Engineering Thermodynamics

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Thermodynamics deals with energy, its transformation, and the properties of the working substance.

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This section builds the vocabulary: systems, properties, equilibrium, the zeroth law and temperature, ideal gases, and the properties of pure substances such as steam, including tables and charts.

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System and Surroundings System Mass transfer Energy transfer Example Closed (control mass) No Yes (heat & work) Gas in piston-cylinder, pressure cooker (before whistle) Open (control volume) Yes Yes Turbine, compressor, nozzle, pump, boiler Isolated No No Universe; ideal thermos flask • Surroundings: everything outside the system.

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Boundary: real or imaginary surface separating them; may be fixed or moving. • Adiabatic boundary allows no heat transfer; diathermic boundary allows heat transfer. • Macroscopic (classical) approach treats matter as a continuum; microscopic (statistical) approach considers molecular behaviour.

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Thermodynamic Properties, State and Path Functions • Property: measurable characteristic defining the state (P, V, T, U, H, S). • Intensive properties are independent of mass: pressure, temperature, density, specific volume, all specific properties.

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Extensive properties depend on mass: volume, mass, U, H, S, total energy.

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Specific (per kg) value of an extensive property is intensive. • State: condition described by properties.

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Process: change of state.

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Path: series of states.

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Cycle: initial state = final state. • Quasi-static (quasi-equilibrium) process: infinitely slow, passes through equilibrium states — can be drawn on property diagrams and is reversible if frictionless. • Point (state) functions depend only on end states:

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P, V, T, U, H, S, G — exact differentials, ∮dX = 0. • Path functions depend on the path: heat (Q) and work (W) — inexact differentials (δQ, δW).

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They are boundary phenomena and not properties.

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Thermodynamic Equilibrium and Zeroth Law • Thermodynamic equilibrium requires simultaneous thermal (uniform T), mechanical (no unbalanced forces, uniform P) and chemical (no reaction/mass transfer) equilibrium. • Zeroth law: if bodies A and B are each in thermal equilibrium with body C, then A and B are in thermal equilibrium with each other.

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It is the basis of temperature measurement (named by R.H.

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Fowler). • Temperature scales:

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K = °C + 273.15; °F = 1.8 °C + 32; °R = °F + 459.67. −40 °C = −40 °F. • Reference: triple point of water = 273.16 K (0.01 °C, 0.611 kPa) — the single fixed point of the Kelvin scale. • Thermometric properties: volume of liquid (mercury thermometer), pressure of gas at constant volume (constant-volume gas thermometer — reference standard), electrical resistance (RTD, thermistor), EMF (thermocouple — Seebeck effect), radiation (pyrometer).

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Ideal Gas • Ideal gas equation:

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Pv = RT (v = specific volume). • Universal gas constant R̄ = 8.314 kJ/kmol·K (same for all gases).

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Characteristic gas constant R = R̄/M (different for each gas).

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M = 28.97, R = 0.287 kJ/kg·K. • Gas laws:

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Boyle (PV = C at constant T), Charles (V/T = C at constant P), Gay-Lussac (P/T = C at constant V), Avogadro (equal volumes of all gases at same P, T contain equal numbers of molecules;

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1 kmol at NTP occupies 22.4 m³). • cp − cv = R; γ = cp/cv.

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Air: cp = 1.005, cv = 0.718 kJ/kg·K, γ = 1.4.

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Monatomic gas γ = 1.67; diatomic 1.4; polyatomic ≈ 1.3. • Real gases approach ideal behaviour at low pressure and high temperature.

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Compressibility factor Z = Pv/RT (Z = 1 for ideal gas).

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(P + a/v²)(v − b) = RT (a = intermolecular attraction, b = molecular volume).

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Pure Substance, Specific Volume and Quality • Pure substance: homogeneous and chemically invariable composition, may exist in more than one phase (water, steam-water mixture, air as long as no phase change). • Specific volume v = V/m (m³/kg) = 1/ρ. • Phase-change states: compressed (subcooled) liquid → saturated liquid → wet (two-phase) mixture → saturated (dry) vapour → superheated vapour. • Saturation temperature rises with pressure (water boils at 100 °C at 101.325 kPa; ~ 90 °C in Kathmandu due to lower pressure).

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Degree of superheat = T − Tsat. • Quality / dryness fraction x = mvapour / (mliquid + mvapour); x = 0 saturated liquid, x = 1 dry saturated vapour.

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Wetness fraction = 1 − x. • Two-phase mixture properties: v = vf + x·vfg; h = hf + x·hfg; s = sf + x·sfg; u = uf + x·ufg. • Dryness fraction is measured with throttling calorimeter (steam must become superheated after throttling), separating calorimeter, combined separating-throttling calorimeter (for very wet steam), electrical calorimeter.

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Point Water data Key fact Triple point 0.01 °C, 0.6113 kPa Solid, liquid and vapour coexist Normal boiling point 100 °C at 101.325 kPa; hfg = 2257 kJ/kg Latent heat decreases as pressure rises Critical point 22.06 MPa, 373.95 °C (≈ 374 °C), vc = 0.003106 m³/kg hfg = 0; no distinct liquid-vapour phase change above it Development of Property Charts and Tables • Steam tables: saturated tables (temperature-based and pressure-based), superheated vapour tables, compressed liquid tables.

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Reference: u and s of saturated liquid = 0 at triple point. • P-v diagram: saturated liquid and vapour lines meet at the critical point forming a dome; isotherms are horizontal inside the dome. • T-s diagram: area under a reversible process curve = heat transferred.

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Constant-pressure lines are horizontal inside the dome. • h-s diagram (Mollier chart): used for steam turbine and nozzle calculations — vertical line = isentropic process, horizontal = throttling (constant h). • P-h diagram: used in refrigeration cycle analysis. • On P-v and T-s diagrams, the region left of the dome is compressed liquid, inside is wet mixture, right is superheated vapour.

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Industrial Applications — Plant Utilities and Energy Accounting • Plant utilities are open systems (control volumes) analysed with steady-flow energy balances: boilers and steam lines, compressed-air systems, chillers and cooling towers, hot water and thermic-fluid heaters, furnaces and ovens. • Energy units and conversions used in energy audits:

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1 toe ≈ 41.87 GJ; calorific values — diesel ≈ 42–45 MJ/kg, furnace oil ≈ 40–42 MJ/kg, LPG ≈ 46 MJ/kg, coal ≈ 20–28 MJ/kg, firewood ≈ 15–16 MJ/kg, rice husk ≈ 13–15 MJ/kg (much used in Nepali boilers). • Specific energy consumption (SEC) = energy used ÷ production (kWh/tonne, MJ/unit) is the standard benchmark for monitoring plant performance; energy cost per unit of product is tracked with the other cost elements (Chapter 7/10.2). • Compressed air and gases are treated as ideal gases (pV = mRT): receiver pressure rises with temperature at constant volume; free air delivered (FAD) is referred to ambient conditions. • Steam quality matters in process heating: wet steam carries less latent heat and causes erosion; steam tables/charts are used to find enthalpy at the working pressure.

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The first law is the law of conservation of energy.

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This section covers internal energy, enthalpy and specific heats, work and heat in the standard processes, and energy balances for steady and unsteady flow devices.

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Conservation of Mass and Energy • Conservation of mass (continuity): ṁ = ρAV = AV/v.

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Steady flow: ṁin = ṁout; for incompressible flow A1V1 = A2V2. • First law for a cycle (Joule's experiment): ∮δQ = ∮δW — net heat = net work. • First law for a process (closed system):

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Q − W = ΔU (sign convention: heat supplied +, work done BY system +). • Energy is a property.

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Energy of an isolated system is constant. • PMM1 (perpetual motion machine of the first kind) — produces work without any energy input — violates the first law. • Free (unrestrained) expansion into vacuum:

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W = 0, Q = 0 (insulated) → ΔU = 0; for ideal gas T stays constant.

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Internal Energy, Enthalpy and Specific Heat • Internal energy (U): energy stored in molecular motion and configuration.

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For an ideal gas it is a function of temperature only (Joule's law): ΔU = m cv ΔT. • Enthalpy H = U + PV; for an ideal gas also a function of T only: ΔH = m cp ΔT. • cv = (∂u/∂T)v; cp = (∂h/∂T)p. cp > cv for gases because at constant pressure some heat does expansion work. • For solids and liquids cp ≈ cv = c.

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Water c ≈ 4.187 kJ/kg·K. • Displacement (boundary) work W = ∫P dV — area under the curve on a P-V diagram.

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Flow work = Pv (energy needed to push fluid across a boundary).

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Work and Heat Transfer in Standard Processes (Ideal Gas) Process Law (n) Work W1-2 Heat Q1-2 ΔU Isochoric (constant V) V = C (n = ∞) 0 m cv(T2 − T1) = Q Isobaric (constant P) P = C (n = 0) P(V2 − V1) m cp(T2 − T1) = ΔH m cvΔT Isothermal PV = C (n = 1) P1V1 ln(V2/V1) = mRT ln(P1/P2) = W 0 Reversible adiabatic (isentropic) PVγ = C (n = γ) (P1V1 − P2V2)/(γ − 1) 0 −W Polytropic PVn = C (P1V1 − P2V2)/(n − 1) W·(γ − n)/(γ − 1) m cvΔT Throttling h1 = h2 0 0 for ideal gas T const. • P-V-T relations for PVn = C:

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T2/T1 = (P2/P1)(n−1)/n = (V1/V2)n−1 (use γ for adiabatic). • On a P-V diagram from the same initial state, the adiabatic curve is steeper than the isothermal curve (slope ratio = γ). • Isothermal compression needs the least work; adiabatic compression needs the most (for compressors).

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Steady Flow Energy Equation (SFEE) and Applications For steady flow (per unit mass): h1 + V1²/2 + g z1 + q = h2 + V2²/2 + g z2 + w (h in J/kg; divide V²/2 by 1000 for kJ/kg).

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Device Assumptions Simplified SFEE Nozzle q = 0, w = 0, Δz = 0 V2 = √(2(h1 − h2) + V1²); if V1 ≈ 0:

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V2 = 44.72√Δh (Δh in kJ/kg) Diffuser q = 0, w = 0 Velocity ↓, pressure/enthalpy ↑ (opposite of nozzle) Turbine (adiabatic) q = 0, ΔKE ≈ ΔPE ≈ 0 w = h1 − h2 (work output) Compressor / pump q ≈ 0 w = h2 − h1 (input); pump w = v(P2 − P1) Boiler w = 0 q = h2 − h1 (heat added) Condenser w = 0 q = h1 − h2 (heat rejected to cooling water) Throttling valve q = 0, w = 0, ΔKE ≈ 0 h1 = h2 (isenthalpic); pressure drops, entropy rises Heat exchanger w = 0, no heat loss Heat lost by hot fluid = heat gained by cold fluid Unsteady (Transient) Flow • Mass and energy inside the control volume change with time (filling or emptying tanks). • Energy balance for tank filling: m2u2 − m1u1 = mihi + Q − W. • Filling an evacuated insulated rigid tank from a supply line: u2 = hline; for ideal gas T2 = γ·Tline (the gas in the tank becomes HOTTER than the line — flow work converts to internal energy). • Examples: charging a gas cylinder or tyre, bottle filling, discharging a compressed air tank (temperature falls).

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Industrial Applications — Energy Balance and Waste-Heat Recovery • An energy audit is a first-law energy balance of a plant or a piece of equipment: energy input (fuel + electricity) = useful output + losses (flue gas, radiation, blowdown, cooling water, leaks).

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Preliminary (walk-through) and detailed audits; instruments — flue-gas analyser, thermometers, power analyser, flow meters, ultrasonic leak detector. • Boiler efficiency: direct (input-output) method η = ṁs(hs − hfw)/(ṁf·CV); indirect (heat-loss) method η = 100 − (dry flue-gas loss + loss due to H2 in fuel + moisture loss + CO loss + radiation and unaccounted losses) — the indirect method shows where the losses are. • Waste-heat recovery: economiser (feed water), air pre-heater, recuperator on furnaces, condensate recovery, flash steam recovery, heat recovery from compressors and refrigeration condensers, waste-heat boilers on DG sets. • Energy conservation measures: insulation of steam lines and vessels, repairing steam-trap and compressed-air leaks, reducing excess air, optimum boiler loading, load management, and recording savings in the plant's monitoring and targeting system. • Simple payback period = investment ÷ annual saving, with NPV/IRR used for larger energy projects (Chapter 10.2).

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The first law says energy is conserved; the second law says in which direction processes can occur and how much heat can be converted into work.

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It introduces heat engines, refrigerators, the Carnot cycle, reversibility and entropy.

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Kelvin-Planck and Clausius Statements • Kelvin-Planck: it is impossible to construct a device operating in a cycle that produces net work while exchanging heat with a single reservoir. → No heat engine can be 100% efficient. • Clausius: it is impossible to construct a device operating in a cycle whose sole effect is transfer of heat from a colder to a hotter body. → A refrigerator needs work input. • Equivalence: violation of one statement implies violation of the other (both statements are equivalent). • PMM2 (perpetual motion machine of the second kind) — 100% efficient engine using one reservoir — violates Kelvin-Planck statement (not the first law).

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Heat Engine, Heat Pump and Refrigerator Device Purpose Performance measure Heat engine Converts heat into work:

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Q1 from source at TH, rejects Q2 to sink η = W/Q1 = 1 − Q2/Q1 (always < 1) Refrigerator Removes heat Q2 from cold space using work W COPR = Q2/W = Q2/(Q1 − Q2) (can be > 1) Heat pump Delivers heat Q1 to warm space using work W COPHP = Q1/W = Q1/(Q1 − Q2) • COPHP = COPR + 1 (for same temperature limits).

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COPHP is always > 1. • Energy balance for all three:

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Reversible and Irreversible Processes • Reversible process: system and surroundings can both be restored to initial states without leaving any trace.

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Idealisation — gives maximum work output / minimum work input. • Causes of irreversibility: friction, unrestrained (free) expansion, heat transfer across a finite temperature difference, mixing of fluids, electrical resistance (I²R), inelastic deformation, combustion/chemical reaction, throttling. • Internally reversible: no irreversibility within the system; externally reversible: heat exchange with reservoirs across infinitesimal ΔT.

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Carnot Cycle and Carnot Theorem • Carnot cycle = two reversible isothermal processes + two reversible adiabatic (isentropic) processes: isothermal heat addition → isentropic expansion → isothermal heat rejection → isentropic compression.

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It appears as a rectangle on a T-s diagram. • ηCarnot = 1 − TL/TH (T in kelvin).

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Depends ONLY on reservoir temperatures, not the working fluid. • COPR,Carnot = TL/(TH − TL);

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COPHP,Carnot = TH/(TH − TL). • Carnot theorem:

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(1) no engine working between two reservoirs can be more efficient than a reversible engine;

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(2) all reversible engines between the same two reservoirs have the same efficiency. • To increase η: raise TH or lower TL; lowering TL by ΔT is more effective than raising TH by ΔT. • Carnot cycle is impractical: isothermal processes need very slow motion, adiabatic ones very fast; work ratio is low. • Thermodynamic (Kelvin) temperature scale:

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Q1/Q2 = T1/T2 for reversible engines — independent of any substance.

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Entropy and Entropy Relations • Clausius inequality: ∮δQ/T ≤ 0 (= 0 for reversible cycle, < 0 for irreversible). • Entropy: dS = (δQ/T)rev; unit kJ/K (specific entropy kJ/kg·K).

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It is a property (point function) — a measure of molecular disorder and energy unavailability. • Principle of increase of entropy: ΔSisolated = ΔSsystem + ΔSsurroundings ≥ 0.

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Entropy of the universe always increases in real processes; = 0 only for reversible processes.

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Entropy generation Sgen ≥ 0. • Tds relations:

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T ds = dh − v dP. • Ideal gas: Δs = cv ln(T2/T1) + R ln(v2/v1) = cp ln(T2/T1) − R ln(P2/P1). • Isothermal ideal gas: ΔS = mR ln(V2/V1).

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Solids/liquids: ΔS = mc ln(T2/T1).

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Phase change: ΔS = m·hfg/Tsat. • Third law (Nernst): entropy of a pure perfect crystalline substance at absolute zero is zero. • Available energy (exergy) of heat Q at T:

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AE = Q(1 − T0/T); unavailable energy = T0·ΔS.

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Isentropic Process and Isentropic Efficiency • Isentropic process = reversible + adiabatic (s = constant) → vertical line on T-s and h-s diagrams.

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An adiabatic but irreversible process always INCREASES entropy. • Turbine: ηT = actual work / isentropic work = (h1 − h2a)/(h1 − h2s). • Compressor/pump: ηC = isentropic work / actual work = (h2s − h1)/(h2a − h1). • Nozzle: ηN = actual exit KE / isentropic exit KE = (h1 − h2a)/(h1 − h2s).

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Industrial Applications — Availability, Refrigeration and Efficiency Limits • The second law explains why low-temperature waste heat has little work value: the maximum work available from heat Q at temperature T with surroundings at T0 is Q(1 − T0/T).

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Exergy (availability) analysis therefore ranks losses by quality, not just quantity — a key tool in industrial energy management. • Thermal power plants (≈ 30–42% efficient) and DG sets (≈ 35–42%) reject most fuel energy as heat; cogeneration (CHP) uses that rejected heat for process steam and raises overall utilisation to ≈ 70–85%. • Refrigeration in industry: cold storage for fruits, vegetables and dairy (important for Nepal's agro-industry), ice plants, process chilling, air conditioning.

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COP = refrigerating effect ÷ work input;

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1 tonne of refrigeration (TR) = 3.5 kW (3 024 kcal/h); a lower condensing temperature and higher evaporating temperature raise the COP (clean condensers, correct charge, good maintenance). • Heat pumps (COP > 1) are used for industrial drying and hot water; absorption chillers driven by waste heat or steam replace electric compression chillers where cheap heat is available. • Entropy generation in throttling, mixing, heat transfer across large temperature differences and unrestrained expansion identifies avoidable irreversibilities.

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Practical power and refrigeration plants are modelled by ideal cycles.

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This section covers the air-standard cycles (Otto, Diesel, Brayton), the Rankine steam cycle, and vapour compression and vapour absorption refrigeration cycles, with their efficiency and COP.

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Air-Standard Assumptions • Working fluid is air behaving as an ideal gas with constant specific heats; combustion is replaced by heat addition from an external source; exhaust by heat rejection; all processes internally reversible. • Compression ratio r = Vmax/Vmin = (Vs + Vc)/Vc.

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Cut-off ratio ρ = V3/V2.

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Pressure ratio rp = P2/P1. • Mean effective pressure (MEP) = net work ÷ swept volume — used to compare engines of different sizes.

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Power Cycles Cycle Processes Efficiency Application Carnot 2 isothermal + 2 isentropic 1 − TL/TH Ideal benchmark Otto (constant volume) Isentropic compression, constant-V heat addition, isentropic expansion, constant-V heat rejection η = 1 − 1/rγ−1 SI (petrol) engines; r = 6–10 (limited by knock) Diesel (constant pressure) Isentropic compression, constant-P heat addition, isentropic expansion, constant-V heat rejection η = 1 − (1/rγ−1)·[(ργ − 1)/(γ(ρ − 1))] CI (diesel) engines; r = 14–22 Dual (mixed/limited pressure) Heat added partly at constant V and partly at constant P Between Otto and Diesel Modern high-speed diesel engines Brayton (Joule) Isentropic compression, constant-P heat addition, isentropic expansion, constant-P heat rejection η = 1 − 1/rp (γ−1)/γ Gas turbines, jet engines Stirling / Ericsson 2 isothermal + 2 constant-V (Stirling) or constant-P (Ericsson) with regenerator Equal to Carnot (with perfect regeneration) Stirling engines, cryocoolers • Otto efficiency increases with compression ratio r and γ; independent of heat supplied. • Diesel efficiency increases with r but DECREASES with increasing cut-off ratio ρ (i.e., with load).

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Bracket term > 1 so Diesel < Otto for the same r. • Same compression ratio & heat input: ηOtto > ηDual > ηDiesel. • Same maximum pressure & temperature (and heat rejection): ηDiesel > ηDual > ηOtto — which is why diesel engines are more efficient in practice (they use much higher r). • Brayton: efficiency depends on pressure ratio only.

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Optimum rp for maximum net work = (Tmax/Tmin)γ/2(γ−1).

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Back-work ratio (compressor work/turbine work) is high, 40–80%. • Gas turbine improvements: regeneration (exhaust heats compressed air), intercooling (reduces compressor work), reheating (increases turbine work).

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Open cycle (aircraft) vs closed cycle.

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Rankine Cycle (Steam Power Plant) • Processes:

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1-2 isentropic expansion in turbine → 2-3 constant-pressure heat rejection in condenser → 3-4 isentropic compression in pump → 4-1 constant-pressure heat addition in boiler. • ηRankine = (WT − WP)/Qin = [(h1 − h2) − (h4 − h3)]/(h1 − h4).

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Pump work = vf(Pboiler − Pcond) — very small, often neglected. • Why not Carnot for steam: compressing a wet mixture in a pump is impractical and the maximum temperature is limited to saturation temperature. • Ways to improve efficiency: raise boiler pressure, superheat steam, lower condenser pressure (vacuum), reheating, regenerative feed heating (bleed steam heats feedwater). • Raising boiler pressure or lowering condenser pressure increases moisture at turbine exit → blade erosion; turbine exhaust dryness should be ≥ ~0.88–0.9.

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Reheating mainly reduces exit moisture (and slightly improves η); regeneration raises mean temperature of heat addition. • Work ratio of Rankine is high (pump work small) compared with Brayton.

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Refrigeration Cycles • Vapour compression refrigeration (VCR):

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1-2 isentropic compression (compressor) → 2-3 constant-pressure heat rejection (condenser) → 3-4 throttling, h3 = h4 (expansion valve/capillary) → 4-1 constant-pressure heat absorption (evaporator). • COPVCR = refrigerating effect / compressor work = (h1 − h4)/(h2 − h1).

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Analysed on a P-h chart. • Subcooling the liquid after condenser increases refrigerating effect and COP.

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Lower condenser pressure or higher evaporator pressure increases COP. • Capacity:

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1 TR (ton of refrigeration) = 3.517 kW = 211 kJ/min (heat to freeze 1 US ton of water at 0 °C in 24 h). • Vapour absorption refrigeration (VAR): compressor replaced by absorber + pump + generator; driven mainly by HEAT (waste heat, solar, steam) with small pump work.

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Lower COP (≈ 0.5–0.8 single effect) but uses low-grade energy. • VAR pairs:

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NH3-water (ammonia = refrigerant, water = absorbent; for sub-zero cold storage) and water-LiBr (water = refrigerant, LiBr = absorbent; above 0 °C, large air-conditioning).

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Electrolux refrigerator uses NH3-H2O-H2, no moving parts/pump. • Ideal VAR COP = [(TG − T0)/TG] × [TE/(T0 − TE)]. • Air refrigeration (Bell-Coleman / reversed Brayton): used in aircraft cabin cooling; low COP but light weight.

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Feature Vapour compression Vapour absorption Energy input Mechanical work (compressor) Heat (generator) + small pump work Moving parts Compressor — noise, wear Only pump (none in Electrolux) — quiet COP High (2–5) Low (0.5–1.2) Capacity at part load Falls Nearly constant Best when Electricity is available Waste heat / cheap heat is available Industrial Applications — Plant Power and Refrigeration Cycles • Rankine cycle plants supply electricity and process steam in sugar, cement, paper and textile industries; back-pressure turbines (exhaust steam used for process) and extraction-condensing turbines are the usual cogeneration arrangements; bagasse and rice husk are common biomass fuels in South Asia. • Brayton (gas-turbine) cycle with waste-heat recovery boilers forms combined-cycle plants;

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Otto and Diesel cycles apply to gensets (2.5). • Vapour-compression cycle: compressor, condenser, expansion device, evaporator — the standard for cold stores, chillers and air conditioning.

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Vapour-absorption cycle (LiBr-water or ammonia-water) replaces the compressor with a generator, absorber and pump — driven by waste heat, steam or gas, so it consumes very little electricity (COP ≈ 0.6–1.2 single effect) and reduces peak electrical demand. • Cycle efficiency improvements (reheat, regeneration, higher pressure) are evaluated against capital cost using engineering-economics methods (Chapter 10.2).

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IC engines burn fuel inside the cylinder.

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This section covers spark-ignition and compression-ignition engines, four-stroke and two-stroke operation, the main components and their functions, and basic performance terms.

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Engine Terminology • Bore (D) = cylinder diameter; stroke (L) = distance between TDC and BDC = 2 × crank radius. • TDC/BDC: top/bottom dead centre — extreme piston positions. • Swept volume Vs = (π/4)D²L; clearance volume Vc; compression ratio r = (Vs + Vc)/Vc. • Indicated power IP = pmi·L·A·n·k/60 (n = N/2 for four-stroke, N for two-stroke; k = number of cylinders).

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Brake power BP = 2πNT/60.

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Friction power FP = IP − BP. • Mechanical efficiency ηm = BP/IP.

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Brake thermal efficiency = BP/(ṁf × CV).

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Brake specific fuel consumption (BSFC) = fuel per kWh.

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Volumetric efficiency = actual air inducted / swept volume (at intake conditions). • Four-cylinder in-line engine firing order:

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1-3-4-2 (or 1-2-4-3).

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Spark Ignition vs Compression Ignition Engines Feature SI (petrol) engine CI (diesel) engine Ideal cycle Otto (constant volume) Diesel (constant pressure) Fuel Petrol (high volatility, high self-ignition temperature) Diesel (low volatility, low self-ignition temperature) Charge inducted Air-fuel mixture (carburettor / port or direct injection) Air only; fuel injected at end of compression Ignition Spark plug Heat of compressed air (self-ignition) — fuel injector Compression ratio 6–10 (up to ~12 in modern GDI) 14–22 Load control Quantity governing (throttle valve) Quality governing (fuel quantity varies, air constant) Thermal efficiency Lower Higher (higher r) Weight / speed Lighter, high speed Heavier (high pressures), lower speed Knocking At END of combustion — auto-ignition of end charge At START — long ignition delay Fuel rating Octane number (iso-octane = 100, n-heptane = 0) Cetane number (n-cetane = 100, α-methylnaphthalene = 0) Applications Cars, motorcycles, small generators Trucks, buses, tractors, ships, locomotives, gensets • Reducing knock in SI: high-octane fuel, lower compression ratio, retard spark, lower intake temperature, good turbulence. • Reducing knock in CI: high-cetane fuel, HIGHER compression ratio, higher intake temperature (to shorten ignition delay) — the factors are OPPOSITE to SI. • Additives:

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TEL (tetraethyl lead, now banned) raised octane; amyl nitrate raises cetane number.

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Four-Stroke and Two-Stroke Cycle of Operation • Four-stroke: suction (intake valve open) → compression (both closed) → power/expansion (both closed) → exhaust (exhaust valve open).

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One power stroke every 2 crankshaft revolutions (720°).

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Camshaft runs at half crankshaft speed. • Two-stroke: suction, compression, power and exhaust completed in one crankshaft revolution.

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Uses ports (inlet, transfer, exhaust) uncovered by the piston instead of valves; crankcase compression and scavenging (fresh charge pushes out exhaust). • Valve timing (4-stroke): inlet opens before TDC and closes after BDC; exhaust opens before BDC and closes after TDC.

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The period when both valves are open near TDC is called valve overlap.

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Feature Four-stroke Two-stroke Power strokes 1 per 2 revolutions 1 per revolution (theoretically 2× power) Valves Poppet valves + valve mechanism Ports — simpler, fewer parts Flywheel Heavier (uneven torque) Lighter (more uniform torque) Thermal efficiency, fuel economy Higher, better Lower (loss of fresh charge in scavenging) Lubrication Separate oil sump Oil mixed with fuel (petroil) in small SI engines Emissions / wear Lower Higher HC emissions, more wear Use Cars, trucks, most motorcycles Mopeds, chainsaws, outboard motors, very large marine diesels Major Components and Their Functions Component Function Cylinder block Houses cylinders; made of cast iron or aluminium alloy; cooling jackets/fins Cylinder head Closes top of cylinder; holds valves, spark plug/injector; forms combustion chamber Piston Transmits gas force to connecting rod; usually aluminium alloy (light, good conductivity) Piston rings Compression rings (top) seal gas; oil-control/scraper ring (bottom) controls lubricating oil Gudgeon (wrist) pin Connects piston to small end of connecting rod Connecting rod Converts reciprocating motion of piston into rotary motion of crank;

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I-section, forged steel Crankshaft Converts reciprocating to rotary motion and delivers output torque Camshaft Operates valves through cams, tappets/rocker arms; half engine speed in 4-stroke Flywheel Stores energy during power stroke, releases it during idle strokes — reduces speed fluctuation Valves Poppet valves: inlet (larger) and exhaust (made of heat-resistant steel) Spark plug / injector Ignites mixture in SI / sprays atomised fuel at high pressure in CI Carburettor Mixes air and petrol in correct ratio using a venturi (older SI engines) Fuel injection pump Meters and pressurises diesel for injectors Crankcase & sump Encloses crankshaft; stores lubricating oil Governor Controls speed by regulating fuel supply according to load Supercharger / turbocharger Increases intake air density → more power; turbocharger is driven by exhaust gas • Cooling: air cooling (fins — motorcycles) or water cooling (radiator, pump, thermostat — cars).

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Lubrication: splash, pressure (forced) or petroil systems. • Stoichiometric air-fuel ratio for petrol ≈ 14.7 :

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CI engines run with excess air (lean overall).

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Industrial Applications — Engines and Captive Power • Diesel generating (DG) sets provide captive and standby power in industries, hospitals and commercial buildings — essential during grid outages and load-shedding in Nepal; they are also the costliest source of power per kWh. • Genset sizing and loading: rated kVA chosen from connected load, diversity and starting (inrush) currents of the largest motor;

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DG sets should be run at 60–80% load for best specific fuel consumption (≈ 0.25–0.30 L/kWh for medium sets); lightly loaded sets suffer poor efficiency and wet stacking. • Operating cost per kWh = (fuel cost + lubricating oil + maintenance + depreciation) ÷ units generated; compare with grid tariff and with solar/battery hybrids in energy-source decisions. • Engine maintenance (air filters, injectors, valve clearance, cooling) is planned preventive maintenance (Chapter 8); exhaust emission and noise limits apply (Chapter 9).

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Applied thermodynamics puts the laws into practice:

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HVAC systems, steam boilers, air compressors, refrigerants and their properties, and psychrometry — the study of moist air used in air-conditioning.

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Basic HVAC System • HVAC = Heating, Ventilation and Air Conditioning.

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Air conditioning controls temperature, humidity, air purity (filtration) and air motion/distribution simultaneously. • Human comfort conditions: ≈ 22–26 °C DBT, 40–60% RH, air velocity ≈ 0.1–0.25 m/s.

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Effective temperature combines DBT, humidity and air motion. • Components: compressor, condenser, expansion device, evaporator/cooling coil, air-handling unit (AHU), fans/blowers, ducts, dampers, filters, humidifier, heater, diffusers, thermostat/controls, cooling tower (large plants). • Summer air-conditioning: cooling + dehumidification.

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Winter air-conditioning: heating + humidification.

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Year-round AC does both. • System types: window AC, split AC, packaged unit, central plant (all-air, all-water, air-water with chilled water and AHUs/FCUs), VRF/VRV multi-split systems. • Cooling-coil bypass factor = fraction of air passing without contact with coil; contact factor = 1 − BPF.

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ADP = apparatus dew point (effective coil surface temperature). • Cooling load components: sensible (conduction, solar, occupants, lights, equipment) and latent (moisture from people, infiltration).

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Boilers Feature Fire-tube boiler Water-tube boiler Principle Hot flue gases flow INSIDE tubes; water surrounds tubes Water flows INSIDE tubes; hot gases outside Pressure / capacity Low pressure (up to ≈ 25 bar), small capacity High pressure (up to 250+ bar, supercritical), large capacity Water volume / steaming Large water volume; slow steam raising; handles load fluctuation Small water volume; rapid steam raising Explosion risk More severe (large stored energy) Less severe (only a tube bursts) Examples Cochran (vertical), Cornish (1 flue), Lancashire (2 flues), locomotive, Scotch marine Babcock & Wilcox, Stirling; high-pressure:

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La Mont (forced circulation), Benson (supercritical, no drum), Loeffler, Velox Boiler mountings (safety & control — compulsory) Boiler accessories (efficiency improvement) Safety valve (spring loaded, dead weight, lever) — releases excess pressure Economiser — preheats FEEDWATER using flue gases Water level indicator (gauge glass) Air preheater — heats COMBUSTION AIR using flue gases Pressure gauge (Bourdon tube) Superheater — converts wet/saturated steam into superheated steam Fusible plug — melts to extinguish fire if water level drops too low Feed pump / injector — supplies water to boiler Steam stop valve, feed check valve (non-return) Steam separator, steam trap Blow-off cock — removes sludge and sediments Draught fans • Flue gas path order: furnace → superheater → economiser → air preheater → chimney. • Equivalent evaporation 'from and at 100 °C' = ms(h − hf)/2257 — compares boilers on a common basis. • Boiler efficiency = ms(h − hfw) / (mf × CV). • Draught: natural (chimney; hw = 353H(1/Ta − 1/Tg) mm water) or artificial — forced (fan before furnace, pressure above atmospheric), induced (fan near chimney, pressure below atmospheric), balanced (both). • Boiler water treatment prevents scale, corrosion, priming and foaming.

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Indian Boiler Regulations (IBR) govern boiler safety in the region.

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Compressors Type Examples Characteristics Reciprocating (positive displacement) Single/multi-stage piston compressors High pressure ratio, low flow; pulsating delivery; clearance affects volumetric efficiency Rotary positive displacement Screw, sliding vane, lobe (Roots blower — no internal compression) Medium pressure, continuous smooth flow Dynamic — centrifugal (radial) Impeller + diffuser High flow, moderate pressure ratio per stage; surging at low flow Dynamic — axial Multiple rotor/stator blade rows Very high flow, high efficiency: gas turbines, jet engines; stalling & surging • Compression work order: isothermal (minimum) < polytropic < adiabatic (maximum).

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Isothermal work = P1V1 ln(P2/P1).

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Isothermal efficiency = isothermal work ÷ actual work. • Multistage compression with intercooling: reduces work, lowers delivery temperature, improves volumetric efficiency and lubrication.

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Perfect intercooling: air cooled back to initial temperature. • Two-stage optimum intermediate pressure:

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P2 = √(P1P3) (equal pressure ratio and equal work in each stage).

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N stages: stage ratio = (Pfinal/P1)1/N. • Volumetric efficiency of reciprocating compressor: ηv = 1 + C − C(P2/P1)1/n (C = clearance ratio) — decreases as pressure ratio or clearance increases. • FAD (free air delivery): volume of delivered air reduced to intake (atmospheric) conditions. • Surging: unstable flow reversal at low flow rate in centrifugal/axial compressors; stalling: flow separation on blades.

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Refrigerants and Their Properties • Numbering for halocarbons CmHnFpClq:

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R-(m − 1)(n + 1)(p).

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E.g., CCl2F2 → R-12;

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Inorganic refrigerants:

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R-700 + molecular weight — NH3 = R-717, water = R-718, CO2 = R-744.

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Hydrocarbons: propane R-290, isobutane R-600a. • Desirable properties: low boiling point, high latent heat (large refrigerating effect), high critical temperature, low freezing point, moderate positive condenser/evaporator pressures, low specific volume of vapour, non-toxic, non-flammable, non-corrosive, chemically stable, zero ODP, low GWP, easy leak detection, low cost. • CFCs (R-11, R-12) — high ozone depletion potential — phased out under the Montreal Protocol (1987).

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HCFC R-22 being phased out.

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HFCs (R-134a, R-410A) — zero ODP but high GWP — phase-down under the Kigali Amendment (2016).

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HFOs (R-1234yf) and natural refrigerants (R-717, R-744, R-290, R-600a) are low-GWP alternatives. • Leak detection: halide torch or electronic detector for halocarbons; sulphur candle (white fumes) or litmus for ammonia; soap solution for any refrigerant.

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Refrigerant Key facts Typical use R-717 Ammonia (NH3) Highest refrigerating effect per kg, cheap, zero ODP/GWP; toxic, slightly flammable; attacks copper (use steel) Cold storage, ice plants, large industrial plants, VAR R-12 (CCl2F2) CFC; safe, non-toxic but high ODP — banned Old domestic refrigerators, car AC R-22 (CHClF2) HCFC; low ODP; being phased out Older window/split AC R-134a (CH2FCF3) HFC; zero ODP, GWP ≈ 1430; replaced R-12 Domestic refrigerators, car AC Refrigerant Key facts Typical use R-410A HFC blend; zero ODP; higher pressures Modern split AC, heat pumps R-600a Isobutane / R-290 Propane Hydrocarbons; very low GWP; flammable Domestic refrigerators, small AC R-744 CO2 GWP = 1, non-toxic; very high pressures, low critical temp (31 °C) Transcritical systems, supermarkets, heat pumps R-718 Water Cheap, safe; only above 0 °C Refrigerant in LiBr absorption chillers Psychrometrics • Moist air = dry air + water vapour.

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Ptotal = pa + pv. • DBT (dry-bulb temp.) — ordinary thermometer;

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WBT (wet-bulb temp.) — thermometer with wet wick;

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DPT (dew-point temp.) — temperature at which vapour starts condensing when cooled at constant pressure. • For unsaturated air:

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For saturated air (RH = 100%):

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Wet-bulb depression = DBT − WBT (zero for saturated air). • Specific humidity (humidity ratio) ω = 0.622 pv/(P − pv) kg vapour per kg dry air. • Relative humidity φ = pv/ps (at same DBT).

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Degree of saturation μ = ω/ωs. • Enthalpy of moist air: h = 1.005 t + ω(2500 + 1.88 t) kJ/kg dry air. • Psychrometric chart:

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DBT on horizontal axis, specific humidity on vertical axis; curved RH lines; inclined WBT/enthalpy lines.

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Instruments: sling psychrometer, hygrometer. • Sensible heat factor SHF = sensible heat / total heat.

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Psychrometric process What changes Example Sensible heating DBT ↑, ω constant (horizontal line), RH ↓ Heating coil Sensible cooling DBT ↓, ω constant, RH ↑ (coil above DPT) Cooling coil without condensation Cooling & dehumidification DBT ↓, ω ↓ (coil below DPT) Summer air conditioning Heating & humidification DBT ↑, ω ↑ Winter air conditioning Evaporative (adiabatic) cooling WBT constant, DBT ↓, ω ↑, RH ↑ Desert cooler — best in hot, dry climate Chemical dehumidification ω ↓, DBT ↑ (latent heat released) Silica gel, activated alumina Adiabatic mixing Mixed state lies on straight line joining the two states Mixing fresh and recirculated air Industrial Applications — Boilers, Compressed Air and HVAC in Plants • Steam system: boiler (fire-tube for small/medium process plants, water-tube for high pressure), feed-water treatment (softening, deaeration) to prevent scale and corrosion, blowdown control (TDS), steam distribution with proper insulation, steam traps (thermodynamic, thermostatic, mechanical) to remove condensate, condensate and flash-steam recovery, pressure-reducing stations.

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Boiler operation is regulated for safety (pressure-vessel inspection, safety valves, water-level controls). • Compressed air is the most expensive common utility (only ≈ 10–20% of input electrical energy becomes useful air power): reciprocating, screw and centrifugal compressors; measures — fix leaks (a 3 mm hole at 7 bar can waste several kW), lower the set pressure (≈ 6–8% power saving per 1 bar), cool and dry the air, take suction from cool outside air (every 4 °C fall in intake temperature ≈ 1% power saving), use correct pipe sizes and receivers, avoid using compressed air for cleaning. • HVAC and psychrometry in industry: comfort air conditioning for offices, and process air conditioning where humidity must be controlled — textile spinning (≈ 50–60% RH to prevent yarn breakage), printing, pharmaceuticals and electronics (clean rooms), tea and food drying, storage of grain and seeds.

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Psychrometric processes: sensible cooling/heating, cooling with dehumidification, humidification, evaporative cooling, mixing of air streams. • Industrial ventilation and local exhaust for fumes, dust and heat; cooling towers for condenser and process cooling water. • Refrigerants: phase-out of CFC/HCFC (R-12, R-22) under the Montreal Protocol and HFCs under the Kigali Amendment; ammonia (R-717) remains common in large cold stores and ice plants — efficient and cheap but toxic, so safety systems are required.