Mechanically timed ignition Ignition system

top of distributor cap wires , terminals



rotor contacts inside distributor cap

most four-stroke engines have used mechanically timed electrical ignition system. heart of system distributor. distributor contains rotating cam driven engine s drive, set of breaker points, condenser, rotor , distributor cap. external distributor ignition coil, spark plugs , wires linking distributor spark plugs , ignition coil. (see diagram below)

the system powered lead-acid battery, charged car s electrical system using dynamo or alternator. engine operates contact breaker points, interrupt current induction coil (known ignition coil).

the ignition coil consists of 2 transformer windings — primary , secondary. these windings share common magnetic core. alternating current in primary induces alternating magnetic field in core , hence alternating current in secondary. ignition coil s secondary has more turns primary. step-up transformer, produces high voltage secondary winding. primary winding connected battery (usually through current-limiting ballast resistor). inside ignition coil 1 end of each winding connected together. common point taken capacitor/contact breaker junction. other, high voltage, end of secondary connected distributor s rotor.

ignition circuit diagram mechanically timed ignition

the ignition firing sequence begins points (or contact breaker) closed. steady current flows battery, through current-limiting resistor, through primary coil, through closed breaker points , battery. current produces magnetic field within coil s core. magnetic field forms energy reservoir used drive ignition spark.

as engine crankshaft turns, turns distributor shaft @ half speed. in four-stroke engine, crankshaft turns twice ignition cycle. multi-lobed cam attached distributor shaft; there 1 lobe each engine cylinder. spring-loaded rubbing block follows lobed portions of cam contour , controls opening , closing of points. during of cycle, rubbing block keeps points closed allow current build in ignition coil s primary winding. piston reaches top of engine s compression cycle, cam s lobe high enough cause breaker points open. opening points causes current through primary coil stop. without steady current through primary, magnetic field generated in coil collapses. high rate of change of magnetic flux induces high voltage in coil s secondary windings causes spark plug s gap arc , ignite fuel.

the spark generation story little more complicated. purpose of ignition coil make spark jumps spark plug s gap, might 0.025 inches (0.64 mm) (it has jump rotor-to-distributor-post gap). @ moment points open, there smaller gap, 0.00004 inches (0.001 mm), across points. must done prevent points arcing separate; if points arc, drain magnetic energy intended spark plug. capacitor (condenser) performs task. capacitor temporarily keeps primary current flowing voltage across points below point s arcing voltage. there race: voltage across points increasing primary current charges capacitor, @ same time points separation (and consequent arcing voltage) increasing. ultimately, point separation increase such 0.015 inches (0.38 mm), maximum separation of points.

in addition staying below arcing voltage, ignition system keep voltage across points below breakdown voltage air gap prevent glow discharge across points. such glow discharge transition arc, , arc prevent spark plug firing. minimum voltage glow discharge in air 320 v. consequently, capacitor value chosen keep voltage across points less 320 v. keeping points arcing when separate reason ignition coil includes secondary winding rather using simple inductor. if transformer has 100:1 ratio, secondary voltage can reach 30 kv.

the ignition coil s high voltage output connected rotor sits on top of distributor shaft. surrounding rotor distributor cap. arrangement sequentially directs output of secondary winding appropriate spark plugs. high voltage coil s secondary (typically 20,000 50,000 volts) causes spark form across gap of spark plug in turn ignites compressed air-fuel mixture within engine. creation of spark consumes energy stored in ignition coil’s magnetic field.

the flat twin cylinder 1948 citroën 2cv used 1 double ended coil without distributor, , contact breakers, in wasted spark system.

some two-cylinder motorcycles , motor scooters had 2 contact points feeding twin coils each connected directly 1 of 2 sparking plugs without distributor; e.g. bsa thunderbolt , triumph tigress.

high performance engines 8 or more cylinders operate @ high r.p.m. (such used in motor racing) demand both higher rate of spark , higher spark energy simple ignition circuit can provide. problem overcome using either of these adaptations:

two complete sets of coils, breakers , condensers can provided - 1 set each half of engine, typically arranged in v-8 or v-12 configuration. although 2 ignition system halves electrically independent, typically share single distributor in case contains 2 breakers driven rotating cam, , rotor 2 isolated conducting planes 2 high voltage inputs.
a single breaker driven cam , return spring limited in spark rate onset of contact bounce or float @ high rpm. limit can overcome substituting breaker pair of breakers connected electrically in series spaced on opposite sides of cam driven out of phase. each breaker switches @ half rate of single breaker , dwell time current buildup in coil maximized since shared between breakers. lamborghini v-8 engine has both these adaptations , therefore uses 2 ignition coils , single distributor contains 4 contact breakers.

a distributor-based system not different magneto system except more separate elements involved. there advantages arrangement. example, position of contact breaker points relative engine angle can changed small amount dynamically, allowing ignition timing automatically advanced increasing revolutions per minute (rpm) or increased manifold vacuum, giving better efficiency , performance.

however necessary check periodically maximum opening gap of breaker(s), using feeler gauge, since mechanical adjustment affects dwell time during coil charges, , breakers should re-dressed or replaced when have become pitted electric arcing. system used universally until late 1970s, when electronic ignition systems started appear.