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Boost control for twin turbo set up
Sunday 20th October 2013
I'm starting to rough wire my tt v8 and I'm wondering wether there is an issue with running both wastegate signal lines from a single solenoid , base boost is 6psi and I don't have any room for an intercooler so I'm using a water meth system with a controller which has a map sensor input so doesn't need a trigger from the ecu .
Sunday 20th October 2013
I'm starting to rough wire my tt v8 and I'm wondering wether there is an issue with running both wastegate signal lines from a single solenoid , base boost is 6psi and I don't have any room for an intercooler so I'm using a water meth system with a controller which has a map sensor input so doesn't need a trigger from the ecu .
There are no issues with a single solenoid. I would prefer a single over solenoid per turbo/wastegate anyway. Less variables.Sunday 20th October 2013
You want to aim for each turbo to be doing the same amount of work, and that in turn means the same back pressure per bank. Assuming you have a common intake plenum boost pressure will get effectively 'averaged' by the plenum volume, but the turbo work can be unbalanced. This results in unbalanced pre turbine exhaust back pressure and hence different intake mass flow per bank (and potentially different AFR per bank - not good.) (and in really bad cases, potential for turbo overspeed too)
So, a good solution is a pair of control solenoids, driven from a common (electrical pwm)source, that are fed with compressor bleed air taken from as close to each of the compressor outlets as possible. In that way, the negative feedback of each compressor/wastegate system will naturally tend to correct excessive imbalances. Of course, you need to carefully set up the wastegates for preload etc to ensure a decent balance is achieved.
Only using one solenoid means you are forced to only take bleed air from a single common point, and hence cannot take advantage of the negative feedback of each wastegate loop.
So, a good solution is a pair of control solenoids, driven from a common (electrical pwm)source, that are fed with compressor bleed air taken from as close to each of the compressor outlets as possible. In that way, the negative feedback of each compressor/wastegate system will naturally tend to correct excessive imbalances. Of course, you need to carefully set up the wastegates for preload etc to ensure a decent balance is achieved.
Only using one solenoid means you are forced to only take bleed air from a single common point, and hence cannot take advantage of the negative feedback of each wastegate loop.
Monday 21st October 2013
Surely as the MAP reading will be coming from a single point...ie the intake manifold.
Your boost control setup controlling the solenoid/s will be doing the same thing anyway ?
Having 2 solenoids stands more chance of each turbo doing different amounts of work ?
Your boost control setup controlling the solenoid/s will be doing the same thing anyway ?
Having 2 solenoids stands more chance of each turbo doing different amounts of work ?
Monday 21st October 2013
To clarify its a 1uzfe Lexus v8 on stock 10-1 compression . No way of intercooling without altering the front grill position which would spoil the look of the car .
My ecu won't run two solenoids but has various outputs so I can trigger the meth off iat in low boost and trigger it when running higher boost which would only be 10psi .
My ecu won't run two solenoids but has various outputs so I can trigger the meth off iat in low boost and trigger it when running higher boost which would only be 10psi .
Monday 21st October 2013
Correct. However, although the basic 'target' boost control signal from the ecu to the solenoids will be the same for both turbos (and hence the average boost pressure in the common plenum will be correct) each individual turbo has a basic pneumatic/mechanical negative feedback pressure loop to itself that works in conjunction.
Lets say you have more preload on the LHS wastegate actuator (or a stickier wastegate, or more efficient turbine or whatever):
So, if you just take a single common pressure from the plenum to each w/g actuator, the LHS turbo will on average have a more 'closed' wastegate, and hence will need to build more pre turbine pressure to vent the same exhaust mass flow. This means it is doing more work than the RHS turbo with the softer preload, and that imbalance in turbo work and back pressure results in AFR distribution errors etc
Option 2, taking each individual compressor exit pressure (pre intercooler, from as close to the scroll exit as possible) through two separate control valves and to their respective w/g creates a second feedback loop that acts to balance the turbo work. In our imbalanced wastegate preload example above, the LHS turbo will produce a higher compressor outlet pressure than the RHS (because it will have a larger post comp intake pressure loss due to the higher massflow), that increased pressure will act purely on its own w/g and push it open a bit more, reducing boost.
What you find is that you end up with several boost control factors all interelating at different frequencies. The overall plenum pressure gets controlled by the ECU (measures MAP, commands bstCnt sols to an average position) and each turbo acts to balance it's own work.
If you measure turbo shaft speed (or wastegate actuator position) you find that you get a heterodyning effect, where the average speed (or position) oscillates up and down based on the ecu control response, and the individual speed (or position) oscillates in antiphase to each other around that average position!
You end up with turbo speeds (and hence work) that looks something like this (although not as nicely sinusoidal obviously)
Balanced:
Unbalanced:
The minor differences between control valves are less important that the major differences due to w/g preload and the actual efficiencies of the turbochargers themselves.
Lets say you have more preload on the LHS wastegate actuator (or a stickier wastegate, or more efficient turbine or whatever):
So, if you just take a single common pressure from the plenum to each w/g actuator, the LHS turbo will on average have a more 'closed' wastegate, and hence will need to build more pre turbine pressure to vent the same exhaust mass flow. This means it is doing more work than the RHS turbo with the softer preload, and that imbalance in turbo work and back pressure results in AFR distribution errors etc
Option 2, taking each individual compressor exit pressure (pre intercooler, from as close to the scroll exit as possible) through two separate control valves and to their respective w/g creates a second feedback loop that acts to balance the turbo work. In our imbalanced wastegate preload example above, the LHS turbo will produce a higher compressor outlet pressure than the RHS (because it will have a larger post comp intake pressure loss due to the higher massflow), that increased pressure will act purely on its own w/g and push it open a bit more, reducing boost.
What you find is that you end up with several boost control factors all interelating at different frequencies. The overall plenum pressure gets controlled by the ECU (measures MAP, commands bstCnt sols to an average position) and each turbo acts to balance it's own work.
If you measure turbo shaft speed (or wastegate actuator position) you find that you get a heterodyning effect, where the average speed (or position) oscillates up and down based on the ecu control response, and the individual speed (or position) oscillates in antiphase to each other around that average position!
You end up with turbo speeds (and hence work) that looks something like this (although not as nicely sinusoidal obviously)
Balanced:
Unbalanced:
The minor differences between control valves are less important that the major differences due to w/g preload and the actual efficiencies of the turbochargers themselves.
Monday 21st October 2013
My ecu won't run two solenoids
Check the current capacity of the PWM output driver in the ecu, it will probably run two solenoids in parallel. If not, then one TIP102 transistor and a 1k resistor are all you need to beef up the output current (although that will invert the signal, if you can't set in s/w the effective 'direction' for more boost, you will need to use two transistors!)Monday 21st October 2013
Both blowers are new and have a direct action onto the wastegate valve so should behave the same , as I'm not going anywhere near the limit of what the turbos can do I should be ok .
I still don't see how I can get a meaningful measurement on individual output as it will most likely spool too quick to see any difference in pressure between the two turbos
Edited by mighty kitten on Monday 21st October 13:48
Monday 21st October 2013
To clarify its a 1uzfe Lexus v8 on stock 10-1 compression . No way of intercooling without altering the front grill position which would spoil the look of the car .
My ecu won't run two solenoids but has various outputs so I can trigger the meth off iat in low boost and trigger it when running higher boost which would only be 10psi .
Any ecu will run 2 solenoids. Even boost controllers just pair the solenoids if you want to run two.My ecu won't run two solenoids but has various outputs so I can trigger the meth off iat in low boost and trigger it when running higher boost which would only be 10psi .
I dont think Max is suggesting 2 independently controlled solenoids ? as that would be very extreme IMO.
Thursday 24th October 2013
Forgive me if wrong, but my understanding of feedback loops says that there is no point in having two boost solenoids if they are connected to the same control signal from the ECU. The PWM command will be common irrespective of which turbo is over/under boosting?
Thursday 24th October 2013
Forgive me if wrong, but my understanding of feedback loops says that there is no point in having two boost solenoids if they are connected to the same control signal from the ECU. The PWM command will be common irrespective of which turbo is over/under boosting?
I thought so, but I can see what Max is saying. Although valve duty is the same, airflow and pressure through the valve may not be on each side. SO it does still help things.Thursday 24th October 2013
My thinking is that as long as the gates which are still on the factory setting should be close enough to work fine with a single solenoid and back pressure should be pretty close as the compressors feed a shared plenum .
Anyway the consensus in the shop is to run both lines into an old purge valve then into an mbc so we can have a big boost button on the gear stick
Anyway the consensus in the shop is to run both lines into an old purge valve then into an mbc so we can have a big boost button on the gear stick
Friday 25th October 2013
nobles run a single solonoid to control the twin turbos.
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