# 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.
