Internal Combustion Engines (ME-501) - Unit 1 Short Notes

1.0 FUNDAMENTAL THERMODYNAMIC CYCLES & ANALYSIS

1.1 Air-Standard Analysis Assumptions & Limitations

• Assumptions:

  1. Working fluid is ideal gas (obeys $$\displaystyle PV = mRT $$).

  2. Specific heats ($$\displaystyle C_v, C_p $$) are constant.

  3. All processes are internally reversible.

  4. Compression and expansion are isentropic.

  5. Heat addition and rejection are at constant volume (Otto) or constant pressure (Diesel).

  6. No heat loss to surroundings, no friction.

• Limitations: Ignores actual gas behavior, variable specific heats, heat transfer, friction, pumping work, and exhaust blowdown. Results are ideal upper limits.

[!TIP] Exam Focus: Air-standard analysis is the foundation for all cycle efficiency derivations. Always state the assumptions before solving numericals.

1.2 Otto Cycle (Spark Ignition / Constant Volume Heat Addition)

• Processes (1-2-3-4-1):

  1. 1-2: Isentropic compression ($$\displaystyle V_1/V_2 = r $$, compression ratio).

  2. 2-3: Constant volume heat addition ($$\displaystyle Q_{in} $$).

  3. 3-4: Isentropic expansion (power stroke).

  4. 4-1: Constant volume heat rejection ($$\displaystyle Q_{out} $$).

• Diagrams:

  DiagramSEARCH: "Otto cycle PV TS diagram"

• Thermal Efficiency Derivation:

$$Q_{in} = C_v(T_3 - T_2), \quad Q_{out} = C_v(T_4 - T_1)$$

Using isentropic relations: $$\displaystyle T_2/T_1 = (V_1/V_2)^{\gamma-1} = r^{\gamma-1} $$ and $$\displaystyle T_3/T_4 = r^{\gamma-1} $$.

$$\eta_{th} = 1 - \frac{Q_{out}}{Q_{in}} = 1 - \frac{T_4 - T_1}{T_3 - T_2}$$

Substituting and simplifying:

$$\boxed{\eta_{otto} = 1 - \frac{1}{r^{\gamma-1}}}$$

• Numerical Analysis Steps:

  1. Find $$\displaystyle T_2 $$ from isentropic compression: $$\displaystyle T_2 = T_1 \cdot r^{\gamma-1} $$.

  2. Given $$\displaystyle Q_{in} $$ or $$\displaystyle P_3/T_3 $$, find $$\displaystyle T_3 $$: $$\displaystyle T_3 = T_2 + Q_{in}/C_v $$.

  3. Find $$\displaystyle T_4 $$ from isentropic expansion: $$\displaystyle T_4 = T_3 / r^{\gamma-1} $$.

  4. Net work: $$\displaystyle W_{net} = Q_{in} - Q_{out} $$.

  5. Mean Effective Pressure (MEP): $$\displaystyle MEP = \frac{W_{net}}{V_1 - V_2} = \frac{W_{net}}{V_s} $$ (per kg, $$\displaystyle V_s $$ = swept volume).

• Effect of Compression Ratio: Efficiency increases with $r$. Higher $r$ → higher $$\displaystyle T_2 $$ → risk of detonation in SI engines.

• Loss due to Variable Specific Heats: At high $$\displaystyle T_3 $$, $$\displaystyle C_v $$ increases. This reduces the effective temperature rise for same $$\displaystyle Q_{in} $$, lowering $$\displaystyle T_3 $$ and $$\displaystyle T_4 $$ compared to constant $$\displaystyle C_v $$ case. Efficiency drops slightly.

  DiagramCANVAS: "PV diagram showing two Otto cycles: one with constant gamma (higher area), one with variable gamma (lower area)"
  .

1.3 Diesel Cycle (Compression Ignition / Constant Pressure Heat Addition)

• Processes (1-2-3-4-1):

  1. 1-2: Isentropic compression.

  2. 2-3: Constant pressure heat addition ($$\displaystyle Q_{in} $$).

  3. 3-4: Isentropic expansion.

  4. 4-1: Constant volume heat rejection.

• Diagrams:

  DiagramSEARCH: "Diesel cycle PV TS diagram"

• Thermal Efficiency Derivation:

$$Q_{in} = C_p(T_3 - T_2), \quad Q_{out} = C_v(T_4 - T_1)$$

Isentropic relations: $$\displaystyle T_2 = T_1 r^{\gamma-1} $$, $$\displaystyle T_4 = T_3 / r^{\gamma-1} $$.

Define **cut-off ratio** $$\displaystyle \rho = V_3/V_2 $$.

From constant pressure process: $$\displaystyle T_3/T_2 = V_3/V_2 = \rho $$.

$$\eta_{diesel} = 1 - \frac{1}{r^{\gamma-1}} \cdot \frac{\rho^{\gamma} - 1}{\gamma(\rho - 1)}$$

• Numerical Analysis: Given $r$, $\rho$, $$\displaystyle T_1 $$, find $$\displaystyle T_2 $$, $$\displaystyle T_3 = \rho T_2 $$, $$\displaystyle T_4 = T_3 / r^{\gamma-1} $$, then $\eta$, $$\displaystyle W_{net} $$.

• Comparison with Otto (same $r$, same $$\displaystyle Q_{in} $$):

  • Diesel cycle has lower efficiency because heat addition at constant pressure starts earlier (at higher $$\displaystyle T_2 $$) and ends later.

  • Diesel cycle has higher work output per cycle due to larger area in PV diagram.

  • Cut-off ratio $\rho$ critically affects efficiency: $\eta \downarrow$ as $\rho \uparrow$.

1.4 Dual Cycle (Limited Cycle)

• Processes: Combustion occurs partly at constant volume (2-2') and partly at constant pressure (2'-3). More realistic for modern CI engines.

• Efficiency Expression: Between Otto and Diesel. Qualitative: For same $r$ and $$\displaystyle Q_{in} $$, $$\displaystyle \eta_{Otto} > \eta_{Dual} > \eta_{Diesel} $$.

• Relevance: Models combustion in engines with finite combustion duration.

1.5 Brayton Cycle (Gas Turbine - Brief Reference)

• Processes: Isentropic compression, constant pressure heat addition, isentropic expansion.

• Efficiency: $$\displaystyle \eta = 1 - 1/(r_p)^{(\gamma-1)/\gamma} $$, where $$\displaystyle r_p $$ = pressure ratio.

• Used in gas turbines, not in automotive reciprocating engines.

1.6 Comparison of Otto, Diesel, and Dual Cycles

2.0 ENGINE TYPES, CONSTRUCTION & BASIC PERFORMANCE PARAMETERS

2.1 Classification of I.C. Engines

2.2 Key Performance Parameters & Definitions

[!TIP] Common Pitfall: MEP is not an actual pressure in the cylinder; it's an average pressure that would produce the net work if constant throughout the stroke.

2.3 Engine Testing & Performance Evaluation

• Engine Trial: Measures $BP$ (dynamometer), fuel consumption, air/fuel flow, temperatures (exhaust, cooling water, lub oil), pressures.

• Heat Balance Sheet (on minute/hour basis):

  | Heat Input | Heat Output | | :--- | :--- | | 1. Heat in fuel = $$\displaystyle \dot{m}_f \cdot CV $$ | 1. Heat equivalent of $BP$ | | | 2. Heat to cooling water = $$\displaystyle \dot{m}_w C_w \Delta T_w $$ | | | 3. Heat to exhaust gases = $$\displaystyle \dot{m}_{ex} C_{ex} \Delta T_{ex} $$ | | | 4. Heat to radiation & unaccounted (by difference) |

• Morse Test (Multi-cylinder I.P. determination):

  • Method: Run engine at constant speed & throttle. Measure $BP$ with all cylinders working. Then cut out one cylinder at a time (by stopping its fuel supply/ignition) and measure $BP$.

  • Assumptions:

    1. Frictional losses of the cut-out cylinder remain same when it is working.

    2. Friction of other cylinders & external friction (water pump, lub oil pump) constant.

  • Calculation:

    $$\displaystyle IP_{total} = BP_{all} + \sum (BP_{all} - BP_{cut\ i}) $$

    $$\displaystyle \eta_{mech} = \frac{BP_{all}}{IP_{total}} $$

  • Example (Nov 2022): $$\displaystyle BP_{all}=32 $$ kW, $$\displaystyle BP_1=21.6 $$, $$\displaystyle BP_2=22.3 $$, $$\displaystyle BP_3=22.5 $$, $$\displaystyle BP_4=23 $$ kW.

    $$\displaystyle \sum (BP_{all}-BP_i) = (10.4 + 9.7 + 9.5 + 9) = 38.6 $$ kW.

    $$\displaystyle IP = 32 + 38.6 = 70.6 $$ kW.

    $$\displaystyle \eta_{mech} = 32 / 70.6 = 0.453 $$ or 45.3%.

3.0 FUEL SYSTEMS (SI & CI)

3.1 Fuel Requirements

• SI (Petrol): High volatility, high octane number (anti-knock), good vaporization, clean burning.

• CI (Diesel): Good ignition quality (high cetane number), proper viscosity for injection, low sulfur.

3.2 Carburetion (SI Engines)

• Function: To prepare homogeneous air-fuel mixture of correct strength for varying engine speeds/loads.

• Theory: Based on Bernoulli's principle. Velocity increase in venturi → pressure drop → fuel drawn from jet.

• Simple Carburetor:

  DiagramSEARCH: "simple carburetor diagram"

  • Construction: Float chamber, venturi, fuel jet, throttle valve.

  • Working: Air flows through venturi, pressure at fuel jet tip drops below atmospheric, fuel lifted into air stream.

  • Limitations: Provides correct mixture only at one speed (full throttle). Fails at part-load (lean) and high speed (rich) due to lag.

• Types: Solex, Zenith (multiple venturis), SU (constant depression/variable choke).

• Multi-Point Fuel Injection (MPFI): Injector near each intake valve. Advantages: Better fuel metering, reduced emissions, higher power, no throttle losses. Disadvantages: Cost, complexity.

• Numerical (Carburetor):

  • Air flow rate: $$\displaystyle \dot{m}_a = C_d A_t \rho_a V_t $$ (with $$\displaystyle V_t $$ from energy eqn: $$\displaystyle \frac{V_t^2}{2} = \frac{\gamma}{\gamma-1} R T_1 \left[1 - \left(\frac{P_2}{P_1}\right)^{(\gamma-1)/\gamma}\right] $$).

  • Fuel flow rate: $$\displaystyle \dot{m}_f = C_f A_f \sqrt{2 \rho_f (P_{atm} - P_{fuel} - \frac{\rho_f V_f^2}{2})} $$ (with/without nozzle lip correction).

  • $$\displaystyle AFR = \dot{m}_a / \dot{m}_f $$.

3.3 Fuel Injection System (CI Engines)

• Function: Atomize fuel, control timing & rate, distribute fuel in combustion chamber.

• Types of Injection Systems:

  • Air Injection: Fuel atomized by high-pressure air. Obsolete.

  • Solid (Mechanical) Injection: Fuel injected directly by high-pressure pump. Types: Common Rail, Unit Injector, Distributor Pump, In-line Pump.

• Fuel Injector (Nozzle):

  DiagramSEARCH: "diesel fuel injector nozzle types"

  • Construction: Nozzle body, needle valve, spring, pressure chamber.

  • Working: Pump pressure lifts needle, fuel sprays through holes. Spring closes needle when pressure drops.

• Types of Nozzles:

  • Single-hole: Low pressure, large engines.

  • Multi-hole: 4-8 holes, common in automotive.

  • Pintle type: Good atomization at low pressure, used in pre-combustion chambers.

  • Throttle type: For air-blast injection (obsolete).

• Injection Timing & Rate: Advanced timing → higher efficiency but more noise. Rate should be high initially for good mixing, then reduce to limit pressure rise.

3.4 Fuel Injection in CI Engines (General)

• Stages:

  1. Injection Delay (Ignition Delay): Fuel atomizes, vaporizes, mixes with air, reaches auto-ignition temperature.

  2. Uncontrolled Combustion (Rapid): Fuel accumulated during delay burns rapidly → sharp pressure rise.

  3. Controlled Combustion (Afterburning): Remaining fuel burns as it mixes.

• Factors: Fuel properties (cetane, viscosity), injection pressure, nozzle design, chamber design, engine speed/load.

4.0 IGNITION SYSTEMS (SI ENGINES)

4.1 Battery (Coil) Ignition System

• Components: Battery, ignition coil (step-up transformer), contact breaker (points), capacitor, distributor, spark plug.

• Working Principle:

  DiagramSEARCH: "battery ignition system diagram"

  1. Current flows in primary circuit (battery → switch → points → coil primary → ground).

  2. Points open → primary current collapses → high voltage induced in secondary.

  3. Distributor routes high voltage to correct spark plug.

  4. Capacitor suppresses arcing at points, speeds collapse.

• Advantages: Simple, inexpensive.

• Disadvantages: Contact wear, maintenance, limited spark energy at high speed, timing drift.

4.2 Electronic Ignition & Microprocessor-Based Systems

• Advantages: No contact wear, precise timing control, higher spark energy, better performance/emissions, adaptive to engine conditions (knock control, idle stabilization).

4.3 Spark Plug

• Construction: Insulator (ceramic), central electrode, ground electrode, shell, gasket.

• Requirements:

  • Insulation: Withstand high voltage.

  • Heat Range: Hot plug (longer insulator nose, slower heat transfer) for low-speed engines. Cold plug (shorter nose, faster transfer) for high-speed/high-compression engines. Must keep tip hot enough to burn off deposits but not cause pre-ignition.

  • Gap: Typically 0.6-1.2 mm. Too large → weak spark. Too small → poor ignition.

• Location: Central in combustion chamber for shortest flame travel.

4.4 Ignition Lag (Ignition Delay) in SI Engines

• Definition: Time interval between spark discharge and start of pressure rise due to combustion.

• Stages:

  1. Physical Delay: Atomization, vaporization, mixing of fuel-air (0.1-0.5 ms).

  2. Chemical Delay: Chemical reactions forming radicals, chain initiation (0.2-1.0 ms).

• Effect of Engine Variables:

  | Variable | Effect on Ignition Lag | Reason | | :--- | :--- | :--- | | ↑ Compression Ratio | ↓ | Higher $$\displaystyle T_2 $$, better vaporization | | ↑ Engine Speed | ↓ (in °CA) | More turbulence, better mixing | | Rich Mixture | ↓ | More fuel vapor, easier ignition | | ↑ Turbulence | ↓ | Faster mixing | | ↑ Electrode Gap | ↓ | Larger kernel, faster growth | | Advanced Spark Timing | ↓ (in °CA) | Higher $$\displaystyle T_2 $$ at spark | | ↑ Inlet Temp/Pressure | ↓ | Higher $$\displaystyle T_2 $$ |

5.0 COMBUSTION IN SPARK IGNITION (SI) ENGINES

5.1 Stages of Combustion (P-θ Diagram)

1. Ignition Delay (Preparation Phase): Spark → flame kernel formation. Pressure rise slow.

2. Flame Propagation: Flame front moves across chamber. Pressure rises rapidly but smoothly.

3. Afterburning (Tail Flame): Burn-out of near-wall gases. Pressure rise slows.

5.2 Flame Propagation

• Normal Flame Speed: 20-50 m/s (at 1 atm). Not same as burning velocity (laminar, ~0.5 m/s).

• Factors Affecting Flame Speed:

  • Turbulence: ↑ turbulence → ↑ flame speed (most significant).

  • Mixture Strength: Slightly rich (φ≈1.1) gives max speed.

  • Compression Ratio: ↑ $r$ → ↑ $$\displaystyle T_2 $$, ↑ pressure → ↑ speed.

  • Spark Location: Central → shortest travel distance.

  • Inlet Temp/Pressure: ↑ → ↑ density → ↑ speed.

  • Engine Speed: ↑ speed → ↑ turbulence → ↑ speed.

5.3 Abnormal Combustion: Detonation (Knock)

• Phenomenon: Uncontrolled, explosive combustion of end-gas. High-frequency pressure waves → "pinging" sound, piston/head damage.

• Theories:

  1. Auto-ignition Theory: End-gas (unburned mixture) compressed & heated by flame front → auto-ignites → pressure spike.

  2. Detonation Wave Theory: Supersonic shock wave initiates reactions.

• Factors Affecting Detonation:

  • ↑ Compression Ratio (most critical)

  • ↑ Engine Load

  • Retarded Ignition Timing (increases pressure at end of combustion)

  • Lean Mixture (higher $$\displaystyle T_{max} $$)

  • ↑ Inlet Temperature

  • Poor Combustion Chamber Design (large surface/volume ratio, hot spots)

  • Low Octane Number Fuel

• Control & Prevention:

  • Retard ignition timing.

  • Enrich mixture (slightly).

  • Reduce compression ratio.

  • Use high-octane fuel / knock inhibitors (TEL, MTBE, aromatics).

  • Improve cooling (water jacket, sodium-filled valves).

  • Optimize chamber design (swirl, squish).

5.4 Pre-ignition

• Definition: Ignition of mixture before spark plug fires. Caused by hot spot (spark plug tip, exhaust valve, carbon deposit).

• Difference from Detonation: Occurs before spark, during compression. Often leads to detonation.

• Causes: Lean mixture, advanced timing, hot spots, low heat range plug.

• Remedies: Use cold plug, clean deposits, enrich mixture, retard timing.

5.5 Combustion Chambers for SI Engines

• Desirable Characteristics: High volumetric efficiency, good scavenging, high thermal efficiency, low knock tendency, adequate turbulence, easy manufacture.

• Types (with sketches):

  1. T-head / L-head (Side valve): Valves in block. Simple, poor breathing, low CR. Obsolete.

  2. I-head / OHV: Valves in head, pushrods. Good balance of cost/performance.

  3. Hemispherical (Hemi):

     DiagramSEARCH: "hemispherical combustion chamber"

     • Advantages: Large valve area, good breathing, high CR possible, central spark plug.

     • Disadvantages: Complex head, expensive, high heat loss.

  4. Wedge:

     DiagramSEARCH: "wedge combustion chamber"

     • Advantages: Simple, good volumetric efficiency, low heat loss.

     • Disadvantages: Less swirl than hemi.

  5. Pent-roof:

     DiagramSEARCH: "pent roof combustion chamber"

     • Advantages: Excellent breathing (4 valves), high CR, good swirl/tumble.

     • Disadvantages: Complex head.

• Squish & Swirl/Tumble:

  • Squish: Piston near head at TDC → air forced radially → turbulence → faster flame.

  • Swirl/Tumble: Intake port design → rotary motion → better mixing.

6.0 COMBUSTION IN COMPRESSION IGNITION (CI) ENGINES

6.1 Stages of Combustion (P-θ Diagram)

1. Ignition Delay Period: Fuel injection → first pressure rise. Physical (atomization, vaporization, mixing) + Chemical (pre-flame reactions).

2. Uncontrolled Combustion (Rapid): Fuel accumulated during delay burns rapidly → sharp pressure rise (diesel knock if too long).

3. Controlled Combustion (Afterburning): Remaining fuel burns as it mixes → gradual pressure rise.

6.2 Diesel Knock

• Definition: Rough combustion, high vibration & noise due to excessive ignition delay. Large amount of fuel accumulates → rapid burn → high pressure rise rate.

• Difference from SI Knock: SI knock = auto-ignition of end-gas. CI knock = too rapid burn of first-injected fuel.

6.3 Factors Affecting Ignition Delay & Combustion

• Fuel Properties:

  • Cetane Number (CN): Higher CN → shorter delay. CN = % by volume of cetane (C₁₆H₃₄) in reference fuel that matches ignition quality.

  • Viscosity, volatility (affect atomization, vaporization).

• Engine Variables:

  • ↑ Compression Ratio → ↓ delay (higher $$\displaystyle T_2 $$).

  • ↑ Engine Speed → ↓ delay (in °CA) due to turbulence.

  • ↑ Load → ↑ injection pressure → better atomization → ↓ delay.

  • ↑ Inlet Temp/Pressure → ↓ delay.

  • Injection Timing: Advanced → higher $$\displaystyle T_2 $$ at injection → ↓ delay.

  • Nozzle Design: Smaller holes → better atomization → ↓ delay.

6.4 Combustion Chambers for CI Engines

• Desirable: Good air utilization, high rate of pressure rise (controlled), low heat loss, low soot/NOx.

• Types:

  1. Open Combustion Chamber (Direct Injection - DI):

     DiagramSEARCH: "direct injection diesel combustion chamber"

     • Fuel injected directly into piston bowl.

     • Requirements: High injection pressure (150-250 bar), multi-hole nozzle.

     • Advantages: Simple, no heat loss in swirl chamber, high efficiency.

     • Disadvantages: Requires high pressure pump, sensitive to fuel properties.

  2. Indirect Combustion Chambers:

     • Pre-combustion Chamber:

       DiagramSEARCH: "precombustion chamber diesel"

       • Auxiliary chamber connected by orifice. Fuel injected into chamber → initial combustion → pressure blows burning mixture into main chamber.

       • Advantages: Smooth running, less noise, lower injection pressure needed.

       • Disadvantages: Heat loss in chamber → lower efficiency (5-10% loss).

     • Swirl Chamber:

       DiagramSEARCH: "swirl chamber diesel"

       • Tangential intake port creates strong swirl. Fuel injected into swirling air.

       • Similar pros/cons to pre-chamber.

• Air Motion:

  • Squish: Piston to head clearance → radial air motion.

  • Swirl: Rotary motion about cylinder axis (from inclined intake port).

  • Tumble: Rotary motion about transverse axis (from vertical port).

7.0 FUELS & FUEL PROPERTIES

7.1 General Chemical Formulas & Structures

7.2 Fuel Rating

• SI Engines - Octane Number (ON):

  • Definition: % by volume of iso-octane (2,2,4-trimethylpentane, ON=100) in reference mixture with n-heptane (ON=0) that matches knock resistance of test fuel.

  • Research Octane Number (RON): Mild test (low speed). Motor Octane Number (MON): Severe test (high speed, load). Anti-Knock Index (AKI) = (RON+MON)/2 (pump octane).

• CI Engines - Cetane Number (CN):

  • Definition: % by volume of cetane (C₁₆H₃₄, CN=100) in reference mixture with heptamethylnonane (CN=0) that matches ignition quality of test fuel.

  • Significance: Higher CN → shorter ignition delay → smoother combustion. Minimum CN required: 45-55 for automotive diesels.

7.3 Desirable Fuel Characteristics

7.4 Fuel Additives (Dopes)

• Why mixed? Improve existing properties (octane, cetane), impart new ones (detergency, anti-corrosion).

• Requirements of Good Additive: Effective at low concentration, stable, non-corrosive, non-toxic, compatible.

• Types:

  • Anti-knock: Tetraethyl lead (TEL - phased out), MTBE, aromatics, ethanol.

  • Oxidation Inhibitors: Prevent gum formation.

  • Detergents: Keep injectors/carburetor clean.

  • Pour Point Depressants: Improve cold flow (diesel).

  • Cetane Improvers: Alkyl nitrates (e.g., 2-ethylhexyl nitrate).

7.5 Alternative Fuels

8.0 FORCED INDUCTION: SUPERCHARGING & TURBOCHARGING

8.1 Need & Effect on Performance

• Need: Increase mass of air per cycle → more fuel → more power.

• Effects:

  • ↑ Power Output (by 30-50% for same engine size).

  • ↓ Specific Fuel Consumption (SFC) at full load (more air → better combustion).

  • ↑ Volumetric Efficiency (>100% possible).

  • ↑ Mean Effective Pressure.

• Limitations:

  • SI Engines: ↑ tendency for detonation (higher $$\displaystyle T_2 $$, pressure).

  • Both: ↑ thermal & mechanical stress, ↑ NOx emissions (higher $$\displaystyle T_{max} $$).

8.2 Supercharging vs. Turbocharging

8.3 Methods of Supercharging

1. Mechanically Driven: Roots blower (positive displacement), Vane type.

2. Exhaust Gas Driven (Turbocharger):

   DiagramSEARCH: "turbocharger diagram"
   • Turbine wheel (exhaust gas) → compressor wheel (induces air) on same shaft.

3. Pressure-Wave Supercharger (Comprex): Uses pressure waves in exhaust to compress intake air.

8.4 Turbocharging Systems

• Constant Pressure Charging: All cylinders exhaust into common manifold → steady pressure at turbine. Most common.

• Pulse Charging: Individual exhaust pipes → turbine sees pressure pulses → better energy extraction at low speed.

• Advantages/Disadvantages: Pulse charging better low-end torque but complex piping. Constant pressure simpler, better high-speed.

8.5 Intercooling

• Purpose: Cool compressed air from supercharger/turbo before entering engine.

• Benefits: ↑ air density → more mass per volume. ↓ $$\displaystyle T_2 $$ → ↓ detonation tendency (SI), ↓ NOx.

• Types: Air-to-air, air-to-water.

9.0 COOLING SYSTEM

9.1 Need for Cooling

• Prevent piston seizure, maintain clearances.

• Avoid lubricant breakdown.

• Control detonation (SI).

• Prevent material strength loss.

9.2 Types of Cooling Systems

1. Air Cooling: Fins on cylinder/head. Forced by fan. Used in motorcycles, small engines.

2. Liquid Cooling (Most Common): Coolant (water+antifreeze) circulated by pump.

   • Forced Cooling System:

     DiagramSEARCH: "liquid cooling system diagram"

     • Components: Radiator, water pump, thermostat, cooling fan, hoses, water jacket.

     • Working: Hot coolant from engine → radiator (cooled by air flow) → thermostat (controls flow) → water pump → engine.

9.3 Cooling System Components

• Radiator: Types: Cellular (air passes through tubes), Tubular (water in tubes). Pressure cap raises boiling point.

• Thermostat: Regulates coolant flow to maintain optimum temp (~80-90°C). Types: Wafer (wax), Bellows, Piston.

• Water Pump: Centrifugal type, belt-driven.

• Cooling Fan: Clutch-driven (thermal/viscous) or electric. Operates when coolant temp high.

9.4 Coolants & Additives

• Anti-freeze: Ethylene glycol (lower freezing point, raise boiling point). Glycerin (less toxic).

• Corrosion Inhibitors: Phosphates, silicates, borates (protect metal).

• Rust Preventives: Oiliness additives.

9.5 Cooling Fins

• Design: Thin, closely spaced, tapered (thick at base, thin at tip). Direction of airflow along fins.

• Effectiveness Factors: Fin material (Al, Cu), surface area, airflow velocity, temperature gradient.

10.0 LUBRICATION SYSTEM

10.1 Functions of Lubrication

• Reduce friction & wear (boundary, hydrodynamic).

• Seal piston rings against cylinder wall.

• Cool (carry heat away).

• Clean (carry contaminants to filter).

• Prevent corrosion (oil additives).

10.2 Properties of Lubricating Oil

• Viscosity: Most important. Must be sufficient at high temp (film strength) but not too high at low temp (cold start). Measured by SAE grades (e.g., 10W-40).

• Pour Point: Lowest temp at which oil flows.

• Flash Point: Temp at which vapors ignite (safety).

• Carbon Residue: Tendency to form deposits.

• Acidity: Should be neutral (low TAN).

• Demulsibility: Ability to separate from water.

10.3 Types of Lubrication Systems

1. Mist Lubrication: Oil mixed with air (2-stroke engines). Simple, oil burned with fuel.

2. Splash Lubrication: Crankshaft dips in oil sump → splashes oil. For small engines.

3. Pressure Feed Lubrication:

   • Wet Sump: Oil pan (sump) stores oil. Pump draws from sump → filters → main bearings → galleries → other parts → back to sump.

     DiagramSEARCH: "wet sump lubrication system"

   • Dry Sump: Separate oil tank. Scavenger pumps return oil from sump to tank. Pressure pump supplies from tank. Used in racing/aero engines (no oil surge).

4. Filtration:

   • Full Flow: All oil passes through filter before bearings.

   • Bypass: Only part of oil filtered, rest bypasses (used with centrifuge).

10.4 Lubrication of Critical Components

• Main Bearings: Pressure feed through galleries.

• Connecting Rod Bearings: Oil from main bearing gallery → through crankshaft drillings.

• Piston & Cylinder: Oil from cylinder wall (splash) or dedicated nozzle (oil jet).

• Valve Mechanism: Oil from camshaft bearings or pushrods (drip/splash).

• Camshaft: Pressure feed or splash.

11.0 OTHER SYSTEMS & ADVANCED CONCEPTS

11.1 Valve Timing & Port Timing

• Theoretical vs Actual: Actual timing differs due to valve inertia, dynamics. Valves open/close before/after TDC/BDC.

• Events (4-Stroke):

  • IO (Inlet Open): Before intake stroke (TDC exhaust).

  • IC (Inlet Close): After intake stroke (BDC).

  • EO (Exhaust Open): Before exhaust stroke (BDC).

  • EC (Exhaust Close): After exhaust stroke (TDC intake).

• Overlap: Period when both inlet & exhaust open. Helps scavenging (exhaust pulse draws fresh charge).

• Effect: Optimizes breathing, scavenging, volumetric efficiency.

11.2 Scavenging (2-Stroke Engines)

• Definition: Clearing exhaust gases & filling cylinder with fresh charge.

• Types:

  1. Cross-flow: Inlet & exhaust on opposite sides. Simple, poor scavenging (short-circuiting).

  2. Loop Scavenging:

     DiagramSEARCH: "loop scavenging 2-stroke"

     • Transfer ports angled → creates loop → prevents direct short-circuit.

     • Better than cross-flow.

  3. Uniflow Scavenging:

     DiagramSEARCH: "uniflow scavenging 2-stroke"

     • Inlet at bottom, exhaust valve at top (or ports). One-way flow.

     • Best scavenging efficiency, used in large marine diesels.

11.3 Exhaust Gas Recirculation (EGR)

• Purpose: Reduce NOx emissions by lowering peak combustion temperature.

• Types:

  • Internal EGR: Retain some exhaust in cylinder by valve overlap.

  • External EGR: Exhaust gas routed back to intake via valve & cooler.

    DiagramSEARCH: "external EGR system diagram"

• Advantages: Significant NOx reduction.

• Limitations: ↑ soot, HC, CO; ↓ power; needs control to avoid low-load instability.

11.4 Wankel (Rotary) Engine

• Construction & Working:

  DiagramSEARCH: "wankel engine diagram"

  • Rotor (triangular) in epitrochoid housing. Three chambers: intake, compression, expansion, exhaust.

  • One power stroke per revolution of rotor (3 power strokes per shaft revolution).

• Advantages: Compact, smooth, high power/weight, fewer parts.

• Disadvantages: Poor fuel economy, high emissions, sealing problems (apex seals), low thermal efficiency.

11.5 Engine Emissions & Control (Brief)

• Sources:

  • CO, HC: Incomplete combustion (rich, poor mixing, crevices).

  • NOx: High $$\displaystyle T_{max} $$ (thermal NOx).

  • PM (Soot): Rich zones, poor mixing (CI).

• Control Methods:

  • Catalytic Converter: Oxidizes CO, HC; reduces NOx (3-way cat for SI).

  • EGR: Reduces NOx.

  • Fuel Injection Control: Precise metering.

  • Diesel Particulate Filter (DPF): Traps soot.

12.0 NUMERICAL PROBLEM SOLVING (INTEGRATED WITH TOPICS)

• Air-Standard Cycles: Follow steps in 1.2 & 1.3. Use correct specific heat ratio ($$\displaystyle \gamma=1.4 $$ for air unless given $$\displaystyle C_v, R $$). For Diesel, find $\rho$ from given volumes or pressures.

• Performance Parameters from Trial Data:

  • $$\displaystyle BP = \frac{2\pi N T}{60} $$

  • $$\displaystyle bsfc = \frac{\dot{m}_f}{BP} $$ (ensure consistent units: kg/s, kW).

  • $IP$ (if Morse test data): $$\displaystyle IP = BP_{all} + \sum (BP_{all} - BP_{cut\ i}) $$.

  • $$\displaystyle IMEP = \frac{IP \cdot 60}{L A N} $$.

• Heat Balance Sheet: Calculate each heat output term (BP, cooling water, exhaust) in kW or kJ/min. Heat input = $$\displaystyle \dot{m}_f \cdot CV $$. Unaccounted = Input - Sum(outputs).

• Carburetion: Use Bernoulli for air velocity, then mass flow. Fuel flow from orifice equation. $$\displaystyle AFR = \dot{m}_a / \dot{m}_f $$. Remember discharge coefficients.

• Morse Test: As shown in 2.3.

[!TIP] Final Exam Strategy: For numericals, draw the cycle diagram first, label states 1,2,3,4. Write given data clearly. Use consistent units (kJ, kg, kPa, m³). For performance problems, list all knowns, identify unknowns, select correct formula. For comparison questions (Otto vs Diesel), state assumptions clearly (same $r$, same $$\displaystyle Q_{in} $$).