[Diy_efi] RE: Throttling intake air

[Perry Harrington]

> 
> You caught me.  Neither fluid flow not turbos are my
> field of expertise.
> 
> To what degree does cavitation have an impact? 
> Realistically, I would imagine there being SOME
> decrease in turbine speed when throttling the intake. 
> Or does cavitation take over completely and allow
> freewheeling?  I do see the point in the vacuum
> reference; the motor obviously speeds up when you plug
> the intake off entirely.
> 
> I imagine you can educate me on these points, since
> you pointed them out, Perry.  ;)

>From my understanding the turbo would actually speed up in RPM
due to the reduced load on the compressor.  This is why throttling
the turbo with the wastegate is more advantageous.  Also,
with all this talk of turbo throttling, I don't recall if
anyone pointed out that this only works when above 1 atmosphere
absolute.  If you have, for instance, a Volvo 2.4L R engine,
it's NA specific output is between 170 and 194 HP (low vs high
compression, don't know which).  The turbo then adds anywhere
from 67 to 89 HP.  Your throttling of the turbo would, assuming
no conventional throttle, only be considered for large power
outputs.

> Yes, but there would still be considerable boost in
> the intake manifold, yes?  Would that not offset most

No, because there is a closed circuit.   You will get high pressure
in the exhaust system prior to the brake, and a vacuum in the intake
side.

> of the loss of combustible gases from the described
> throttle?  Granted, you would then have the piston
> using more work in removing the exhaust gases, but how
> much more?

The only way to really implement such a system would be to have
a non-interference valve engine which has it's valve followers
disengaged.  This way the gases inside the cylinder are contained
to a certain degree.

> 
> Seems to me like a throttle BEFORE the turbine would
> be much better at achieveing the desired brake effect,
> yes, Perry?

The after turbine throttles are only used on diesel engines because of
the lack of a conventional throttle.  The reason why they are placed
aft of the turbo is efficiency.  You want as much thermal mass to pass
through the turbine.  The downpipe then becomes the next best place to
install such a beast.  On a Ford powerstroke these can generate between
300 and 500 lb/ft of braking torque.

Having a throttle before the compressor is analgous to a positive
displacement SC or suck through turbo setup.  The major differences
are that you get vacuum in the compressor and don't need a vacuum
operationed recirculation (blow off) valve.

I believe there is a non-trivial efficiency associated with a throttle
before the compressor.  All types of compressor designs would benefit
from that as the compressor then draws little power from it's source,
whether that be turbine or crank.  The other benefit I could see, WRT
turbos is that you will get a higher shaft speed at lower loads, so
boost upon WOT is faster.

> 
> =====
> | Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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