This is a sequel to our last post, “How the Wastegate Works”. If you didn’t read it, you may want to go back now.
So, now that we have the basic operation of the wastegate down, we’ll want to add a boost controller and get some control over the boost.
So, what is a boost controller?
The system as described so far, has a direct feedback from the boost source to the wastegate actuator. So, boost pressure attained is based solely on the spring in the wastegate diaphragm. Set it to 7 psi and you get 7 psi, no more. However, most turbo systems have an electronic or manual boost controller. The control is simply a “leak’ between the boost pressure signal and the wastegate. We could in theory, punch a hole in the line running from the boost source (either the compressor outlet, or the manifold system) and “bleed” off some of the pressure, which would allow for more boost in the intake manifold, than would be present at the wastegate actuator. In practice however, you must place a restriction in the line, before the leak. Otherwise, there will be more flow from the boost source, than can be overcome by the leak. In other words, you will be filling a bucket with a 1 inch hose, that has a 1/4 inch leak. You need to restrict the flow, so the pressure on the other side of the leak, drops. This is accomplished by a change in hose diameter, or a restrictor pill. The restrictor pill is simply a hose coupler, with a small passage through the center. If you were to add a needle valve to the “leak” you would create an adjustable system, as shown below.
Note that the image also shows a check valve. While not needed on a boost only source (such as the turbo outlet), this wastegate is sourced from the intake manifold. The intake sometimes has a vacuum condition, which would result in a vacuum leak. We’ll go into this in detail, in our E-Book. For now, we only need to be aware of it.
How the Boost Control Solenoid Works:
On modern factory systems, the wastegate control circuit uses a solenoid valve, controlled by the ECU (Electronic/Engine Control Unit–the brain). Under certain conditions, this solenoid valve opens, “bleeding” off a portion of the boost signal. The net result is exactly the same, but instead of opening and closing a needle valve, manually, the solenoid does it automatically.
You can think of the needle valve as an “analog” system, where there is always a portion of boost pressure being “bled” off. The solenoid is more of a “digital” system, where the valve is either open or closed. By pulsing this action, you mimic the needle valve. That’s IT! That’s all the factory system does to control the boost. So, when someone tells you that a needle valve bleed will destroy your engine, they are at best, under-informed. Both systems function in a similar fashion. Each has minor advantages and disadvantages. But they all are some variation of the following illustration, with the “going to the solenoid” line leading the the bleed/leak:
The third type of boost control is the checkvalve system. That is our personal favorite, but we will discuss it in the next post.
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