Estandarización del control positivo

Two different types of ignition systems are used to control the spark that’s delivered to an engine’s cylinders: distributor-type systems and direct-fire systems. In a distributor-type ignition system, a single ignition coil powers all the spark plugs in the engine. A device called a distribu-tor is used to direct the high voltage from the ignition coil to the spark plugs. Remember that in most engines, each cylinder ignites at a differ-ent time so that the engine runs more smoothly. Therefore, the dis-tributor directs the high voltage to the cylinder that’s currently on its compression stroke and ready to have the air-and-fuel mixture ignited to produce power.

In contrast, a direct-fire system uses a single battery and triggering de-vice, and separate coils to control the spark at each engine cylinder. A computer control system takes the information from the triggering de-vice and uses it to fire each cylinder at the proper time.

You’ll learn about direct-fire ignition systems in detail later in the study unit. For now, let’s concentrate on the ignition systems that use distributors. A simplified view of a distributor-type ignition system is shown inFigure 23. or-der for a General Motors V-6 engine; Figure 22B arrange-ment and firing order for a General Motors V-8

The main parts of the distributor are the housing, gear, shaft, cap, and rotor. In some systems, the ignition coil and the triggering device are both housed inside the distributor.

The distributor itself consists of several different components, as shown inFigure 24. The distributor’s outer shell is called the housing.

The distributor’s top covering is called the distributor cap. The dis-tributor shaft runs through the middle of the disdis-tributor. The distribu-tor gear is attached to the end of the distribudistribu-tor shaft. The distribudistribu-tor is usually mounted to an engine with its housing placed in a hole in the engine block or cylinder head.

FIGURE 23—A typical distributor-type ignition system for an eight-cylinder engine is shown here.

When a distributor is installed in an engine, the gear on the end of the distributor shaft is driven by a similar gear that’s attached to the en-gine’s camshaft. Therefore, whenever the engine is running, the dis-tributor shaft turns with the camshaft at the same speed. Therefore, one camshaft rotation results in one distributor rotation.

Distributor caps are made from heat-resistant plastic and are heavily insulated (Figure 25A). The distributor cap fits snugly over the top of the distributor housing. In this figure, note that the distributor cap con-tains several points called towers. The towers that are arranged evenly around the outer edge of the distributor cap are called the spark towers.

The tower in the center of the distributor cap is called the coil tower.

Note that a distributor cap contains one spark tower for each of the gine’s cylinders. Therefore, the distributor cap in an eight-cylinder en-gine has eight spark towers, and the cap in a six-cylinder enen-gine has six spark towers. Remember that a complete circle contains 360 de-grees. So, if a distributor cap contains four towers, each tower is posi-tioned 90 degrees apart from its neighboring towers on the distributor (360 ¸ 4 = 90). If a distributor contains six towers, the towers are posi-tioned 60 degrees apart (360 ¸ 60 = 6). In a distributor with eight towers, the towers are 45 degrees apart (360 ¸ 8 = 45). The distributor

FIGURE 24—The components of a typical distributor are shown here. Note that the ignition coil is mounted inside this particular distributor.

cap shown inFigure 25Ahas eight spark towers that are positioned 45 degrees apart.

Figure 25Bshows the underside of the distributor cap. Metal inserts are cast into each tower in the cap. These metal inserts extend down-ward into the cap as shown in the figure. The metal contact in the cen-ter of the cap is inserted into the coil tower. This cencen-ter contact is called the rotor button.

Each of the engine’s spark plug wires is fastened to the spark towers.

The opposite ends of the spark plug wires are then fastened to the spark plugs. Each spark plug wire is attached to a spark tower and a spark plug. Spark plug wires are made in different lengths, depending on how far the wire must travel between the spark tower and the spark plug.

Note that the spark plug wires are attached to the spark towers in the same order as the firing order. For example,Figure 26shows a typical automotive spark plug wire arrangement for a six-cylinder engine. Re-member this is only an example—the actual spark plug wire arrange-ment and firing order depend on the vehicle design. In this example, the distributor rotor turns clockwise and the firing order of the engine is 1-4-2-5-3-6. The spark plug wires are installed around the edge of the distributor cap in that order.

FIGURE 25—Figure 25A shows an external view of a typical automotive distributor cap. Figure 25B shows the metal contacts on the underside of the cap.

A separate wire called a coil wire leads from the engine’s ignition coil to the coil tower on the distributor cap. The coil wire is similar in con-struction to a spark plug wire, but the coil wire is a different length and the terminals in its ends are shaped differently.

A component called a rotor is attached to the top of the distributor shaft. As the distributor shaft rotates, the rotor also rotates. The rotor’s function is to direct the high voltage from the ignition coil to the spark plugs.

Two typical distributor rotors are shown inFigure 27. Rotors are usu-ally constructed of materials that have a very high insulating quality.

Note that a conducting metal strip runs from the center of a rotor to its outer tip. This metal strip on the rotor touches the rotor button on the inside of the distributor cap. The metal strip on the rotor doesn’t touch the spark tower contacts, however. Instead, a small air gap is be-tween the end of the rotortip and the spark tower contacts inside the distributor cap.

Now, let’s discuss the basic operation of the distributor and rotor in more detail. InFigure 28on page 38 you can see the operation of a distributor-type ignition system for a four-cylinder engine. In the fig-ure, note how the spark plug wires are attached to the four towers on the distributor cap. The firing order of the cylinders in this engine is 1-3-4-2, so the spark plug wires are installed around the edge of the dis-tributor cap in that order. Also note how the coil wire is connected be-tween the ignition coil and the coil tower on the distributor cap.

FIGURE 26—This illustration shows a spark plug wire arrangement for a six-cylinder engine. The spark plug wires are installed around the distributor cap in the direction of rotor rotation. The rotor rotates clockwise, and the firing or-der of the engine is 1-4-2-5-3-6.

As the engine operates, the engine’s camshaft rotates and causes the distributor shaft to rotate inside the distributor cap. When the distribu-tor shaft rotates, the rodistribu-tor also rotates. At this time, the engine’s igni-tion coil is also operating. Current from the secondary winding of the ignition coil passes through the coil wire to the coil tower in the center of the distributor cap. Current passes down through the coil tower, through the rotor button, and into the contact strip on the rotor. (Note that in some ignition systems, the ignition coil is located inside the dis-tributor. If an ignition coil is located inside a distributor, no separate coil wire is needed. Instead, the ignition coil output travels directly to the rotor.)

As the rotor rotates, it passes under each of the spark towers. Each time the rotor passes under a spark tower, the high voltage jumps across the air gap to the spark plug tower contact. The spark travels through the distributor tower, through the spark plug wire attached to that tower, and then to the engine cylinder on the other end of the spark plug wire. No more than 2,000 or 3,000 volts is required to carry the current across the air gap, so almost all of the voltage produced by the ignition coil reaches the spark plug.

Remember that the firing order of this engine is 1-3-4-2. Therefore, as the rotor rotates, it passes under the spark tower for Cylinder 1 first, and Cylinder 1 receives a spark. The rotor then passes under the spark tower for Cylinder 3, and Cylinder 3 receives a spark. Next, the rotor passes under the spark tower for Cylinder 4, and Cylinder 4 receives a spark. Finally, the rotor passes under the spark tower for Cylinder 2, and Cylinder 2 receives a spark.

FIGURE 27—Shown here are two common types of rotors. Rotors are attached to the distributor shaft and transfer the high voltage from the coil to the proper spark plug wire.

The relationship between the movement of the rotor and the position of the spark tower contacts is critical. At the exact time a spark is pro-duced by the ignition coil, the rotor tip must pass under one of the spark tower contacts inside the distributor cap. If the rotor tip isn’t properly aligned with the spark tower contact inside the cap, the air space between the rotor tip and the spark tower contact will be too large. If the air gap is too large, more voltage is needed to complete the circuit to the spark plugs. For this reason, it’s very important to select the proper replacement rotor recommended by the manufacturer.

FIGURE 28—The spark plug wires are installed around the distributor cap in their proper firing order, the direction of rotor rotation.