Internal Combustion Engines (ME-501) - Unit 4 Short Notes

UNIT 4: Internal Combustion Engines - Comprehensive Notes

1.0 Air Standard Cycles and Analysis

1.1 Otto Cycle (Spark Ignition)

Processes (4-stroke, closed cycle):

1. 1-2: Isentropic compression (V₁ → V₂, r = V₁/V₂)

2. 2-3: Constant volume heat addition (Q_in)

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

4. 4-1: Constant volume heat rejection (Q_out)

P-V & T-S Diagrams:

Thermal Efficiency Derivation:

For air as ideal gas with constant specific heats:

$$ \eta_{\text{Otto}} = 1 - \frac{1}{r^{\gamma-1}} $$

where \( r \) = compression ratio, \( \gamma = C_p/C_v \).

Work & Heat:

• Net work: \( W_{\text{net}} = Q_{\text{in}} - Q_{\text{out}} \)

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

Mean Effective Pressure (MEP):

$$ \text{MEP} = \frac{W_{\text{net}}}{V_1 - V_2} = \frac{Q_{\text{in}} \left(1 - \frac{1}{r^{\gamma-1}}\right)}{V_1 \left(1 - \frac{1}{r}\right)} $$

Effect of Compression Ratio:

Efficiency increases with \( r \). Practical limit set by knock in SI engines.

High-Frequency Problem Type:

Given: \( r \), \( P_1, T_1 \), \( Q_{\text{in}} \)

Find: \( T_3 \), \( \eta \), \( W_{\text{net}} \), \( Q_{\text{out}} \)
Steps:

1. \( T_2 = T_1 \cdot r^{\gamma-1} \)

2. \( T_3 = T_2 + Q_{\text{in}}/C_v \)

3. \( T_4 = T_3 / r^{\gamma-1} \)

4. \( \eta = 1 - T_1/T_2 \) (or formula)

5. \( W = Q_{\text{in}} - C_v(T_4 - T_1) \)

[!TIP] Common Pitfall: Forgetting that \( Q_{\text{in}} \) is per kg of air, not per cycle. Use consistent units (kJ/kg).

Loss Due to Variable Specific Heats:

Real cycle efficiency is lower than air-standard because \( C_v, C_p \) increase with temperature, reducing the effective \( \gamma \).

1.2 Diesel Cycle (Compression Ignition)

Processes:

1. 1-2: Isentropic compression

2. 2-3: Constant pressure heat addition (Q_in)

3. 3-4: Isentropic expansion

4. 4-1: Constant volume heat rejection

P-V & T-S Diagrams:

Thermal Efficiency:

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

where \( \rho = V_3/V_2 \) = cut-off ratio.

Cut-off Ratio Effect:

For same \( r \), efficiency decreases as \( \rho \) increases (longer constant pressure heat addition).

High-Frequency Problem Type:

Given: \( r \), \( \rho \) (or % of stroke), find \( \eta \).

Direct substitution in formula.

Comparison with Otto Cycle:

At same \( r \), Otto cycle is more efficient because heat addition at constant volume gives higher \( T_{\text{max}} \) but less heat rejection. Diesel allows higher \( r \) without knock, so real diesel engines often have higher practical efficiency.

1.3 Ideal Cycle Analysis

Salient Points: 1 (start compression), 2 (end compression), 3 (end heat addition), 4 (end expansion).

Given: \( P_1, T_1 \), \( r \), \( P_3 \) (peak pressure) or \( Q_{\text{in}} \).
Find: All \( P, T, V \) at points 1,2,3,4.

Assumptions:

• Air is ideal gas: \( PV = mRT \)

• Constant \( C_v, C_p, \gamma \)

• All processes reversible.

Key Relations:

• Isentropic: \( P_2/P_1 = (V_1/V_2)^\gamma = r^\gamma \), \( T_2/T_1 = r^{\gamma-1} \)

• Constant volume: \( P_3/T_3 = P_2/T_2 \)

• Constant pressure: \( V_3/T_3 = V_2/T_2 \)

• Isentropic expansion: \( T_4/T_3 = (V_4/V_3)^\gamma = (1/r)^\gamma \) (since \( V_4 = V_1 \))

2.0 Combustion in Spark Ignition (SI) Engines

2.1 Stages of Combustion

1. Ignition Lag (Pre-flame phase): Spark initiates chemical reactions; no visible flame.

2. Flame Propagation: Flame front moves from spark plug across chamber.

3. Afterburning: Combustion of remaining gases behind flame front; completes by ~20° ATDC.

Pressure-Crank Angle Diagram:

2.2 Flame Propagation

Normal Combustion: Flame travels uniformly at speed 10-50 m/s.

Factors Affecting Flame Speed:

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

• Compression ratio: ↑ CR → ↑ pressure/temperature → ↑ flame speed

• Spark location: Centered → shorter travel distance

• Mixture composition: Stoichiometric (~λ=1) gives fastest flame; very lean/rich slows flame.

• Engine speed: Indirectly via turbulence.

2.3 Abnormal Combustion

Detonation (Knock):

• Definition: Spontaneous auto-ignition of end-gas ahead of flame front, causing pressure waves ("pinging").

• Theories:

  • Chemical: Chain reaction in end-gas.

  • Thermodynamic: End-gas reaches auto-ignition temperature due to compression by flame.

• Effects: Piston/head damage, power loss, overheating.

Variables Affecting Knock:

• Compression ratio (↑ CR → ↑ knock tendency)

• Spark timing (advanced → ↑ knock)

• Inlet temperature/pressure (↑ → ↑ knock)

• Altitude (↓ pressure → ↓ knock)

• Fuel octane number (↑ → ↓ knock)

• Engine design (combustion chamber shape, cooling)

Pre-ignition:

• Definition: Fuel ignites before spark (from hot spots: glowing deposits, exhaust valve).

• Causes: Lean mixture, overheating, sharp edges.

• Remedy: Better cooling, fuel additives, decarbonizing.

[!TIP] Knock vs Pre-ignition: Knock occurs after spark; pre-ignition occurs before spark. Both damaging but different mechanisms.

2.4 Ignition Lag

Definition: Time interval between spark discharge and start of flame propagation (measured in crank angle degrees).

Significance: Affects pressure rise rate, knock tendency. Shorter lag → more controlled combustion.

Effect of Engine Variables:

2.5 Combustion Chamber Design for SI Engines

Desirable Characteristics:

• Compact shape → short flame travel, high flame speed.

• High surface-to-volume ratio? Actually, low surface-to-volume ratio reduces heat loss, but some designs use swirl to enhance turbulence.

• Swirl/tumble generation → turbulence for faster flame.

• Avoid crevices → reduce unburned HC emissions.

• Proper spark plug location → central or near intake valve.

Types with Sketches:

3.0 Combustion in Compression Ignition (CI) Engines

3.1 Stages of Combustion

1. Ignition Delay Period: Fuel injected but not yet burning.

   • Physical delay: Atomization, vaporization, mixing.

   • Chemical delay: Pre-flame reactions.

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

3. Controlled Combustion: Burning at approximately constant pressure as injection continues.

4. Afterburning: Late combustion of fuel droplets; completes by ~40° ATDC.

Pressure-Crank Angle Diagram:

3.2 Ignition Lag/Delay Period

Definition: Time between start of injection and start of combustion (or first detectable pressure rise).

Importance: Longer delay → more fuel accumulates → higher rate of pressure rise → diesel knock.

Factors Affecting Delay Period:

3.3 Diesel Knock

Definition: High rate of pressure rise due to prolonged ignition delay causing violent combustion of accumulated fuel.

Relation to Ignition Delay: Directly proportional. Longer delay → more fuel injected before ignition → larger premixed flame → higher dP/dθ.

Control Methods:

• Use high cetane fuel/additives.

• Retard injection timing.

• Increase turbulence (pintle nozzles, helical ports).

• Multiple pilot injections (modern common rail).

3.4 Fuel Injection System

Functions:

1. Metering: Correct fuel quantity per cycle.

2. Timing: Start/end of injection at proper crank angle.

3. Atomization: Break fuel into fine droplets.

4. Distribution: Proper spray pattern in chamber.

Types:

• Air injection: (Historical) Use compressed air to atomize fuel. Pros: Good atomization. Cons: Complex, parasitic power, not used now.

• Solid injection: Fuel at high pressure injected directly.

  • Pump-line-nozzle: Individual pump per cylinder, high-pressure line, nozzle. Common in older engines.

  • Distributor type: Single pump, distributor rotates to deliver fuel.

  • Common rail: Common high-pressure accumulator, electronically controlled injectors. Modern standard.

Fuel Injector (Nozzle) Construction & Working:

• High-pressure fuel from pump enters pressure chamber.

• Overcomes needle spring force → needle lifts → fuel sprays through orifices.

• Spray characteristics depend on nozzle type.

3.5 Nozzles for CI Engines

Types:

Spray Characteristics:

• Cone angle: Determined by orifice design, pressure.

• Penetration: Distance spray travels; increases with pressure.

• Atomization: Droplet size; finer with higher pressure, smaller orifice.

Nozzle Flow Equation:

$$ \dot{m}_f = C_d \cdot A_o \cdot \sqrt{2 \rho_f (P_{\text{inj}} - P_{\text{cyl}})} $$

where \( C_d \) = discharge coefficient, \( A_o \) = total orifice area, \( \rho_f \) = fuel density, \( P_{\text{inj}} \) = injection pressure, \( P_{\text{cyl}} \) = cylinder pressure.

4.0 Fuel Systems: Carburetion and Injection

4.1 Carburetion in SI Engines

Definition: Mixing air and fuel in correct proportion (air-fuel ratio) for combustion.

Simple Carburetor Principle:

Components:

• Float chamber: Maintains constant fuel level; vented to atmosphere.

• Venturi (choke): Throat where velocity ↑, pressure ↓ (Bernoulli).

• Fuel orifice: Metering jet; fuel drawn due to pressure difference.

• Throttle valve: Controls air-fuel mixture flow (engine load).

Why Float Chamber Vented to Atmosphere?

To maintain atmospheric pressure in chamber, so fuel flow depends only on venturi depression.

Air-Fuel Ratio Concepts:

• Rich mixture: λ < 1 (excess fuel). e.g., cold start, full load.

• Stoichiometric mixture: λ = 1 (theoretical perfect combustion). λ = (A/F)actual / (A/F)stoich.

• Lean mixture: λ > 1 (excess air). e.g., cruising.

Carburetor Sizing Calculation (High-Frequency):

Given: Air flow \( \dot{m}_a \), velocity at throat \( V_t \), \( C_d \), \( P_1, T_1 \).

Throat area: \( A_t = \frac{\dot{m}_a}{\rho_t V_t} \), where \( \rho_t \) from isentropic flow.

For compressible flow:

$$ \dot{m}_a = C_d A_t P_1 \sqrt{\frac{\gamma}{R T_1} \left( \frac{2}{\gamma+1} \right)^{\frac{\gamma+1}{\gamma-1}} } $$

if \( P_t/P_1 \leq \left( \frac{2}{\gamma+1} \right)^{\gamma/(\gamma-1)} \).

Air-Fuel Ratio from Carburetor:

Without nozzle lip correction:

$$ \frac{\dot{m}_a}{\dot{m}_f} = \frac{C_d A_t \sqrt{2 \rho_a \Delta P}}{C_{df} A_f \sqrt{2 \rho_f \Delta P}} = \frac{C_d A_t}{C_{df} A_f} \sqrt{\frac{\rho_a}{\rho_f}} $$

With nozzle lip correction (fuel head \( h \)):

$$ \Delta P = P_{\text{atm}} - P_t = h \rho_f $$

4.2 Multi-Point Fuel Injection (MPFI)

Definition: Fuel injected into intake port (or directly into cylinder) near each intake valve, controlled by ECU.

Components: Injectors, sensors (MAF, MAP, O₂, TPS), ECU.

Merits over Carburetor:

• Accurate fuel metering per cylinder.

• Better fuel economy, lower emissions.

• No throttle losses (in direct injection).

• Better cold start, altitude compensation.

Demerits:

• Higher cost, complexity.

• Maintenance of sensors/ECU.

4.3 Fuel Injection in CI Engines

Solid Injection Systems Overview:

• In-line pump: Separate pump element per cylinder; mechanical governor.

• Distributor pump: Single pump element, rotating distributor; compact.

• Common rail: High-pressure accumulator; electronic control of injection timing/rate; multiple injections possible.

Injection Timing:

Start of injection measured BTDC (usually 15-25° BTDC). Retarding ↓ power, ↑ fuel consumption, ↓ smoke; advancing ↑ NOx, knock risk.

5.0 Fuel Properties, Ratings, and Chemistry

5.1 Fuel Properties for SI and CI Engines

5.2 Fuel Ratings

Octane Number (ON):

• Definition: Percentage by volume of iso-octane (2,2,4-trimethylpentane) in reference mixture with n-heptane that matches knocking tendency of test fuel in CFR engine.

• Higher ON → better knock resistance.

• Measurement: CFR single-cylinder engine at specified speed/compression ratio.

Cetane Number (CN):

• Definition: Percentage by volume of cetane (n-hexadecane) in reference mixture with heptamethylnonane that matches ignition delay of test fuel.

• Higher CN → shorter ignition delay.

• Measurement: CFR engine, measure ignition delay.

Other Ratings:

• Diesel Index: Approximate cetane from physical properties.

• Smoke Point: Max fuel grade without smoke in laminar flame.

5.3 Fuel Chemistry

General Formulas:

Octane/Cetane Relation:

• High ON: Aromatics, branched paraffins, olefins.

• High CN: Straight-chain paraffins, naphthenes.

5.4 Alternative Fuels

Gaseous Fuels:

• CNG (Compressed Natural Gas): Mainly methane. High ON (120-130), clean, low power density (compression needed). Modifications: Higher CR, hardened valves.

• LPG (Liquefied Petroleum Gas): Propane/butane. High ON, liquid at moderate pressure. Similar mods to CNG.

• Hydrogen: Very high ON, wide flammability, low density. Challenges: Knocking (high flame temp → NOx), pre-ignition, storage (cryogenic/compressed), material compatibility (embrittlement). Mods: Lean operation, EGR, direct injection.

Biofuels:

• Ethanol (E10, E85): High ON, oxygenated, hydrophilic. Corrosive, lower energy content. Flex-fuel vehicles needed for high blends.

• Biodiesel (B100): From vegetable oils/animal fats. High CN, lubricity, but higher viscosity, NOx increase, cold flow issues.

5.5 Fuel Additives (Dopes)

Why Mixed?

• Improve combustion (octane/cetane improvers).

• Reduce emissions (detergents, smoke suppressants).

• Prevent deposits, improve stability, anti-corrosion.

Requirements of Good Additive:

• Effective at low concentration.

• Compatible with fuel/engine materials.

• Non-corrosive, stable, economical.

Examples:

• Octane improvers: Aromatic amines (e.g., aniline), organometallics (TEL - banned), MTBE (oxygenate).

• Cetane improvers: Alkyl nitrates (2-ethylhexyl nitrate).

• Detergents: Polymeric amines (keep injectors clean).

• Anti-oxidants: Prevent gum formation.

6.0 Supercharging and Turbocharging

6.1 Introduction

Definitions:

• Supercharging: Forced induction by mechanically driven compressor (from engine crankshaft).

• Turbocharging: Forced induction by exhaust gas turbine driving compressor.

Purpose:

• Increase power output (more air → more fuel → more power).

• Maintain power at altitude (aircraft).

• Improve fuel economy at high loads (by reducing throttling losses in SI).

Effect on Performance:

• Power: ↑ up to 50-100% (with proper fueling/ cooling).

• Fuel Consumption: Brake specific fuel consumption (BSFC) may ↓ at high load due to better volumetric efficiency.

• Knock tendency (SI): ↑ due to higher T,P.

6.2 Methods of Supercharging

Mechanical Superchargers:

• Roots type: Positive displacement, meshing lobes. Simple, linear flow with speed. Disadv: Parasitic loss, noise.

• Vane type: Rotating vanes in offset housing. Disadv: Wear, limited pressure ratio.

• Centrifugal type: Impeller increases velocity → diffuser converts to pressure. Pressure ratio ↑ with speed. Disadv: Lag at low speed.

Turbochargers:

• Exhaust gas turbine (radial or axial) drives centrifugal compressor.

• Constant pressure turbocharging: All cylinders exhaust into common manifold → steady flow.

• Pulse turbocharging: Individual exhaust pipes → utilize pulse energy; better low-speed response.

Pressure Wave Supercharging: (Brief) Uses exhaust pulse energy to compress intake air via rotating cell; high efficiency but complex.

6.3 Thermodynamic Cycle of Supercharged Engines

• Increased mass of air per cycle → higher indicated power.

• Cycle analysis: Compression starts at higher pressure (P₁' > atmospheric).

• Efficiency may ↓ due to higher compression work and heat transfer, but overall brake thermal efficiency can ↑ due to reduced pumping loss (SI) or better combustion (CI).

6.4 Limitations of Supercharging

6.5 Need for Turbocharging in Aircraft Engines

• Altitude → air density ↓ → power loss.

• Turbocharger maintains sea-level pressure in intake (by compressing thinner air).

• Allows engine to maintain rated power up to critical altitude.

7.0 Engine Cooling Systems

7.1 Need for Cooling

• Prevent overheating (material strength ↓, lubrication fails).

• Control emissions (NOx ↑ with T).

• Prevent knocking (SI).

• Maintain clearances.

7.2 Types of Cooling Systems

Air Cooling: Fins on cylinder/head, forced air flow. Used in motorcycles, small engines, aircraft. Adv: Simple, no leakage. Disadv: Uneven cooling, noisy.

Liquid Cooling: Coolant (water+antifreeze) circulates in water jackets. Forced Cooling System (with pump):

Components:

• Radiator: Heat exchanger to ambient.

• Water pump: Circulation (usually centrifugal).

• Thermostat: Regulates flow to maintain operating temperature (~80-90°C).

• Cooling fan: Draws air through radiator (engine-driven or electric).

• Water jackets: Surround cylinder/head.

7.3 Cooling Fins

Purpose: Increase surface area for convective heat transfer.

Design Considerations:

• Fin density: More fins → more area, but ↓ airflow → optimal.

• Fin thickness: Thicker → better conduction, but heavier.

• Material: Aluminum (good conductivity) common.

• Attachment: Integrated casting or bonded.

7.4 Anti-freeze Solutions

Types:

• Ethylene glycol: Common, toxic.

• Propylene glycol: Less toxic, used in food/ RV applications.

Properties:

• Lower freezing point (colligative property).

• Raise boiling point (with pressurization).

• Corrosion inhibitors (phosphates, silicates).

• Anti-foaming.

Necessity: Prevent freeze-up in cold climates, avoid corrosion, maintain boiling point above operating.

8.0 Engine Lubrication Systems

8.1 Functions of Lubrication

• Reduce friction & wear.

• Cool (carry heat away).

• Seal (piston rings).

• Clean (carry contaminants to filter).

• Prevent corrosion.

• Cushion (bearings).

8.2 Properties of Lubricating Oil

• Viscosity: SAE grades (e.g., 10W-30). Must be correct at operating T.

• Flash point: >200°C (safety).

• Pour point: Lowest T at which oil flows.

• Carbon residue: Indicates tendency to form deposits.

• Neutralization number: Acidity (from oxidation).

8.3 Types of Lubrication Systems

Mist lubrication: (2-stroke) Oil mixed with fuel; simple but high oil consumption.

Wet Sump Systems:

• Splash lubrication: Crankshaft dips in oil → splashes. Simple, low speed.

• Pressurized lubrication: Oil pump draws from sump → filter → galleries to bearings, cam, etc. Returns to sump.

Dry Sump System: Separate oil tank; scavenge pumps remove oil from sump; pressure pump supplies. Used in racing, aircraft. Adv: No oil surge, lower CG. Disadv: Complex, expensive.

8.4 Lubrication Points

Main bearings, connecting rod bearings, piston pins, cylinder walls (oil squirters), valve train (pushrods, rockers), timing gears/belt.

9.0 Engine Performance and Testing

9.1 Key Definitions and Parameters

9.2 Engine Testing Methods

Morse Test (for multi-cylinder engines):

• Method: Run engine at constant speed, cut out cylinders one by one (by stopping fuel/ignition), measure BP each time.

• Assumptions:

  1. Frictional power (F.P.) same for all cylinders.

  2. No inter-cylinder interference (exhaust/induction).

• Calculations:

  Let \( BP_0 \) = BP with all cylinders, \( BP_i \) = BP with cylinder i cut out.

  Then \( IP_i = BP_i + F.P. \), and total \( IP = \sum IP_i \).

  Since F.P. constant: \( IP = \sum (BP_i + F.P.) = \sum BP_i + n \cdot F.P. \)

  But \( IP = BP_0 + F.P. \) (since all cylinders working).

  Equate: \( BP_0 + F.P. = \sum BP_i + n F.P. \)

  → \( F.P. = \frac{\sum BP_i - BP_0}{n-1} \)

  → \( IP = BP_0 + F.P. \)

  → \( \eta_m = BP_0 / IP \)

[!TIP] Morse test assumes constant friction; not valid if cutting a cylinder changes cooling or pumping significantly.

9.3 Heat Balance Sheet

Purpose: Account for input fuel energy (CV) into useful work and losses.

Preparation:

1. Input: \( \dot{E}_{\text{in}} = \dot{m}_f \cdot CV \) (kJ/min or kW).

2. Outputs:

   • Brake Power (BP): Measured.

   • Coolant loss: \( \dot{m}_w C_{pw} (T_{w,\text{out}} - T_{w,\text{in}}) \)

   • Exhaust loss: \( \dot{m}_e C_{pe} (T_e - T_{\text{amb}}) \) (often approximate \( \dot{m}_e \approx \dot{m}_a + \dot{m}_f \))

   • Unaccounted: Radiation, convection, etc. = Input - (BP + coolant + exhaust).

3. Percentage: Each output / Input × 100%.

High-Frequency Problem: Given trial data (fuel cons., air cons., torque, temps, flow rates), compute BP, SFC, heat balance.

Sample Calculation Structure:

• BP = \( \frac{2\pi N T}{60} \)

• Fuel energy = \( \dot{m}_f \times CV \)

• Coolant heat = \( \dot{m}_w C_w \Delta T_w \)

• Exhaust heat = \( \dot{m}_e C_e \Delta T_e \)

• Unaccounted = Fuel energy - (BP + coolant + exhaust)

• All in consistent units (e.g., kJ/min).

9.4 Factors Affecting Volumetric Efficiency

• Valve timing: Overlap, inlet valve closing (IVC) after BDC helps at high speed.

• Intake/exhaust system design: Runner length/diameter, manifold tuning.

• Scavenging effectiveness (2-stroke): Good scavenging → higher η_v.

• Air temperature/pressure: Cooler, denser air → ↑ η_v.

• Throttling (SI): Part load → ↓ η_v due to throttle loss.

10.0 Engine Design, Components, and Scavenging

10.1 Valve Timing

Theoretical vs Actual:

• Theoretical: Inlet open at BDC, close at TDC; exhaust open at TDC, close at BDC.

• Actual: Dynamic effects require:

  • Inlet valve opens (IVO) before TDC (intake stroke start).

  • Inlet valve closes (IVC) after BDC (use momentum of incoming air).

  • Exhaust valve opens (EVO) before BDC (early release).

  • Exhaust valve closes (EVC) after TDC (use exhaust momentum to help scavenging).

  • Overlap period: Both valves open (between EVO and IVO).

Actual Valve Timing Diagram for 4-stroke SI:

Reasons for Differences:

Gas dynamics: Inertia of air/fuel mixture (inlet) and exhaust gases; need to maximize cylinder filling and scavenging.

10.2 Scavenging in Two-Stroke Engines

Definition: Process of clearing exhaust gases and filling cylinder with fresh charge.

Types:

10.3 Wankel Engine

Construction:

• Rotor: Equilateral triangle shape.

• Housing: Epitrochoid curve.

• Eccentric shaft: Rotor rotates around fixed center, orbiting.

Working Principle:

• Three chambers formed between rotor and housing.

• Each chamber undergoes: intake (expansion), compression, ignition, exhaust in one shaft revolution (vs two revolutions for 4-stroke).

• Four phases per chamber per revolution.

Advantages:

• Compact, smooth (no reciprocating parts), high power/weight.

• Fewer parts.

Disadvantages:

• Apex seal wear, leakage.

• Poor fuel economy, high emissions.

• Low thermal efficiency (large surface/volume ratio).

• Limited to small displacement.

10.4 Cylinder Arrangement

10.5 Spark Plug Requirements

• Correct heat range: Cold plug (fast heat transfer) for high-speed; hot plug for slow speed/rich mixture. Must self-clean at ~450-500°C tip.

• Proper gap: Typically 0.6-1.0 mm; affects spark energy.

• Insulator material: Alumina (high T strength, dielectric).

• Electrode design: Copper/nickel core for heat dissipation; precious metals (platinum, iridium) for longevity.

• Resistance: 5-15 kΩ to prevent RF interference.

• Resistance to fouling/erosion.

11.0 Emissions and Control Systems

11.1 Exhaust Gas Recirculation (EGR)

Principle: Recirculate part of exhaust gas (5-15%) to intake, diluting charge.

Advantages:

• Reduces NOx significantly (lowers peak combustion temperature by absorbing heat and reducing oxygen concentration).

Limitations:

• Increases soot/particulates (especially in diesel).

• Reduces efficiency (less air, lower T).

• Need control (valve, cooler) to optimize.

• Can cause intake deposit formation.

EGR System Sketch:

11.2 Knock Inhibitors

Definition: Chemicals added to fuel to increase octane number.

Mechanism: Interfere with chain reactions in abnormal combustion (scavenge free radicals like OH·, H·).

Examples:

• Aromatic amines: Aniline, toludine.

• Alcohols: Ethanol (oxygenate, high ON).

• Organometallics: Tetraethyl lead (TEL - banned), MMT (methylcyclopentadienyl manganese tricarbonyl).

• Oxygenates: MTBE, ETBE, TAME.

12.0 Ignition Systems

12.1 Battery Ignition System

Components & Working:

• Battery (6V/12V): Low voltage source.

• Ignition coil: Step-up transformer (primary: 200-300 turns; secondary: 15,000-20,000 turns).

• Contact breaker (points): Opens/closes primary circuit; determines timing.

• Capacitor (condenser): Prevents arcing, boosts secondary voltage.

• Distributor: Rotating cam opens points; rotor directs high voltage to correct spark plug.

• Spark plugs: Generate spark in cylinder.

Operation:

1. Points closed → current in primary → magnetic field builds.

2. Points open → field collapses → high voltage in secondary.

3. Voltage jumps spark plug gap.

Advantages: Simple, reliable, cheap. Disadvantages: Points wear, condenser failure, voltage drops at high speed, maintenance.

12.2 Electronic/Transistorized Ignition

Advantages over contact breaker:

• No mechanical points → no wear, no adjustment.

• Higher/consistent spark energy at high speed.

• Precise timing (no bounce).

• Longer spark duration.

Microprocessor-Based Control (ECU):

• Sensors: CKP, CMP, MAP, MAF, TPS, O₂, knock.

• ECU: Calculates optimal spark advance based on load, speed, temperature, knock.

• Ignition module/coil-on-plug: Drives ignition coil(s).

Advantages:

• Optimized performance, fuel economy, emissions.

• Diagnostics, adaptive control.

• Coil-on-plug → no distributor losses.

Disadvantages:

• Complexity, cost.

• EMI/RFI issues.

• Requires trained technicians.

13.0 Two-Stroke vs. Four-Stroke Engines

Trade-offs: Two-stroke: simpler, more power dense, but higher emissions/oil consumption. Four-stroke: more efficient, cleaner, but heavier, more complex.

END OF UNIT 4 NOTES
Focus on understanding derivations (Otto, Diesel efficiency, MEP), combustion stages, factors affecting ignition delay/knock, carburetor calculations, heat balance, Morse test, and diagram-based questions (combustion chambers, valve timing, scavenging types, EGR, ignition systems).