IGNITION SYSTEMS FOR IC ENGINES-NOTES

Ignition Systems

Energy requirements for ignition

1) A spark energy below 10 millijoules for A/F 13:1 to initiate combustion
2) Duration of few micro seconds
3) Critical or breakdown voltage – below which no sparking would occur
4) For automotive engines the spark energy of 40 millijoules and duration of 0.5
millisecond is sufficient

Requirements

1) Adequate storage of secondary voltage & ignition energy
2) Should consume minimum power
3) Conversion efficiency should be high
4) Sufficiency spark duration
5) Ability to produce an ignition spark when a short
6) Good performance at high speeds
7) Longer life of breaker points & spark plug
8) Good starting
9) Good reproducibility of secondary voltage rise & maximum rise
10) Adjustment of spark advance with speed & load

Basic sources of electrical energy

Battery & Generator – 6 V to 12 V DC current
Magneto – AC current at high voltage

Ignition systems

1) Battery ignition system
a. Conventional
b. Transistor assisted

2) Magneto ignition system
a. Low tension
b. High tension

3) Electronic ignition system







Battery or Coil Ignition System

Construction

1) Battery
2) Ignition switch
3) Induction coil
4) Circuit / contact breaker
5) Condenser
6) Distributor

Advantages

1) Better sparks at low speeds, starting & for cranking purposes.
2) Initial cost of the system is low
3) Reliable
4) Maintenance required is negligible
5) High speed engine drive is simpler than magneto drive
6) Adjustment of spark timing has no effect

Disadvantages

1) with increasing speed sparking voltage drops
2) Battery – unreliable component


Magneto ignition system

Advantages

1) More reliable
2) Suitable for medium and very high speed engines
3) Very light & compact units can be made

Disadvantages

1) Voltage is very low at low speeds
2) Spark timing adjustment create effects on spark voltage
3) Burning of the electrodes at high engine speeds


Firing 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 cycle
Factors affecting ignition timing

1) compression ratio
2) engine speed
3) mixture strength
4) CC design
5) Throttle opening
6) Engine tempr
7) Type of fuel

Spark Advance

1) vacuum spark advance
2) centrifugal spark advance

spark plug : it has to conduct the high potential from the ignition system into the combustion chamber

Requirements of a spark plug

1) must withstand high pressures up to atleast 55 bar
2) 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 operations
4) Should give maximum resistance to erosion burning away of the spark points irrespective of nature of fuel
5) Should have high heat resistance to avoid preignition of the charge
6) Insulating material must withstand the chemical reaction effects
7) Gas tight joints between insulator and metal parts are essential

Factors affecting establishment of the arc across the air gap

1) air gap length
2) gap geometry
3) mixture density
4) electrode temperature
5) insulator’s leakage resistance
6) voltage increase rate at the gap
7) F/A ratio
8) Electrode material
9) Pressure of ionized gases in the gap



Limitations of conventional ignition

1) frequent servicing & replacement of contact points
2) weaker sparks
3) misfiring
4) inefficient at low speeds
5) poor starting ability
6) poor reproducibility of the secondary voltage rise, peak voltage & firing time

Electronic ignition system

1) Transistorised coil ignition (TCI) system

Advantages

1) reduced wear of the components
2) burning lean mixtures no problem
3) reduced maintenance
4) increased reliability
5) increased spark plug life

2) Capacitive discharge ignition (CDI) system

Advantages

1) 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 increased
4) increased low speed efficiency & easier cold starting
5) less chances of misfiring