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Boost Control: Boost Control Solenoids

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Boost Control Solenoids

04.57

00:00 - In the last few modules I've talked about pneumatic boost control, which will be the situation we'd achieve with a manual boost tee, or bleed valve.
00:09 However, this course focuses on electronic control, and to achieve this we need an actuator that the ECU can operate to control the pressure signal to the wastegate actuator.
00:20 This is achieved with a boost control solenoid.
00:24 These solenoids are a valve that's opened or closed by a magnetic coil, and the passage of air through the solenoid will depend on whether the coil is energised or not.
00:35 Switching the solenoid on or off will direct the boost pressure to the wastegate, and hence we can alter the boost pressure.
00:44 Just switching the solenoid on or off is not much use though, as the boost pressure in the system will change very quickly.
00:51 To achieve good control the ECU provides a PWM, or Pulse Width Modulated signal to the solenoid.
01:00 A PWM signal is a square wave that consists of periods when it's in an "On" state and periods when it's in a "Off" state.
01:09 When talking about PWM waveforms we also need to know the cycle time, which is the amount of time it takes to go through a full cycle of On and Off.
01:21 The output from the ECU might be a waveform that looks something like this.
01:26 You could think of it like a switch that the ECU is repeatedly turning on and off very quickly.
01:33 Often, when we're discussing the output to the boost solenoid, we will use the term "duty cycle".
01:39 And this refers to the time the waveform is on, as a percentage of the total cycle time.
01:46 I'll explain in more detail shortly, but first, there's another term that's important, and this is the frequency of the signal.
01:55 A PWM output operates at a specific frequency which is the number of cycles per second.
02:02 A boost control solenoid might typically operate at a frequency of 20 Hz for example, and this means that the ECU provides a signal that goes from the On state to the Off state, 20 times in one second.
02:16 If the frequency was 20 Hz, for example, this means that the cycle time, or the time taken for a single cycle, is 1 divided by 20, which is 0.05 seconds, or 50 milliseconds.
02:32 Now we know this, we can discuss duty cycle.
02:36 As I already mentioned, duty cycle is the percentage of time that the ECU is commanding the solenoid to be on.
02:44 Let's say for example, the waveform was in an energised state for 20 milliseconds.
02:49 In this case, the pulse width being supplied to the solenoid is 20 milliseconds, and 20 divided by 50 equals 0.4, or 40%.
03:00 So the duty cycle is 40%.
03:04 By adjusting the pulse width supplied to the solenoid the ECU can adjust the percentage of time that the solenoid is on.
03:13 Now, if we were just to switch the pressure to the wastegate actuator on and off, this would result in very erratic boost pressure.
03:21 Since this is all happening 20 times a second though, this has the effect of averaging the pressure signal reaching the wastegate actuator, and hence delivering smooth boost pressure.
03:33 Now that we know how the ECU controls the solenoid, we need to discuss the types of solenoid available.
03:40 This can be broken down into 2-port and 3-port.
03:45 A 2-port solenoid has an inlet and an outlet and these are connected or blocked, depending on whether the solenoid's energised.
03:53 These are quite common in an OEM application, and are used to bleed pressure away from the wastegate actuator.
04:01 For our purposes in the aftermarket they aren't ideal though, and the 3-port solenoid is usually preferable.
04:09 A 3-port solenoid has 3 ports that are labelled COM, or Common, NO, or Normally Open, and NC, or Normally Closed.
04:20 With this type of solenoid, the airflow through the solenoid is directed between the ports, depending whether the solenoid is energised or not.
04:30 When the solenoid isn't energised, the Common and Normally Open ports are connected together.
04:36 And when the solenoid is energised, the Common and Normally Closed ports are connected.