ENGINE LOAD

The load on an engine is related to the work it must do. Driving up hills or pulling extra weight increases engine load. Under load there is resistance on the crankshaft, therefore the pistons have a harder time moving through their strokes. This is evident by the low measured vacuum during heavy loads. Under light loads and with the throttle plate(s) partially opened, a high vacuum exists in the intake manifold. The amount of air/fuel mixture drawn into the manifold and cylinders is small. On compression, this thin mixture produces less combustion pressure and combustion time is slow. To complete combustion by 10 degrees ATDC, ignition timing must be advanced. Under heavy loads, when the throttle is opened fully, a larger mass of air/fuel mixture can be drawn in, and the vacuum in the manifold is low. High combustion pressure and rapid burning results. In such a case, the ignition timing must be retarded to prevent complete burning from occurring before 10 degrees ATDC.

Firing Order

Up to this point, the primary focus of discussion has been ignition timing as it relates to any one cylinder. However, the function of the ignition system extends beyond timing the arrival of a spark to a single cylinder. It must perform this task for each cylinder of the engine in a specific sequence. Each cylinder of an engine must produce power once in every 720 degrees of crankshaft rotation. Each cylinder must have a power stroke at its own appropriate time during the rotation. To make this possible, the pistons and rods are arranged in a precise fashion. This is called the engine’s firing order. The firing order is arranged to reduce rocking and imbalance problems. Because the potential for this rocking is determined by the design and construction of the engine, the firing order varies from engine to engine. Vehicle manufacturers simplify cylinder identification by numbering each cylinder.

Regardless of the particular firing order used, the # 1 cylinder always starts the firing order, with the rest of the cylinders following in a fixed sequence. The ignition system must be able to monitor the rotation of the crankshaft and the relative position of each piston to determine which piston is on its compression stroke. It must also be able to deliver a high-voltage surge to each cylinder at the proper time during its compression stroke. How the ignition system does these things depends on the design of the system.

BASIC CIRCUITRY

All ignition systems consist of two interconnected electrical circuits: a primary (low voltage) circuit and a secondary (high voltage) circuit. Depending on the exact type of ignition system, components in the primary circuit include the following:

Battery

Ignition switch

Ballast resistor or resistance wire (some systems)

Starting bypass (some systems)

Ignition coil primary winding

Triggering device

Switching device or control module. The secondary circuit includes these components.

Ignition coil secondary winding

Distributor cap and rotor (some systems)

Ignition (spark plug) cables

Spark plugs

Primary Circuit Operation

When the ignition switch is on, current from the battery flows through the ignition switch and primary circuit resistor to the primary winding of the ignition coil. From here it passes through some type of switching device and back to ground. The switching device can be electronically or mechanically controlled by the triggering device. The current flow in the ignition coil’s primary winding creates a magnetic field. The switching device or control module interrupts this current flow at predetermined times. When it does, the magnetic field in the primary winding collapses. This collapse generates a high-voltage surge in the secondary winding of the ignition coil. The secondary circuit of the system begins at this point. Some ignition systems have a ballast resistor connected in series between the ignition switch and the coil positive terminal. This resistor supplies the correct amount of voltage and current to the coil. In some ignition systems, a calibrated resistance wire is used in place of the ballast resistor. Today, many ignition systems are not equipped with the resistor. These systems supply 12 volts directly to the coil. There are also some ignition systems that do not require a ballast resistor. For instance, some control units directly regulate the current flow through the primary of the coil. Hall-effect and optical systems do not require ballast resistors either. The signal voltage is not changed by the speed of the distributor because it is in an inductive magnetic signal generating system.