Comprehensive Study Notes on Internal Combustion Engine Theory, Maintenance, and Exploitation

General Internal Combustion (IC) Engine Theory

• Learning Objectives: Study of theoretical cycles (Otto, Diesel, Dual) and thermodynamic principles.
• Introduction to Cycles:
  • Thermodynamic Cycle: A series of processes where the working fluid returns to its original state. It implies a closed system with no matter exchange with surroundings.
  • IC Engine Cycle Reality: Strictly, an IC engine is an "open cycle" (an oxymoron) because it continuously brings in new fluid at one condition and releases it at another.
• Common Theoretical Cycles:
  • Ideal or Air Standard Cycle.
  • Fuel-Air Cycle.
  • Carnot Cycle.
• Purpose of Theoretical Analysis: Indicates performance upper limits, shows effects of changing variables (compression ratio, inlet pressure/temp), and allows for engine comparison.
• Air Standard Cycle Assumptions:
  • Medium is a perfect gas following $PV = mRT$.
  • Fixed mass of air in a closed system or constant flow circuit.
  • Physical constants match standard air: Molecular weight = 29, $C_p = 0.24$, $C_v = 0.17$, and \frac{C_p}{C_v} = \text{gamma} (\text{̳}) = 1.4.
  • Specific heats are constant.
  • No chemical changes occur.
  • Reversible heat supply/rejection (can be instantaneous).
  • Compression/expansion is reversible adiabatic (constant entropy).
  • Kinetic/potential energy and friction are neglected.

The Otto, Diesel, and Dual Cycles

• Otto Cycle (Constant Volume):
  • Theoretical cycle for Spark Ignition (SI) engines.
  • Process 1-2: Reversible adiabatic compression from Bottom Dead Centre (BDC) to Top Dead Centre (TDC).
  • Process 2-3: Constant volume heat addition (equivalent to instantaneous fuel burning).
  • Process 3-4: Reversible adiabatic expansion.
  • Process 4-1: Constant volume heat rejection.
  • Efficiency Formula: ̳ = 1 - \frac{1}{r^{\text{̳}-1}} where $r$ is the compression ratio ($\frac{\text{Swept Volume} + \text{Clearance Volume}}{\text{Clearance Volume}}$).
  • Efficiency depends only on $r$ and the ratio of specific heats (\text{̳}).
  • Practical Limits: High compression ratios (e.g., above 12-16) offer marginal gains and cause structural issues, bearing reliability problems, and knocking.
• Diesel Cycle (Constant Pressure):
  • Theoretical cycle for slow-speed Compression Ignition (CI) engines.
  • Process 2-3: Constant pressure heat addition ($\text{Volume } v_2 \text{ to } v_3$). At point 3 (cut-off point), fuel supply stops. The ratio ́ = \frac{v_3}{v_2} is the cut-off ratio.
  • Process 4-1: Constant volume heat rejection (blow down).
  • Efficiency Formula: ̳ = 1 - \frac{1}{r^{\text{̳}-1}} \times \frac{́^{\text{̳}}-1}{\text{̳}(́-1)}.
• Dual Combustion (Limited Pressure) Cycle:
  • Model for modern high-speed diesel engines. Heat addition is partially at constant volume and partially at constant pressure.
  • The constant volume part increases efficiency; the constant pressure part limits peak pressure.
  • Efficiency is intermediate between Otto and Diesel cycles for the same compression ratio.

IC Engine Terminology and Parts

• Heat Engine: Transforms chemical energy of fuel into heat, then into mechanical work.
  • External Combustion (ECE): Heat derived outside the engine (e.g., Steam engine/boiler).
  • Internal Combustion (ICE): Heat derived within the engine cylinder (e.g., Diesel engine).
• Advantages of ICE over ECE: Mechanical simplicity, higher power-to-weight ratio, lower initial cost, less water required, higher overall efficiency.
• Key Definitions:
  • Cylinder Bore: Nominal inner diameter.
  • Dead Centres: TDC (farthest from crankshaft), BDC (nearest to crankshaft).
  • Stroke: Distance from TDC to BDC.
  • Swept Volume ($V_s$): Volume covered by piston during one stroke.
  • Clearance Volume ($V_c$): Volume above piston at TDC.
  • Compression Ratio ($r$): $\frac{V_s + V_c}{V_c}$.
  • Indicated Horse Power (IHP): Theoretical power developed in cylinders.
  • Brake Horse Power (BHP): Power available at output shaft ($\text{BHP} = \text{IHP} - \text{friction losses}$).
  • Mechanical Efficiency: $\frac{\text{BHP}}{\text{IHP}} \times 100$.
• Engine Parts Detail:
  • Liners: Dry (thin sleeve, no water contact) vs. Wet (thicker, water in direct contact, uses O-rings for sealing).
  • Piston: Must be light, strong, and gas-tight. Crown shapes help form combustion chambers or deflect gases.
  • Piston Rings: Compression rings (top 2-4, including "fire ring") and Oil Scraper rings (bottom 2-3).
  • Gudgeon Pin (Piston Pin): Connects piston to small end of connecting rod (Fully floating vs. Semi-floating).
  • Connecting Rod: Transmits power. Small end (phosphor bronze bush), Big end (shell type, two halves).
  • Crankshaft: Converts reciprocating to rotary motion. Uses journal, main, and thrust bearings.

Combustion in Spark Ignited (SI) Engines

• Stages of Combustion:
  1. Preparation Phase (Ignition Lag): Passage of spark (A) to the detectable pressure rise (B). Mostly chemical.
  2. Propagation Phase: Flame spreads from point B to peak pressure (C). Mechanical process driven by turbulence.
  • After Burning: Continued burning of remaining fuel after the flame front passes.
• Detonation (Knocking):
  • Normal combustion involves a flame front moving at $15 \text{ m/s}$.
  • Abnormal: End charge auto-ignites before the flame front arrives due to reaching critical temperature. It creates a high-intensity pressure wave striking cylinder walls.
  • Effects: Noise, structural damage, carbon deposits, increased heat transfer, efficiency loss.
  • Control Design: Lower compression ratio, increased turbulence, multiple spark plugs, additives (Octane).
  • Operator Control: Retarding spark, throttling (reducing inlet pressure), making ratio too rich/lean.

Combustion in Compression Ignited (CI) Engines

• Fundamentally Different: High compression ratio ($11:1$ to $22:1$) raises air temperature above fuel auto-ignition point (approx. 450-550 \text{ ^{∘}C} at $30-40 \text{ kg/cm}^2$). Fuel is injected at $110-200 \text{ kg/cm}^2$.
• Stages of Combustion:
  1. Ignition Delay: Physical (atomization/internal mixing) and Chemical (pre-flame reactions).
  2. Rapid/Uncontrolled Combustion: Rapid pressure rise from fuel accumulated during delay.
  3. Controlled Combustion: Fuel burns as it enters based on injection rate.
  4. After Burning: Late-stage burning of remaining particles during expansion.
• Diesel Knock: Caused by long ignition delay; too much fuel accumulates and auto-ignites at once.
• Combustion Chambers:
  • Open: Simplest, needs multiple-orifice injectors ($175-1000 \text{ kg/cm}^2$).
  • Swirl/Turbulent: Tangential throat creates rotary air movement; uses single-hole pintle nozzles.
  • Pre-combustion Chamber: Small chamber connected by holes; burns 20-30 \text{ %} of charge first.
  • Lanova (Energy Cell): Uses an air cell (10-15 \text{ %} volume) to create turbulence.

Carburetion and Fuel Injection

• Carburetion (SI): Preparing fuel-air mixture outside the cylinder.
  • Simple carburetor parts: Float chamber, Venturi (throat), Fuel jet, Throttle valve.
  • A/F Ratios: Idling ($12.5:1$ - Rich); Normal ($16.7:1$ - Lean for economy); Power ($14.3:1$ - Rich).
• Fuel Injection System (CI):
  • Types: Air injection (uses HP air pump/blast - complex) vs. Solid (Airless) injection (Individual pump, Common rail, Unit injector, or Distributor systems).
  • Fuel Injection Pump (FIP) Operation: Uses a plunger with a vertical channel and a helical groove. Rotating the plunger alters when the helix uncovers the "spill port," varying fuel delivery while the start of injection remains constant.
  • Anti-dribble Device: Delivery valve with a small piston that increases pipe volume upon closing, snapping the nozzle valve shut.
  • Nozzle Types: Single/Multi-hole, Pintle (hollow conical spray), Pintaux (auxiliary hole for cold starts).
  • Injector Testing: Back leakage (pressure drop timing), Pressure setting, Seat tightness (wetness check), Spray form (atomization check).

Supercharging and Scavenging

• Supercharging: Inducing air at pressure higher than atmospheric to increase power-to-weight ratio.
  • Roots Blower: Two rotors (usually 2-lobe) running with $0.1-0.2 \text{ mm}$ clearance.
  • Vane Type: Eccentric rotor with sliding vanes; operates at $4000-5000 \text{ RPM}$.
  • Centrifugal: Rotor at $15,000-80,000 \text{ RPM}$. Imparts kinetic energy converted to static pressure.
  • Turbocharging: Uses wasted exhaust energy to drive a gas turbine coupled to a centrifugal compressor. Can gain 20 \text{ %} thermal efficiency.
  • Exhaust Grouping: Connecting cylinders to avoid exhaust wave interference during scavenging.
  • Turbocharging Methods: Constant Pressure (simple, efficient at high loads) vs. Pulse (recover blow-down energy, better response/acceleration).
• Scavenging (Two-stroke):
  • Direct: Ports on opposite sides; angled ports and shaped pistons control flow.
  • Loop: Inlet and exhaust on the same side; charge forms a loop.
  • Uniflow: Inlet ports around cylinder base, exhaust valve in cylinder head; charge moves in one direction.

Valve Timing

• Practical Timing:
  • Inlet Valve Open (IVO): Opens before TDC so it is fully open as the piston descends.
  • Inlet Valve Closed (IVC): Closes after BDC to maximize air induction via momentum.
  • Exhaust Valve Open (EVO): Opens before BDC in the power stroke for blow-down.
  • Exhaust Valve Closed (EVC): Closes after TDC for scavenging.
• Valve Overlap: Period when both valves are open (e.g., $60 \text{ to } 140 \text{ degrees}$). Longer in supercharged engines.
• Improper Timing Effects: Loss of compression, carbonization, poor combustion.

Engineering Maintenance and Checks

• Crankshaft Deflection: Measured using a dial gauge between webs. Indicates stress and alignment. Readings taken at positions A through E.
• Crankcase Explosion:
  • Cause: Lub oil vapour + Hot Spot (from bearing failure or blow-by).
  • Mechanism: Primary explosion creates pressure cracks; air rushes in for a massive Secondary explosion ($\text{velocity up to } 3000 \text{ m/s}$).
  • Protection: Explosion doors (set at $1/15 \text{ bar}$), Flame traps (gauze elements), Oil Mist Detectors, CO2 Drenching.
• Clearances:
  • Tappet Clearance: Taken when cold; ensures expansion room and positive valve closure.
  • Bumping Clearance: Distance from piston to head at TDC; determines compression ratio.
  • Nip Clearance: Interference fit of bearing shells.
• Load Trials: Conducted monthly (1 hr near full power), six-monthly (external load tank), and after major repairs.
• Governor: Speed-sensitive device adjusting fuel rack.
  • Droop: RPM drop from no-load ($n_0$) to full-load ($n_1$). $\text{Droop} = \frac{n_0 - n_1}{n_0} \times 100$.

Petroleum Oil Lubricants (POL) Hygiene and Properties

• Fuel Hygiene: Heavy High Speed Diesel (LSHSD) must be water-free. Water causes pitting on pistons, low power, and corrosion.
• Lubricating Oil Properties:
  • Viscosity: Resistance to flow; decreases with temperature.
  • Flash Point: Temp where it ignites (150-230 \text{ ^{∘}C}).
  • Fire Point: Spontaneous ignition (315 \text{ ^{∘}C}).
  • TBN (Total Base Number): Additives used to neutralize acids (acidity leads to corrosion).
  • Detergency: Keeps particles in suspension.
• Lub Oil Degradation:
  • Dilution: Fuel mixing with oil (max 5 \text{ %}). Lowers viscosity and pressure.
  • Contamination: Water mixing (Sea or Fresh); causes cloudiness and corrosion.
  • Emulsification: Whipping water and oil into a stable mixture; causes foaming.
• Tests:
  • Viscosity comparator: Compares flow time against reference oil.
  • Spot Test: Drops oil on filter paper to estimate carbon content.
  • Crackling Test: Dropping oil on a hot plate; "crackling" sound indicates water.
  • Speedy Moisture Tester: Uses calcium hydride to react with water and produce gas pressure.

Appendix: Protection Devices and Exhaust Temperatures

• Low L.O. Pressure Alarm/Trip: Tested using a 'T' piece to drain pressure locally while the engine is running.
• High F/W Temperature: Probe tested in an external heat bath (electric kettle) while monitoring with a thermometer.
• Overspeed Trip: Tested by actually overspeeding or by simulating the tachogenerator signal with a drill/electronic check panel.
• Exhaust Temperature Significance:
  • High Exhaust Temp: Indicates engine overload, faulty injectors (sticking needle), late injection timing, or blocked air filters. Can cause cylinder head cracking or turbocharger damage.
  • Low Exhaust Temp: Indicates loss of power, fuel starvation (blocked filters), or scored fuel pump plungers.
  • Limits: Typical maximum limit is 650 \text{ ^{∘}C}.