Chapter 8 Ignition Systems

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.

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.

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

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

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

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

Points Open and Closed

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)

Ignition Coil Assembly

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

Sequence of Ignition Events (Cont.) Points closed

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

Sequence of Ignition Events (Cont.) Peak voltage

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

Sequence of Ignition Events (Cont.) Spark jumps gap

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

Toggle Switch

Stop Switch Designs

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

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

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

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

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)

SCR Ignition System Diagram

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

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

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

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

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

TCI System Diagram

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

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

CDI System Diagram

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Cold Plug and Hot Plug

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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