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EFI Tuning Fundamentals: Wave Forms

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Wave Forms

07.49

00:00 - When it comes to the ECU used to run your engine, often the inputs don't rely on simple DC, or direct current voltages.
00:07 Likewise, the outputs used to the control ancillary devices may not be a simple on-off condition.
00:13 Many of the inputs that are critical to the operation of the ECU are AC waveforms, while many of the outputs rely on pulse width modulated, also known as PWM, signals.
00:24 The important thing to understand about these sorts of waveforms is that we can't use a basic multimeter or digital voltmeter to understand what's going on in the circuit.
00:35 The voltage in these waveforms is constantly changing with relation to time, and to visualise this we need to use an oscilloscope.
00:42 I know many tuners don't really understand these sorts of signals, but don't get scared off, we're going to have a look at these terms now, and explain what they mean.
00:52 Let's start by talking about an AC waveform.
00:55 The term AC stands for alternating current.
00:58 What this means is that the direction the current is flowing in the circuit is constantly changing back and forth.
01:06 Remember though that voltage is relative to current, so this meatns that the voltage, too, is constantly changing from positive to negative with relation to time, as we can see here.
01:17 This particular waveform is what we could expect to see from a magnetic, or reluctor sensor, which is commonly used to tell the ECU how fast the engine's rotating.
01:27 With this sort of sensor, we see the voltage begin to rise as the tooth on the trigger wheel comes close to the reluctor sensor.
01:34 As the tooth passes the centre of the sensor, the voltage falls back through zero and becomes negative.
01:39 And, as the tooth moves away, the voltage decays back towards zero.
01:44 A square wave like the one shown here, on the other hand, is the sort of signal we'd see produced by a digital sensor such as a Hall sensor, or optical sensor.
01:54 These are also commonly used for inputs like engine RPM, or cam position, but, unlike an AC wave, their output switches between a high-voltage and a low-voltage state.
02:05 Pulse width modulated outputs, or PWM for short, are very common in an EFI system, and are used to control many outputs such as the fuel injectors, cam control solenoids, boost control solenoids, and many more.
02:20 They use a square wave where the output is switched between a high-voltage and zero-volts, or ground, many times per second.
02:27 The output that we want to operate can be controlled by varying the amount of time that the output is in the on state compared to when it's in the off state.
02:36 You can think of a PWM output a little like a light dimmer.
02:40 By varying the amount of time that the output is in the on state, we can control the light all the way from completely off to completely on, and anywhere in between.
02:49 As I've mentioned, when it comes to these complex waveforms, getting a real picture of what's happening is not possible with a simple digital voltmeter.
02:58 We need to be able to actually see how the voltage is changing in relation to time.
03:02 To do this, we use an oscilloscope.
03:05 An oscilloscope gives us a visual display of the voltage in the circuit, and how it's changing with regard to time, allowing us to visualise these waveforms.
03:15 Most oscilloscopes will also offer dual inputs so that we can overlay and examine two waveforms at the same time.
03:22 Now that we've seen what these waveforms look like, we're going to learn about the components that make them up, and the terms that are used to describe them.
03:31 The first term we'll discuss is amplitude.
03:34 Amplitude is measured in volts, and represents the peak voltage level reached in a waveform.
03:39 However, there are two ways of discussing the amplitude in a waveform, depending on what we want to measure.
03:45 Peak amplitude is the maximum voltage of the waveform measured from the zero-volt, or equilibrium point in the waveform.
03:54 Peak to peak amplitude, on the other hand, is useful in an AC waveform where the voltage is alternating between positive and negative values.
04:02 Peak to peak voltage expresses the total change in voltage between the maximum positive and negative peak values.
04:09 When we look at a pulse width modulated square wave output from the ECU, there are a few more parameters to consider.
04:16 First, we have the actual pulse width, which is the amount of time that the output is in the on state.
04:22 Pulse width's are typically very short amounts of time, and are often expressed in milliseconds.
04:27 A millisecond is simply one thousandth of a second, or to put it another way, there are 1,000 milliseconds in one second.
04:35 To put this in context, when we're discussing injector opening times, we'd normally express these as a pulse width in milliseconds.
04:42 Next, we have the cycle time.
04:44 This is the total length of time it takes to complete one waveform.
04:48 Regardless, whether we're discussing a reluctor input to the ECU, or a pulse width modulated output, we'll have a waveform that repeats itself over, and over in regard to time, and the cycle time details how long one complete cycle takes.
05:02 Cycle times are usually also very short, and also commonly expressed in milliseconds.
05:07 The next component is duty cycle, which is the amount of time an output is in the on state compared to the cycle time.
05:15 For example, if we have a pulse width modulated output with a cycle time of 20 milliseconds, and it's in the on state for 5 milliseconds, then this output is on for 1/4 of the available cycle time.
05:27 This means the duty cycle is 25%.
05:30 To calculate the duty cycle, we simply divide the time that the signal is in the on state with the time for complete cycle.
05:39 For this example, five divided by 20 equals 0.25, or 25%.
05:45 The last component we need to understand is frequency, which is the number of complete waves or cycles which occur per second.
05:53 Depending on what we're measuring, this could be anywhere from 10 to 20 cycles per second through to many thousands or even millions of cycles per second.
06:02 Frequency is expressed in hertz, but with many of the outputs we're interested in, the frequency may be expressed in kilohertz, or even megahertz.
06:11 These terms represents thousands of cycles per second and millions of cycles per second, respectively.
06:18 If we know how long a single cycle takes, we can easily calculate the frequency, or, alternatively, if we know the frequency, we can calculate the cycle time.
06:27 All we need to do is use the inverse function on our calculator.
06:30 For example, if we have a frequency of 20 hertz, then the inverse of this is one divided by 20, which equals 0.05 which means the cycle time is 0.05 seconds.
06:43 Remember that we normally discuss cycle times in milliseconds, and if we multiply 0.05 seconds by 1,000, this will convert the units to milliseconds since there are 1,000 milliseconds in a second, so the result of our calculation is 50 milliseconds.
06:59 Likewise, if we had a cycle time of 5 milliseconds, then we can find the frequency by finding the inverse.
07:06 Again, we need to express the cycle time in seconds so we divide five by 1,000, which equals 0.005.
07:14 Now, we find the inverse, which is one divided by 0.005, and the answer is 200 hertz.
07:21 At this point, you should have a thorough understanding of waveforms, as well as the components that make them up.
07:26 In particular, it's important to understand how the cycle time, frequency, pulse width, and duty cycle relate to the waveform, and how to calculate these parameters.