Internal Combustion Engines (ME-501) - Unit 3 Short Notes

1.0 THERMODYNAMIC CYCLES (AIR STANDARD ANALYSIS)

1.1 Otto Cycle

• Processes: 1-2: Isentropic compression, 2-3: Constant volume heat addition, 3-4: Isentropic expansion, 4-1: Constant volume heat rejection.

• Air Standard Efficiency:

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

where $r$ = compression ratio, $\gamma$ = ratio of specific heats.

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

• Work Output: $$\displaystyle W_{net} = Q_{in} - Q_{out} = C_v(T_3 - T_2) - C_v(T_4 - T_1) $$

• Mean Effective Pressure (M.E.P.):

$$M.E.P. = \frac{W_{net}}{V_s} = \frac{p_1 r^\gamma (r - 1)}{(\gamma - 1) r^\gamma} \left[ \frac{\rho^\gamma - 1}{\rho - 1} \right]$$

where $$\displaystyle \rho = \frac{T_3}{T_2} $$ = heat addition ratio.

• Effect of Compression Ratio: Efficiency increases with increase in $r$. Higher $r$ → higher thermal efficiency, but limited by knocking in SI engines.

• Loss due to Variation of Specific Heats: At high temperatures, $$\displaystyle C_v $$ and $$\displaystyle C_p $$ increase, reducing efficiency. Represented by a modified P-V diagram where the constant volume lines are not vertical due to variable specific heats.

[!TIP] Exam Focus: Numerical problems on Otto cycle are very frequent. Always convert temperatures to Kelvin. Use $$\displaystyle C_p - C_v = R $$ and $$\displaystyle \eta = 1 - \frac{1}{r^{\gamma-1}} $$ for quick checks.

1.2 Diesel Cycle

• Processes: 1-2: Isentropic compression, 2-3: Constant pressure heat addition, 3-4: Isentropic expansion, 4-1: Constant volume heat rejection.

• Key Parameter - Cut-off Ratio: $$\displaystyle r_c = \frac{V_3}{V_2} $$ (volume at end / start of heat addition).

• Air Standard Efficiency:

$$\eta_{diesel} = 1 - \frac{1}{r^{\gamma-1}} \left[ \frac{r_c^\gamma - 1}{\gamma (r_c - 1)} \right]$$

\boxed{\eta_{diesel} = 1 - \frac{1}{r^{\gamma-1}} \left[ \frac{r_c^\gamma - 1}{\gamma (r_c - 1)} \right]}

• Comparison with Otto:

  • For same $r$, Diesel cycle has lower efficiency because heat addition at constant pressure is less efficient.

  • Diesel cycle has higher compression ratio possible (no knocking), so actual efficiency can be higher.

  • Diesel cycle has longer heat addition period → higher NOx? (Consider later).

1.3 Dual Combustion Cycle (Mixed Cycle)

• Processes: 1-2: Isentropic compression, 2-3: Constant volume heat addition, 3-4: Constant pressure heat addition, 4-5: Isentropic expansion, 5-1: Constant volume heat rejection.

• Relevance: More accurately represents real SI engine cycle where combustion is not instantaneous at constant volume; part of heat addition occurs at constant pressure due to finite combustion time.

• Efficiency: Depends on $r$, $$\displaystyle r_c $$ (cut-off), and $$\displaystyle r_{v} $$ (volume ratio for constant volume part).

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

2.1 Stages of Combustion

1. Normal Combustion:

   • Flame initiation at spark plug.

   • Flame propagation across combustion chamber.

   • Pressure-crank angle diagram shows smooth rise.

2. Abnormal Combustion (Knocking/Detonation):

   • Phenomenon: Uncontrolled, rapid combustion of end-gas (unburned mixture ahead of flame front) after normal flame passage. Creates high-frequency pressure oscillations → metallic knocking sound.

   • Effects: Power loss, overheating, piston/ring damage, pre-ignition risk.

3. Pre-ignition:

   • Cause: Ignition of charge by a hot spot (spark plug electrode, exhaust valve, carbon deposit) before spark.

   • Consequence: Runaway condition, severe engine damage.

   • Remedy: Use cooler spark plugs, adjust ignition timing, decarbonize.

2.2 Flame Propagation

• Mechanism: Laminar flame front moves through homogeneous mixture. Turbulence increases flame speed by wrinkling flame front.

• Factors Affecting Flame Speed:

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

  • Compression Ratio: ↑ CR → ↑ temperature/pressure → ↑ flame speed.

  • Fuel-Air Ratio: Stoichiometric (~14.7:1 for gasoline) gives max flame speed. Rich/lean mixtures reduce speed.

  • Ignition Timing: Retarding timing reduces effective flame travel time.

  • Chamber Shape: Shorter flame travel path → faster combustion.

2.3 Ignition Lag (Ignition Delay)

• Definition: Time interval between spark discharge and start of pressure rise due to combustion (not flame initiation). Includes:

  1. Physical Delay: Vaporization, mixing, heating to ignition temp.

  2. Chemical Delay: Pre-flame reactions (chain initiation).

• Effect of Engine Variables:

  • ↑ Compression Ratio → ↓ ignition lag (higher T, P).

  • ↑ Inlet Temperature/Pressure → ↓ lag.

  • Fuel with higher flame speed → ↓ lag.

  • ↑ Turbulence → ↓ lag.

  • Spark Plug Location: Central location minimizes lag.

2.4 Detonation Theories & Factors

• Theories:

  1. Surface Ignition Theory: Hot spots (carbon, valves) ignite end-gas.

  2. Abnormal Combustion Theory: End-gas undergoes rapid, explosive combustion after auto-ignition due to high P & T.

• Variables Affecting Detonation:

  • Compression Ratio: ↑ CR → ↑ tendency.

  • Ignition Timing: Advanced timing → ↑ tendency.

  • Fuel Quality: Low Octane Number → ↑ tendency.

  • Inlet Temperature: ↑ inlet temp → ↑ tendency.

  • Engine Load: Part load (throttled) → ↑ tendency (lower T, slower flame).

  • Combustion Chamber Design: High surface area/volume ratio, sharp corners → ↑ tendency.

2.5 Combustion Chamber Design for S.I. Engines

• Desirable Characteristics:

  • High volumetric efficiency.

  • High flame speed, short flame travel.

  • Low knock tendency.

  • Good cooling, easy manufacture.

• Types:

  • Turbulent Chambers: Create swirl/turbulence.

    • Wedge: Simple, good swirl.

    • Swirl: Tangential intake port creates strong swirl.

    • Turbulent Pot: Pre-chamber with small orifice.

  • Squish Chambers: Piston crown close to head creates squish area → turbulence.

  • Hemispherical (HeMi): Large valves, central spark plug → short flame path, high CR, but expensive.

[!TIP] Exam Focus: Be ready to sketch at least one chamber (Hemispherical, Swirl, or Wedge) and label parts. Compare types in a table: Flame travel, CR, knock tendency, cost.

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

3.1 Stages of Combustion

1. Delay Period (Ignition Lag):

   • Time between start of injection and start of pressure rise.

   • Phases:

     • Physical Delay: Fuel atomization, evaporation, mixing with air.

     • Chemical Delay: Pre-flame reactions (low-temperature oxidation).

   • Importance: Controls rate of pressure rise. Long delay → more fuel accumulates → rapid combustion → high pressure rise → "diesel knock".

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

3. Controlled Combustion: After ignition, remaining fuel burns at controlled rate.

4. Afterburning: Late combustion phase in crevices; incomplete.

3.2 Factors Affecting Delay Period

• Fuel Properties:

  • Cetane Number: ↑ CN → ↓ delay period (shorter ignition lag).

  • Volatility, Viscosity: ↑ volatility → ↓ delay.

• Engine Variables:

  • ↑ Compression Ratio → ↑ T, P → ↓ delay.

  • ↑ Inlet Air Temp/Pressure → ↓ delay.

  • ↑ Load → ↑ T, P → ↓ delay.

  • ↑ Turbulence → better mixing → ↓ delay.

  • Injection Timing: Advanced timing → delay period occurs at higher T, P → ↓ effective delay.

  • Nozzle Design: Good atomization → ↓ delay.

3.3 Diesel Knock

• Definition: Rough, noisy combustion due to long delay period. Not same as SI knock (which is end-gas detonation).

• Cause: Long delay → large fuel accumulation → rapid, uncontrolled burning → high rate of pressure rise.

• Remedy: Increase cetane number, advance injection timing, increase compression ratio, improve atomization.

3.4 Combustion Chambers for C.I. Engines

• Open Combustion Chamber (Direct Injection - DI):

  • Fuel injected directly into piston bowl.

  • Types: Piston bowl (swirl generated by piston shape), square bowl, toroidal bowl.

  • Merits: High thermal efficiency, no heat loss in passage, simple.

  • Demerits: Requires high injection pressure, sensitive to fuel properties.

• Divided Combustion Chamber (Indirect Injection - IDI):

  • Fuel injected into pre-combustion chamber or swirl chamber connected to main cylinder by orifice.

  • Swirl Chamber (Ricardo, Comet): Swirl generated by tangential passage. Good cold start, quieter.

  • Pre-combustion Chamber: Turbulence generated by fuel jet. Better mixing.

  • Merits: Lower injection pressure needed, smoother operation, better cold start.

  • Demerits: Heat loss to walls → lower efficiency (5-10% loss), higher compression ratio needed.

[!TIP] Exam Focus: Direct vs Indirect Injection comparison is crucial. DI = higher efficiency, IDI = smoother, better cold start. Know sketches of piston bowl (DI) and swirl chamber (IDI).

4.0 FUEL SYSTEMS & CARBURETION

4.1 Fuel Injection Systems in C.I. Engines

• Functions: Metering, Timing, Atomization, Distribution.

• Types:

  1. Common Rail: High-pressure accumulator (rail) supplies all injectors. Electronic control of pressure & injection. Merits: Precise control, multiple injections. Demerits: Costly, high pressure.

  2. Unit Injector: Pump and nozzle integrated per cylinder, cam-actuated. Merits: High pressure, precise timing. Demerits: Mechanical complexity.

  3. Pump-Line-Nozzle: Separate pump, high-pressure line, nozzle. Merits: Simple, robust. Demerits: Pressure fluctuations, limited control.

• Fuel Injector (Nozzle):

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

  • Working: Fuel under pressure lifts needle → spray through orifices.

  • Types:

    • Single Orifice: Simple, for IDI.

    • Multi-hole: For DI, better atomization.

    • Pintle Type: Long spray, low pressure.

    • Throttle Type: For air-blast injection.

  • Requirements: Proper atomization (Sauter Mean Diameter), no dribbling, no coking.

4.2 Carburetion in S.I. Engines

• Definition: Mixing air and fuel in correct proportion (air-fuel ratio) for SI engines.

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

• Simple Carburetor:

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

  • Working: Air flow through venturi creates depression → fuel lifted from float chamber → mixes with air.

  • Limitations: Provides correct mixture only at one speed/load. Fails at:

    • Starting (needs rich mixture).

    • Idling (low air velocity).

    • Acceleration (needs extra fuel).

    • High load (needs richer mixture).

  • Float Chamber Vented to Atmosphere: To maintain atmospheric pressure on fuel surface, ensuring fuel flow depends only on venturi depression.

4.3 Multi-Point Fuel Injection (MPFI)

• System: One injector per cylinder, near intake valve. Electronic control unit (ECU) controls timing/duration.

• Merits over Carburetor:

  • Better fuel metering → improved fuel economy, power.

  • Reduced emissions (precise control).

  • No icing, better acceleration.

  • No throttle losses (in some systems).

• Demerits: Higher cost, complexity, maintenance.

[!TIP] Exam Focus: Carburetor numericals are common. Use Bernoulli's equation for air flow:

$$m_a = C_d A_t \sqrt{2 \rho_a \Delta p}$$

and fuel flow:

$$m_f = C_f A_f \sqrt{2 \rho_f (p_{float} - p_{venturi})}$$

. Air-fuel ratio = $$\displaystyle m_a / m_f $$.

5.0 ENGINE PERFORMANCE & TESTING

5.1 Key Performance Parameters

5.2 Engine Testing & Heat Balance

• Morse Test (Cut-Off Method):

  • For: Multi-cylinder engines to find I.P. and $$\displaystyle \eta_m $$.

  • Assumptions: Frictional power of each cylinder is same when running; Power of running cylinders proportional to their I.P.

  • Procedure: Run all cylinders → note BP. Cut off one cylinder → note BP (which is sum of I.P. of remaining cylinders minus their F.P.). Repeat.

  • Calculation: $$\displaystyle IP_{total} = BP_{all} + \sum (BP_{all} - BP_{cut}) $$. $$\displaystyle \eta_m = \frac{BP_{all}}{IP_{total}} $$.

• Heat Balance Sheet:

  • Purpose: Energy audit → shows distribution of heat input.

  • Preparation (per minute/hour):

    1. Heat Input: $$\displaystyle Q_{in} = \dot{m}_f \times CV $$ (kJ/min).

    2. Useful Output (BP): $BP$ (kJ/min).

    3. Heat in Cooling Water: $$\displaystyle m_w C_p \Delta T_w $$.

    4. Heat in Exhaust Gases: $$\displaystyle m_{ex} C_p \Delta T_{ex} $$.

    5. Heat Unaccounted: $$\displaystyle Q_{in} - (BP + Q_{cool} + Q_{ex}) $$.

  • Representation: Tabular form with percentages.

5.3 Numerical Problems

• From Engine Trial: Given torque, speed, fuel/water/exhaust data → calculate BP, BSFC, heat balance.

• From Carburetor: Given venturi/fuel orifice dimensions, coefficients, fuel head → compute A/F ratio.

6.0 SUPERCHARGING & TURBOCHARGING

6.1 Definitions & Need

• Supercharging: Forced induction to increase density of intake charge using a compressor driven by engine (mechanical supercharger) or exhaust gas (turbocharger).

• Turbocharging: Specific type of supercharging where a turbine (driven by exhaust gases) drives a compressor.

• Need for Aircraft Engines: Maintain power output at high altitudes where air density is low.

6.2 Effect on Performance

• Power Output: ↑ Significantly (more air → more fuel → more power).

• Fuel Consumption:

  • Specific Fuel Consumption (SFC): May ↓ (better volumetric efficiency, less throttling loss).

  • Brake Specific Fuel Consumption (BSFC): May ↑ at full load due to higher friction & pumping work.

• Other Effects:

  • ↑ Thermal & mechanical stresses.

  • S.I. Engines: ↑ Tendency to detonate (higher T, P).

  • C.I. Engines: ↑ Smoke (more fuel, less mixing time), ↑ peak pressure.

6.3 Methods of Supercharging/Turbocharging

• Positive Displacement Superchargers:

  • Roots Blower: Two meshing lobes. Merits: Good low-end boost. Demerits: Noisy, parasitic loss.

  • Vane Type: Rotor with sliding vanes. Smoother.

• Dynamic Compressors:

  • Centrifugal Supercharger/Turbocharger: Radial flow turbine/compressor. Merits: Efficient at high speed, compact. Demerits: Lag (turbo lag), poor low-end.

• Methods of Turbocharging:

  1. Constant Pressure: Exhaust manifold at constant pressure (common for C.I.). All cylinders feed common manifold.

  2. Pulse Turbocharging: Exhaust pulses from individual cylinders/group hit turbine → better low-speed response (for high-speed engines).

6.4 Limitations of Supercharging

• S.I. Engines: Detonation, high heat rejection, high mechanical stress, increased NOx.

• C.I. Engines: Increased smoke, thermal stress, higher peak cylinder pressure, turbocharger turbine overheating.

• General: Increased cost, complexity, parasitic power loss (for mechanically driven).

7.0 FUELS, ADDITIVES & ALTERNATE FUELS

7.1 Fuel Classification & Chemistry

7.2 Fuel Requirements

• S.I. Engine Fuels: High octane number, good volatility, smooth burning, low deposits, low sulfur.

• C.I. Engine Fuels: Good volatility, high cetane number, proper ignition delay, low smoke, low sulfur.

7.3 Fuel Rating

• Octane Number (ON): % iso-octane in reference mix (iso-octane + n-heptane) that matches test fuel's knocking tendency. Higher ON → better knock resistance.

• Cetane Number (CN): % cetane in reference mix (cetane + heptamethylnonane) that matches test fuel's ignition quality. Higher CN → shorter ignition delay.

• Knock Inhibitors: Additives (TEL, aromatics, oxygenates like MTBE, ethanol) that increase ON by slowing chain reactions.

7.4 Dopes/Additives

• Why mixed?: Improve existing properties (knock resistance, stability) or impart new ones (detergency, anti-freeze).

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

• Types:

  • Anti-knock (TEL, aromatics).

  • Antioxidants (prevent gum formation).

  • Detergents (clean injectors/valves).

  • Cetane improvers (alkyl nitrates).

  • Metal deactivators (prevent catalytic oxidation).

7.5 Alternate Fuels

• Gaseous:

  • CNG: High ON (120-130), clean, but low energy density, high-pressure cylinders.

  • LPG: High ON, clean, but requires vaporizer, safety issues.

  • Hydrogen: Very high efficiency, clean (H₂O only), but problems: backfire, pre-ignition, low density, NOx formation, storage.

• Liquid:

  • Alcohols (Ethanol, Methanol): High ON, renewable, but low energy density, material compatibility, cold start.

  • Biodiesel: Renewable, lubricity, but higher NOx, cold flow issues, stability.

  • Vegetable Oils: High viscosity, poor atomization → needs preheating or transesterification.

8.0 COOLING SYSTEMS

8.1 Need for Cooling

• Prevent overheating & seizure.

• Maintain clearances (piston ring, valve).

• Avoid pre-ignition (SI) and knocking.

• Ensure lubrication (oil viscosity).

8.2 Types of Cooling Systems

8.3 Cooling System Components

• Cooling Fins:

  • Purpose: Increase surface area for heat transfer.

  • Types: Straight, pin, wavy (wavy better for turbulence).

• Anti-Freeze Solutions:

  • Purpose: Lower freezing point, raise boiling point, prevent corrosion/scale.

  • Common: Ethylene glycol (most common), glycerol.

  • Properties Required: Low freezing, high boiling, non-corrosive, non-volatile, stable.

[!TIP] Exam Focus: Sketch Forced Circulation Liquid Cooling System with labels: radiator, thermostat, water pump, cooling fan, water jacket, hoses. Explain thermostat function: closes at cold start to reach operating temp quickly.

9.0 LUBRICATION SYSTEMS

9.1 Functions of Lubrication

• Reduce friction & wear.

• Seal piston rings (gas sealing).

• Cool (carry away heat).

• Clean (carry contaminants to filter).

• Prevent corrosion.

9.2 Properties of Lubricating Oil

• Viscosity: Most important. Must be correct at operating temp (multigrade oils).

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

• Fire Point: Temp at which it continues to burn.

• Carbon Residue: Tendency to form deposits.

• Pour Point: Lowest temp at which it flows.

• Neutralization Number: Acidity/alkalinity (indicates oxidation).

9.3 Types of Lubrication Systems

10.0 OTHER ENGINE SYSTEMS & TYPES

10.1 Scavenging (for 2-Stroke Engines)

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

• Types:

  • Cross-Flow: Inlet and exhaust on opposite sides. Simple, but incomplete scavenging.

  • Loop Scavenging: Inlet and exhaust ports on same side; incoming charge deflects upward → loop. Better scavenging.

  • Uniflow Scavenging: Exhaust valve at head, inlet at bottom (or piston-controlled). Best scavenging (unidirectional flow). Used in large marine diesels.

10.2 Exhaust Gas Recirculation (EGR)

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

• Working: Exhaust gas recirculated to intake manifold → dilutes charge → reduces flame temperature.

• Advantages: Effective NOx reduction.

• Limitations: ↑ particulate matter, ↓ efficiency, potential for engine knock (SI), need precise control.

10.3 Wankel (Rotary) Engine

• Construction: Rotor (triangular) in epitrochoid housing. Eccentric shaft.

• Working: Rotor's rotation creates expanding/contracting chambers → intake, compression, power, exhaust in continuous cycles.

• Advantages: Fewer parts, smooth (no reciprocating), high power/weight, compact.

• Disadvantages: Sealing problems (apex seals), high emissions, poor fuel economy historically, high HC emissions.

10.4 Valve Timing & Valve Overlap

• Theoretical vs Actual: In theory, IV opens at BDC, closes at TDC; EV opens at TDC, closes at BDC. Actual timing differs to utilize gas dynamics.

• Actual Timing:

  • Inlet Valve Opens (IVO): Before TDC (intake stroke) → to utilize inertia of incoming air.

  • Inlet Valve Closes (IVC): After BDC → to allow more air due to momentum.

  • Exhaust Valve Opens (EVO): Before BDC (power stroke) → to reduce pumping work.

  • Exhaust Valve Closes (EVC): After TDC → to utilize exhaust pulse energy.

• Valve Overlap: Period when both IV and EV are open (between EVO and IVC). Effect: Improves scavenging at high speed, but can cause reversion at low speed.

[!TIP] Exam Focus: Draw actual valve timing diagram for 4-stroke engine, showing IVO, IVC, EVO, EVC relative to TDC/BDC of intake/exhaust strokes. Explain purpose of each deviation.