UNIT 5: INTERNAL COMBUSTION ENGINES - SHORT NOTES

I. THERMODYNAMIC CYCLES & ANALYSIS

A. Otto Cycle (Spark Ignition / Constant Volume Heat Addition)

Processes (on P-V & T-S diagrams):

1. 1-2: Isentropic compression

2. 2-3: Constant volume heat addition

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

4. 4-1: Constant volume heat rejection

Air-Standard Efficiency:

Assumptions: Working fluid is air, behaves as an ideal gas, constant specific heats, no friction, perfect gas processes.

Derivation from first principles for a reversible cycle:

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

Using isentropic relations: $$\displaystyle T_2/T_1 = (v_1/v_2)^{\gamma-1} = r^{\gamma-1} $$ and $$\displaystyle T_3/T_4 = r^{\gamma-1} $$.

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

Key Parameters:

• Maximum Temperature: $$\displaystyle T_3 $$ (occurs at end of constant volume heat addition).

• Work Done per Cycle/kg: $$\displaystyle W_{net} = Q_{in} - Q_{out} = C_v[(T_3 - T_2) - (T_4 - T_1)] $$

• Heat Rejected: $$\displaystyle Q_{out} = C_v(T_4 - T_1) $$

• Mean Effective Pressure (MEP): Hypothetical constant pressure that would produce the same net work as the actual cycle.

$$ \text{IMEP} = \frac{W_{net}}{(v_1 - v_2)} = \frac{W_{net}}{v_1(1 - 1/r)} $$

Effect of Compression Ratio (r):

• Efficiency increases with increase in r (non-linear relationship).

• Higher r → Higher thermal efficiency → Better fuel economy.

• Practical limit due to detonation in SI engines.

[!TIP] Exam Focus: Numerical problems always involve finding T2, T3, T4 using isentropic relations first, then efficiency, work, MEP. Remember $$\displaystyle P_2 = P_1 \cdot r^{\gamma} $$.

Losses in Real Engines vs. Air-Standard Cycle:

• Variation of Specific Heats: $$\displaystyle C_v $$ and $$\displaystyle C_p $$ increase with temperature. This reduces the peak temperature $$\displaystyle T_3 $$ for a given heat input, slightly lowering efficiency compared to constant specific heat assumption.

• Effect on Diagrams: The actual P-V and T-S diagrams show a "rounded" constant volume heat addition process and a lower area (net work) than the ideal cycle.

B. Diesel Cycle (Compression Ignition / Constant Pressure Heat Addition)

Processes:

1. 1-2: Isentropic compression

2. 2-3: Constant pressure heat addition

3. 3-4: Isentropic expansion

4. 4-1: Constant volume heat rejection

Key Ratio - Cut-off Ratio (α):

$$ \alpha = \frac{v_3}{v_2} $$

It is the ratio of volumes at the end and start of constant pressure heat addition.

Air-Standard Efficiency:

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

\boxed{\eta_{diesel} = 1 - \frac{1}{r^{\gamma-1}} \cdot \frac{\alpha^{\gamma} - 1}{\gamma(\alpha - 1)}}

Effect of Cut-off Ratio (α):

• For a fixed compression ratio r, efficiency decreases as α increases.

• Higher α means more heat addition at lower pressure, increasing $$\displaystyle Q_{in} $$ more than $$\displaystyle W_{net} $$.

• Work output initially increases with α (up to an optimum) then decreases.

Comparison with Otto Cycle:

[!TIP] Common Pitfall: When comparing Otto vs. Diesel for same maximum pressure/temperature, Diesel cycle can have a higher efficiency because it can operate at a much higher compression ratio.

C. Dual Combustion Cycle (Mixed Cycle)

Processes: Combines features of both Otto and Diesel cycles.

1. 1-2: Isentropic compression

2. 2-3: Constant volume heat addition (part of fuel burns instantly)

3. 3-4: Constant pressure heat addition (remaining fuel burns)

4. 4-5: Isentropic expansion

5. 5-1: Constant volume heat rejection

Relevance: Represents the actual combustion process in CI engines more accurately. Initial part of fuel burns at near constant volume (after ignition delay), followed by diffusion combustion at approximately constant pressure.

Efficiency Formula (for reference):

$$ \eta_{dual} = 1 - \frac{1}{r^{\gamma-1}} \cdot \frac{\rho \alpha^{\gamma} (\beta - 1) + \gamma (\alpha - \beta)}{\gamma (\alpha - 1)(\beta - 1)} $$

Where $$\displaystyle \beta = v_3/v_2 $$ (constant volume heat addition ratio), $$\displaystyle \alpha = v_4/v_3 $$ (cut-off ratio).

D. Performance Parameters & Calculations

Mean Effective Pressure (MEP):

• Indicated MEP (IMEP): $$\displaystyle \frac{IP}{n \cdot L \cdot A \cdot N} $$ or $$\displaystyle \frac{W_{net}}{v_s} $$ (per cylinder per cycle). Represents cylinder pressure effectiveness.

• Brake MEP (BMEP): $$\displaystyle \frac{BP}{n \cdot L \cdot A \cdot N} $$. Relates to usable power.

• n = number of cylinders, L = stroke, A = piston area, N = RPM (rev/min for 4-stroke, rev/2min for 2-stroke), $$\displaystyle v_s $$ = swept volume.

Work Output & Power:

• Indicated Power (IP): Power developed inside cylinder.

$$ IP = \frac{IMEP \cdot v_s \cdot N \cdot n}{60} \quad (\text{for 4-stroke}) $$

• Brake Power (BP): Net usable power at output shaft.

$$ BP = \frac{2\pi NT}{60} \quad (T \text{ in N-m}) $$

• Frictional Power (FP): $$\displaystyle FP = IP - BP $$

• Mechanical Efficiency: $$\displaystyle \eta_m = \frac{BP}{IP} $$

II. COMBUSTION IN SPARK IGNITION (S.I.) ENGINES

A. Stages of Combustion (with Pressure-Crank Angle Diagram)

1. Ignition Lag (Preparation Phase): Time (or crank angle) between spark discharge and start of significant pressure rise. (~0.001 sec). Flame nucleus develops.

2. Flame Propagation (Main Combustion): Flame front travels across combustion chamber. Pressure rises rapidly to peak.

3. Afterburning: Completion of combustion in the later part of expansion stroke. Pressure drop is slower.

[!DIAGRAM: CANVAS] Sketch a typical pressure vs. crank angle diagram for SI engine. Label: Spark timing (TDC), ignition lag, rapid combustion phase (steep slope), peak pressure, afterburning.

B. Flame Propagation

Mechanism: Flame is a deflagration (subsonic). Heat transfer from burned to unburned gas raises temperature above ignition point. Turbulence greatly enhances flame speed by increasing flame surface area.

Factors Affecting Flame Speed:

C. Abnormal Combustion: Detonation (Knock)

Phenomenon: Spontaneous, violent auto-ignition of end-gas ahead of flame front. Creates high-frequency pressure waves ("knock" sound). Causes piston damage, power loss, overheating.

Theories:

1. Auto-ignition Theory: End-gas temperature/pressure exceeds critical limit → chemical reaction → sudden pressure rise.

2. Detonation Wave Theory: A detonation wave (supersonic) propagates through end-gas.

Variables Affecting Detonation:

• Increase tendency: Higher compression ratio, advanced spark timing, high inlet temperature, lean mixtures (slightly), low engine speed (more time for reaction), poor combustion chamber design (hot spots), low octane fuel.

• Decrease tendency: Retarded spark, rich mixture, cooled inlet charge, high speed, good chamber design (swirl/squish), high octane fuel.

Control Measures:

• Retard spark timing.

• Enrich mixture (slightly).

• Use high octane fuel/additives.

• Cool intake charge (intercooler, water injection).

• Optimize combustion chamber design (swirl, quench areas).

• Reduce compression ratio.

D. Pre-ignition

Definition: Ignition of compressed charge by a hot spot before the spark plug fires. Causes rough running, loss of power, possible engine damage.

Difference from Detonation:

Causes & Remedies:

• Causes: Glowing carbon deposits, overly hot spark plug (wrong heat range), lean mixture, ignition timing too far advanced.

• Remedies: Use colder spark plug, decarbonize, enrich mixture, retard spark.

E. Combustion Chamber Design for S.I. Engines

Desirable Characteristics:

• High turbulence/swirl.

• Short flame travel distance.

• Good scavenging.

• High volumetric efficiency.

• Anti-knock properties (quench areas, reduced hot spots).

• Adequate cooling.

Common Types (with Sketches):

Swirl & Squish:

• Swirl: Organized rotary motion of charge. Promoted by offset intake valves or helical ports. Increases flame speed.

• Squish: Radial outward flow of air as piston approaches TDC, squeezing charge into combustion chamber. Creates turbulence and helps cool piston crown. Increases flame speed and reduces knock.

III. COMBUSTION IN COMPRESSION IGNITION (C.I.) ENGINES

A. Stages of Combustion (with Pressure-Crank Angle Diagram)

1. Ignition Delay Period (Preparation Phase): Time between start of injection and start of combustion. Physical (atomization, evaporation, mixing) + Chemical (pre-flame reactions) delay. Pressure rise is slow.

2. Uncontrolled (Rapid) Combustion: Due to accumulated fuel from delay period burning at near constant volume. Sharp pressure rise.

3. Controlled (Diffusion) Combustion: Fuel injected continuously burns as it mixes with air. Rate controlled by injection rate. Pressure rises more slowly.

4. Afterburning: Completion of combustion in expansion stroke.

[!DIAGRAM: CANVAS] Sketch pressure vs. crank angle for CI engine. Label: Injection start, ignition delay, rapid combustion (sharp rise), diffusion combustion (gradual rise), peak pressure.

B. Ignition Lag / Delay Period in C.I. Engines

Definition: Time interval (in ms or °CA) between start of fuel injection and start of combustion (or rapid pressure rise).

Importance: Longer delay → more fuel accumulates → more violent "diesel knock" (uncontrolled combustion). Affects noise, smoke, and pressure rise rate.

Factors Affecting Delay Period:

C. Fuel Injection System & Components

Functions:

1. Metering: Deliver correct fuel quantity per cycle.

2. Timing: Start and end injection at correct crank angle.

3. Atomization: Break fuel into fine droplets.

4. Distribution: Place fuel spray correctly in combustion chamber.

Types of Injection Systems:

1. Air-blast Injection: Fuel atomized by high-pressure air. Used in large marine engines. Complex, parasitic power loss.

2. Solid (Mechanical) Injection: Fuel injected directly at high pressure (200-2000 bar). Types:

   • Inline Pump: One pump element per cylinder.

   • Distributor (Rotary) Pump: Single pump element, distributes fuel.

   • Unit Injector: Pump and nozzle integrated per cylinder.

   • Common Rail: High-pressure pump supplies common rail, electronic control of injectors. (Most modern).

Fuel Injector (Nozzle) Construction:

• Body: High-strength steel, screwed into cylinder head.

• Nozzle Pin/Needle: Opens against spring force at high pressure.

• Orifices: Holes through which fuel sprays.

• Spring: Adjusts opening pressure.

Types of Nozzles:

Spray Characteristics:

• Droplet Size: Smaller → faster evaporation, better mixing.

• Penetration: Distance spray travels. Must reach air but not hit walls.

• Spray Cone Angle: Determines spray pattern (narrow for deep penetration, wide for air swirl).

Injection Timing & Rate:

• Timing: Specified as °BTDC (Before TDC). Affects power, smoke, knock.

• Rate of Injection: Shape of injection curve. Modern systems control this to manage pressure rise rate (reduce noise).

IV. FUEL PREPARATION & DELIVERY SYSTEMS

A. Carburetion in S.I. Engines

Definition: Process of forming a homogeneous air-fuel mixture outside the cylinder (in intake manifold).

Theory of Carburetion (Bernoulli's Principle):

• Air flows through venturi (constriction). Velocity increases → static pressure decreases.

• Pressure drop in venturi throat sucks fuel from nozzle into air stream.

• Fuel atomizes and mixes with air.

Simple Carburetor:

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

• Working: Float maintains constant fuel level. Air flow through venturi creates suction, draws fuel. Throttle controls air-fuel mixture quantity.

• Limitations: Only correct A/F ratio at one engine speed/load (full throttle). Fails at part load (lean) and acceleration (rich needed).

Compensating Devices (to maintain A/F ratio):

• Emulsion Tubes: Air bleed into fuel passage.

• Auxiliary Ports/Valves: Additional air/fuel ports.

• Power Valve: Enriches mixture at high load.

• Accelerator Pump: Provides extra fuel during sudden throttle opening.

Problems:

• Rich during acceleration: Sudden throttle opening → air flow increases faster than fuel flow → momentary lean → accelerator pump squirts fuel.

• Lean at high speed: Venturi effect insufficient to draw enough fuel.

Numerical (Air-Fuel Ratio):

$$ \dot{m}_a = C_d A_t \frac{P_0}{\sqrt{T_0}} \sqrt{\frac{2\gamma}{R(\gamma-1)}\left[\left(\frac{P_t}{P_0}\right)^{\frac{2}{\gamma}} - \left(\frac{P_t}{P_0}\right)^{\frac{\gamma+1}{\gamma}}\right]} $$

Where $$\displaystyle C_d $$ = discharge coeff., $$\displaystyle A_t $$ = venturi area, $$\displaystyle P_0, T_0 $$ = atmospheric, $$\displaystyle P_t $$ = throat pressure.

Fuel flow: $$\displaystyle \dot{m}_f = C_{df} A_f \sqrt{2 \rho_f (P_f - P_t)} $$ (considering fuel head $$\displaystyle P_f $$).

Nozzle Lip Effect: Actual fuel pressure drop is less than $$\displaystyle P_f - P_t $$ due to fuel momentum. Effective head = $$\displaystyle P_f - k \frac{\dot{m}_a^2}{2 \rho_f A_f^2} $$.

B. Multi-Point Fuel Injection (MPFI) / Electronic Fuel Injection (EFI)

Principle: One injector per cylinder, sprays fuel into intake port (port injection) or directly into cylinder (direct injection). Controlled by ECU using sensors (MAP, TPS, O2, etc.).

Merits vs. Carburetor:

• Better fuel metering → lower emissions, better fuel economy.

• No icing/choking (no venturi).

• Better throttle response, power.

• Can optimize for all operating conditions.

• Demerits: Higher cost, complexity, maintenance.

V. IGNITION SYSTEMS

A. Spark Plug

Construction:

• Insulator: Ceramic (alumina), electrical insulation.

• Electrode: Central (connected to HT cable) and ground (threaded shell).

• Shell: Threaded for cylinder head, seals combustion chamber.

• Sealing: Gaskets/plating to prevent gas leakage.

Requirements:

1. Heat Range: Ability to dissipate heat. "Hot" plug (long insulator nose) runs hotter, prone to fouling. "Cold" plug (short nose) runs cooler, used in high-performance engines. Must be self-cleaning (500°C+) but not cause pre-ignition (<850°C).

2. Correct Gap: Typically 0.6-1.2 mm. Affects spark energy and voltage requirement.

3. Resistance: 5-15 kΩ to suppress RF noise.

4. Durability: Withstand high T, P, corrosion.

Fouling & Erosion:

• Fouling: Carbon/oil deposits short-circuit gap. Caused by rich mixture, low T, oil leak.

• Erosion: Electrode wear due to spark. Gap increases → need adjustment/replacement.

B. Battery (Coil) Ignition System

Components:

• Battery, Ignition Switch, Ignition Coil (step-up transformer), Contact Breaker (points), Capacitor (condenser), Distributor (rotor + cap), Spark Plugs.

Working Principle:

1. Closing of points: Current flows from battery → coil primary → points → ground. Magnetic field builds in coil.

2. Opening of points: Current interrupted → collapsing magnetic field induces high voltage (~20 kV) in secondary.

3. High voltage distributed by distributor rotor to appropriate spark plug → spark.

Advantages:

• Simple, reliable, inexpensive.

• High voltage available even at low engine speed.

Disadvantages:

• Contact breaker wear, requires maintenance (adjustment, replacement).

• Voltage limited by coil saturation.

• Poor performance at high speed (points bounce, insufficient time).

• No spark advance/retard control (mechanical/centrifugal advance only).

C. Ignition Lag in S.I. Engines

Definition: Time interval (or crank angle) between spark discharge and the start of pressure rise (not just flame initiation). Typically 0.001-0.002 sec or 10-20° CA.

Factors Affecting Ignition Lag:

VI. ENGINE PERFORMANCE, TESTING & PARAMETERS

A. Key Definitions & Parameters

B. Engine Testing Methods

1. Morse Test (for multi-cylinder engines):

• Purpose: Find IP and mechanical efficiency without indicator.

• Assumptions: Frictional power is constant when one cylinder is cut out (FP same as all cylinders running).

• Procedure:

  1. Run all cylinders, measure BP.

  2. Cut out one cylinder (disable spark/fuel), measure BP' (this is BP of remaining cylinders).

  3. IP of cut-out cylinder = BP(all) - BP' (since FP assumed constant).

  4. Repeat for each cylinder. Sum all individual IPs = Total IP.

  5. $$\displaystyle \eta_m = BP_{total} / IP_{total} $$.

2. Willan's Line Method:

• Plot BP (y-axis) vs. Fuel consumption rate (x-axis).

• Extrapolate line to BP=0 → gives frictional power (fuel consumption at zero load).

• FP = (Fuel rate at BP=0) × CV.

• IP = BP + FP.

C. Heat Balance Sheet (Engine Trial)

Purpose: Account for all heat energy input from fuel. Basis: Per unit time (usually 1 hour or 1 minute).

Numerical Example (from NOV 2023):

Given: Torque=186 Nm, N=1900 rpm, m_f=10.2 kg/h, CV=43890 kJ/kg, m_w=15.5 kg/min, ΔT_w=36°C, m_exh≈m_air+m_f, T_exh=410°C, Cp_exh=1.17 kJ/kgK, T_amb=20°C.

1. BP: $$\displaystyle BP = \frac{2\pi \times 1900 \times 186}{60 \times 1000} = 37.1 $$ kW.

2. BSFC: $$\displaystyle BSFC = \frac{10.2}{37.1} = 0.275 $$ kg/kW-hr.

3. Heat Balance (per minute):

   • Heat Input: $$\displaystyle (10.2/60) \times 43890 = 7460 $$ kJ/min.

   • Heat to BP: $37.1 \times 60 / 4.187$ (convert kJ to kcal) ≈ 531 kcal/min (or 2223 kJ/min).

   • Heat to water: $$\displaystyle 15.5 \times 1 \times 36 = 558 $$ kcal/min (2335 kJ/min).

   • Heat to exhaust: $$\displaystyle (\text{m_exh}) \times 1.17 \times (410-20) $$. m_exh ≈ m_air + m_f = (3.8 kg/min) + (0.17 kg/min) = 3.97 kg/min. → $$\displaystyle 3.97 \times 1.17 \times 390 = 1812 $$ kcal/min (7580 kJ/min). Note: Check units consistency.

   • Unaccounted: Balance.

D. Valve Timing

Theoretical vs. Actual:

• Theoretical: Inlet valve opens/closes at BDC/TDC, exhaust at TDC/BDC. Simple, but poor breathing.

• Actual: Deviations due to inertia of gas columns and valves.

  • Inlet Valve: Opens before BDC (lead), closes after BDC (lag).

  • Exhaust Valve: Opens before BDC (lead), closes after TDC (lag).

  • Overlap: Period when both valves are open (inlet open before exhaust closes). Helps scavenging.

[!DIAGRAM: CANVAS] Draw typical 4-stroke valve timing diagram (crank angle on x-axis, valve lift on y-axis). Label: IVO, IVC, EVO, EVC, overlap.

VII. FORCED INDUCTION: SUPERCHARGING & TURBOCHARGING

A. Need & Effect on Performance

• Increase Power Output: More air → more fuel → more power per cycle (up to 50-100%).

• Effect on SFC: Often improves at high loads (better volumetric efficiency, less throttling loss). May worsen at part load.

• Effect on Detonation (SI): Increases tendency (higher T & P).

• Effect on CI: Helps combustion, reduces smoke.

• Limitations: Increased heat (charge heating), mechanical stress, knocking (SI), smoke limitation (CI), cost, complexity.

B. Supercharging

Definition: Supplying air at pressure > atmospheric using a power-driven compressor (engine-driven).

Methods:

C. Turbocharging

Definition: Using exhaust gas energy to drive a turbine which drives a compressor. No direct parasitic loss.

Methods of Turbocharging:

1. Constant Pressure Charging (CI engines): Exhaust manifold at nearly constant pressure. Single turbine inlet. Most common.

2. Pulse Charging (SI engines): Utilizes kinetic energy of exhaust pulses. Smaller, separate turbine nozzles for each cylinder/group. Better low-speed response.

Merits:

• Uses waste energy → no parasitic power loss.

• Can achieve high boost pressures.

• Compact.

Demerits:

• Turbo lag: Delay in boost build-up (inertia, exhaust energy buildup).

• Turbine/Compressor Matching: Critical for efficiency and surge/overload.

• High Exhaust Back-pressure: Can increase pumping losses.

• Heat: Turbocharger gets very hot, needs cooling/lubrication.

D. Factors Limiting Degree of Super/Turbocharging

VIII. ENGINE COOLING & LUBRICATION SYSTEMS

A. Cooling Systems

Need: Maintain optimal operating temperature (~80-100°C). Prevent seizure, ensure durability, control emissions.

Types:

1. Air Cooling:

   • Fins: Increase surface area for heat dissipation.

   • Design Factors: Fin area (more is better), thickness (thin for conduction), pitch (close for area, far for airflow).

   • Forced/Induced Draft: Fan (engine-driven or electric) forces air over fins.

2. Water Cooling (Forced Circulation - most common):

   • Components: Radiator, water pump, thermostat, fan, water jackets, hoses, expansion tank.

   • Working: Pump circulates hot coolant to radiator → cooled by airflow → returns to engine. Thermostat regulates flow to reach operating temp quickly.

   • Thermosyphon System: Natural circulation due to density difference (no pump). Obsolete.

Coolants:

• Water + Additives: Ethylene glycol (lower freezing, raise boiling), Glycerol, Methyl alcohol.

• Purpose: Anti-freeze, anti-boil, corrosion inhibition, seal conditioning.

B. Lubrication Systems

Functions:

1. Reduce friction & wear.

2. Seal piston rings.

3. Cool (carry away heat).

4. Clean (carry particles to filter).

5. Prevent corrosion.

Required Properties of Lubricating Oil:

• Viscosity & Index: Correct thickness at operating T. High viscosity index (small change with T).

• Flash & Fire Point: Must be high enough to avoid vaporization/ignition.

• Pour Point: Must flow at lowest ambient T.

• Carbon Residue: Low (less coke).

• Neutralization Number: Low acidity.

• Detergency: Keep parts clean.

Types of Lubrication Systems:

Crankcase Ventilation (PCV Valve):

• Purpose: Recirculate blow-by gases (unburned fuel, exhaust) from crankcase to intake to reduce emissions and prevent oil dilution.

• PCV Valve: Controls flow rate based on engine load (more vacuum at idle → less flow).

IX. FUELS, RATINGS & ALTERNATIVES

A. Chemical Composition & Classification

B. Fuel Ratings

Need for Rating: To compare knocking tendency of different fuels under standardized conditions.

S.I. Engine Fuels - Octane Rating:

• RON (Research Octane Number): Mild test (low speed, 600 rpm). Higher than MON.

• MON (Motor Octane Number): Severe test (high speed, 900 rpm, higher T).

• Anti-knock Index (AKI): (RON + MON)/2. Pump octane.

• Definition: Percentage by volume of iso-octane (2,2,4-trimethylpentane, ON=100) in mixture with n-heptane (ON=0) that matches fuel's knock intensity.

C.I. Engine Fuels - Cetane Number:

• Definition: Percentage by volume of cetane (n-hexadecane, CN=100) in mixture with heptamethylnonane (CN=0) that matches fuel's ignition delay.

• Higher CN → Shorter ignition delay → Smoother, quieter operation.

• Typical: Diesel ~45-55, Jet fuel ~40-50.

C. Alternative Fuels

Gaseous Fuels:

• CNG (Compressed Natural Gas): Mainly methane. High octane (~120), clean burning. Requires high-pressure cylinders, engine modifications (lower compression ratio or turbo, hardened valves).

• LPG (Liquefied Petroleum Gas): Propane/Butane mix. High octane, clean. Liquid at moderate pressure. Similar mods as CNG.

• Hydrogen (H₂):

  • Properties: Very wide flammability limits (4-75%), high flame speed, low density, high specific energy.

  • Methods of Use:

    1. Induction manifold injection (mixed with air).

    2. Direct injection (into cylinder).

    3. Dual-fuel (pilot diesel ignition).

  • Advantages: Clean (only H₂O), high efficiency possible.

  • Problems: Severe knocking/pre-ignition (high T), NOx formation, backfire, storage (high pressure/low temp), low energy density.

Biofuels:

• Ethanol (C₂H₅OH): From biomass. High octane, oxygenated. Can be blended (E10, E85). Corrosive, lower energy density.

• Biodiesel (FAME): From vegetable oils/animal fats. Used in CI engines. Good lubricity, renewable. Can cause filter clogging, material compatibility issues.

D. Fuel Additives (Dopes)

Why Mixed? To improve specific properties: anti-knock, oxidation stability, detergency, cold flow, smoke suppression.

Requirements of a Good Additive:

• Effective at low concentration.

• Stable under storage and engine conditions.

• Non-corrosive.

• Compatible with other additives and fuel system materials.

• Economical.

Types:

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

• Antioxidants: Prevent gum formation (e.g., phenolic compounds).

• Detergents: Keep injectors/valves clean (polymeric amines).

• Pour Point Depressants: Improve cold flow (wax modifiers).

• Metal Deactivators: Bind trace metals that catalyze oxidation.

E. Knock Inhibitors

Definition: Additives that raise octane number by suppressing knocking.

Examples:

• Tetraethyl Lead (TEL): Historical. Scavenges free radicals in knock chain reaction. Phased out due to lead pollution.

• Alternatives: MTBE, ETBE, TAME, Aromatics (benzene, toluene, xylene), Oxygenates (ethanol).

Mechanism: Radical Scavenging. Knock involves chain-branching reactions producing peroxides. Lead/other additives react with chain carriers (H·, OH·) to terminate reactions.

X. ADVANCED TOPICS & SPECIAL ENGINES

A. Scavenging (2-Stroke Engines)

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

Types:

1. Cross-flow: Inlet and exhaust ports on opposite sides. Simple but poor scavenging (short-circuiting possible).

2. Loop-flow (Schnuerle): Transfer ports angled to create looped gas flow, minimizing direct short-circuiting. Better.

3. Uniflow: Exhaust valve at head, inlet at crankcase/base. Most efficient (unidirectional flow). Used in large marine 2-strokes.

Comparison: Uniflow > Loop-flow > Cross-flow in scavenging efficiency.

B. Exhaust Gas Recirculation (EGR)

Principle: Recirculate a portion (5-15%) of exhaust gas to intake. Dilutes charge, increases specific heat, lowers peak combustion temperature → reduces NOx formation.

Advantages:

• Significant NOx reduction (primary method for diesel NOx control).

Limitations:

• Increased particulate matter (PM) and HC/CO.

• Power loss (reduced oxygen).

• Requires cooling (cooled EGR) to increase density.

• System complexity, potential for soot/ash buildup.

[!DIAGRAM: CANVAS] Sketch of EGR system: Exhaust manifold → EGR valve → cooler → intake manifold. Label components.

C. Cylinder Arrangement

D. Wankel Engine (Rotary Engine)

Construction & Working:

• Housing: Epitrochoid shape.

• Rotor: Triangular, rotates on eccentric shaft.

• Working: Three chambers formed between rotor and housing. Each chamber goes through intake, compression, expansion, exhaust as rotor turns. One power stroke per chamber per revolution (vs. 2 strokes for 4-stroke piston).

Advantages:

• Fewer parts (no valves, pistons, connecting rods).

• Very smooth, high power/weight ratio.

• Compact, high RPM.

Disadvantages:

• Sealing: Apex seals wear, cause leakage → high oil consumption, emissions.

• Fuel Consumption: Poor thermal efficiency (large surface area/volume ratio → heat loss).

• Emissions: HC emissions high.

• Durability: Seal wear.

E. Comparative Phenomena

Knock in SI vs. CI Engines:

Flame Propagation (SI) vs. Diffusion Combustion (CI):

END OF UNIT 5 NOTES