LECTURE NOTES : IC ENGINE ,IGNITION SYSTEM

Ignition SystemsEnergy requirements for ignition1) A spark energy below 10 millijoules for A/F 13:1 to initiate combustion2) Duration of few micro seconds3) Critical or breakdown voltage – below which no sparking would occur4) For automotive engines the spark energy of 40 millijoules and duration of 0.5 millisecond is sufficient Requirements1) Adequate storage of secondary voltage & ignition energy 2) Should consume minimum power3) Conversion efficiency should be high4) Sufficiency spark duration5) Ability to produce an ignition spark when a short6) Good performance at high speeds7) Longer life of breaker points & spark plug8) Good starting9) Good reproducibility of secondary voltage rise & maximum rise10) Adjustment of spark advance with speed & loadBasic sources of electrical energyBattery & Generator – 6 V to 12 V DC currentMagneto – AC current at high voltageIgnition systems1) Battery ignition systema. Conventionalb. Transistor assisted2) Magneto ignition systema. Low tensionb. High tension3) Electronic ignition systemBattery or Coil Ignition SystemConstruction1) Battery 2) Ignition switch3) Induction coil4) Circuit / contact breaker5) Condenser6) DistributorAdvantages1) Better sparks at low speeds, starting & for cranking purposes.2) Initial cost of the system is low3) Reliable4) Maintenance required is negligible5) High speed engine drive is simpler than magneto drive6) Adjustment of spark timing has no effect Disadvantages1) with increasing speed sparking voltage drops2) Battery – unreliable componentMagneto ignition systemAdvantages1) More reliable2) Suitable for medium and very high speed engines3) Very light & compact units can be madeDisadvantages1) Voltage is very low at low speeds2) Spark timing adjustment create effects on spark voltage3) Burning of the electrodes at high engine speedsFiring order : it is the order in which various cylinders of a multi cylinder engine fire.Ignition timing : is the correct instant for the introduction of spark near the end of compression stroke in the cycleFactors affecting ignition timing1) compression ratio2) engine speed3) mixture strength4) CC design5) Throttle opening6) Engine tempr7) Type of fuelSpark Advance1) vacuum spark advance2) centrifugal spark advancespark plug : it has to conduct the high potential from the ignition system into the combustion chamberRequirements of a spark plug1) must withstand high pressures up to atleast 55 bar2) to prevent short circuiting it must provide suitable insulation between two electrodes.3) Must be capable of withstanding high temperatures of 2000 to 2500 deg cel. over long periods of operations4) Should give maximum resistance to erosion burning away of the spark points irrespective of nature of fuel5) Should have high heat resistance to avoid preignition of the charge 6) Insulating material must withstand the chemical reaction effects7) Gas tight joints between insulator and metal parts are essentialFactors affecting establishment of the arc across the air gap1) air gap length2) gap geometry3) mixture density4) electrode temperature5) insulator’s leakage resistance6) voltage increase rate at the gap7) F/A ratio8) Electrode material9) Pressure of ionized gases in the gapLimitations of conventional ignition1) frequent servicing & replacement of contact points2) weaker sparks3) misfiring4) inefficient at low speeds5) poor starting ability6) poor reproducibility of the secondary voltage rise, peak voltage & firing timeElectronic ignition system1) Transistorised coil ignition (TCI) systemAdvantages1) reduced wear of the components2) burning lean mixtures no problem3) reduced maintenance4) increased reliability5) increased spark plug life2) Capacitive discharge ignition (CDI) systemAdvantages1) capacitor stores several thousand times energy than in TCI system hence permits for high output voltage even at high spark rates (speeds)2) internal resistance is very small hence faster voltage rise is obtained 3) frequent maintenance is avoided hence life of contact points increased4) increased low speed efficiency & easier cold starting 5) less chances of misfiring