Contents: Amateur work and modern ignition… ↳ Always in a "double pack" - the… ↳ The CKP sensor supplies the main… ↳ The transmission control unit (PCM)… ↳ Optimum ignition timing depending on… ↳ Timing Shift - Ignition and… ↳
Elements of the ignition system: 1 - ignition wire; 2 - ignition coil; 3 - spark plugs
Touch control: The ignition system in the Focus is an electronic high-performance module that converts the on-board voltage of 12 volts into an ignition voltage of over 30,000 volts. The ignition voltage is supplied to the spark plugs with angular degree accuracy. Work on electronic ignition systems must be carried out with extreme caution - high voltage peaks can be life-threatening.
Traditional coil ignition systems, consisting of a spark plug coil, distributor, condenser, distributor pin, breaker contacts and spark plug wires, provided, with their relatively limited flexibility, the operation of gasoline engines of the old design: the ignition coil supplied the transferable voltage, the mechanical distributor supplied the spark to the spark plugs. In the upper rpm range, this occurred with a modest adjustment of the ignition timing, but usually before the piston, which was currently on the compression stroke, reached top dead center. In other words: the peripheral components of the engine, primarily the air-fuel mixture preparation and valve train, were more or less dependent on the static performance of the ignition system. Coil ignition systems distributed sparks with the flexibility of a matchbox.
In the era of electronically controlled engines, this has changed: behind the scenes, completely invisible to the untrained eye, electronic components create the connections that provide ignition and flexibility.
Amateur work and modern ignition systems
For DIYers and ambitious amateur mechanics, this means: ignition systems, tightly integrated into engine management, provide almost no application points for the "rational activity of an amateur" without special equipment. Under the hood of the Focus, "black boxes" also "rule the roost" - you can penetrate their inner life only with expert knowledge and highly sensitive test programs.
In all Focus petrol engines, ignition timing is calculated individually for each cylinder based on characteristics stored in the memory of the transmission control unit (PCM), as well as depending on the operating condition of the engine at the moment.
Always in a "double pack" - the spark of ignition
By design, the two ignition coils always deliver sparks to the spark plugs in a "double pack": one spark ignites the fresh air-fuel mixture in the cylinder on the compression stroke, the other spark jumps off in the opposite cylinder during the exhaust stroke. In other words: cylinders 1 and 4, as well as cylinders 3 and 4, always receive their spark at the same time.
The CKP sensor supplies the main parameters for each ignition spark
The signal from the crankshaft angle sensor (CAS) serves as the calculation base. Its signal, previously converted into digital form by the PCM, controls the primary winding of the ignition coil. To do this, the PCM briefly interrupts the voltage supply to the primary winding of the ignition coil. This creates high voltage (ignition voltage), which is discharged through the ignition wire on the spark plugs.
The transmission control unit (PCM) with various characteristics ensures the timely appearance of the ignition spark
The correct coordination of the ignition spark is ensured by the PCM. Among other things, its memory stores the theoretical basic parameters of the various ignition moments. In order to adapt the dead theory to practice with maximum efficiency, the control unit evaluates the current information from the peripheral units of the engine. For example, it analyzes the signals of the TDC sensor and - in Zetec-SE engines - additionally the information of the knock sensor (KS). This current information must be constantly compared with the theoretical information on the hard disk at each crankshaft rotation. Before each individual change in the composition of the working mixture, the black box communicates with the injection system, the lambda probe, the speed sensor, as well as with various temperature sensors and the air mass meter under the hood of the Focus.
Touch Control: The ignition system in the Focus is an electronic high-performance module that converts 12 volts of on-board voltage into ignition voltage in excess of 30,000 volts. Ignition voltage is delivered to the spark plugs with angular degree precision in a 1-3-4-2 sequence via the spark plug wire and spark plug connector.
Optimum ignition timing depending on load
The fuel-air mixture burns in the combustion chambers at different speeds depending on the load state (idle, partial, full load) and the quality of the fresh air. In order to make the most of the fuel energy, the black box changes the ignition timing according to the load state for each individual cylinder. The best option is when the fresh gas ignites at the moment of maximum compression. In a four-stroke engine, this is the moment when the piston wants to go from the rise of the compression stroke to the return movement of the power stroke.
Timing Shift - Ignition and Combustion
However, the ignition moment is not exactly at the top dead center (TDC), since it takes about three thousandths of a second for the mixture to ignite. In this regard, the ignition spark gets the green light even during the piston's upward movement. On the contrary, the maximum compression pressure occurs when the piston just passes TDC. Since it always takes the same amount of time to ignite the fuel-air mixture, the ignition moment shifts further and further from TDC toward the advance as the engine speed increases.