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Author Topic: How WasteGates work WG  (Read 3560 times)
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« on: December 11, 2009, 06:31:46 AM »


My goal with this thread is to take all of the mystery out of one of the fundamental turbo controls, the wastegate. I am the designer of the Synchronic wastegate, so any answers from me will be straight from the horse's mouth.

For those of you that were out at SCSN, I was the guy floating around Ed Thornton's pits and we had a little booth at the vendor midway too. Thanks to Ed, Doug, Tom and the team for letting me dig in and learn a thing or two about the first hand, and also being able to dig into the data. After some good conversations with Kenny Duttweiller, I'm confident that some of my views on wastegating in this thread are on the same page as his. And he's a guy with lots of engine dyno time playing around with wastegates.

Now, onto wastegates. There's a couple of basics to understand:

1) You can't spool a turbo faster than when a wastegate is completely closed
2) You can make more horsepower with a wastegate once it is open and controlling boost.

A turbo works by having the exhaust gas energy spin a wheel (turbine) on a common shaft that spins a wheel that compresses air into the engine. A wastegate works by taking away exhaust energy from that turbine once the desired pressure is achieved. Without a wastegate, the turbo will keep spinning faster to produce more pressure until the compressor runs out of airflow.

Wastegates are misunderstood by many as a basic device that just gets you to your target boost pressure. But a wastegate can take any turbo, efficient or not, and really make big difference in horsepower. More on that later.

A wastegate is a basic mechanical valve as easy to understand as a throttle body or pump. The valve moves based on a basic principle, pressure acting on a given surface area. So, let's say that you have an area that is 12 cu" and you apply 10 psi to it, you will have 120 pounds of force.

Your old-technology wastegate has 2 surface areas, top of the diaphragm and the bottom of the diaphram. The top of the diaphragm keeps the WG closed and the bottom opens it up. Usually, you have a ratio between the two of 1:0.9 depending on how much area the valve stem takes up.

Now, here's the part that everyone completely neglects. You also have to factor in the amount of surface area of the valve face when trying to figure out when the WG is going to open. The bigger the valve face, the harder it will be to keep the WG closed on the two step, or the harder it will be to keep it closed under high backpressures. Think of the area of the valve face like it is a pressure sensor that is reading the backpressure in the header. The position of valve is directly related to the amount of pressure in the header and the boost pressure that is opening up the bottom part of the diaphragm. That is why it doesn't work to just put shop pressure into the diaphragm on the bench and try to figure out when it will open, because you have to account for the pressure acting on the valve also.

For the sake of simplicity, you have to add together both areas A & B in Green to determine how much the valve is going to lift. So, areas A & B are the only areas that open the WG valve.

Knowing your backpressure is very important, because this is one of the factors in determining why you are or are not building boost, detonating, or even making the most horsepower that your motor can make.

Now, how high your valve will lift will be determined by how much spring pre-load that A+B are fighting against. The more spring pre-load, the higher the boost pressure that will be produced by the system.

A: Actuator area to open the valve
B: Valve face area to open the valve
C: Spring pressure to keep the valve closed, pressure to overcome

(A+B) - C = Indicated Manifold Boost Pressure (some call it IMEP)

We're going to use the my 50mm WGs to do the math in this example:

A= 4.4 cu" of surface area to open the valve
B= 3.0 cu" of surface area to open the valve
C= 80 psi of spring force keeping the valve closed

Let's assume 10 psi of intake manifold pressure, and 15 psi of exhaust manifold pressure.

A: 10*4.4) + B: 15*3.0) - C:80 = 9 psi, valve is open
A:44 psi + B:45 psi - C:80 psi = 9 psi, valve is open

If you chart this out on a graph, you'll notice that the valve will be closed until you exceed C. So you will keep building boost pressure until the valve opens and C is overcome by the pressures in A+B.

At it's most basic, this is how wastegates work. You always have to factor in the valve face area to really determine what is going on with the wastegate. The WG is the most important control in determining how much boost your turbo will make. It is the end all and be all that controls your boost level.

The First, DASTEK UNICHIP TECHNICALLY THE BEST; If you need more POWER, you need more AIR(cmf)
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