2. Chapter 8
Ignition Systems

3. Objectives
Explain the operation of mechanical breaker ignition (MBI) system.
Explain the operation of silicon-controlled rectifier (SCR) ignition systems.
Explain the operation of capacitor discharge ignition (CDI) systems.

4. Objectives (Cont.) Understand variable slope timing.
Identify the parts of a spark plug.
Recognize the types of switches commonly used in handheld two-stroke outdoor power equipment.

5. Ignition System
Ignition systems produce spark to ignite air-fuel mixture in cylinder
Emission controlled engines have variable ignition timing
Electronic ignition systems produce electricity to make spark and run microprocessor

6. Technicians and Ignitions
Modern technician must know about
Common electrical principles
Some basic electrical components
How electricity produces and times spark

7. How Ignition Systems Work
Ignition system creates spark
Induction from rotating magnet builds magnetic field in ignition coil
Older systems used points and condenser system

8. Points and Condenser System
Breaker points, condenser, and ignition coil
Also called mechanical breaker ignition (MBI)
Points
Mechanical contacts on switch
Connect coil’s primary winding to ground
Condenser
Electron storage device that protects points

9. Points Open and Closed

10. Ignition Coil Windings around iron core Primary winding
About 150 turns of heavy wire
Secondary winding
80–100 times more wire turns than primary
Very fine (small diameter)

11. Ignition Coil Assembly

12. Sequence of Ignition Events
Flat spot on crankshaft allows breaker points to close, completing circuit
Flywheel magnet approaches coil
Current induced by magnet flows through points and primary winding to ground
Builds magnetic field around primary winding

13. Sequence of Ignition Events (Cont.)
Points closed

14. Sequence of Ignition Events (Cont.)
Crankshaft flat spot ends and crankshaft opens points
Circuit through primary winding breaks
Magnetic field around primary collapses, inducing current in secondary winding
Low voltage is converted to high voltage
Difference in number of wire turns in windings

15. Sequence of Ignition Events (Cont.)
Peak voltage

16. Sequence of Ignition Events (Cont.)
Additional voltage in secondary winding
Condenser releases electrons to increase primary winding magnetic field
South end of magnet reverses current flow in primary winding
Voltage in secondary winding is high enough to jump across air gap of spark plug
Spark ignites air-fuel mixture

17. Sequence of Ignition Events (Cont.)
Spark jumps gap

18. Kill Switch/Stop Switch
Spark is stopped to stop engine
Operator closes switch that grounds primary winding
Switch bypasses points
Magnetic field in primary winding cannot collapse
High voltage is no longer created in secondary winding

19. Toggle Switch

20. Stop Switch Designs

21. Momentary Switch Spring-loaded Remains in one position until pressed
Reverts back to that position after release
Connected to set of circuits that manage engine ignition

22. Mechanical Breaker Point Limitations
No way to adjust spark timing
Limits power and economy
Violates emission laws
Points wear out

23. Electronic Ignition Systems
Coil design is same as point-type systems
Primary winding induces high voltage in secondary winding
Electronic components manage current flow
Timing can be adjusted

24. SCR Ignition System
Magnetic field from rotating flywheel passes through trigger coil
Voltage is induced in coil
Voltage signal goes to silicon-controlled rectifier (SCR)
SCR allows electrons to flow when it receives correct polarity voltage

25. SCR Ignition System (Cont.)
SCR turns on when it receives voltage
Allows current to pass to one of two transistors
Transistor
Acts as switch (makes electrical connection)
Acts as amplifier (multiplies voltage)

26. SCR Ignition System Diagram

27. First and Second Transistors
First transistor switches on
Multiplies voltage
Sends voltage to second transistor
Second transistor switches on
Completes circuit through primary winding

28. SCR Current Interruption
Turning flywheel magnet sends opposite polarity magnetic field to trigger coil
Trigger coil sends opposite polarity pulse to SCR
SCR stops signal voltage to first transistor
First transistor turns off, turning off second transistor

29. SCR Current Interruption (Cont.)
Second transistor opens primary winding circuit
Primary winding magnetic field collapses
High-voltage is induced in secondary winding
High-voltage spark is created

30. TCI System Transistor controlled ignition (TCI)
Transistors in ignition module create normally closed circuit to primary winding
Flywheel magnetic field cuts through primary winding
Voltage is created in winding

31. TCI System (Cont.)
When voltage reaches 200 volts, primary winding circuit is opened
Magnetic field around primary winding collapses through secondary winding
Induces voltage and creates spark
TCI circuit action is advanced or retarded, depending on engine speed

32. TCI System Diagram

33. CDI System Capacitor discharge ignition (CDI)
Capacitor stores electrons
Turning flywheel causes magnetic fields to pass through exciter coil
Exciter coil sends electrons to capacitor
Capacitor charges and voltage builds in circuit

34. CDI System (Cont.) When voltage is high enough, it turns on SCR
SCR completes circuit between capacitor and primary winding
Electrons from capacitor discharge through primary winding, creating magnetic field
Induces large voltage in secondary winding
Creates spark

35. CDI System Diagram

36. Variable Ignition Timing
Timing is advanced (occurs earlier) as
Engine speed increases
Engine load decreases
Timing is retarded (occurs later) as
Engine speed decreases
Engine load increases

37. Variable Slope Timing (VST)
Spark timing at different engine speeds and loads is engineered for specific
Engine series
Engine usage
Spark plug

38. Electronic Governor Control
Senses engine speed
Limits maximum speed
Usually by controlling throttle opening

39. Spark Plug Must provide spark
Must keep itself clean through varying engine conditions
Manufacturers carefully select specific spark plugs for engine series, application, and usage

40. Spark Plug Components Terminal (nut or solid)
Insulator top with corrugations
Hexagon drive
Threads
Gasket
Insulator nose (tip)
Center electrode
Round electrode
Spark plug gap

41. Terminal End of plug connected to spark plug wire
Solid metal extends through center of spark plug as center electrode
Some terminals are threaded and use nut

42. Insulator and Insulator Nose
Surrounds and insulates center electrode
Corrugations (flash ribs)
Prevent voltage from traveling down outside of insulator top to chassis ground
Insulator nose
Insulator portion exposed to combustion chamber
Carries heat from combustion chamber to spark plug body

43. Hexagon Drive and Gasket
Used to tighten spark plug into spark plug hole
Standard hexagon drive sizes are 9/16″, 5/8″, 3/4″, 13/16″
Gasket
Washer that seals between spark plug and spark plug hole

44. Center Electrode and Ground Electrode
Carries high voltage from terminal to plug gap
Exposed end is cylindrical with square edge
Ground electrode
Attached to spark plug threads
When spark plug is installed in engine, ground electrode is connected to chassis ground

45. Spark Plug Gap Distance between center and ground electrodes
Adjustable
Must be set correctly

46. Spark Plug Operating Temperature
Operating temperature—temperature at which spark plug cleans itself during engine operation
840°F to 1,550°F range
Carbon and other deposits are burned off
Below 840°F
Carbon is not completely burned off and plug fouls
Combustion deposits coat center and ground electrodes

47. Spark Plug Operating Temperature (Cont.)
Above 1,550°F
Insulator nose and center electrode retain too much heat
Could preignite air-fuel charge

48. Combustion Chamber Heat
Combustion chamber temperature determines plug operating temperature
Lean mixture produces more heat
Rich mixture cools combustion chamber and spark plug
High compression engines raise spark plug temperature

49. Combustion Chamber Heat (Cont.)
Advanced timing increases combustion chamber temperature
Armature air gap greater than normal retards spark timing
Increases compression and combustion chamber pressure
Damages spark plug

50. Using the Right Plug Engine designs and applications vary
Combustion temperatures vary
Spark plug operating conditions vary
Spark plugs cannot be interchanged between engines

51. Plug Heat Range
Ability of insulator nose to transmit heat from combustion chamber to cooling system
Cold plug
Quickly transmits heat to engine cooling system
Very little heat is retained in insulator nose
Hot plug
Transmits heat more slowly to cooling system
More heat is retained in insulator nose

52. Cold Plug and Hot Plug

53. Spark Plug Condition Determine cause of bad plug
Check brand and number of removed plug to verify that it is correct plug for engine
Check spark plug gap

54. Reading Plug Condition
Plug in good condition
No center electrode and insulator nose carbon fouling
Light tan ash deposits
Square edge on center electrode
Correct gap

55. Electrode Condition
As engine is used, arcing erodes edges of center electrode
Underside of ground electrode erodes, increasing spark plug gap

56. Reading Electrode Condition
Edge of center electrode should be square
Should be minimal wear on ground electrode
Gap should be correct for engine application

57. Oil Fouled Plug Tar-like carbon buildup caused by Incorrect premix oil
Excessive oil content

58. Overheated Plug Body
Shiny coating on metal between hex drive and threads becomes dull when overheated
Dull finish indicates engine overheating, plug overheating, or both

59. Carbon Fouled Plug
Insulator nose and center electrode coated with unburned carbon
Intermittent spark or no spark results
Causes:
Excessive fuel
Restricted air intake
Cold plug
Overcooled engine

60. Abrasive Ingestion
Abrasives and oil that coats plug fuse during combustion, creating gray crust
Build up of deposits eventually foul plug

61. Abrasive Ingestion Build up of dust deposits on plug
Eventually causes fouling
Plug already coated with premix oil
Normally burned off at operating temperature
Abrasives stick to oil
Abrasives and oil fuse during combustion
Create gray crust

62. Safety Switches Switches used to
Ensure safe starting and operating conditions
Shut off engine when operator leaves equipment
Operator actuates switches by pressing or holding lever or trigger

63. Switch Design Switches are identified by
Poles
Throws
Process of actuation
Pole is circuit controlled by switch
Single pole (SP) switch controls only one circuit
Double pole (DP) switch controls two circuits

64. Switch Throw Single throw (ST) switch Double throw (DT) switch
Simple toggle switch
Input to switch connects to single output
Double throw (DT) switch
Has input that can connect to either one of two outputs

65. Toggle Switch Common two-stroke engine on-off switch SPST switch
Connects or disconnects circuit
Switch is either closed or open

66. SPDT Switch Single-pole double-throw switch Single input
Switch selects one of two output circuits

67. Momentary Switches
Spring-loaded actuator keeps switch in closed or open position
Normally closed (NC)
SPST switch that is closed until switch is actuated
Normally open (NO)
SPST switch that is open until switch is actuated