers → higher octane than straight-chain.

   • Aromatic Content: Improves octane but increases emissions.

   • Sulfur Content: Causes corrosion, SOₓ emissions, catalyst poisoning.

2. Fuel Rating

1. Gasoline Octane Rating:

   • Measures resistance to knocking/pre-ignition.

   • Research Octane Number (RON): Test at low speed, mild conditions.

   • Motor Octane Number (MON): Test at high speed, severe conditions.

   • Anti-Knock Index (AKI): Average of RON and MON = $(RON + MON)/2$ (US pump rating).

   • Iso-octane: Defined as 100 octane (good knock resistance).

   • n-heptane: Defined as 0 octane (poor knock resistance).

2. Diesel Cetane Rating:

   • Measures ignition quality (ease of auto-ignition).

   • Cetane Number (CN): Higher = shorter ignition delay.

   • Cetane index: Calculated from density and distillation points.

   • Standard reference fuels:

     • Cetane (n-hexadecane): CN = 100

     • Alpha-methylnaphthalene: CN = 0 (now heptamethylnonane: CN = 15)

3. Fuel Additives:

   • Gasoline:

     • Anti-knock agents (leaded: TEL, unleaded: oxygenates)

     • Detergents (clean injectors, valves)

     • Antioxidants (prevent gum formation)

     • Corrosion inhibitors

   • Diesel:

     • Cetane improvers (alkyl nitrates)

     • Lubricity improvers (important after desulfurization)

     • Cold flow improvers (prevent wax crystallization)

     • Stabilizers

4. Alternative Fuel Ratings:

   • CNG: Methane number (similar to octane for gases).

   • Ethanol: High octane (~109 RON) but lower energy content.

   • Biodiesel: Cetane number typically 45-65.

3. Fuel and Lubrication Systems

1. Fuel Supply Systems:

   • Gasoline Systems:

     • Carburetor (older): Mixes air and fuel by venturi principle.

     • Port Fuel Injection (PFI): Injectors in intake manifold.

     • Gasoline Direct Injection (GDI): Fuel injected directly into cylinder.

   • Diesel Systems:

     • Direct Injection (DI): Fuel into main combustion chamber.

     • Indirect Injection (IDI): Fuel into pre-chamber.

     • Common Rail Direct Injection (CRDI): High-pressure rail supplies all injectors.

2. System Components:

   • Fuel tank (with baffles, vent)

   • Fuel pump (mechanical/electric)

   • Fuel filter (critical for diesel injection systems)

   • Fuel lines (high pressure for modern systems)

   • Injectors (spray pattern, atomization quality)

   • Fuel pressure regulator

3. Lubrication Systems:

   • Purpose: Reduce friction, cool components, clean contaminants, seal, protect.

   • Types:

     • Wet sump: Oil reservoir in crankcase (most common).

     • Dry sump: Separate oil tank, scavenge pumps (high-performance).

   • Components:

     • Oil pump (gear, rotor, or vane type)

     • Oil filter (full-flow or bypass)

     • Oil cooler (for high-performance engines)

     • Pressure relief valve

   • Circulation Path: Sump → pump → filter → main gallery → bearings → drain back.

4. Lubricating Oil Properties:

   • Viscosity: Resistance to flow, graded by SAE numbers (0W-20, 10W-40).

   • Viscosity Index: Rate of viscosity change with temperature.

   • Pour Point: Lowest temperature at which oil flows.

   • Flash Point: Temperature at which vapors ignite.

   • Total Base Number (TBN): Reserve alkalinity to neutralize acids.

4. Specific Fuel Consumption

1. Definitions:

   • Specific Fuel Consumption (SFC): Fuel consumed per unit power output.

   • Brake Specific Fuel Consumption (BSFC): $BSFC = \frac{\dot{m}_f}{P_b}$

     • $\dot{m}_f$ = fuel mass flow rate (kg/h)

     • $P_b$ = brake power (kW)

     • Units: kg/kWh or g/kWh

   • Indicated Specific Fuel Consumption (ISFC): $ISFC = \frac{\dot{m}_f}{P_i}$

     • $P_i$ = indicated power

2. Relationship to Efficiency:

   • Brake Thermal Efficiency ($\eta_b$): $\eta_b = \frac{P_b}{\dot{m}_f \times CV} = \frac{1}{BSFC \times CV}$

     • $CV$ = calorific value of fuel (kJ/kg)

   • Lower BSFC = higher efficiency.

   • Typical values: Gasoline SI: 250-350 g/kWh, Diesel CI: 180-250 g/kWh.

3. Factors Affecting SFC:

   • Engine Design: Compression ratio, combustion chamber design, injection timing.

   • Operating Conditions: Load, speed, AFR, ignition timing (SI) or injection timing (CI).

   • Fuel Properties: Calorific value, volatility, cetane/octane number.

   • Auxiliary Losses: Friction, pumping, accessory loads.

4. Performance Maps:

   • BSFC contours plotted against engine speed and torque.

   • Shows "islands" of best efficiency.

   • Used for transmission matching and operating strategy.

5. Comparison Between Engines:

   • Diesel vs Gasoline: Diesel typically has 20-30% lower BSFC (higher efficiency).

   • Modern Trends: Downsizing + turbocharging reduces BSFC at part load.

   • Hybrid Systems: Engine operates at optimal BSFC points more frequently.

6. Measurement Methods:

   • Fuel flow measurement: Gravimetric, volumetric, Coriolis flow meters.

   • Dynamometer testing at various load-speed combinations.

   • Standard test cycles for certification.