I have a question that is probably too dumb. Here goes:

say a truck engine uses a turbo and the wastegate is not integral, it's separate. the spring inside needs to know the boost output to justify opening and diverting excess boost away from the throttle body. that to me means it must be located after the turbo, before the throttle body, or else how would it know how much boost is occurring.

If it is after the turbo, why do I sometimes read that it diverts exhaust away from the turbine wheel..to accomplish, it would need to provide an outlet in FRONT of the turbo, not BEHIND.

I can only imagine it sits after the turbo and therefore, ALL the force of the exhaust is applied on turbine ALL the time. Is this true? Can someone explain?

No need to explain BOVs, they're self explanatory.

 

Thread moved to General Diesel Discussion.

-Matt

 

The wastegate passage is a port that begins upstream of the exhaust wheel housing, goes through the wastegate valve and dumps downstream of the exhaust wheel. It's purpose is to lower the drive pressure of the exhaust stream. In that way it controls the amount of boost that the turbo is able to generate. Open the wastegate valve and some of the exhaust pressure is vented so that there is less pressure available to spin up the turbine.
Hope this helps.

 

but if it's ahead of the turbo, how does it know when there is excess boost (eg. more than 10 psi). i would think it could only know about excess backpressure in the exhaust system ahead of the turbine wheel

 

Wastegates are hooked into the vacuum system and have a calibrated (if you want to call it that) spring inside so that it only opens at a certain boost level. The spring can be changed out to achieve different boost levels.

Blowoff valves work much the same way, actually.

 

one thing to remember is a diesel does not have a throttle body (or any type of valve that opens and closes) it uses an open "un throttled" intake system. the wastegate has a sensor in the intake track that measures boost and tells the wastegate actuator when to start opening. many people dissconnect the line that goes from the sensor to the wastegate actuator to delay opening of the wastegate, at that point the only thing holding the wastegate shut is the spring in the actuator housing. once that spring pressure is overcome by pressure in the exhaust side turbine housing it will push the wastegate open. the spring pressure is adjustible by changing the length of the actuator rod (longer = less tension, shorter = more tension)

 

This might help:
http://auto.howstuffworks.com/turbo6.htm

 

So it physically sits ahead ahead of the turbo, but has a spring -- or at least a sensor for a spring -- after the turbo, before the engine? That makes sense, I just want to make sure I understand. I dont think that website does not get detailed enough.

 

I'm not sure how wastegates are exactly hooked up on a diesel engine, but on turboed gas engines, like I said the wastegate is hooked directly into the vacuum system. Since the entire vacuum system sees boost while the turbo is boosting, this same amount of boost reaches the wastegate and once it hits the predetermined boost level the wastegate starts to open and eventually opens completely. Of course, that's on a regular vacuum operated wastegate. Do some diesels use electrically-operated wastegates?

 

on a diesel it is not ahead of ,or in front of, or what ever of the turbo. there is a hole in the exhaust side compressor housing that has a valve on it that opens and closes...thats it

 

Our 7.3 has electronically controlled WG valve.
The 6.0 has electronically controlled variable vane, no wastegate. MUCH better system.
Just about every modern diesel engine and some gas, all have computer controls for the wastegate or exhaust housing vanes. They control it by manipulating the air pressure to the canisters that operate the wastegate or the vanes.

 

Well, that's an internal wastegate...he was asking about an external wastegate which is "before" the turbo, or actually between the exhaust manifold and the turbo.

 

do make note the 6.0 vanes are controled by manipulating oil pressure to it.

 

Ahh, thank you. I was not aware of that.

 

Thanks to all for the input. You've told me what I couldn't get from the turbo "intros" Ive read.