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.
