Marcus - big kudos to you for building this and trying it out.
It looks like you are building what we refer to in the turbo engineering world as a "series sequential" system. By implementing the turbine bypass valve between the two turbine stages, you are allowing the larger low pressure (LP) turbo to be shut off at low loads and low engine speeds, and allowing the smaller high pressure (HP) turbo to operate on its own. Once there's enough exhaust energy to power the larger LP turbo then you open the valve and allow both the turbines and compressors to operate in series, giving you higher boost than would be possible from either turbo individually.
If instead you wish to completely switch between the LP and HP stages at some point in engine operation, then you are talking about a pure sequential system, i.e. the two turbos are operating in timed sequence, one after the other, but never one
into the other - never in series.
Can you clarify which type of system you're going for here? Or maybe planning for either possibility?
A few thoughts after looking at your system photos:
- If you open the wastegate which vents to atmosphere after the HP turbine, then you may be essentially shutting down the LP turbo completely (depends on flow capacity of LP turbine and of the wastegate).
- If you open both the atmospheric wastegate and the inter-stage wastegate, then you are bypassing both turbine stages.
- If you want to run this as a series-sequential system (first type described above) then you will want a compressor bypass valve between the HP and LP stages. This can be a passive spring loaded flapper valve, but it's needed because without it, when your turbine bypass valve is open and you want the LP to run on its own, then the LP compressor will by trying to force-feed the HP compressor which won't be actively working, so it will effectively just be a restriction. Adding a compressor bypass valve would let the LP boost pressure dump directly into the intercooler and on to the engine. This valve would need to be about the size of your charge plumbing between the LP compressor and intercooler, not any smaller.
- Again if you want to run in series, an inter-stage cooler between the two compressors is an absolute must-have in my opinion. Without it you will likely overheat the HP compressor and kill efficiency, because the air coming into it will already be quite hot after being compressed by the LP compressor.
In general terms, the reason the large compressor feeds into the smaller one is a function of how radial compressors work. They just multiply the inlet pressure via pressure ratio to get to an outlet pressure. They really don't care what the value of the inlet pressure is... whether it's atmospheric or already boosted (by the LP stage) doesn't matter much because the air flow correction accounts for that. If you look at a compressor map, the flow axis is always corrected flow, not actual physical flow. At atmospheric conditions these are essentially the same. But when you pre-compress the inlet air with another turbo, the corrected flow rate actually goes down, meaning a smaller compressor is a better match for this already-boosted airflow. If you swapped the two turbos around then they would both be very poorly matched for the conditions they would see, and performance would be terrible. The smaller turbo would also then act as an inlet restriction which would severely limit your total power and negate the reason for using a larger LP turbo in the first place.