1. Changing an engine’s ignition timing so that the spark plug fires earlier is called _______
the ignition timing.
2. Changing an engine’s ignition timing so that the spark plug fires later is called _______ the ignition timing.
3. A _______ is a device that adjusts ignition timing in relation to the load on the engine.
4. True or False? A distributor shaft may rotate either clockwise or counterclockwise, depend-ing on the engine design.
Check your answers with those on page 100.
In a direct-fire system, the computer, ignition module, and ignition coils work together to control the spark at the cylinders. Various sen-sors positioned in the engine send information about different engine conditions to the computer. The computer then processes the informa-tion and determines the ideal spark timing for the current engine con-ditions. The computer tells the ignition module when to trigger the primary windings in the ignition coils. In this type of system, the igni-tion module takes the place of the distributor.
In order for a direct-fire ignition system to work properly, the com-puter must know the exact crankshaft position at all times so that it can determine when the cylinders should be fired. For this reason, a spe-cial crankshaft position sensor is used to monitor the crankshaft position.
As the crankshaft rotates, the crankshaft position sensor monitors its position and sends signals about the crankshaft position to the com-puter. The computer uses these signals to determine when to fire the various cylinders.
As you can see inFigure 48, the crankshaft position sensor is usually mounted in the engine block close to the crankshaft. A special rotating wheel on the crankshaft is then used to trigger the sensor. Depending on the manufacturer, this wheel may be called a trigger wheel, a trigger ring, a reluctor, an interrupter ring, or a pulse ring. The trigger wheel may be an integral part of the crankshaft, or it may be attached to the crank-shaft.
The exact design of the trigger wheel varies depending on the engine make and model. In all cases, though, the outer edge of the trigger wheel contains some type of notches or holes that represent crankshaft positions. One common type of trigger wheel is shown inFigure 49.
This trigger wheel has four notches cut out of its edge. Each notch in the trigger wheel represents a one-quarter turn—90 degrees—of crank-shaft rotation.
FIGURE 48—A typical direct-fire ignition system is shown here.
How exactly does a crankshaft position sensor work? One common type of crankshaft position sensor contains a permanent magnet and a wire winding. When the sensor’s winding is energized, a magnetic field develops between the crankshaft position sensor and the surface of the metal trigger wheel. As the trigger wheel rotates, the notches in the trigger wheel cause the strength of this magnetic field to change.
Each time a notch passes by the crankshaft position sensor, the mag-netic field changes and an electrical signal is sent to the computer. The computer then uses the signals from the crankshaft position sensor to determine the current crankshaft position.
Remember that this trigger wheel has four notches, one for every 90 degrees of crankshaft rotation. Each time a notch passes the crankshaft position sensor, the sensor sends a voltage pulse to the computer. The computer simply counts the voltage pulses to figure out the correct crankshaft position. The first pulse tells the computer that the crank-shaft is at 90 degrees of rotation. The second pulse tells the computer that the crankshaft is at 180 degrees of rotation. The third pulse tells the computer that the crankshaft is at 270 degrees of rotation. The fourth pulse tells the computer that the crankshaft has rotated a com-plete 360 degrees and is back where it started. The computer continu-ally counts the voltage pulses from the crankshaft position sensor as the engine operates.
Once the computer knows the current position of the crankshaft, it can determine when to fire the engine’s cylinders. For example, suppose an engine contains four cylinders that fire in a 1-2-3-4 firing order.
When the computer receives the first signal from the crankshaft posi-tion sensor, at 90 degrees of crankshaft rotaposi-tion, the computer knows that Cylinder 1 is at TDC on its compression stroke and is ready to be fired. The computer signals the ignition module to fire the spark plug attached to Cylinder 1. When the computer receives the second signal from the crankshaft position sensor—at 180 degrees of crankshaft rota-tion—it knows that Cylinder 2 is ready to be fired. The computer sig-nals the ignition module to fire the spark plug at Cylinder 2. At the third signal from the crankshaft position sensor, at 270 degrees of crankshaft rotation, the computer fires Cylinder 3. Finally, at the fourth signal from the crankshaft position sensor, at 360 degrees of crankshaft
FIGURE 49—This trigger wheel has four notches cut into its edge. Each notch represents one-quarter turn—90 degrees—of crankshaft rotation. As the trigger wheel rotates, each time one of the notches passes the crankshaft posi-tion sensor, the sensor sends a signal to the com-puter.
In addition to firing the engine cylinders, the computer in a direct-fire ignition system also controls the ignition timing and timing advance.
A variety of different sensors in the engine monitor conditions, such as engine speed and temperature, and send information about current conditions to the computer. The computer then determines how many degrees of crankshaft rotation before TDC the spark should occur.
Once this is determined, the computer waits until the crankshaft is at that exact degree of rotation. At that moment, the computer signals the ignition module to trigger the ignition coil for that cylinder. The tion module shuts off the voltage in the primary winding of that igni-tion coil, the magnetic field in the coil collapses, and a spark is sent to the spark plug for that cylinder.
This ignition system process repeats thousands of times every minute.
This isn’t always easy, considering how fast an engine operates. How-ever, the computers used in direct-fire ignition systems can make deci-sions very quickly and can control engine operation efficiently even at high speeds.
Note that this is a very basic description of the operation of a direct-fire system. Many individual system variations are possible, depending on the engine design, number of engine cylinders, and type of ignition sys-tem components used. For example, there are many different styles of trigger wheels. Also, some direct-fire systems may use more than one crankshaft position sensor to signal the computer. However, the basic principles of operation are the same in all systems—a rotating trigger wheel and sensor work together to tell the computer when to fire the en-gine cylinders. Remember that all of the ignition timing and firing func-tions in a direct-fire system are performed electronically, without a distributor. Because electronic components are long-lasting and require no adjustments, direct-fire ignition systems are very reliable.
Now, let’s discuss some of the different components that may be used in direct-fire ignition systems.