Rusty,
I am totally new to all of this, but have to learn. I don't want
much boost (maybe 35" or so MAP) for takeoff, but want to turbo normalize
to around 15k'. I know detonation is a problem with high boost and
I will have limited intercooler resources. With that in mind, how
does this sound ....
I would have a manual internal waste gate. The waste gate would
be normally open (dumping all exhaust past the turbo). On takeoff,
I would run WOT (say 29"), no boost. I could close the waste gate
a little and get 33" for example for a hot day or short field. As
I climb out at WOT, I gradually close the waste gate to maintain 30" MAP.
Since I'm running essentially NA at low altitude (where the atmosphere
is warmer) I don't need much in the way of an intercooler. As I climb
and progressively close the waste gate to maintain 30", I need more intercooling,
but this need is reduced by the cooler atmospheric temps. At altitude,
say 15k' - 17k', 30" has me going like a bat out of hell, and the cooler
air going into the system requires less intercooling.
Basically, I'm running WOT most of the time and controlling the power
with the waste gate, using the least amount of turbocharging I can to get
the power I need. I need to know what's wrong with this line of reasoning
before I start committing resources.
Just a theory .... Jim S.
Russell Duffy wrote:
I
was reading the installation and operation manual that came with my MicroTech
electronic engine management system, and under "setting up" there is a
section entitled BstCUT, which says "This feature prevents turbo
engines from overboosting by cutting all fuel supply when bost pressure
exceeds the limit set in theBSTcut screen, i.e: if you set this value
to 10 psi, all fuel supply will be immediately shut off when boost is 11
psi or higher." Does this sound like a useful tool in installing
and setting up a turbo on my 13b? Please don't flame me, as I know
less about turbos than I know about understanding women. Does this
mean I would not need a wastegate? Thanks in advance for your replies.
Paul Conner, Mobile, ALHi
Paul,OK,
here's my best turbo 101 theory:In
all stock turbo applications, there's a wastegate to control the boost.
This is just a "valve" of sorts to let exhaust gas go around the turbine
(less boost), rather than through it (more boost). It can be built
into the turbo housing as in most stock applications, and called an internal
wastegate, or it can be a separate unit called an external wastegate.
This valve is physically actuated by the wastegate actuator, which is a
big vacuum pot looking thing with a long arm that connects to the wastegate
lever. The actuator itself has a spring that's set to a specific
pressure, which is usually below the max pressure that you want.
In other words, if you plan to run 10 psi of boost, the spring might be
6 psi. The actuator is generally controlled by pressure on a hose
from the turbo compressor itself. This hose can also be Teed into
an electronic control of some type. Basically,
here's how it works. Say you have a turbo, with an internal wastegate,
and an actuator with a spring pressure of 6 psi. The wastegate is
held closed by the spring, which causes all the exhaust to flow through
the turbine, and makes maximum boost. At idle and low throttle settings,
there isn't enough exhaust flow to produce any significant boost, so the
wastegate stays closed. When you open the throttle, the extra exhaust
flow spins up the turbine, and starts producing boost on the compressor
side. As the boost increases the pressure increases in the actuator
line as well. Once the pressure in the line gets higher than the
spring pressure in the actuator (6psi for our example) the pressure starts
acting against the spring in the actuator to open the wastegate.
As the wastegate opens, less exhaust flows through the compressor, which
reduces the boost. If the wastegate opens too much, the boost is
reduced below 6 psi, and the spring is able to close the wastegate again,
which starts the cycle over. Keep in mind that this isn't a binary,
open/closed operation in reality, the amount the wastegate is open varies
the boost, accordingly. The spring in the actuator also doesn't just
snap completely open when the boost hits 6.1 psi. It starts to open
at 6, and opens more at 7, more at 8, etc. That
was the most basic (automatic) way of controlling the wastegate.
Remember that you wanted 10 psi, so if the wastegate is big enough, and
the spring is set to 6 psi, how will you ever get there? The answer
is to bleed some of the boost out of the wastegate actuator pressure line.
You can do something as simple as putting a needle valve in the boost actuator
line, and open it some. By varying the opening amount, you're changing
the pressure that the actuator sees. Now you can have 10 psi of boost,
by allowing a 4 psi leak of the air in the actuator line. (Turbo
201 begins here) In stock vehicles, you tee this line into a solenoid,
and let the computer control the opening and closing of the solenoid.
Since this is a binary open/closed situation, the computer sends a square
wave pulse to the solenoid, and varies the pulse width to control the amount
of time the solenoid is open. Now,
as for the question of using the boost fuel cut. This is a safety
device, and not a controller of any sort. You do not ever want this
to have to kick in. Imagine you've just lifted off, running full
throttle (the only time you're going to have too much boost), and your
engine quits. The computer saved your engine, while trying to kill
you. IF you can verify how this feature behaves, and test it on the
ground, it might be worth setting up, otherwise, I'd leave it off, or set
it to something like 30 psi so it never comes on. I would want
to know how quickly it reacts. In other words, say you set it at
5 psi, then go to full throttle where it sees 6 psi almost immediately.
It will cut the fuel, but when does it turn it back on? Boost will
immediately drop due to loss of power, so does it come back on when it
gets back below 5 psi? If so, you could use it, but would have to
teach yourself to react to an engine failure by pulling the throttle back
some. One
other word about wastegates. You can control the wastegate manually,
as I am trying to do. I just have a cable running to the wastegate
lever, rather than actually having an actuator. Depending on how
much boost you need, and how effective your wastegate is, you could possibly
just wire the wastegate wide open. For example, I know that I can
make more than 3 psi of boost (over ambient pressure) with the wastegate
wide open, because that's where mine is, and I have a relief valve that's
dumping the excess above that. The relief valve is also a safety
item, that I'll eventually get rid of, but I'm using it now to keep me
from blowing the engine while I prove that I can control boost via the
throttle. So far, boost is easily controllable with the throttle
alone, though it does add to the workload. I
hope that answered some turbo questions.Cheers,Rusty
--
Jim Sower
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T
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