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Boost Control: Wastegate Operation

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Wastegate Operation


If you need further clarification of how a Turbo System works, watch this video below:



00:00 - As I've mentioned in the last module, this course will focus specifically on boost control using a traditional pneumatic wastegate actuator.
00:09 I find that one of the most common causes of boost control issues is due to incorrect connection of the wastegate and boost control solenoid, so a solid understanding of how the wastegate functions is the first place to start.
00:23 You can rely on the plumbing instructions that come with a boost controller, or are provided by your ECU manufacturer.
00:30 But if you actually understand the fundamentals, then you can work through this yourself and it will make it easier to fault find issues with your plumbing.
00:39 The wastegate can be broadly broken down into internal wastegates, which are housed inside the exhaust housing of the turbo, and external wastegates that are located remotely to the turbo charger on the exhaust manifold.
00:53 Regardless which type of wastegate you're using, the operational principles are the same.
00:58 The wastegate is a valve located somewhere in the exhaust flow that controls and directs the flow of exhaust gas.
01:06 When the wastegate's closed, all of the exhaust flow is directed into the exhaust housing of the turbo charger, and hence maximum energy is provided to the turbine wheel.
01:17 When the wastegate valve is opened, some portion of the exhaust flow is bypassed around the turbo charger.
01:24 This wastegate flow may either be directed back into the exhaust system downstream of the turbo charger, or alternatively dumped straight to atmosphere.
01:33 But either way, the effect is that the energy being provided to the turbine wheel is reduced.
01:41 By controlling the amount of exhaust flow bypassing the turbine wheel, the rotational speed, and hence boost pressure, can be controlled.
01:50 All things being equal, the more the wastegate closes, the higher the boost pressure, while the more the wastegate is opened, the lower the boost pressure.
02:01 As well as the actual valve located in the exhaust flow, the wastegate assembly also includes a chamber and a diaphragm, which is acted on by boost pressure.
02:12 The diaphragm in the wastegate actuator is connected to the wastegate valve so that as the position of the diaphragm moves, the valve is opened or closed.
02:22 The diaphragm is also acted on by a spring that naturally pushes the wastegate into the closed position.
02:30 Now we have a basic understanding of the components of a wastegate, we'll discuss how the wastegate actuator functions.
02:39 Let's start by considering a simple installation where an internal wastegate actuator is pneumatically connected directly to boost pressure.
02:50 Under normal conditions, the spring pressure acting on the diaphragm will force the wastegate valve shut to allow full exhaust flow to reach the turbine wheel.
03:01 As the turbo begins to produce boost pressure, this pressure will be supplied to the wastegate actuator and will start acting on the diaphragm.
03:10 At some point, the pressure acting on the diaphragm will be sufficient to overcome the spring pressure and the diaphragm will move, in turn opening the wastegate valve.
03:22 When this happens, some of the exhaust gas will bypass the turbine wheel, controlling the turbo charger's speed, and hence, boost.
03:31 As the boost pressure increases, the wastegate will open further, bypassing more flow from the turbine.
03:38 At some point, the wastegate will reach an equilibrium point where sufficient exhaust flow is being bypassed through the wastegate to maintain consistent boost pressure.
03:48 Now let's look at how we can influence the boost pressure.
03:52 If we were to reduce the amount of boost pressure reaching the wastegate actuator, the result would be that less pressure was available to overcome the spring, and the wastegate actuator would not open as far.
04:04 In turn, less exhaust flow would bypass the turbo charger and hence the turbine speed and boost pressure would be increased.
04:13 If we take this to extremes and remove the pressure source altogether, there's no pressure available to open the wastegate valve and boost pressure would rise uncontrollably.
04:24 Actually, as we'll see in the next module, boost pressure in this situation wouldn't rise indefinitely, but either way, it's likely to be dangerous.
04:34 The effect of boost control, either pneumatic or electronic, is to manipulate the pressure signal reaching the wastegate to raise or lower the boost pressure reaching the engine.

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