[Diy_efi] RE: Throttling intake air

[Mike]

At 09:55 AM 14/1/2003 -0600, you wrote:
>I would rather look at it from a thermodynamics problem.

Yeah sure, makes far more sense :)

>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?

OK, at moment, we can buy an off the shelf turbo with wastegate cast
which is cheaper than a turbo without wastegate *plus* the cost
of adding an ECU or using a flybywire ecu with a 'boost' feed.

BUT, the central point is "...from a control systems perspective..."
it makes more sense to do it differently.

>> 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.

Hey - I'm not the one confused here !

"... from a control systems perspective...", remember...

>If we are doing X horsepower, and don't need any more horsepower, harnessing
>extra exhaust doesn't buy anything.

Thats exactly right, so why dump the power into exhaust via wastegate
when it makes more sense to not feed that extra into the engine in
the first place...

>  EBP is not free, the combustion process
>provided that power, and increasing EBP reduces engine volumetric
>efficiency.

I take it 'EBP' means Exhaust Back Pressure.

Ok - I hear what you say, but this is where *you* my friend are
confused. There is no need to keep adding fuel/air when
you want to produce less output by virtue of fact that you are
limiting boost pressure by throttling the input to the engine.

Surely an engine that is producing say 15psi boost pressure
and venting via the wastegate will consume more fuel than
one which is 'held' to 15psi by controlling the throttle...

mmmm  ?

>  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.

<groan> this is really tangential and arises from confused thinking,
sorry dont mean to be mean (;) but its a bit of waffle not on the
point because you are focussed on the back pressure issue which
doesnt really arise does it, since you have control of boost and
power output by throttling inlet, in fact more back pressure
when venting via wastegate (unless its a separate wastegate exhaust).

>> 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.

Sure, and understandable for smaller planes where ICE are used that
its cheaper and more reliable (from past electronics experience)
coupled with complexity to use turbos with wastegates - dime a dozen,
but doesnt invalidate the point I made initially, "... from a
control systems perspective..."

Ah ha ! As you are interested in the aero perspective:-

Well from that aero perspective - do they throttle the exhaust of a
turbine on say a 747 or do they control inlet, I dont see a large
wastegate on the engines on passenger or military jets for that
matter !  Grumble ;)

I think any avionics engine designer would think you were nuts
and completely confused if you wanted to put a wastegate on
a aviation turbine !

>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.

OK, quite traditional and understandable, but from a control systems
perspective its not the most efficient.

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

Maybe, is that really a valid assumption for large changes in
intake air pressures where we are using MAF and not MAP and where
a different set of sizing issues arise if one does throttle inlet
expecting we run at close to STP for the most part, not the case
in aviation requirements so, no, not the same at all.

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

If you werent generating heat and if you werent concerned with
exhaust energy loss and turbulence and fuel consumption then that
'might' be the case. ie. For a compressed air engine thats probably
completely true - but we don't have that, we are stealing a little
exhaust energy to provide inlet compression - different set of
dynamics I would have thought. Actually I just realised your
statement above doesnt hold water when you go from wastegate closed
to wastegate open, because as soon as you open the wastegate the
turbine sees more backpressure. In fact I recall some tech in
a magazine sponsored trial testing different turbos, backpressure
always goes up when the wastegate opens.
With an inlet throttle boost pressure control
system that doesnt happen *and* you have less turbulent flow
after the exhaust turbine if the housing doesnt include the
wastegate area *and* it depends just where you measure highest
pressure on the intake - which should be at the atmosphere not at the
inlet manifold post TB - because the turbo is a power transfer
device - it complicates your statement above in a comparative
study of throttling inlet vs wastegate due to power transfer back
to inlet with all the P1V1/T1 = P2V2/T2 issues...

>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.

Well, <cough> isnt this how a diesel works !

In fact if I recall the SAE did some work/report on diesel fuel control
where the fuel injected qty was reduced on turbo engines instead
of using a wastegate as a boost control measure...

Anyone on the SAE subscription or know what I'm referring to ?



Regards

Mike Massen
Hm 08 9444 8961,  Mb 0438 048961

Some power stuff here: http://www.iinet.net.au/~erazmus

Ancient Sufi saying:
        "Should your God save you from adversity, choose another God"


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