page_title: Ignition system
section_title: Mechanically timed ignition
hierarchical_section_title: Ignition system / Modern ignition systems / Mechanically timed ignition
context_page_description: An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, rocket engines, etc. The widest application for spark ignition internal combustion engines is in petrol road vehicles such as cars and motorcycles.
  Compression ignition Diesel engines ignite the fuel-air mixture by the heat of compression and do not need a spark. They usually have glowplugs that preheat the combustion chamber to allow starting in cold weather. Other engines may use a flame, or a heated tube, for ignition. While this was common for very early engines it is now rare.
  The first electric spark ignition was probably Alessandro Volta's toy electric pistol from the 1780s.
  Siegfried Marcus patented his "Electrical igniting device for gas engines" on 7 October 1884.
context_section_description: Most four-stroke engines have used a mechanically timed electrical ignition system. The heart of the system is the distributor. The distributor contains a rotating cam driven by the engine's drive, a set of breaker points, a condenser, a rotor and a distributor cap. External to the distributor is the ignition coil, the spark plugs and wires linking the distributor to the spark plugs and ignition coil. (see diagram Below)
  The system is powered by a lead-acid battery, which is charged by the car's electrical system using a dynamo or alternator. The engine operates contact breaker points, which interrupt the current to an induction coil (known as the ignition coil).
  The ignition coil consists of two transformer windings — the primary and secondary. These windings share a common magnetic core. An alternating current in the primary induces an alternating magnetic field in the core and hence an alternating current in the secondary. The ignition coil's secondary has more turns than the primary. This is a step-up transformer, which produces a high voltage from the secondary winding. The primary winding is connected to the battery (usually through a current-limiting ballast resistor). Inside the ignition coil one end of each winding is connected together. This common point is taken to the capacitor/contact breaker junction. The other, high voltage, end of the secondary is connected to the distributor's rotor.
  The ignition firing sequence begins with the points (or contact breaker) closed. A steady current flows from the battery, through the current-limiting resistor, through the primary coil, through the closed breaker points and finally back to the battery. This current produces a magnetic field within the coil's core. This magnetic field forms the energy reservoir that will be used to drive the ignition spark.
  As the engine crankshaft turns, it also turns the distributor shaft at half the speed. In a four-stroke engine, the crankshaft turns twice for the ignition cycle. A multi-lobed cam is attached to the distributor shaft; there is one lobe for each engine cylinder. A spring-loaded rubbing block follows the lobed portions of the cam contour and controls the opening and closing of points. During most of the cycle, the rubbing block keeps the points closed to allow a current to build in the ignition coil's primary winding. As a piston reaches the top of the engine's compression cycle, the cam's lobe is high enough to cause the breaker points to open. Opening the points causes the current through the primary coil to stop. Without the steady current through the primary, the magnetic field generated in the coil immediately collapses. This high rate of change of magnetic flux induces a high voltage in the coil's secondary windings that ultimately causes the spark plug's gap to arc and ignite the fuel.
  The spark generation story is a little more complicated. The purpose of the ignition coil is to make a spark that jumps the spark plug's gap, which might be 0.025 inches (0.64 mm) (it also has to jump the rotor-to-distributor-post gap). At the moment the points open, there is a much smaller gap, say about 0.00004 inches (0.001 mm), across the points. Something must be done to prevent the points from arcing as they separate; if the points arc, then they will drain the magnetic energy that was intended for the spark plug. The capacitor (condenser) performs that task. The capacitor temporarily keeps the primary current flowing so the voltage across the points is below the point's arcing voltage. There is a race: the voltage across the points is increasing as the primary current charges the capacitor, but at the same time the points' separation (and consequent arcing voltage) is increasing. Ultimately, the point separation will increase to something such as 0.015 inches (0.38 mm), the maximum separation of the points.
  In addition to staying below the arcing voltage, the ignition system keep the voltage across the points below the breakdown voltage for an air gap to prevent a glow discharge across the points. Such a glow discharge would qu
caption: Ignition circuit diagram for mechanically timed ignition