MeOughtta

First post on MT. Flame away!

Several ECUs support throttle position sensitive boost control (TPS BC). These ECUs have a built-in boost controller for which the boost target is determined by he throttle position. (The boost target increases with the throttle position.)

But the minimum boost is still determined by the can spring. Right? So where I might wish I could have 0 boost until, say, 33% throttle then ramp up to 15psi at full throttle, I can't do it. If I have a 4psi spring on the can, then the what I'd get would be a 4psi boost target up to 33% throttle then ramping up to 15psi at full throttle. Is that correct?

My question: Is there any way to make TPS BC work with, effectively, no spring? Or is the boost control just not stable without some base level boost target?

M.

JasonC SBB

There has to be some spring and some minimum boost.

Joe Perez

iTrader: (8)

Given a conventional wastegate actuator, the spring is what actually holds the valve closed. If you removed the spring, then the wastegate itself would simply flop open all the time. In fact, very weak springs are sometimes inadequate to hold the valve fully closed when running small turbos at high load. Many of the folks on this board using the TD04H-15G turbo have found it necessary to add an additional "helper" spring to the wastegate arm to hold it closed.

Now, if you were to invent some mechanism other than a pneumatic servo to operate the wastegate arm, such as an electronic actuator, then you could indeed do whatever you wanted with regard to target boost.

All that being said however, I find that even with a simple MBC, the gas pedal itself does a thoroughly adequate job of regulating boost. If I press it down 1/2 way, I get maybe 4 or 5 PSI. If I press it down fully, I get 13 PSI. No fancy electronics necessary.

paul

iTrader: (21)

All of us aren't so lucky though to have a TD04H-15G. Some of us have to deal with faster spooling turbos and can see 8 psi by 40% throttle with no EBC help.

Wasn't Lazzer working on some electronic wastegate actuator? It would be sweet to have a switch to turn boost off. I think I might have to wire open my wastegate this winter.

Braineack

iTrader: (62)

my arm fell off yesterday morning on a 45 minute drive through a backroad....it was not fun.

MeOughtta

Are there any electric drive actuators that would be tough enough to live under the hood and fast/strong enough to EBC?

The thread has a #1 in it because I want to design and build the ultimate boost control setup. I don't think what I end up with will be practical -- it may not even be workable -- but it would be a real interesting exercise and if it did work would give the best of all worlds. 1) More linear throttle, 2) Reduced lag, 3) reduced backpressure (i.e. less heat and more MPG).

Here's what I'm thinking:
1) boost target determined by throttle position.
2) hit the boost target using servos on the blow-off valve as well as the wastegate -- by opening the blow-off instead of the wastegate, the turbo should stay better spooled during shifts.

what I think I need:
a) some way of sensing turbo RPMs so I can open the wastegate before overrevs.
b) some way of controlling a blow-off valve -- maybe by using a modified wastegate valve as a blow-off valve?
c) some computer which can accept the necessary sensors and produce the necessary output signals.

Ideas?

Joe Perez

iTrader: (8)

Welcome back, Lazzer.

JasonC SBB

MeOughtta,

I've been thinking about this for a bit too. But not because I want the boost to be TPS controllable to below 5 psi.

The whole reason for TPS controlled boost is so you don't get an unwanted wad of (full) torque at say 50% throttle. That would make throttle steering difficult.

I have a 5 psi wastegate can, and TPS controlled boost. So around say, 3500 RPM, I get 5 psi with something like 40% throttle. My boost control is set up go from 5 to 10 psi from 50% to 90% throttle linearly. The dead band from 40-50% is so my right foot can easily find 5 psi for puttering around without me having to look at the boost gauge. I have the AFR's fairly lean at 5 psi, and this gives me MPGs.

5 psi of boost isn't *that* torquey that it makes the car hard to control. We're talkin' a 1.8L motor after all. I have a supercharged M3, and that car's torque to weight ratio makes its throttle more sensitive than the miata. And the miata's throttle pedal has more stroke and is more controllable. I realized this when I autox'ed the M3. The miata was much easier to throttle modulate.

What I've been thinking of is to open the wastegate under vacuum. BMW's new 335i does this, in order to improve cruise MPG's. One way to do it is to have a 2nd actuator that overcomes the wastegate can when vacuum is under a certain amount. It could be some kind of solenoid connected by a long stiffish spring, sitting near the firewall so it doesn't receive much radiant heat from the turbo/manifold.

An external wastegate has both side of the diaphragm available. One side pushes the wastegate open with boost, the other side pushes the wastegate open with vacuum. You could use a weak (say 3 psi) spring and a boost control solenoid on each side, so that when you need the wastegate shut when you have some boost, you use boost on one side, and when you need it open during vacuum, you apply vacuum on it. Obviously if you need it to open during boost you apply boost on the other (usual) side. You could simply apply opposite signals to the 2 solenoids...


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During shifts, in a standard setup, the wastegate will still regulate at its can pressure. It doesn't open except for a very brief period that the boost in the compressor outlet spikes because the throttle has shut abruptly and the BOV hasn't quite opened yet.

You can buy a cheap 12V operated solenoid valve from the plumbing section of a hardware store to act as a BOV.

I do know there are sensors in some pricey hi end turbos that sense shaft RPM and the signal is useful for the boost control strategy.

MeOughtta

That is the main reason, but I'm also going for max economy (= maximum efficiency = minimum heat loading) It is just a mental exercise right now, but ...... Oh hell let me just go ahead and reveal how whacked out my thinking is. I want to come as close as possible to a drive-by-wire system that incorporates two modes.

In economy mode, the throttle plate would go from 0 to 100% (and the wastegate would remain OPEN) in the first 60% of throttle pedal travel. Only as the throttle plate nears full open would we even try to add boost. Over the last 40% of throttle travel, the boost target would increase from 0 to MAX_BOOST all while the throttle plate remains wide open.

In "sport" mode, things would act much as you have described your setup. The throttle plate would go from 0 to 100% open as the throttle pedal goes from 0 to 100% depressed. I'd ramp the boost target (measured in the intake plenum) from 0 TO MAX_BOOST over the first 70% of so of throttle travel so that the last 40% is just throttle-modulated MAX_BOOST.
Yes. I think I first learned that from you and thought it sounded like an excellent idea. How to do all this reliably -- and handle the transitions between different states (e.g. vacuum and boost) smoothly is the hard bit.
Very good -- I hadn't thought of this at all. It could get really hairy though trying to combine/control two different control signals created by modulating two different (but interrelated) constantly varying pressure sources. I don't even know that achieving responsive, stable control throughout the entire operating range would be possible, but it is surely worth some thought. Thank you.
I agree. I should have been more clear. What I wanted to do was open the blow-off valve during shifts so that the turbo sees less pressure at the compressor, thus avoiding stalling and maintaining max turbo RPMs. (Even better would be no blow-off valve and instead a second throttle plate before the compressor intake so that the compressor spun in a vacuum during the shift, but there are even more practical problems with that idea than what we're discussing here.