[Diy_efi] RE: Throttling intake air

[Brian Michalk]

To clarify, just in case we are thinking about two different animals; when I
say wastegate, I'm talking about a device that routes exhaust gasses around
the turbo, not through it.

> 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.

Okay, even if it cost less to throttle the intake, the operating costs over
the life of the engine would outweigh the operational costs.
Given the mythical 2000 hour TBO (time between overhaul) for an aviation
engine.  Assume 10 gallons per hour.  Assume throttling the intake gave 60%
efficiency, and throttling the wategate gave 61% efficiency.  Given a cost
of $2.00 per gallon, that comes to 20,000 gallons burned over TBO, costing
$40,000.
at 60% efficiency: $24000 for useful work, the rest wasted.
at 61% efficiency: $24400 for useful work, the rest wasted.

I would rather have the more efficient engine.  I think I can buy a
wastegate that cost within $400 of an intake throttle.

> >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...

Okay, because opening the wastegate bypasses the turbo, causing it to spin
slower, which in turn causes the compressor to spin slower, therefore not
making a higher MAP than the engine wants for a certain power setting.

> 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...

I disagree.  I never said that throttling the intake would not work.  It's
just less efficient.  If I had access to my books on the subject, I could
provide some references with respect to compressor efficiency, pressure
ratios and air flow.  I'll see what I can find tonight and post the relevant
equations.

> 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 ;)

No, we are talking about Internal Combustion Engines here.
Kinda interesting to think about it though.  Take a piston engine, and add a
turbo to increase the power, now let's say we want to put a reduction on the
turboshaft and put a generator on it.  Need a bigger generator?  Increase
the turbo.  Eventually you reach a point where the piston engine is only
there to supply the expanded exhaust gas ... and it looks a lot like a
turbine engine.  Check out:
http://www.gas-turbines.com/nt5/nt5.htm

> 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 !

Then why do Cessna, et. al.  do it this way?

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

Why?  Please give me a source to back up your claims?  If I'm mistaken I
seriously want to know why.

> 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.

If you remember which magazine, I would like a reference.

> 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

What if the wastegate has it's own pipe to atmosphere?  Then there can be no
added turbulence after the turbine.

> >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 !

Exactly!  So why don't the diesel guys jack up their intake pressure to
28PSI, and then throttle it back down to whatever they need?  They don't.
They actuate the wastegate to get the MAP they want, and when power requres
less than one atmosphere, they run lean.

> 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 ?

I'll get my book references tonight, but in the meantime, I would like to
refer you to:
http://www.avweb.com/news/columns/182146-1.html
and search down the page for "turbo".  A lot of it is basic, but there are
bits about efficiency in there such as:

"Early general aviation turbo systems sometimes had a simple manual control,
just like an additional throttle. The wastegate would be fully open for
takeoff, and as the altitude increased, and the pilot wanted more manifold
pressure, the "additional throttle" would be added to produce the desired
manifold pressure. "


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