Aerochargers are better than a twin screw!! hahaha ;-)

[Greg Hermann]

I will admit to being in a bit of a growly mood meself, Steve.

That bear nickname ain't there for nothing. :-)

Fact is that the Lysholm screw compressors do have rather poor VE at low
rotational speeds. Been there and studied the curves that Whipple publishes
for the ones they sell. The VE curve for them looks pretty similar to an
engine's torque curve--starts out pretty low, I presume due to blow-by,
reaches a peak, and then falls off, I presume (again) due to flow
inefficiencies.

The AE curves, IIRC, peak at about 60%, maybe 65%.

The rpm range over which a screw will give reasonable VE--from when the
curve starts to come up to when the recommended max rpm for the unit has
been reached--is maybe 2.5 to 1.

A well selected conventional turbo will make boost over maybe a 2:1 range
of engine speeds, while a VATN turbo can do 3 :1 or perhaps even 3.5: 1.
SO--if flexibility is what you crave, figuring out how to make a VATN turbo
work reliably is an attractive approach.

The big problem of note with VATN turbos is sticking of the movable nozzle
vanes. The scuttlebutt I have heard is that this problem is much worse when
a VATN is used on a spark ignition engine--and that this is most likely the
case because of the higher EGT's which occur in a spark engine. Supposedly
vane sticking is not a particularly frequent problem on VATN turbo
applications on diesels (which, as noted earlier, have significantly lower
EGT's than spark motors do).

The problem noted with the Aerocharger design is bearing failures, and it
seems to have been related to high boost. The Aerochargers 1. use ball
bearings and 2. have both wheels (compressor and turbine) overhung to one
side of the bearings, so as to keep the bearings in the relatively cool
inlet air stream.

Neat ideas, both. BUT--1. Overhung bearings and shafts NEVER behave as well
as non-overhung or symmetrical ones. and 2. Overspeeding causes failure of
ball bearings due to the centrifugal load of the balls against the outer
race.

Hopefully we all know that it takes higher rotational speed for a
centrifugal compressor to generate a higher pressure ratio. So--I don't
think it would take Sherlock to figure out why  going to higher than design
boost with an Aerocharger will cause bearing failures !

The thing that Aerocharger has that IS _really_ slick is their controller.
Pure analog, mechanical unit. But--it opens the vanes up wide at light
load--thus giving low back pressure for cruising and such, and therefore
improved fuel economy. Crack the throttle open a bit, and it instantly
closes down the vanes, so as to make boost. As boost comes up to the
set-point, it begins to re-open the vanes, thus regulating boost.  It would
be pretty interesting to use this controller to operate a conventional
waste gate on a conventional turbo--but one has to wonder whether the valve
in even the best of the conventional waste-gates would live with such a
high percentage of its time spent "off seat"!

It seems fairly obvious that, with this kind of a control, the vanes in an
Aerocharger move a lot more frequently than they would with a more
conventional boost controller. So--maybe this controller is the reason
Aerochargers don't have trouble with sticking vanes?

Might be pretty interesting to see whether using an Aerocharger controller
on a Garrett or IHI VATN turbo might cure the sticking vane problems???

My own thought is not only to do that, but also use full time water
injection to not only allow more mechanical compression in the motor,  but
ALSO to knock the EGT's down to diesel levels or below, thus eliminating
the other reason for sticking vanes.

Another, maybe even bigger, benefit with the H2O injection is that peak
cycle pressures are lowered substantially without any loss in power--thus
helping out the engine's  bearings. (Not to mention the ability to get by
with cheaper fuel.)

Greg

At 10:18 PM 10/14/01, Stephen Andersen wrote:
>Alright, it looks like I stepped into a royal pissing
>match here.
>
>All I was freaking trying to say was that Aerocharger's
>boost curve chart for comparison was a crock of doo-doo.


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