Fuel Supply System and Ignition System

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This section covers fuels for spark-ignition engines and their properties, the air-fuel mixture requirements of SI engines under different operating conditions, and the simple carburettor with its limitations and the auxiliary systems used in practical carburettors.

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Fuels for Spark-Ignition Engines • Main SI fuel: petrol (gasoline) — hydrocarbons ≈ C5–C10, boiling range ≈ 30–200 °C, density ≈ 0.72–0.75 kg/L, LCV ≈ 43–44 MJ/kg.

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LPG, CNG, ethanol/methanol (and blends E10, E20, E85), hydrogen, biogas. • Requirements of a good SI fuel: high octane number (resistance to knock); suitable volatility — volatile enough for easy cold starting and good distribution but not so volatile as to cause vapour lock and evaporative loss; high calorific value; low gum and deposit formation; low sulphur; good storage stability; safe handling. • Volatility is judged by the ASTM distillation curve:

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10% point — cold starting and vapour lock;

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50% point — warm-up and acceleration;

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90% point — crankcase dilution, deposits and fuel economy.

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Reid vapour pressure (RVP) indicates front-end volatility. • Octane number:

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RON (milder test) > MON; aromatics and iso-paraffins give high octane (see ME 9.5).

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Air-Fuel Ratio and Mixture Requirements • Stoichiometric (chemically correct) A/F for petrol ≈ 14.7 :

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Petrol-air mixtures burn only within about 7 :

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1 (rich limit) to 20 :

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1 (lean limit). • Mixture strength: λ = actual A/F ÷ stoichiometric A/F (λ > 1 lean, λ < 1 rich); equivalence ratio φ = 1/λ. • The 'fish-hook' curve (bsfc vs bmep for various A/F at constant speed and throttle) shows the best power mixture (≈ 12.5–13 :

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1, slightly rich) and the best economy mixture (≈ 16–17 :

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Operating condition Mixture required Reason Cold starting Very rich (as rich as ≈ 3–5 :

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1 at carburettor) Only light fractions vaporise in a cold engine; enough vapour must reach the cylinder Warm-up Rich, gradually leaner Engine and manifold still cold Idling and low load Rich (≈ 10–12 :

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1) High dilution by residual exhaust gas at low throttle opening Cruising (normal power range) Lean (≈ 15–17 :

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1) Maximum economy Maximum power (full throttle) Rich (≈ 12.5–13 :

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1) Maximum power; also cools valves and pistons, reduces knock Acceleration Momentary enrichment Fuel lags behind air (fuel film on manifold walls) when throttle is opened suddenly • Modern engines with three-way catalysts operate at λ ≈ 1 in closed loop most of the time, enriching only for cold start and full load.

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Simple Carburettor • Parts: float chamber with float and needle valve (keeps fuel level constant, slightly below nozzle tip), fuel strainer, metering (main) jet, discharge nozzle in the throat of the venturi (choke tube), throttle valve (butterfly, after the venturi — controls quantity of mixture and hence power), choke valve (before the venturi — enriches mixture for cold start). • Working: air flowing through the venturi accelerates and its pressure falls (Bernoulli); the pressure difference between float chamber (atmospheric) and throat draws fuel through the jet, where it is atomised, partly vaporised and mixed with air. • Approximate A/F from a simple carburettor:

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A/F ≈ (CdaAa/CdfAf)·√(ρa/ρf) (neglecting nozzle lip height and compressibility). • Limitations of the simple carburettor: the mixture becomes progressively richer as air flow (speed/throttle opening) increases — it cannot give a lean cruise and rich idle; delivers no fuel at very low air flow (idling) because throat depression is too small; no enrichment for cold start, acceleration or full power; carburettor icing in cold humid weather; poor distribution among cylinders; affected by altitude and temperature.

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Auxiliary Systems of a Practical Carburettor System / device Purpose and working Main metering system with compensation Keeps mixture nearly constant over cruising range — compensating jet (Zenith), air-bleed / emulsion tube, back-suction control, metering pin Idling (slow-running) system Idle port below the throttle plate with idle mixture screw, plus progression (transfer) holes for smooth transition — supplies rich mixture at idle Power enrichment / economiser Metering rod or vacuum-operated power valve opens an extra jet at full throttle for maximum power Accelerating pump Plunger or diaphragm pump squirts extra fuel when the throttle is opened suddenly — prevents 'flat spot' or hesitation Choke (cold-start) system Choke plate closes air entry to give very rich mixture; manual or automatic (bimetal-controlled); fast-idle cam Other devices Anti-dieseling solenoid, altitude compensator, hot-idle compensator, anti-icing heating • Types: up-draught, down-draught (most common in cars — gravity assists flow), side-draught/horizontal; constant-choke (fixed venturi — Solex, Zenith, Carter) and constant-vacuum / variable-venturi (SU, Stromberg CD; motorcycle CV carburettors — a piston varies venturi area); single- and multi-barrel (twin-choke) carburettors.

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The syllabus heading reads 'Injection system for Diesel Engine', but its listed topics — drawbacks of the carburettor, single-point and multipoint injection and gasoline direct injection — concern PETROL (SI) engines; diesel injection is covered in 6.4.

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This section covers why injection replaced carburettors, types of petrol injection, electronic fuel injection components and control, and GDI.

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Drawbacks of the Carburettor • Non-uniform distribution of mixture to different cylinders (unequal A/F, especially in multi-cylinder engines). • Venturi restricts air flow → lower volumetric efficiency and power. • Imprecise A/F control over the whole speed/load range and with changes in altitude and temperature → higher fuel consumption and emissions; cannot maintain λ = 1 closed-loop control needed by three-way catalysts. • Poor atomisation at low speeds; fuel film on manifold walls (wall wetting) → poor transient response; icing; vapour lock; backfire risk; flooding when tilted; difficult cold starting.

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Advantages of Petrol Fuel Injection • Accurate metering of fuel by the ECU for every condition; better atomisation; uniform distribution to all cylinders (multipoint); higher volumetric efficiency (no venturi; manifold can be tuned for air flow only); better cold starting and throttle response; lower emissions and fuel consumption with closed-loop λ control; automatic altitude compensation; fuel cut-off on overrun (deceleration); no icing.

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Types of Petrol Injection Basis Types Injector location Single-point / throttle-body injection (TBI, central injection) — one or two injectors above the throttle valve; multipoint / port fuel injection (MPFI) — one injector per cylinder in the intake port, spraying on the back of the inlet valve; direct injection (GDI) — injector in the combustion chamber Injection timing Continuous (e.g., mechanical K-Jetronic); intermittent — simultaneous (all injectors together), grouped (banks), sequential (each injector timed to its cylinder's intake stroke — best) Control Mechanical (K-Jetronic); electronic — L-Jetronic (air-flow metering), D-Jetronic (manifold pressure / speed-density), Motronic (combined injection and ignition control) Feature Single-point (TBI) Multipoint (MPFI) Injectors One (or two) at the throttle body One per cylinder near inlet valve Manifold 'Wet' — carries fuel-air mixture 'Dry' — carries only air Distribution Similar to carburettor (less uniform) Uniform to all cylinders Cost / complexity Lower Higher Performance / emissions Better than carburettor Best of port systems; allows tuned manifolds Electronic Fuel Injection (EFI) System • Fuel system: tank → electric in-tank fuel pump (≈ 3–4 bar) → fuel filter → fuel rail → solenoid injectors; fuel-pressure regulator keeps a constant pressure difference across the injectors (returns excess fuel to tank, or returnless design).

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Injected quantity ∝ injector opening time (pulse width). • Air system: air filter → air-flow meter (hot-wire/hot-film MAF) or MAP sensor → throttle body with throttle position sensor (TPS) → idle-air control valve or electronic throttle (drive-by-wire) → intake plenum and runners. • Sensors to ECU:

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MAF/MAP (load), intake air temperature (IAT), engine coolant temperature (ECT) — cold enrichment, TPS — acceleration enrichment and full-load, crankshaft position (CKP) — speed and TDC, camshaft position (CMP) — cylinder identification for sequential injection, oxygen (λ) sensor in exhaust (zirconia switching or wideband), knock sensor (piezoelectric), vehicle speed. • Actuators: injectors, ignition coils, idle-air control/electronic throttle, fuel-pump relay, EGR valve, canister purge valve. • Open-loop control during cold start, warm-up, acceleration and full load; closed-loop control around λ = 1 at idle and cruise using O2-sensor feedback.

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Gasoline Direct Injection (GDI) • Fuel is injected directly into the combustion chamber at high pressure (≈ 50–200 bar, newer systems up to ≈ 350 bar) by a cam-driven high-pressure pump, common rail and solenoid or piezo injectors. • Homogeneous mode: injection during the intake stroke → uniform λ = 1 mixture at high load; fuel evaporating inside the cylinder cools the charge → higher volumetric efficiency and knock resistance → higher compression ratio (≈ 10.5–13). • Stratified-charge mode: late injection during compression → ignitable rich mixture near the spark plug while the overall mixture is very lean (λ up to ≈ 3); engine runs largely unthrottled → lower pumping loss, better part-load economy.

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Mixture guidance: wall-guided, air-guided, spray-guided. • Advantages: better fuel economy and torque, higher compression ratio, quick response, reduced wall wetting and better cold-start HC. • Disadvantages: higher particulate emissions (gasoline particulate filters needed), NOx in lean stratified operation (lean NOx trap), carbon deposits on intake valves (no fuel washing), costly high-pressure components, injector deposits and noise.

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Dual-injection systems (port + direct) reduce valve deposits.

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This section covers the requirements and working of ignition systems (battery-coil, magneto, electronic and distributorless), spark-advance mechanisms, and the combustion phenomenon in SI engines — ignition delay, flame propagation, pressure-crank angle diagram, abnormal combustion, auto-ignition, detonation and knocking, and the factors affecting them.

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Requirements of an Ignition System • Produce a high voltage (≈ 10–25 kV, up to ≈ 40 kV in modern systems) to jump the spark-plug gap under compression pressure (breakdown voltage rises with pressure and gap). • Deliver adequate spark energy and duration to ignite the mixture reliably, including lean or diluted mixtures. • Fire each cylinder in the correct firing order at the correct timing for every speed and load (spark advance). • Work reliably at all speeds (including cranking); long life, low maintenance, minimal radio interference. • Spark plug: centre electrode, ground electrode, alumina ceramic insulator, steel shell; gap ≈ 0.6–1.1 mm.

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Heat range: hot plug (long insulator nose, slow heat dissipation) for low-speed/light-load engines to burn off deposits; cold plug (short nose, rapid heat dissipation) for high-performance engines to avoid pre-ignition.

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Plug tip should run ≈ 450–850 °C (fouling below, pre-ignition above).

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Platinum/iridium tips give long life.

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Battery (Coil) Ignition System • Components: battery (12 V), ignition switch, ballast resistor, ignition coil (step-up transformer: primary ≈ 200–300 turns of thick wire, secondary ≈ 20,000 turns of fine wire), contact-breaker (CB) points opened by a cam on the distributor shaft (lobes = number of cylinders), condenser (capacitor) across the points, distributor (rotor and cap) running at camshaft speed (half crank speed), high-tension leads and spark plugs. • Working: when the points close, current flows in the primary and builds a magnetic field; when the cam opens the points, the field collapses rapidly and a high voltage is induced in the secondary by mutual induction; the rotor directs it to the correct spark plug. • Condenser: prevents arcing and burning of the CB points and speeds the collapse of the primary current, raising secondary voltage. • Dwell angle: cam angle during which the points remain closed (coil charging time).

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At high speeds dwell time falls, the coil does not saturate and spark voltage decreases. • Drawbacks: point wear and pitting, need for periodic adjustment, weak spark at high speed, limited primary current.

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Magneto Ignition System • Generates its own electrical energy — a permanent magnet rotates relative to a coil (rotating-magnet or rotating-armature type) — so no battery is required;

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CB points (or electronics), condenser and secondary winding produce the high voltage. • Spark voltage increases with engine speed: weak at cranking (harder starting) but strong at high speed. • Used in motorcycles and scooters (older), small engines (lawnmowers, chainsaws), racing engines and aircraft piston engines (dual magnetos for reliability).

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Feature Battery (coil) ignition Magneto ignition Source of current Battery Self-generated by magneto Spark at starting / low speed Good (strong spark) Weak — starting more difficult Spark at high speed Weakens (less dwell time) Improves Battery maintenance Required; fails if battery is flat Not required Feature Battery (coil) ignition Magneto ignition Space, cost Cheaper, more compact wiring Costlier magneto unit Typical use Cars, buses, trucks (older) Two-wheelers, small engines, aircraft, racing Electronic and Distributorless Ignition Systems • Electronic (breakerless / transistorised coil ignition, TCI): mechanical points replaced by a pulse generator — magnetic (variable reluctance: reluctor and pick-up coil), Hall-effect or optical sensor — which triggers a power transistor in the ignition module to switch the coil primary.

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Advantages: no point wear or adjustment, constant high spark energy at all speeds (electronic dwell control), higher voltage, better cold starting and emissions, reliability. • Capacitor discharge ignition (CDI): a capacitor is charged to ≈ 300–400 V (from a charging coil or DC-DC converter) and discharged through a thyristor (SCR) into the coil primary → very fast voltage rise, strong spark even with fouled plugs, short spark duration; widely used in two-wheelers, outboard and racing engines (AC-CDI, DC-CDI). • Programmed (digital) ignition / electronic spark advance: the ECU calculates timing from speed, load (MAP/MAF), temperature and knock-sensor feedback using stored spark maps — mechanical advance units are eliminated. • Distributorless ignition system (DIS): no distributor; the ECU fires coils directly using crank/cam position sensors.

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(i) Waste-spark system — one double-ended coil for each pair of cylinders whose pistons move together (e.g., 1 & 4, 2 & 3); both plugs fire together — one on compression (useful spark), the other on exhaust (wasted spark needing little voltage).

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(ii) Coil-on-plug (direct ignition) — an individual coil on each plug, fired sequentially. • DIS advantages: no rotor/cap wear or HT-lead losses, less radio interference, more accurate timing, longer dwell and higher energy, better reliability.

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Spark Advance Mechanisms • Combustion takes a roughly fixed time, so the spark must occur earlier (more degrees before TDC) at higher speed for peak pressure to occur at ≈ 10–15° after TDC (MBT — minimum advance for best torque).

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At part load the lean/diluted charge burns slower → more advance; at full load less advance (to avoid knock). • Centrifugal (mechanical) advance: flyweights in the distributor move outward with speed and rotate the cam ahead of the drive shaft → advance increases with engine speed. • Vacuum advance: a diaphragm connected to manifold/carburettor vacuum rotates the breaker plate against cam rotation → advance increases at part load (high vacuum) and reduces at full throttle. • Electronic spark advance:

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ECU maps with knock-sensor feedback (retard when knock is detected). • Initial (static) timing (e.g., ≈ 5–10° bTDC) is set with a timing light.

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Over-advance → knock, high NOx, loss of power; excessive retard → loss of power and economy, overheating and high exhaust temperature.

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Combustion Phenomenon in SI Engines • Stages (Ricardo):

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(I) ignition lag / preparation phase — from the spark to a perceptible pressure rise above the motoring curve; growth of the flame kernel (chemical process, depends on fuel, mixture strength — shortest for slightly rich — temperature, pressure, residual gas, turbulence);

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(II) flame propagation (main combustion) — a turbulent flame front sweeps the chamber, pressure rises rapidly to a peak ≈ 10–15° after TDC;

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(III) after-burning — combustion of the remaining charge near walls during expansion. • Flame speed: laminar ≈ 0.3–0.5 m/s; turbulent flame speed in engines is much higher and increases roughly in proportion to engine speed (turbulence ∝ speed), so combustion duration in crank degrees stays nearly constant. • Factors affecting flame speed: turbulence (most important — squish, swirl, tumble), mixture ratio (maximum ≈ 10% rich), intake temperature and pressure, compression ratio, engine speed, residual gas/EGR (dilution slows it), engine size (longer flame path), spark-plug location. • Pressure-crank angle (p-θ) diagram: shows the motoring curve (no combustion) and the firing curve; the firing curve departs from the motoring curve after the ignition lag, rises steeply during flame propagation and peaks after TDC.

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Over-advanced timing gives an early, very high peak (knock risk); retarded timing gives a late, low peak.

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Abnormal Combustion:

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Auto-Ignition, Detonation and Knocking • Auto-ignition: spontaneous ignition of a fuel-air mixture without a flame or spark when it is held above its self-ignition temperature for longer than its ignition delay. • Knock (detonation): after the spark, the unburned end gas ahead of the flame front is compressed and heated by the expanding burned gas and radiation; if it auto-ignites before the flame arrives, a very rapid pressure rise sets up pressure waves (≈ 5–10 kHz) → metallic 'pinking' noise. • Effects of knock: loss of power and efficiency, overheating, erosion of piston crown and ring lands, head-gasket and bearing damage, can trigger pre-ignition. • Surface ignition: ignition by hot surfaces (glowing carbon deposits, overheated plug or exhaust valve) — pre-ignition (before the spark) or post-ignition (after the spark but before flame arrival); also run-on (dieseling) and rumble.

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Knock is detected by a knock sensor (piezoelectric accelerometer).

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Factor Effect on knock tendency Temperature factors — higher compression ratio, supercharging, higher intake and coolant temperature, hot spots, spark advance INCREASE knock (end gas hotter) Density factors — higher intake pressure, wide-open throttle (full load) INCREASE knock Time factors — larger bore / longer flame path, off-centre spark plug, low turbulence, low engine speed INCREASE knock (more time for end gas to auto-ignite) Compact chamber, central spark plug, high turbulence (squish), cool end-gas region, higher engine speed REDUCE knock Composition factors — higher octane fuel, very lean or very rich mixture (knock is maximum near slightly rich), EGR, humidity, water injection REDUCE knock Control in practice Retard spark (knock sensor + ECU), high-octane fuel, lower CR, intercooling, EGR, combustion-chamber design

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This section covers CI engine fuels, the air induction system, diesel fuel-injection requirements and systems (in-line, distributor and unit injection pumps, injectors, nozzles and common-rail direct injection), the combustion phenomenon in CI engines, and types of combustion chambers.

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Fuels for CI Engines • Diesel (high-speed diesel, HSD) for automotive engines; light diesel oil (LDO) for slow/medium-speed stationary engines.

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Density ≈ 0.82–0.85 kg/L, LCV ≈ 42.5–43 MJ/kg. • Important properties: cetane number (ignition quality; typically 40–55) — high CN → short ignition delay; viscosity (too high → poor atomisation; too low → leakage and pump wear); volatility; cloud point, pour point, CFPP (cold-weather waxing and filter blocking); flash point (safe storage); sulphur content (ultra-low-sulphur diesel ≤ 10–15 ppm for modern after-treatment); carbon residue, ash, water and sediment; lubricity (protects injection pumps). • Alternatives: biodiesel (FAME;

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CN ≈ 50–60), straight vegetable oils (high viscosity), DME, HVO/GTL (paraffinic diesels).

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Air Induction System • Air cleaner (dry paper element; oil-bath type for very dusty conditions — tractors, earth-moving machines), intake manifold, turbocharger and intercooler (in most modern diesels). • CI engines have no throttle for load control (quality governing) — only the fuel quantity is varied. • Helical/directed intake ports create induction swirl for fuel-air mixing; glow plugs or intake-air heaters assist cold starting (especially IDI engines).

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Requirements of Diesel Fuel Injection • Meter the correct quantity of fuel per cycle according to load and equally to every cylinder; time the start and end of injection correctly; control the rate of injection (to control rate of pressure rise); atomise fuel into fine droplets; ensure adequate penetration and distribution in the chamber air (matched to air swirl); sharp beginning and cut-off with no dribbling or secondary injection.

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Fuel Injection Systems System Features Use Air injection Fuel blasted in by compressed air (Rudolf Diesel's original) Obsolete Individual pump / in-line (jerk) pump One plunger-and-barrel element per cylinder in a common housing, driven by a cam at half crank speed; high-pressure pipes of equal length to each injector Heavy-duty trucks, buses, tractors — robust Distributor (rotary) pump Single pumping element with a rotating distributor feeds all cylinders in turn (e.g., Bosch VE, Lucas/CAV DPA) Small multi-cylinder car and light-vehicle diesels — compact, light, cheaper Unit injector Pump and nozzle combined in one unit in the cylinder head, operated by an engine camshaft/rocker; no high-pressure pipes; very high pressure (≈ 2000 bar) Heavy trucks, some car engines (pump-düse) Common rail (CRDI) High-pressure pump feeds an accumulator rail; electronically controlled injectors Modern cars, SUVs, trucks • Jerk-type (Bosch) pump element: plunger with a helical groove (helix) and vertical slot reciprocates with a constant stroke in a barrel with inlet/spill ports.

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Delivery begins when the plunger top closes the ports and ends when the helix uncovers the spill port.

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Quantity is varied by rotating the plunger through a control rack and pinion/sleeve (changing effective stroke); zero delivery when the vertical slot faces the spill port. • Delivery valve (with retraction piston): gives sharp cut-off, prevents after-dribble and keeps residual pressure in the high-pressure line. • Governor (mechanical centrifugal — idle-and-maximum-speed or all-speed; pneumatic; electronic) moves the control rack; automatic timing advance device advances injection with speed. • Low-pressure side: fuel tank, feed (lift) pump, primary and secondary filters with water separator, priming pump, bleed screws, leak-off (return) lines.

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Fuel Injector and Nozzle Types • Injector (nozzle holder): body, pressure spring (sets opening/injection pressure — ≈ 150–250 bar in mechanical DI systems, lower for pintle nozzles; adjusted by shims/screw), spindle, nozzle needle valve and nozzle body.

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Fuel pressure acting on the needle's differential area lifts it against the spring → spray; when pressure falls, the spring snaps the needle shut.

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Leak-off fuel lubricates the needle and returns to tank.

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Nozzle Features Application Single-hole One orifice; narrow spray Early open-chamber engines Multi-hole 4–10 small holes (≈ 0.1–0.2 mm); high injection pressure; good distribution with little swirl Direct-injection (open chamber) engines Pintle Pin projects through the hole → hollow-cone spray; self-cleaning; lower opening pressure Indirect-injection (swirl/pre-chamber) engines Pintaux Pintle with an auxiliary hole that sprays toward the hot chamber at cranking Easy cold starting in IDI engines Solenoid / piezo electronic injectors ECU-controlled; piezo is faster — more, smaller injections Common-rail systems Common Rail Direct Injection (CRDI) • Components: low-pressure feed pump and filters → engine-driven high-pressure radial-piston pump → common rail (accumulator storing fuel at ≈ 1000–2500 bar, damping pressure pulsations) with rail-pressure sensor and pressure-control/metering valve → high-pressure lines → solenoid or piezo injectors;

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ECU with crank/cam, accelerator-pedal, MAF, boost and temperature sensors; return line. • Key feature: injection pressure is independent of engine speed and load, and injection timing and quantity are fully flexible. • Multiple injections per cycle: pilot (small pre-injection — shortens the delay period of the main injection → lower noise, knock and NOx), main, post (soot oxidation, DPF regeneration). • Advantages: quieter and smoother running, lower PM and NOx, better fuel economy, higher low-speed torque, easy compliance with Euro/BS emission norms.

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Disadvantages: high cost, sensitivity to fuel contamination (water, dirt), complex diagnostics.

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Combustion Phenomenon in CI Engines Stage Description 1.

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Ignition delay period From start of injection to start of combustion.

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Physical delay (atomisation, vaporisation, mixing) + chemical delay (pre-flame reactions).

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Typically ≈ 1 ms (≈ 10–20° crank angle) 2.

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Rapid (uncontrolled / premixed) combustion Fuel accumulated during the delay burns almost at once → steep pressure rise.

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The longer the delay, the more fuel accumulates → violent pressure rise = diesel knock 3.

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Controlled (mixing-controlled / diffusion) combustion Fuel burns as it is injected; rate controlled by injection rate and air-fuel mixing; maximum pressure and temperature reached 4.

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After-burning Remaining fuel burns during expansion after injection ends; excessive after-burning causes smoke and efficiency loss • Pressure-crank angle diagram: firing curve departs from the motoring curve after the delay, shows a steep rise during rapid combustion, then a flatter region during controlled combustion; peak pressures in CI engines (≈ 60–150+ bar) are higher than in SI engines. • Factors affecting delay period: compression ratio (higher → shorter delay), intake air temperature and pressure (higher/supercharging → shorter), cetane number (higher → shorter), injection timing (too early injection into cooler air → longer delay), load (higher load → hotter walls → shorter delay), engine speed (delay in crank degrees increases with speed), atomisation and injection pressure, air swirl, coolant temperature, altitude (lower pressure → longer delay). • Diesel knock occurs at the START of combustion (long delay), whereas SI knock occurs at the END (end-gas auto-ignition).

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To reduce CI knock: high cetane fuel, higher compression ratio, higher intake temperature and pressure (supercharging), optimum injection timing, pilot injection, reduced initial injection rate, good atomisation and swirl — broadly the OPPOSITE of the measures for SI knock.

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Types of CI Combustion Chambers Chamber Features Merits / demerits Direct injection (DI) / open chamber Combustion bowl in piston crown (toroidal/'Mexican hat', re-entrant bowl); multi-hole nozzle; high injection pressure; quiescent or swirl-supported Higher efficiency (≈ 10–15% better sfc), easy cold starting (no glow plug usually), simpler head / noisier, needs high injection pressure, more fuel-sensitive Swirl chamber (IDI) — e.g., Ricardo Comet Spherical chamber in head (≈ 50–70% of clearance volume) connected tangentially → strong compression swirl; pintle nozzle Smooth, quiet, good mixing, high speed / higher heat loss, poorer economy, needs glow plugs, CR ≈ 20–24 Pre-combustion chamber (IDI) Small pre-chamber (≈ 25–40% of clearance volume) with narrow throat; partial combustion ejects burning mixture into main chamber Tolerant of fuels, smooth / high heat and throttling losses, hard cold start Air-cell / energy cell (Lanova) Separate air cell in head or piston Smooth combustion / largely obsolete M-system (MAN) Fuel sprayed on hot spherical piston-bowl wall as a film; evaporates progressively Quiet, multi-fuel / higher HC at light load • Air motion in CI chambers: swirl (rotation about cylinder axis — induction swirl via helical ports, compression swirl in swirl chambers), squish (radial inward flow as piston approaches the head), tumble.

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This section covers types of lubricants and their properties, SAE rating of lubricants, types of lubrication systems, the necessity of engine cooling, air and liquid cooling systems and their comparison, and the importance and types of coolants.

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Types of Lubricants and Their Properties • Types: mineral oils (paraffinic, naphthenic), synthetic oils (PAO, esters — better viscosity index, high-temperature stability and cold flow), semi-synthetic blends, vegetable/animal oils (e.g., castor oil in early racing), greases (oil + soap thickener, e.g., lithium grease for wheel bearings and chassis), solid lubricants (graphite, MoS2). • Properties: viscosity (most important — measured by capillary/Redwood/Saybolt viscometers), viscosity index (less change with temperature is better), flash and fire point, pour and cloud point, oiliness/lubricity, oxidation and thermal stability, detergency and dispersancy (keep soot and sludge in suspension), TBN (total base number — neutralises acids formed from fuel sulphur; higher for diesel oils), carbon residue, sulphated ash, foaming resistance, corrosion protection. • Additives: viscosity-index improvers (for multigrades), detergents, dispersants, anti-wear (ZDDP), extreme-pressure (gear oils), antioxidants, corrosion/rust inhibitors, pour-point depressants, anti-foam agents, friction modifiers.

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SAE Rating and Other Classifications • SAE J300 engine-oil viscosity grades: winter grades 0W, 5W, 10W, 15W, 20W, 25W (limits on low-temperature cranking and pumping viscosity — lower number = better cold flow) and hot grades SAE 20, 30, 40, 50, 60 (kinematic viscosity at 100 °C and high-temperature high-shear viscosity). • Multigrade oil, e.g., SAE 15W-40: behaves like 15W when cold (easy starting) and like SAE 40 at 100 °C (adequate film at operating temperature) — achieved with VI-improver polymers.

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Modern cars use thin oils such as 0W-20 or 5W-30 for fuel economy. • Gear oils:

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SAE J306 (75W-90, 80W-90, 85W-140).

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API service classes: 'S' for petrol engines (SJ … SN, SP), 'C' for diesel engines (CF … CI-4, CK-4);

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ACEA (Europe), JASO MA/MA2 (motorcycles with wet clutches), ILSAC.

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Types of Lubrication Systems System Working Use Petroil / mist Oil mixed with petrol (≈ 1 :

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50) or injected separately (auto-lube); oil mist lubricates crankcase parts Two-stroke engines Splash Dippers on big-end caps splash oil from the sump Small single-cylinder and older engines Pressure (forced-feed) Gear/rotor pump draws oil through a strainer and sends it via the filter to the main gallery → main bearings → drilled crankshaft → big ends; also camshaft, rockers, turbocharger; cylinder walls by splash/jets; piston-cooling jets in turbo diesels Most automobile engines Semi-pressure Pressure to main parts, splash to others Some medium engines Dry sump Scavenge pumps return oil to a separate tank; pressure pump feeds engine Racing, off-road, aircraft (no oil starvation, lower engine height) • Components: sump, pick-up strainer, oil pump (gear, rotor/gerotor, vane), pressure-relief valve, oil filter — full-flow (all oil filtered; bypass valve opens when clogged or cold) or bypass (part of flow through a fine filter); oil cooler, pressure gauge/warning lamp, dipstick, PCV system.

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Necessity of Engine Cooling • Combustion gas temperatures reach ≈ 2000–2500 °C; about 25–35% of the fuel energy must be removed to keep cylinder walls, piston, head and valves within safe limits. • Cooling is needed to: maintain the lubricating oil film (oil breaks down at high wall temperature); prevent thermal stresses, distortion and cracking; prevent piston seizure; avoid detonation and pre-ignition in SI engines; prevent valve burning; maintain volumetric efficiency; preserve material strength (aluminium alloys weaken above ≈ 200 °C). • Over-cooling is also harmful: lower thermal efficiency, poor fuel vaporisation and oil dilution, corrosive wear from condensed acids and water, sludge formation, higher friction (cold viscous oil), higher HC/CO emissions and longer warm-up.

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Optimum coolant temperature ≈ 85–95 °C (higher under pressure in modern engines).

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Types of Cooling Systems and Their Comparison • Air cooling: fins on cylinder and head increase surface area; air flow from vehicle motion or a fan/blower with cowling and baffles; fins usually aluminium.

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Used in motorcycles, scooters, small engines, aircraft piston engines, older cars (VW Beetle). • Liquid (water) cooling: water jackets around cylinders and head; thermosyphon (natural circulation by density difference, no pump — old engines) or forced (pump) circulation — centrifugal water pump, radiator (core of tubes and fins; down-flow or cross-flow; tubular or cellular/honeycomb; aluminium or copper-brass), fan (belt-driven, viscous-coupling or thermostatically switched electric fan), thermostat (wax-pellet valve opening at ≈ 80–95 °C; bypass for quick warm-up), pressure cap (pressure valve raises the boiling point to ≈ 120 °C; vacuum valve admits coolant back from the expansion tank on cooling), expansion/overflow tank, hoses, temperature sensor, heater core.

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Feature Air cooling Liquid cooling Construction / weight Simple, light, fewer parts Complex, heavier (radiator, pump, coolant) Cost and maintenance Low; no coolant leakage or freezing Higher; leaks, freezing, corrosion possible Cooling efficiency / uniformity Lower, less uniform (hot spots) Higher, uniform — allows higher compression ratio and output Warm-up Quick Slower (thermostat helps) Noise Noisier (fins ring, no water jacket damping) Quieter Engine size Limited to small engines Any size Dependence Depends on air flow and ambient temperature Independent of vehicle speed (pump, fan) Importance and Types of Coolant • Water alone has high specific heat (4.18 kJ/kg·K) but freezes at 0 °C (expansion can crack the block), boils at 100 °C, and causes corrosion and scale. • Coolant = water + antifreeze + inhibitors: ethylene glycol (most common) or less-toxic propylene glycol, usually 50 :

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50 with water — lowers freezing point (≈ −35 to −37 °C) and raises boiling point; corrosion and cavitation inhibitors (protect aluminium, cast iron, solder, and wet liners from cavitation pitting), anti-foam agents, dye; distilled/de-ionised water recommended. • Functions of coolant: heat transfer, freeze protection, boil-over protection, corrosion and scale prevention, cavitation protection, lubrication of the water-pump seal. • Types by inhibitor technology:

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IAT (inorganic additive technology — silicates/phosphates; traditional, short life ≈ 2 years), OAT (organic acid technology — long-life, ≈ 5 years), HOAT (hybrid organic acid technology) and variants (P-HOAT, Si-OAT).

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Different types should not be mixed; follow manufacturer's specification.

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Alcohol-based antifreezes (methanol) are obsolete (low boiling point, flammable).

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This section covers the objectives, limitations, methods and types of supercharging and turbocharging with their arrangements, and engine exhaust emissions — formation of NOx, HC, CO and particulate matter, constituents of exhaust, and their harmful effects on the environment and human health.

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Objectives and Limitations • Supercharging: increasing the density of the intake charge above atmospheric by compressing it before it enters the cylinder.

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Engine power is roughly proportional to the mass of air inducted per unit time. • Objectives: increase power output of a given engine (higher power-to-weight and power-to-volume) — enables downsizing; compensate for power loss at high altitude (air density falls ≈ 1% per 100 m — important for hill roads and aircraft); improve volumetric efficiency; in CI engines, better combustion, shorter ignition delay (smoother, less knock), lower smoke and better fuel economy (turbocharged). • Limitations: in SI engines increased knock tendency (requires lower compression ratio, higher-octane fuel, intercooling, spark retard); higher mechanical and thermal loads (stronger pistons, bearings, gaskets and better cooling needed); higher NOx; mechanical superchargers consume engine power (parasitic loss); turbo lag; higher cost and complexity; high-speed turbo bearings need good lubrication and cooling.

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CI engines are well suited to supercharging (it reduces diesel knock).

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Methods and Types Method Types / features Mechanically driven supercharger (belt/gear from crankshaft) Roots blower (two/three-lobe rotors; no internal compression; strong low-speed boost; noisy, less efficient at high pressure ratio); twin-screw (Lysholm) (internal compression, efficient); vane type; centrifugal supercharger (boost rises with speed²).

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Immediate response but consumes engine power Turbocharger Exhaust-driven radial-inflow turbine and centrifugal compressor on a common shaft (speeds up to ≈ 200,000 rpm); uses waste exhaust energy → better overall efficiency; suffers turbo lag.

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Boost control by wastegate (bypasses exhaust around turbine) or variable-geometry turbine (VGT/VNT) with adjustable nozzle vanes (common in diesels — reduces lag); twin-scroll turbine housing separates exhaust pulses of cylinder pairs Electric supercharger / e-booster Electric-motor-driven compressor (often 48 V) — instant boost at low speed; electric-assisted turbochargers (e-turbo) Pressure-wave supercharger (Comprex) Exhaust gas compresses intake air directly in rotor cells Ram / tuned-intake and resonance charging Uses pressure waves in tuned intake pipes — no compressor • Constant-pressure turbocharging: exhaust from all cylinders into a large manifold → steady pressure at turbine; good at high load (large marine/stationary diesels).

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Pulse turbocharging: short narrow manifolds deliver exhaust pulses (blow-down energy) to the turbine; better at part load and transients — used in automobiles. • Turbo lag: delay in boost build-up due to rotor inertia and low exhaust energy at low speed — reduced by smaller/lighter turbines, VGT, twin-scroll, sequential turbos, electric assist. • Intercooler (charge-air cooler) — air-to-air or air-to-water — cools compressed air → higher density, lower knock and NOx.

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Blow-off / diverter valve releases boost when the throttle closes suddenly (prevents compressor surge).

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Turbo care: let the engine idle briefly before shutdown; use good-quality oil.

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Arrangements of Turbochargers and Superchargers Arrangement Description Single turbocharger (with intercooler) Most common in cars and trucks Parallel twin turbos Two identical turbos, one per cylinder bank (V engines) — smaller turbos spool faster Sequential twin turbos Small turbo works at low speed; second turbo joins at high speed — reduces lag and extends boost range Two-stage (series) turbocharging Low-pressure and high-pressure turbos in series — very high boost (heavy diesels, high-performance diesels) Twin-charging (compound) Mechanical supercharger for low-speed response + turbocharger for high speed (e.g., VW TSI twincharger) Turbo + electric compressor Electric booster fills in low-speed boost Turbo-compounding Additional power turbine in the exhaust geared to the crankshaft (heavy trucks, aircraft) Engine Exhaust Emissions:

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Constituents • Exhaust mainly contains harmless N2 (≈ 70–75%), CO2 and H2O (≈ 10–13% each in SI exhaust), and O2 (lean-burn and diesel engines have large excess O2). • Pollutants (about 1% or less of SI exhaust): carbon monoxide (CO), unburnt hydrocarbons (HC), oxides of nitrogen (NOx) — mostly NO with some NO2 — particulate matter (PM) (mainly diesel and GDI), SO2, aldehydes, and formerly lead.

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CO2 is a greenhouse gas proportional to fuel burned. • SI engines emit mainly CO, HC and NOx;

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CI engines emit mainly NOx and PM (CO and HC are low due to excess air).

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Other SI sources: crankcase blow-by (HC) and fuel evaporation (HC).

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Formation of Pollutants Pollutant Formation mechanism Influencing factors CO Incomplete oxidation of carbon due to lack of oxygen (rich mixture); also dissociation of CO2 at high temperature and poor mixing Strongly dependent on A/F — rises sharply with rich mixtures (cold start, full load); low in diesels HC Unburnt/partially burnt fuel from flame quenching at cold walls, crevice volumes (piston top-land and ring crevices — major source), absorption in oil films and deposits, misfire/incomplete combustion, valve overlap and short-circuiting in two-strokes; in diesels — over-lean spray edges, nozzle-sac fuel High at cold start, idle, very lean or rich mixtures, misfire, high EGR, two-stroke engines NOx Mainly thermal NO:

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N2 and O2 react at high temperature (above ≈ 1800–2000 K) — Zeldovich mechanism; also prompt and fuel NO;

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NO2 forms in cooler regions (significant in diesels) Increases exponentially with peak temperature, availability of O2 and residence time; maximum slightly LEAN of stoichiometric (λ ≈ 1.05–1.1); more with spark advance, high CR, high load; reduced by EGR, retard, water injection PM (partic ulates) Soot (carbon) formed in fuel-rich zones of the diesel diffusion flame by pyrolysis, partly oxidised later; plus soluble organic fraction (unburnt fuel and lube oil), sulphates (from fuel sulphur) and ash Over-fuelling, poor atomisation, air shortage, low swirl, high sulphur fuel; black smoke — soot, blue smoke — burning oil, white smoke — unburnt fuel/water at cold start Harmful Effects on Environment and Human Health Pollutant Health effects Environmental effects CO Binds with haemoglobin ≈ 200–250× more strongly than O2 (carboxyhaemoglobin) → headache, dizziness, impaired judgement, death in enclosed spaces Contributes to ozone formation HC / VOC Eye and respiratory irritation; some carcinogenic (benzene, PAH, formaldehyde) With NOx and sunlight → photochemical smog, ground-level ozone, PAN NOx NO2 irritates lungs, aggravates asthma, lowers resistance to infection Acid rain (nitric acid), photochemical smog and ozone, secondary nitrate particles, eutrophication PM (PM10, PM2.5) Penetrates deep into lungs and bloodstream → respiratory and cardiovascular disease, lung cancer (diesel exhaust is classified as a Group 1 human carcinogen by IARC) Reduced visibility, soiling of buildings; black carbon adds to global warming and glacier melting SOx Respiratory irritation Acid rain; sulphate particles; poisons catalysts CO2 — Greenhouse gas → global warming and climate change Lead (old leaded petrol) Neurotoxic, especially to children Soil and water contamination; destroys catalytic converters • Vehicles are a major source of PM2.5, NOx and CO in Kathmandu Valley.

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Controls (see ME 9.4): engine design, EGR, three-way catalyst (λ ≈ 1), DOC, DPF, SCR with urea, gasoline particulate filters, cleaner fuels (unleaded, ultra-low-sulphur), emission norms (Euro/Bharat Stage;

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Nepal's vehicle emission standards) and emission testing (gas analysers for petrol vehicles, smoke opacity meters for diesels).