Author Topic: Can VEMS control a variable geometry turbo?  (Read 18380 times)

Offline BimmerDread

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Re: Can VEMS control a variable geometry turbo?
« Reply #15 on: December 08, 2010, 03:25:27 pm »
I suspect it would. I am going to test an old one to see how much leakage occurs. I wonder what pressure would damage the diaphragm. I believe that they are good to ~4bars and it should not see more than 5bars under any driving conditions.

Offline paul_f

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Re: Can VEMS control a variable geometry turbo?
« Reply #16 on: December 08, 2010, 09:30:17 pm »
Early VGTs on diesels all used vacuum and an N75 type valve to control the position.   Surely that would be easier than oil?

Offline Sprocket

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Re: Can VEMS control a variable geometry turbo?
« Reply #17 on: December 09, 2010, 07:17:19 pm »
I thought the purpose of the variable geometry compressor was to keep the compressor in its operating limits at varying mass flow conditions. For example, at low load rpm mass flow is low, but at high load high rpm, mass flow is high. To satisfy the high mass flow you need a big compressor which falls off the map at the low mass flow. Inversely  a compressor that satisfies the low mass flow falls of the map at the high mass flow. Variable geometry compressors allow a much bigger compressor to work at the low mass flow and keep it on the map.

Thats the way at least it works on our VERY large turbo compressors with the variable diffuser opening up as the mass flow increases. It is CRITICAL that the diffuser is in the correct position during operation. Failure to have the diffuser in the correct position results in catastrophic failure of the bearings, which results in damage to the compressor wheel, rendering the £10,000 impellor, scrap! Not so much of a bother with a small turbo charger costing maybe £450 all in. Incorrect diffuser position creates a little know phenomenon called 'Rotating Stall', is a pre curser to surge, and is as damaging if not more so.



I am not saying that the variable geometry turbos work the same way. Awareness of the phisics of why it is variable geometry in the first instance, have the data on the diffuser position at varying mass flows before deciding on how to control it. It could be as simple as an on off actuator as suggested, but I remain sceptical. Having said that, the small turbo chargers appear to be much more tollerable of unsatisfactory conditions than our turbo compressors are.

One of the lads over on Turbo Minis used a VNT turbo and used another actuator to control it. Not really sure if it worked that well.
« Last Edit: December 09, 2010, 07:22:12 pm by Sprocket »

Offline gunni

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Re: Can VEMS control a variable geometry turbo?
« Reply #18 on: December 09, 2010, 11:19:35 pm »
a VGT from Holset is designed to accelerate the exhaust air onto the turbine blades the best possible to create torque on the compressor axle. This is done by varying the area the discharge has on to the blades.


Offline BimmerDread

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Re: Can VEMS control a variable geometry turbo?
« Reply #19 on: December 10, 2010, 01:09:04 am »
Early VGTs on diesels all used vacuum and an N75 type valve to control the position.   Surely that would be easier than oil?
I thought about that as well where boost pressure would be used to switch the system on. However if you look at my function map,there is a case were boost pressure is needed to activate the valve when boost is not available.
« Last Edit: December 10, 2010, 01:31:28 am by BimmerDread »

Offline BimmerDread

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Re: Can VEMS control a variable geometry turbo?
« Reply #20 on: December 10, 2010, 01:41:54 am »
Sprocket, you mention varying the compressor or cold side of the turbo. The VGT turbos vary the size of the turbine housing or hot side. There is till a concern for surge if the there isnt enough flow at 2000rpm to support the suggested 1bar boost level. I will triple check the compressor map :).

Offline z0tya

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Re: Can VEMS control a variable geometry turbo?
« Reply #21 on: December 10, 2010, 08:02:48 am »
This vgt turbos are good for petrol engines? Are they accept the higher egt temp?

Offline BimmerDread

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Re: Can VEMS control a variable geometry turbo?
« Reply #22 on: December 13, 2010, 02:52:44 pm »
They are being used with good success. It seems the additional heat doesn't bother them too much. It is always better to use a tubular manifold to get rid of some of the heat pre-turbo. The issue with the older VGT's is the shaft wearing out earlier than the non VGT's.

I did the calculations and 1 bar of boost @2000rpm will create surge. So I will limit it to 0.5 bar @2k and 1.5 bar for the rest of the range. ;D