Thermal Engg Lab (ME-306) - Unit 1 Short Notes

UNIT 1: FOUNDATIONS OF THERMAL ENGINEERING LABORATORY

1.0 Introduction & Laboratory Orientation

The Thermal Engineering Laboratory provides hands-on experience with thermal systems (IC engines, heat transfer setups, refrigeration) to validate theory, develop measurement skills, and understand performance parameters. The core learning outcome is the safe and accurate execution of standard experiments, data analysis, and professional reporting.

1.1 Purpose & Scope

• To correlate theoretical thermal engineering concepts (thermodynamics, heat transfer, fluid mechanics) with real-world systems.

• To master the use of instruments for measuring temperature, pressure, flow, speed, and load.

• To conduct standard tests on Internal Combustion (IC) engines and heat transfer apparatus.

• To develop skills in data acquisition, error analysis, and technical report writing.

1.2 Laboratory Safety Protocols & Regulations (CRITICAL)

General Rules:

• Wear proper attire: closed shoes, lab coat, safety glasses. No loose clothing.

• No eating/drinking. Know location of emergency exits, showers, and eyewash.

• Never operate equipment without instruction. Ensure all guards are in place.

Specific Hazards & Mitigation:

• High Temperatures: Exhaust manifolds, coolant, engine parts. Use heat-resistant gloves and allow sufficient cool-down.

• Moving Parts: Belts, shafts, couplings. Keep hands/clothing away. Use lockout/tagout before adjustments.

• Fuels & Lubricants: Fire risk. Store in designated areas. No open flames. Spills must be cleaned immediately.

• Electrical Systems: Risk of shock. Ensure equipment is earthed. Do not handle with wet hands.

• Exhaust Gases: Carbon Monoxide (CO) poisoning risk. Ensure exhaust is vented outdoors. Never run engines in enclosed spaces.

Safety Equipment:

• Fire Extinguishers: Know type (CO₂ for electrical/flammable liquids, Dry Powder for general). PASS technique: Pull, Aim, Squeeze, Sweep.

• First-Aid Kits & Emergency Showers/Eyewash: Must be accessible and unobstructed.

Fuel Handling:

• Diesel/Petrol: Use approved containers, ground during transfer, no smoking.

• Chemicals (e.g., for bomb calorimeter): Follow Material Safety Data Sheet (MSDS).

Emergency Procedure:

1. Stop the equipment if safe.

2. Alert instructor and others.

3. Evacuate if fire/gas leak. Assemble at designated point.

[!TIP] Exam Focus: Safety questions are VERY COMMON. Be prepared to list specific hazards for an IC engine test and corresponding safety measures. Always mention CO poisoning risk and lockout/tagout.

2.0 Laboratory Instruments, Tools, and Measurement Systems

2.1 Basic Measurement Fundamentals

• Accuracy: Closeness to true value.

• Precision: Repeatability of measurements.

• Calibration: Comparing instrument to a standard to determine error.

• Error Analysis: Total Error = Systematic Error (bias) + Random Error (scatter). Express as % error.

• Direct Measurement: Length (vernier calipers), temperature (thermocouple).

• Indirect Measurement: Calculated from direct ones (e.g., power from torque & RPM).

2.2 Temperature Measurement

| Instrument | Principle | Key Features | Typical Lab Use |

| :--- | :--- | :--- | :--- |

| Mercury-in-glass | Thermal expansion of Hg | Direct reading, fragile, limited range (-38°C to 356°C) | Simple, local readings |

| Bimetallic Strip | Differential expansion of two metals | Mechanical deflection, robust, slow response | Thermostats, dial thermometers |

| Thermocouple | Seebeck Effect: Two dissimilar metals joined at junction produce voltage proportional to ΔT. | Wide range (-200°C to 2300°C), fast response, requires cold junction compensation. Types: K (Chromel-Alumel), J (Iron-Constantan), T (Cu-Constantan). | Engine exhaust, coolant, oil temp. |

| RTD (Pt100) | Resistance of pure Pt increases linearly with T. | High accuracy & stability, good repeatability, slower than TC, expensive. | Precise, stable temperature measurement. |

| Pyrometer | Measures radiation intensity. | Non-contact. Optical: compares brightness. Infrared: detects IR energy. | Surface temp of hot bodies, furnaces. |

2.3 Pressure Measurement

| Instrument | Principle | Key Features |

| :--- | :--- | :--- |

| Manometer (U-tube, Inclined) | Balance of fluid column height (ΔP = ρgh). | Simple, accurate for low pressure, no calibration needed. |

| Bourdon Tube Gauge | Curved tube straightens under pressure, moving pointer. | Robust, direct reading, for higher pressures (up to ~700 bar). |

| Diaphragm/Bellows Gauge | Pressure deflects thin diaphragm or bellows. | For low/medium pressure, good for dynamic response. |

| Pressure Transducer (Strain Gauge, Piezoelectric, Capacitive) | Converts pressure to electrical signal. | For data acquisition, high frequency response (piezoelectric). |

2.4 Flow Measurement

| Instrument | Principle | Typical Lab Use |

| :--- | :--- | :--- |

| Rotameter (Variable Area) | Float position balances gravity & flow drag. | Visual, low pressure drop, for low flow rates (air, water). |

| Orifice Meter | Pressure drop across a plate with a hole. | Common for pipe flow, requires differential manometer. |

| Venturi Meter | Converging-diverging section causes pressure drop. | Low permanent loss, high accuracy for large flows. |

| Engine Air/Fuel Flow | Air-box method for air. Fuel burette/weighing scale for fuel (mass flow). | Standard for IC engine performance tests. |

2.5 Speed/RPM Measurement

• Mechanical Tachometer: Centrifugal force (hand-held).

• Electrical/Electronic:

- **Magnetic Pickup:** Counts teeth on rotating gear.

- **Optical Encoder:** Slotted disc & light sensor, high precision.

- **Stroboscope:** Non-contact, visual matching.

2.6 Load Measurement

• Dynamometers (Dyno): Absorb engine power.

- **Prony/Rope Brake:** Friction bands, simple but crude, high heat loss.

- **Hydraulic:** Water/fluid shear, smooth load.

- **Eddy Current:** Magnetic drag, precise, electrically controlled.

- **Electric (DC/AC):** Acts as generator, can measure power directly.

• Load Cell: Strain gauge-based, converts force/weight to electrical signal. Used with rope brake or directly.

2.7 Data Acquisition & Recording (DAQ)

• System: Sensor → Signal Conditioning (amplification, filtering) → Analog-to-Digital Converter (ADC) → Computer/PLC.

• Software: LabVIEW, MATLAB, proprietary DAQ software for logging, display, and preliminary analysis.

• Advantage: High-speed, multi-channel, automated data collection, reduces human error.

3.0 Fundamental Experiments on Internal Combustion (IC) Engines

3.1 Engine Components & Terminology

• 4-Stroke Cycle: Intake, Compression, Power, Exhaust.

• Key Dimensions:

- **Bore (D):** Cylinder diameter.

- **Stroke (L):** Piston travel distance.

- **Displacement Volume (V_s):** Total swept volume = (π/4) D² L × No. of cylinders.

- **Clearance Volume (V_c):** Volume at TDC.

- **Compression Ratio (r):** 

$$ r = \frac{V_s + V_c}{V_c} $$

\boxed{r = 1 + \frac{V_s}{V_c}} (Typically 8-12 for petrol, 14-22 for diesel).

3.2 Engine Performance Parameters

| Parameter | Definition | Formula |

| :--- | :--- | :--- |

| Brake Power (BP) | Net usable power at output shaft (measured by dyno). |

$$ BP = \frac{2\pi N T}{60} \text{ (W)} $$

or

$$ BP = \frac{W \times 2\pi N}{60 \times 1000} \text{ (kW)} $$

<br> where W = load (N), N = RPM, T = torque (N·m). |

| Indicated Power (IP) | Power developed inside cylinder on piston (from indicator diagram). | IP = BP + FP |

| Frictional Power (FP) | Power lost to friction (piston, bearings, accessories). | FP = IP - BP |

| Mechanical Efficiency (η_mech) | Ratio of output to indicated power. |

$$ \eta_{mech} = \frac{BP}{IP} \times 100\% $$

|

| Brake Specific Fuel Consumption (BSFC) | Fuel consumed per unit BP per hour. |

$$ BSFC = \frac{\dot{m}_f}{BP} \text{ (kg/kW·hr)} $$

|

| Brake Thermal Efficiency (BTE) | Ratio of BP output to fuel energy input. |

$$ BTE = \frac{BP}{\dot{m}_f \times CV} \times 100\% $$

|

| Indicated Thermal Efficiency (ITE) | Ratio of IP to fuel energy input. |

$$ ITE = \frac{IP}{\dot{m}_f \times CV} \times 100\% $$

|

| Volumetric Efficiency (η_v) | Ratio of actual air intake to theoretical displacement volume at atmospheric conditions. |

$$ \eta_v = \frac{\dot{m}_a \times R \times T}{P \times V_s \times N \times K} \times 100\% $$

<br> (K = no. of intake strokes per rev, 1 for 4-stroke). |

3.3 Standard Engine Test Procedures

3.3.1 Load Test (Performance Test) on 4-Stroke Diesel Engine

Aim: To determine BP, BTE, BSFC, etc. at varying loads.

Apparatus: Diesel engine, eddy current/hydraulic dynamometer with load & speed measurement, fuel measuring apparatus (burette/scale), thermometers (coolant, exhaust, lub oil), pressure gauges (lub oil, fuel), tachometer.

Procedure:

1. Start engine, warm up at no-load.

2. Gradually apply load in steps (e.g., 0%, 25%, 50%, 75%, 100% of rated load).

3. At each load, wait for steady state (constant temp, RPM). Record:

• Time for fuel consumption (or mass) → compute $$\displaystyle \dot{m}_f $$.

• Dynamometer reading (load & spring balance reading) → compute Torque (T) & BP.

• RPM (N).

• Temperatures (coolant inlet/outlet, exhaust gas, lub oil).

• Pressures (lub oil, fuel).

• Air flow (if measured, via air-box method).

4. Tabulate and calculate performance parameters.

3.3.2 Morse Test (Frictional Power of Multi-Cylinder Engine)

Principle: For multi-cylinder engines, run engine with one cylinder cut out (misfiring). The BP measured is the sum of IP of remaining cylinders. The difference between total IP (all cylinders) and sum of IPs of running cylinders gives FP of the cut-off cylinder.

Procedure:

1. Conduct full load test on all cylinders → get Total BP₁ and Total IP₁ (from indicator diagrams).

2. Cut off one cylinder (disable injection/spark). Run at same BP (by increasing load on remaining cylinders). Measure BP₂ (from remaining cylinders).

3. IP of cut-off cylinder (IP_c) = Total IP₁ - (IP of remaining cylinders at same BP₂). Since IP ∝ BP for a given engine under similar conditions, IP_c ≈ BP₁ - BP₂.

4. FP of cut-off cylinder = IP_c - (BP contribution of that cylinder, which is zero when cut off).

5. Total FP = FP of cut-off cylinder × No. of cylinders.

3.3.3 Determination of Calorific Value (Bomb Calorimeter)

Principle: Complete combustion of a known fuel mass in an oxygen-rich, constant-volume chamber (bomb) submerged in a known mass of water. Heat released raises water temperature.

Apparatus: Bomb calorimeter (strong steel vessel), water jacket, thermometer/thermocouple, stirrer, oxygen cylinder, ignition system.

Procedure:

1. Weigh fuel sample (m_f, ~1g).

2. Fill bomb with O₂ (≈25-30 atm). Place in calorimeter with known mass of water (m_w) at initial temp (T₁).

3. Ignite fuel electrically. Stir water. Record max temperature (T₂).

4. Calculate Gross Calorific Value (GCV):

$$ GCV = \frac{(m_w C_w + C_c)(T_2 - T_1)}{m_f} $$

where C_w = sp. heat of water, C_c = water equivalent of calorimeter.

5. Net Calorific Value (NCV) = GCV - Latent heat of vaporization of water in combustion products (≈ 9% of GCV for petrol/diesel).

3.4 Engine Tuning & Valve Timing

3.4.1 Valve Timing Diagram

Aim: To determine Inlet Valve Opening (IVO), Inlet Valve Closing (IVC), Exhaust Valve Opening (EVO), Exhaust Valve Closing (EVC) w.r.t. piston position (crank angle).

Method: Using follower/feeler gauge on valve stem while slowly turning crank, or using dial indicator on valve stem. Mark positions on a protractor attached to crank pulley. Alternatively, use oscilloscope with magnetic pickup on flywheel and valve lift sensor.

Typical 4-Stroke Diesel Timing:

• IVO: ~10° BTDC (Before TDC)

• IVC: ~30° ABDC (After BDC)

• EVO: ~40° BBDC (Before BDC)

• EVC: ~10° ATDC (After TDC)

3.4.2 Engine Tuning

• SI Engine (Ignition Timing): Adjust spark advance (distributor/ECU) for optimum power (usually a few °BTDC at full load). Check by detonation test or power output.

• CI Engine (Injection Timing): Adjust fuel pump timing (advance/retard). Check by fuel consumption and exhaust smoke (too retarded = black smoke, too advanced = high pressure & knock).

4.0 Heat Transfer Fundamentals Experiments (Introductory)

4.1 Conduction

Steady-State Method (Metal Rod):

Aim: Determine thermal conductivity (k) of a metal.

Setup: Heated rod with thermocouples at known distances, insulated sides, steady-state reached.

Principle: Fourier's Law:

$$ Q = -k A \frac{dT}{dx} \approx k A \frac{T_1 - T_n}{(n-1)d} $$

Procedure: Apply constant heat input (Q = V×I). Measure steady-state temperatures at several points along rod. Plot T vs. distance. Slope = ΔT/Δx. Calculate k.

4.2 Convection

Forced Convection (Pipe Flow):

Aim: Determine convective heat transfer coefficient (h).

Setup: Heated pipe with hot fluid (water/air) inside, thermocouples at inlet/outlet, surface temp measurement, flow meter.

Principle:

$$ Q = \dot{m} C_p (T_{out} - T_{in}) = h A_s (T_{s} - T_{b}) $$

where T_s = surface temp, T_b = bulk fluid temp (avg of inlet/outlet).

Procedure: Measure Q (from fluid side), T_s, T_b, A_s. Calculate h.

4.3 Radiation

Stefan-Boltzmann Law Verification:

Aim: Verify $$\displaystyle E \propto T^4 $$ and determine emissivity (ε).

Setup: Radiation source (heated plate) with known T (measured by thermocouple), radiation detector (thermopile) at fixed distance.

Principle:

$$ E = \varepsilon \sigma T^4 $$

where σ = 5.67×10⁻⁸ W/m²K⁴.

Procedure: Vary source temperature. Measure emitted radiation intensity (E). Plot log E vs. log T. Slope should be 4. Intercept gives εσ.

5.0 Refrigeration & Air Conditioning Basics

5.1 Vapour Compression Cycle Components

1. Compressor: Raises pressure & temp of refrigerant vapour.

2. Condenser: Rejects heat, condenses vapour to liquid.

3. Expansion Valve: Throttles, causes temp/pressure drop.

4. Evaporator: Absorbs heat, evaporates liquid to vapour.

5.2 Performance Test on Refrigeration Rig

Aim: Determine Refrigerating Effect, Compressor Work, Coefficient of Performance (COP).

Measurements:

• Refrigerant flow rate ($$\displaystyle \dot{m}_r $$) - via sight glass/flow meter.

• Temperatures & Pressures at inlet/outlet of each component.

• Compressor power input (W) - via wattmeter/voltmeter-ammeter.

• Condenser cooling water flow & ΔT (to find Q_cond).

Calculations:

• Refrigerating Capacity (Q_evap):

$$ Q_{evap} = \dot{m}_r (h_1 - h_4) $$

(use P-h chart).

• Compressor Work (W_comp):

$$ W_{comp} = \dot{m}_r (h_2 - h_1) $$

or measured electrical input.

• COP:

$$ COP = \frac{Q_{evap}}{W_{comp}} \text{ (for refrigeration)} $$

• Heat Rejection (Q_cond):

$$ Q_{cond} = \dot{m}_r (h_2 - h_3) = Q_{evap} + W_{comp} $$

5.3 Psychrometry

• Psychrometer: Dry-bulb & wet-bulb thermometers. Wet-bulb depression gives relative humidity (RH).

• Use psychrometric chart to find humidity ratio, enthalpy, dew point.

6.0 Data Analysis, Error Propagation & Laboratory Reporting

6.1 Processing Raw Data

• Tabulate readings clearly with units.

• Calculate averages for repeated measurements.

• Use correct significant figures (based on instrument least count).

6.2 Error Propagation

For a calculated quantity Z = f(x, y, ...), maximum probable error:

$$ \frac{\Delta Z}{Z} = \sqrt{ \left( n \frac{\Delta x}{x} \right)^2 + \left( m \frac{\Delta y}{y} \right)^2 + ... } $$

where Z = xⁿ yᵐ ...

Example: BP = 2πNT/60 →

$$ \frac{\Delta BP}{BP} = \sqrt{ \left( \frac{\Delta N}{N} \right)^2 + \left( \frac{\Delta T}{T} \right)^2 } $$

6.3 Graphical Representation

• Plot independent variable (e.g., load) on X-axis, dependent variable (e.g., BP, BTE) on Y-axis.

• Use best-fit curve (line, polynomial) to show trend.

• Label axes with quantity & units. Include error bars if possible.

• From graph, determine optimum point (e.g., max BTE load), intercepts.

6.4 Structure of a Laboratory Report/Journal

1. Title: Experiment name, date, equipment.

2. Aim: Objective in one sentence.

3. Theory/Principle: Brief theory, key formulas with definitions.

4. Apparatus & Diagram: List instruments with ranges/least counts. Schematic diagram of setup.

5. Procedure: Step-by-step in past tense, numbered.

6. Observations: Tabulated raw data with units.

7. Calculations: Show sample calculation clearly. Use boxed final result.

8. Results: Tabulate calculated values (BP, BTE, etc.).

9. Graphs: Plot as per 6.3.

10. Discussion: Explain trends, compare with theory, analyze errors (systematic vs. random), suggest improvements.

11. Conclusion: State whether aim achieved, summarize key findings.

[!TIP] Common Pitfalls in Reports:

• Missing schematic diagram or apparatus list.

• Not mentioning instrument ranges/least counts.

• Calculations without sample (just final table).

• Graph without equation of trend line or key points marked.

• Discussion that only describes the graph, doesn't analyze reasons for deviation or error sources.

• Incorrect significant figures in final results.