Ignition Systems
Chapter 12/13 - Class Manual
p379
Basics
•	Must fire a spark on at the “top” of the compression stroke.
•	That spark must be “timed” to the proper cylinder and TDC
•	Must provide power to “light off A/F mixture”
•	Must work at all cylinders, each cylinder is firing at a different time
Terms
•	TDC = Top Dead Center (top of compression stroke)
•	ATDC = After TDC (late - retarded)
•	BTDC = Before TDC (early - adv.)
•	Timing Degrees = 20, 10
•	10 BTDC - 20 ATDC - see top of p381
•	Firing Order = The order of the spark.. p382
Timing needs to change...
•	Timing needs to change due to changes in engine RPM and load
•	Timing is either “advanced” or “retarded”
•	Slower RPM = Retarded (late) timing
•	Faster PRM = Advanced (early) timing
•	Can be controlled by a vacuum, centrifugal device, or computer
When timing changes:
•	High RPM = More Advance
•	Low RPM = Less advance
•	Less Vacuum = Less advance
•	More Vacuum = More Advance

•	But the car makes the most vacuum at a near closed throttle!
Fact
•	Low RPM, Open throttle (low vac) = Starting off from a stop, or powering up a hill with a heavy load
•	High RPM, Open Throttle (again, low vac) = Passing someone on a highway
•	Timing must take all types of driving into account for best power and MPG.
Basic Parts
•	Primary Circuit (low voltage 12v)
•	Secondary Circuit (high voltage 25,000v)
•	Timing Device - Distributor from engine, or computer controlled system
•	p386

Pieces Parts (primary)
•	Battery/Alternator/Generator
•	Ignition Switch
•	Coil Primary Winding (12vDC in)
•	Triggering Device
–	computer, mechanical
•	Older devices
–	Resistor
–	Starter Bypass

Pieces Parts (secondary)
•	Coil secondary winding 
–	15-50,000 vDC
•	Distributor Cap
•	Rotor
•	Spark Plug Wires
•	Spark Plugs

Secondary System
•	Carries High voltage to plugs
•	Timing must be as accurate as possible to get the most economy out of the engine
•	Each cylinder has a plug, a wire, and a terminal off of the distributor
•	- see lab
Primary Circuit
•	Converts 12 volts DC into 15 - 50000 volts DC
•	+12 V DC is sent to one side of the coil (at all times when ign. Is on)
•	The ground side is controlled by a trigger device
•	The “time” that the ground is supplied to the coil is called “Dwell”

The coil...
•	Acts like a capacitor (battery) and “charges up” when the coil has power and ground
•	The primary coil passes power to the secondary coil VIA inductance
•	When the ground is removed, the electrons stored up in the coil must leave - a magnetic field collapses holding the electrons
How its made
•	Two coils of wire (the primary and the secondary) around an iron core
•	Round coils are filled with oil to keep them cool (older)
•	“E” core coils are air cooled (more common today)
•	Can’t fix-em, just test and replace
System Types
•	Conventional DI - Cap/Rotor/Coil/Wires/Distributor
•	Electronic EI - No cap or rotor or distributor - 100% electronic “coil pack”
•	Coil over Plug - Separate coil for each plug on each cylinder
Conventional
Electronic EI
Coil Over Plug
Spark Plugs
•	Made of ceramic (insulator) and metal (conductor), electrode
•	Has an adjustable gap at the bottom of the plug
•	May contain a resistor to prevent RFI (radio frequency interference) on electronic equipment (the car’s processor)
•	Fragile!  Handle with care!

Plug Designs
•	Plug Gap
•	Plug Size
•	Heat ranges - move heat away from plug tip to prevent melting
•	Gaskets (metal)
•	Platinum, Copper, Alloys may be used for the electrode tip
•	Don’t mix plug types on an engine
Plug ratings
•	R = Resistor Type Plug
•	T = Tapered Seat Type
•	C = Alloy used on electrode
•	Other numbers are heat ranges, reach, and product ID codes
•	See Sheet
Plug Wires
•	Must be flexible
•	Must not “leak” voltage
•	Must be heat resistant
•	Must conduct well
•	Must have boots at both ends
Websites:	
•	http://auto.howstuffworks.com/ignition-system.htm
•	http://www.autoshop101.com/autoshop15.html