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. |
| 04:42 | The 3-port solenoid offers some advantages, particularly when dealing with an external wastegate, and in the boost control plumbing module we'll discuss the techniques for connecting the solenoid pneumatically. |
