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

KNOW YOUR ENGINE

Supercharging

Purpose

1) To increase power output
2) To maintain power output of aircraft
3) To reduce weight of an engine per unit power developed such as aircraft & racing cars
4) To reduce bulk of an engine to fit in limited space in marine & locomotive

Factors increasing power output

1) Amount of air taken in cylinder
2) Utilization of this air
3) Speed of engine (limit of friction)
4) Quantity of fuel taken
5) Combustion
6) Thermal efficiency of engine

Supercharging – forced convection – due to higher pressure more air or A/F mixture will be forced in cylinder.

Application

1) Racing cars
2) Marine engines
3) Automotive engines
4) Engines at high altitude

Types

1) Centrifugal type
2) Axial flow
3) Roots blower
4) Vane blower

Basic types

1) Mechanically driven (crankshaft driven)
2) Turbocharger




Effects of supercharging

Positive

1) Higher power output
2) Intake mass of charge increases
3) Better atomization of fuel
4) Better mixing of fuel & air
5) Better scavenging
6) Better torque characteristics
7) Quicker acceleration
8) Complete & smoother combustion
9) Poor fuel can be used
10) Reduction in diesel knock
11) Improved cold starting
12) Reduced exhaust smoke
13) Reduced SFC in turbocharging
14) Mechanical efficiency increased

Negative

1) Detonation increased in S I
2) Increase in thermal stresses
3) Increase in heat losses due to turbulence
4) Gas loading will increase
5) Valve overlap will increase
6) Increased cooling requirements of piston & valves

Limitations

1) increases in thermal load (stresses)
2) higher stresses will develop
3) require heavier engine components
4) increase in heat generation
5) increase in heat transfer
6) exhaust valve seat damages piston crown & exhaust valve *

Arrangements

1) Compressor driven by engine
2) Compressor & turbine driven by exhaust
3) Compressor & turbine driven by exhaust as well as by engine
4) Compressor driven by full power of engine




Methods of turbocharging

1) constant pressure turbocharging

Advantages

1) At higher pressure ratio efficient
2) Better fuel consumption
3) At constant pressure & temperature turbine runs at high efficiency
4) Exhaust piping is simple
5) Engine speed is not limited by the pressure waves in exhaust pipes

Disadvantages

1) pressure requires to keep constant at exhaust
2) due to proper scavenging hence more diameter pipe requires
3) response to load change is poor
4) scavenging is little difficult
5) at part load efficiency of turbine reduces

2) Pulse turbocharging

Advantages

1) recovery of exhaust blowdown is efficient
2) rapid acceleration at higher speed
3) space is less
4) better scavenging

Disadvantages

1) complicated inlet & outlet exhaust pipe
2) poor efficiency *