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Timing is everything with internal combustion engines

Tuesday, May 19, 2015 - 07:00
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As one goes through life, it is said that timing is everything. In the case of the internal combustion engine, this could not be truer. In order for the engine to operate correctly, the event timing and event sequence must be correct. This means the location of the crankshaft and camshaft positions must be known, as well as their relationship to one another. So the physical position of the crankshaft and camshaft must be known.

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Figure 1

In order to determine where in space these shafts’  physical positions are a sensor will be used. A sensor reads the physical quantity and converts this to an electrical signal. This physical quantity or shaft position of the crankshaft and camshaft will be determined with sensors that send an electrical output signal. This output signal will be produced with the interaction between the trigger wheel and the magnetic field of the sensor, as seen in Figure 1. This type of sensor is known as a variable reluctance sensor, but sensors that rectify this analog voltage will all so be used; rectification of an analog signal means that it is converted to a digital signal or square wave. The trigger wheel is mounted on the shaft and will have some type of indexing means, such as a missing tooth, so the shaft’s orientation can be calculated. If the engine is rotating, the interaction between the trigger wheel and sensor will produce a waveform or a voltage that changes over time.

The internal clock
The voltage change produced from a sensor is just that, a voltage change. In order for the Engine Control Module (ECM) to be able to interpret the voltage change, a program must be written. The ECM microprocessor uses an internal clock to run with the software so that the shaft’s position can be calculated. This clock produces pulses that set up the rate at which tasks can be carried out. Each clock pulse sets up a machine cycle that caches the registers that carry out the programming tasks. Clock speed refers to the number of pulses per second generated by an oscillator that sets the tempo for the processor. Clock speed in the automotive computer is usually measured in MHz (megahertz), or millions of pulses per second. In order for the microprocessor to work accurately, the clock will need to have a high oscillator stability, so a quartz crystal oscillator circuit is used.

The quartz crystal used in a quartz crystal oscillator circuit is a thin, small piece of cut quartz. At the ends of the cut quartz, the surfaces are metallized in order to attach electrical connections. When producing the quartz crystal, the size and thickness are important because it affects the fundamental frequency of oscillations. Once the quartz crystal is cut and shaped, the crystal cannot be used at any other frequency. In other words, its size and shape produce an oscillation frequency that is directly proportional to it size.   

When a voltage source is applied to the quartz crystal, it begins to change shape, producing a characteristic known as the Piezoelectric Effect. This Piezoelectric Effect is the property of a crystal by which an electrical charge produces a mechanical force by changing the shape of the crystal and vice versa; a mechanical force applied to the crystal produces an electrical charge. This Piezoelectric Effect produces mechanical vibrations or oscillations that will directly change the voltage. It will be necessary to maintain a very accurate constant supply voltage on the quartz crystal so that the frequency output is maintained. This quartz clock circuit will send a continuous stream of square waves that will set the master clock and system timing within the microprocessor.

This master clock is built into the hardware of the microprocessor. The program that runs the engine is part of the software that is running on the hardware of the microprocessor. The software uses this hardware clock to carry out the instructions that will allow the microprocessor to set up the timing sequence for the internal combustion engine. This is accomplished by the crankshaft position sensor’s electrical signal. The signal is monitored by the software so the crankshaft position and velocity can be calculated. By having the crankshaft sensor indexed, the position of the No. 1 piston can be calculated, and this will allow all of the piston positions to be set by the mechanical configuration of the crankshaft. Additionally, using the clock and the sensor to monitor the rate at which the crankshaft is changing will provide the velocity or speed that the crankshaft is rotating. On a four-stroke engine, the crankshaft rotates two times to complete a fire cycle. This means that the piston is at Top Dead Center (TDC) and Bottom Dead Center (BDC) twice for each fire cycle. By using the crankshaft position sensor, one can calculate the position of the crankshaft and know if the piston is at TDC or BDC. However, with this limited information, one cannot calculate which of the four strokes (intake, compression, power, exhaust) the crankshaft is on. Since the piston is at TDC on compression and exhaust, it will be difficult to calculate which of these strokes the engine is currently on. In order to calculate the crank angle space, a second sensor will be needed; this sensor is the camshaft position sensor. The camshaft position sensor will allow one to calculate which stroke the crankshaft is currently on. By using the software to compare the crankshaft to camshaft position, a timing sequence can be calculated.

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