Thermal Engg Lab (ME-306) - Unit 5 Short Notes

UNIT 5: PERFORMANCE TESTING OF INTERNAL COMBUSTION ENGINES & EMISSIONS ANALYSIS

5.1 Introduction to Engine Performance Parameters

Core Definitions & Significance:

• Brake Power (BP): Actual usable power output at the engine's crankshaft. Measured directly by a dynamometer.

$$BP = \frac{2\pi N T}{60} \quad (\text{in Watts}) \quad \text{or} \quad BP = \frac{W \cdot N}{60} \quad (\text{in metric HP})$$

Where: $N$ = Speed (RPM), $T$ = Torque (N-m), $W$ = Load (kgf).

• Indicated Power (IP): Power developed inside the engine cylinder from combustion pressure. Calculated from an indicator diagram.

• Frictional Power (FP): Power lost in overcoming friction of moving parts. $$\displaystyle FP = IP - BP $$.

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

  • Brake SFC (bsfc): $$\displaystyle bsfc = \frac{\dot{m}_f}{BP} $$ (kg/kWh or g/kWh)

  • Indicated SFC (isfc): $$\displaystyle isfc = \frac{\dot{m}_f}{IP} $$

• Thermal Efficiency ($$\displaystyle \eta_{th} $$): Ratio of output power to energy input from fuel.

  • Brake Thermal Efficiency (BTE): $$\displaystyle \eta_{bth} = \frac{BP}{\dot{m}_f \cdot CV_f} $$

  • Indicated Thermal Efficiency (ITE): $$\displaystyle \eta_{ith} = \frac{IP}{\dot{m}_f \cdot CV_f} $$

• Mechanical Efficiency ($$\displaystyle \eta_m $$): $$\displaystyle \eta_m = \frac{BP}{IP} = 1 - \frac{FP}{IP} $$

• Volumetric Efficiency ($$\displaystyle \eta_v $$): Ratio of actual air intake to theoretical air intake (swept volume) at atmospheric conditions.

Engine Classification:

Purpose of Testing: To determine performance characteristics, calculate efficiencies, diagnose issues, and ensure compliance with standards.

[!TIP] Exam Focus: Be prepared to define all parameters and derive relationships like $$\displaystyle FP = IP - BP $$. Know the key differences between SI and CI engines.

5.2 Experimental Setup and Apparatus

A. Dynamometers (Absorption Type):

B. Fuel Consumption Measurement:

• Burette Method: Measures volume of fuel consumed in a timed interval. $$\displaystyle \dot{V}_f = \frac{\text{Volume (ml)}}{\text{Time (s)}} $$. Needs density to get mass flow.

• Weighing Scale Method: More accurate. Measures mass of fuel consumed in a timed interval. $$\displaystyle \dot{m}_f = \frac{\Delta m}{\Delta t} $$.

C. Air Intake Measurement:

• Orifice Meter with U-tube Manometer: Measures differential pressure ($\Delta h$) across an orifice plate.

$$\dot{m}_a = C_d A_o \sqrt{\frac{2 \rho_a \Delta P}{(1 - \beta^4)}} \quad \text{or from manometer: } \Delta P = \rho_w g \Delta h$$

Where $$\displaystyle C_d $$ = discharge coefficient, $$\displaystyle A_o $$ = orifice area, $\beta$ = diameter ratio.

D. Temperature & Pressure:

• Temperatures: Thermocouples (exhaust gas), RTDs (cooling water, lube oil).

• Pressures: Pressure gauges (lube oil), Indicator Diagram (cylinder pressure vs. crank angle using mechanical/electronic indicator), fuel injection pressure gauge.

E. Exhaust Gas Analyzers:

F. Data Acquisition System (DAS): Collects analog signals (from thermocouples, pressure transducers) and converts to digital for display/storage.

[!TIP] Common Pitfall: Remember the orifice meter formula uses air density $$\displaystyle \rho_a $$ and differential pressure $\Delta P$ (from manometer reading $\Delta h$). Ensure units are consistent (SI preferred).

5.3 Standard Engine Performance Experiments

5.3.1 Load Variation Test (Full Load to No Load)

• Aim: Determine BP, Torque, SFC, $$\displaystyle \eta_{bth} $$ vs. load at constant speed.

• Procedure: Start engine, warm up. Apply load incrementally (0%, 25%, 50%, 75%, 100% of rated). At each step, wait for steady-state and record: Load (kg), Speed (RPM), Fuel consumption (ml/time), Temp (water in/out, exhaust), Pressure (if any).

• Key Calculations:

  • Torque $$\displaystyle T = \frac{W \cdot g \cdot r}{1000} $$ (N-m) [W in kg, r = brake arm radius in m]

  • BP from formula in 5.1.

  • $$\displaystyle \dot{m}_f = \frac{\text{Volume} \times \rho_f}{\text{Time}} $$ (kg/s)

  • $$\displaystyle bsfc = \frac{\dot{m}_f}{BP} $$ (kg/kWh)

  • $$\displaystyle \eta_{bth} = \frac{BP}{\dot{m}_f \cdot CV_f} $$

• Graphs: BP vs Load, Torque vs Load, bsfc vs Load, $$\displaystyle \eta_{bth} $$ vs Load.

• Precautions: Ensure steady-state before reading. Adequate cooling water flow. Ventilation for exhaust fumes.

5.3.2 Morse Test (Multi-Cylinder Engines)

• Aim: Find individual cylinder IP and overall $$\displaystyle \eta_m $$.

• Theory: By cutting off one cylinder's fuel supply and noting the BP drop while maintaining constant speed with remaining cylinders.

• Procedure:

  1. All cylinders firing: Note $$\displaystyle BP_{total} $$ at constant speed $N$.

  2. Cut off cylinder 1, adjust load to keep speed $N$ constant. Note $$\displaystyle BP_{rest} $$.

  3. Repeat for each cylinder.

• Calculations (for n-cylinder engine):

  • IP of cut-off cylinder $i$: $$\displaystyle IP_i = BP_{total} - BP_{rest(i)} $$

  • Total IP: $$\displaystyle IP_{total} = \frac{n}{n-1} \left[ BP_{total} - \frac{1}{n} \sum_{i=1}^{n} BP_{rest(i)} \right] $$

  • $$\displaystyle \eta_m = \frac{BP_{total}}{IP_{total}} $$

[!TIP] Morse Test Formula: The factor $$\displaystyle \frac{n}{n-1} $$ accounts for the fact that when one cylinder is cut, the remaining (n-1) cylinders take up the load. Memorize the IP_total formula.

5.3.3 Heat Balance Sheet

• Aim: Account for all input fuel energy.

• Energy Input: $$\displaystyle Q_{in} = \dot{m}_f \cdot CV_f $$ (W or kJ/s)

• Energy Outputs (at full load):

  1. Useful Work: $BP$ (or $IP$ if available)

  2. Cooling Water Loss: $$\displaystyle Q_{cool} = \dot{m}_w \cdot C_{pw} \cdot (T_{wo} - T_{wi}) $$

  3. Exhaust Gas Loss: $$\displaystyle Q_{ex} = \dot{m}_{ex} \cdot C_{pex} \cdot (T_{ex} - T_{amb}) $$ (approx using avg $$\displaystyle C_p $$)

  4. Radiation & Unaccounted: $$\displaystyle Q_{rad} = Q_{in} - (BP + Q_{cool} + Q_{ex}) $$ (usually 5-10%)

• Verification: $$\displaystyle \% \text{Error} = \frac{Q_{in} - \sum Q_{out}}{Q_{in}} \times 100\% $$. Should be < 10%.

5.3.4 Variable Speed Performance Test (Governor Characteristics)

• Aim: Study effect of speed on performance.

• Procedure: Fix load (or fix throttle). Vary engine speed (using governor/fuel control) and record parameters at each speed.

• Graphs:

  • Torque vs Speed: Shows maximum torque and speed range.

  • Power vs Speed (at constant throttle): Shows peak power and rated speed.

  • bsfc vs Speed: Shows speed for best efficiency.

• Analysis: Governor droop = $$\displaystyle \frac{\Delta N}{N_{full}} $$ for full load change.

5.3.5 Exhaust Emissions Testing

• Aim: Measure CO, HC, NOx concentrations under different conditions.

• Procedure: Warm engine. Sample exhaust gas (after muffler for chassis dyno, before for engine dyno). Record ppm/% from analyzers at various loads/speeds. Measure exhaust flow rate or calculate from air/fuel ratio.

• Calculations:

  • Mass Emission Rate: $$\displaystyle \dot{m}_{pollutant} = \frac{C \cdot \dot{V}_{ex} \cdot \rho_{ex}}{10^6} $$ (g/h) [C in ppm, $$\displaystyle \dot{V}_{ex} $$ in m³/h]

  • Specific Emission: $$\displaystyle SE = \frac{\dot{m}_{pollutant}}{BP} $$ (g/kWh)

• Trends: NOx ↑ with load & temperature; CO & HC ↓ with load (better combustion); HC ↑ at very low load.

[!TIP] Emissions: Always convert ppm to g/kWh using exhaust flow rate. Remember: $$\displaystyle 1\% = 10,000 $$ ppm. Use dry exhaust gas corrections if specified.

5.4 Data Analysis, Reporting, and Error Estimation

• Tabulation: Use structured tables with units, clear headings.

• Graphs: Plot all characteristic curves with labeled axes, units, and legend.

• Error Sources:

  • Instrument precision (least count).

  • Parallax in analog readings.

  • Heat losses to surroundings (unaccounted in heat balance).

  • Incomplete steady-state.

  • Inaccurate fuel/air measurement (density, temperature).

  • Calibration drift in sensors.

• Error Propagation: For $$\displaystyle Z = f(x, y, ...) $$, $$\displaystyle \frac{\Delta Z}{Z} = \sqrt{ \left( \frac{\partial Z}{\partial x} \frac{\Delta x}{Z} \right)^2 + ... } $$. Often simplified for lab reports.

• Lab Report Structure: Aim, Apparatus, Theory, Procedure, Observations (raw), Calculations (sample), Graphs, Results (tabulated), Discussion (compare trends, explain anomalies), Conclusion, Error Analysis, Suggestions.

5.5 Safety Protocols in Engine Testing

• PPE: Safety shoes, gloves (heat/chemical resistant), goggles/face shield, ear plugs/defenders.

• Ventilation: Exhaust extraction system (fume hood) MUST be ON. Ensure lab air changes.

• Fire: CO₂ & dry powder extinguishers nearby. No smoking/open flames. Fuel spills cleaned immediately.

• Electrical: Proper grounding of dynamometer/DAS. Check insulation.

• Engine: Secure mounting on test bed. Rotating parts must be guarded. Emergency stop button accessible.

• Hot Components: Use heat-resistant gloves for exhaust, manifold, coolant lines. Allow cooling before inspection.

• First Aid: Eye wash station, safety shower. Know procedure for fuel ingestion/inhalation/burns.

5.6 Advanced Topics and Applications

• Gas Turbine Testing: Similar parameters (BP, thermal efficiency), but includes compressor work. Net power = Turbine power - Compressor power.

• Alternative Fuels: Compare performance of biodiesel (higher BSFC, lower emissions), ethanol (higher octane, lower energy density), CNG/LPG (lean burn possible, lower CO/HC, may need tuning).

• Forced Induction: Turbocharging/supercharging increases air mass → more power, but increases NOx due to higher combustion temperature. May increase BSFC at high load.

• On-Board Diagnostics (OBD): Reading sensor data (O₂, MAP, MAF, coolant temp) and Diagnostic Trouble Codes (DTCs) via scan tool to diagnose emission-related faults.

• Engine Mapping: 3D contour plots of torque, BSFC, NOx, etc. vs. engine speed and load (or manifold pressure). Used by ECU for fuel injection and ignition timing maps.

• Combustion Analysis: Using cylinder pressure transducer and crank angle encoder to calculate: heat release rate, $P-\theta$ diagram, combustion duration, coefficient of variation (COV) of IMEP.

[!TIP] Advanced Topics: Understand the qualitative impact of turbocharging (↑power, ↑NOx, ↓BSFC at high load) and alternative fuels (e.g., biodiesel → ↑BSFC, ↓CO/HC/PM). Know OBD monitors (e.g., Oxygen Sensor, Catalyst, EVAP).