8.6 Engine Performance and Testing

8.6 Engine Performance and Testing

1. Engine Efficiencies

1. Thermal Efficiency:

   • Ratio of work output to heat input.

   • $\eta_{th} = \frac{W_{net}}{Q_{in}} = 1 - \frac{Q_{out}}{Q_{in}}$

   • Indicated Thermal Efficiency: Based on indicated power (inside cylinder).

   • Brake Thermal Efficiency: Based on brake power (output shaft).

     • $\eta_{bth} = \frac{BP}{m_f \times CV}$ where $m_f$ = fuel flow rate, $CV$ = calorific value.

2. Mechanical Efficiency:

   • Ratio of brake power to indicated power.

   • $\eta_m = \frac{BP}{IP} = \frac{BP}{BP + FP}$

   • Accounts for friction losses (FP = friction power).

3. Volumetric Efficiency:

   • Measure of breathing capacity.

   • $\eta_v = \frac{\text{Mass of air actually inducted}}{\text{Mass of air that could fill displacement volume at intake conditions}}$

   • $\eta_v = \frac{m_a}{\rho_a V_d}$ where $V_d$ = displacement volume, $\rho_a$ = intake air density.

   • Affected by: Valve timing, intake design, engine speed.

4. Relative Efficiency:

   • Ratio of actual thermal efficiency to air-standard efficiency.

   • $\eta_{rel} = \frac{\eta_{actual}}{\eta_{air-standard}}$

5. Air-Standard Efficiencies:

   • Otto Cycle: $\eta_{otto} = 1 - \frac{1}{r^{\gamma-1}}$ where $r$ = compression ratio, $\gamma$ = specific heat ratio.

   • Diesel Cycle: $\eta_{diesel} = 1 - \frac{1}{r^{\gamma-1}} \left[\frac{\rho^\gamma - 1}{\gamma(\rho - 1)}\right]$ where $\rho$ = cut-off ratio.

   • Dual Cycle: Combination of Otto and Diesel cycles.

6. Specific Fuel Consumption:

   • BSFC (Brake Specific Fuel Consumption):

     • $BSFC = \frac{m_f}{BP}$ (kg/kWh)

     • Lower BSFC = better efficiency.

   • ISFC (Indicated Specific Fuel Consumption):

     • $ISFC = \frac{m_f}{IP}$

2. Cooling Systems

1. Cooling Necessity:

   • Remove ~30% of heat from combustion.

   • Maintain optimal operating temperature.

   • Prevent: Overheating, thermal stresses, lubrication breakdown, knocking.

2. Cooling Methods:

   • Air Cooling:

     • Fins on cylinder/head increase surface area.

     • Fan assists airflow.

     • Advantages: Simple, lightweight, no coolant leaks.

     • Disadvantages: Uneven cooling, noisy, limited by ambient temperature.

   • Liquid (Water) Cooling:

     • Coolant circulates through water jackets.

     • Advantages: Uniform cooling, better temperature control, quieter.

     • Disadvantages: Complex, heavier, potential leaks.

3. Liquid Cooling System Components:

   • Radiator: Transfers heat to atmosphere.

   • Water Pump: Circulates coolant.

   • Thermostat: Controls coolant temperature (opens at ~80-90°C).

   • Cooling Fan: Increases airflow through radiator.

   • Pressure Cap: Increases boiling point of coolant.

   • Coolant: Typically water + antifreeze (ethylene glycol).

4. Cooling System Types:

   • Thermosyphon System: Natural convection (older systems).

   • Pump Circulation System: Forced circulation (modern systems).

   • Pressurized System: Operates above atmospheric pressure.

5. Temperature Control:

   • Optimum Temperature: 80-90°C for petrol, 70-80°C for diesel.

   • Effects of Overcooling: Increased friction, poor vaporization, increased emissions.

   • Effects of Overheating: Knocking, reduced power, thermal damage.

3. Knocking and Pre-ignition

1. Knocking (Detonation):

   • Cause: Auto-ignition of end-gas before flame front arrival.

   • Characteristics: Sharp metallic sound, pressure oscillations.

   • Effects: Power loss, overheating, engine damage (piston/ring failure).

   • Factors Promoting Knocking:

     • High compression ratio.

     • Advanced spark timing.

     • Lean air-fuel mixture.

     • Low octane fuel.

     • High cylinder temperature.

2. Pre-ignition:

   • Cause: Premature ignition before spark (hot spots: carbon deposits, hot valves).

   • Characteristics: Smooth power loss, possible backfiring.

   • Effects: Severe overheating, possible engine destruction.

3. Key Differences:

   • Timing: Pre-ignition occurs BEFORE spark; knocking occurs AFTER spark.

   • Sound: Knocking has characteristic pinging; pre-ignition often silent.

   • Severity: Pre-ignition often more destructive.

4. Anti-Knock Measures:

   • Fuel Related:

     • Use higher octane fuel.

     • Octane number: Resistance to knocking (RON = Research, MON = Motor).

   • Engine Design:

     • Lower compression ratio.

     • Optimal spark timing (retarded from MBT).

     • Efficient cooling system.

     • Proper combustion chamber design (compact).

   • Operational:

     • Maintain proper air-fuel ratio.

     • Prevent carbon deposits.

     • Use knock sensors for closed-loop control.

5. Octane Number:

   • RON (Research Octane Number): Measured under mild conditions.

   • MON (Motor Octane Number): Measured under severe conditions.

   • Anti-Knock Index: $AKI = \frac{RON + MON}{2}$ (pump octane in US).

4. Fuel Injection Systems

1. Fuel Injection Advantages over Carburetion:

   • Precise air-fuel ratio control.

   • Better fuel atomization.

   • Improved cold starting.

   • Reduced emissions.

   • Faster engine response.

2. Types of Injection Systems:

   • Port Fuel Injection (PFI/MFI):

     • Injectors in intake ports.

     • Sequential or simultaneous injection.

     • Common in petrol engines.

   • Direct Injection (DI):

     • Fuel injected directly into cylinder.

     • Higher pressure required.

     • Better efficiency, but more complex.

   • Common Rail System (Diesel):

     • High-pressure rail supplies all injectors.

     • Precise electronic control.

     • Multiple injections per cycle possible.

3. Diesel vs Petrol Injection:

   • Diesel:

     • Injection pressure: 1500-2500 bar.

     • Timed injection (critical for combustion).

     • Injection Timing: Before TDC (15-30°).

   • Petrol:

     • Injection pressure: 3-10 bar (PFI), 100-200 bar (DI).

     • Continuous or timed injection.

     • Injection Timing: Less critical (during intake).

4. Electronic Fuel Injection (EFI) Components:

   • ECU (Engine Control Unit): Processes sensor data.

   • Sensors: MAP/MAF, TPS, oxygen, coolant temperature, crank position.

   • Actuators: Fuel injectors, fuel pump, idle air control.

   • Fuel Delivery: Electric fuel pump, pressure regulator, fuel rail.

5. Injection Parameters:

   • Injection Timing: When fuel is injected.

   • Injection Duration: How long injector stays open.

   • Injection Pressure: Affects atomization quality.

   • Spray Pattern: Determines air-fuel mixing.

6. Emission Control via Injection:

   • Multiple Injection Strategies:

     • Pilot injection: Reduces noise (diesel).

     • Main injection: Power production.

     • Post injection: Reduces PM emissions.

   • Lean Burn Strategies: Ultra-lean mixtures for reduced NOx.

   • EGR Compatibility: Better control with precise injection.

7. Performance Testing Parameters:

   • Power Curve: Brake power vs engine speed.

   • Torque Curve: Torque vs engine speed.

   • Specific Fuel Consumption Curve: BSFC vs engine speed.

   • Heat Balance: Distribution of fuel energy (brake power, cooling, exhaust, friction).

   • Emission Levels: CO, HC, NOx, PM measurements.