[Diy_efi] RE: Throttling intake air

[Brian Michalk]

> Its pretty straightforward if you dont drop context or mix definitions
> ;-)

I would rather look at it from a thermodynamics problem.

> Thats not quite what I said, you are twisting the words (just a little),
> I said "... from a control systems perspective... more efficient to
> control the nonamplitude variable..." or some such ~`:o

Okay, not to be mean or anything, but your statements are merely for ease of
building the control system.  If budget was for cost of implementation,
verses budget for max efficiency?

>
> >Do you have a reference?
>
> Its basic control systems theory (and practice I might add) except
> as I already said - not used in automotive apps due to commercial
> issues.
>
> In essence:-
>
> 'Its more efficient to control the power going in than dump whats
> been created and going out when it doesnt do useful work'

Okay, here is where I think you are a little confused.
If we are doing X horsepower, and don't need any more horsepower, harnessing
extra exhaust doesn't buy anything.  EBP is not free, the combustion process
provided that power, and increasing EBP reduces engine volumetric
efficiency.  Harnessing pressure would cost us efficiency, but harnessing
that heat would be useful.  Perhaps some thermoelectric recovery without a
pressure drop?  I could se that as a win.  If we put X BTUs of fuel into the
engine, and absolutely no heat came out of the engine, we would have a 100%
efficient engine.

> Opening a wastegate and dumping all that heat and kinetic
> energy is not particularly efficient, far better is to *size*
> the system (I would have thought) so it never needs to get
> to that point.

At least from an aero perspective, which is my interest; everything is a
trade-off.  Size a turbo for 5000 feet operation, and efficiency goes down
when you go to 15,000 feet.  Size it for 5,000 and you'll never make it to
15,000.

My fly-by-wire setup is as follows; and this is for turbo-normalizing, which
means the deck pressure never exceeds sea level pressure.
Assume a sea level takeoff.  The computer knows that MAP is at sea level,
and maintains an open wastegate.  Now some time later, if the power lever is
still at max throttle, the computer may start closing the wastegate in order
to maintain SLP.  Now I'm screaming along at 200 knots, and decide to
throttle back at some low altitude, like 3,000 feet, and to do this, I need
a MAP of 24 inches H2O.  The wastegate should open completely, as no boost
is needed, and power is regulated by throttle (in front of the compressor).
Now let's put the airplane at 12,000 feet where I want to cruise at 50%
power.  In this regime, MAP at 12,000 feet is what is reguired for 50%
power.  The wastegate is still wide open.  Let's put the airplane at 18,000
feet, at 100% power, 220 horsepower.  According to my spreadsheet
calculations this is my critical altitude.  The wastegate is completely
closed, throttle is wide open.  Climbing above this altitude, I cannot
maintain SLP in the intake manifold, and thus HP will decrease with increase
in altitude.

I know you are not talking about airplanes, but the principles are the same.

Thermodynamically speaking, keeping the highest pressure on the intake, and
the lowest pressure on the exhaust is the most efficient.

If we really wanted an efficient set, we wouldn't have a throttle at all.
Inject only enough fuel to make the power we want.  Of course the EPA
doesn't like all thos NOX emissions, and the flame front propogation gets
more erratic (unstable), leading to other concerns.


_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi