|
|
Posted for "Ed Anderson" <eanderson@carolina.rr.com>:
> I had Turbonetics(Dave Austin) size me a turbo. What do you guys think?
> recommended - TO4E 60 based on:
...snip...
> based on 180hp(?) shows max 231hp w/out aftercooler, 256hp with
aftercooler.
It seems to me that you are looking at 13b hp ratings. A NA 20b will do
250 hp without a turbo.
Hi Barny,
Sizing a turbo is a combination of calculations to help you select the
compressor needed from compressor maps and operating conditions/parameters
based on experience. Selecting the A/R (Area/Radius) ratio for the turbin
exhaust housing is more problematic (meaning you may need to experiment some
in order to get the right A/R size). Mass flow (Lbs/Min) is generally one
parameter used to size the compressor. Various "trims" are specialized for
the fine tuning of the compressor wheel selection. Much of this can be
gained from various Turbo books on the topic.
There is one thing that needs additional consideration for turbo aircraft
selection. That is "Compressor Surge". It turns out (particularly if your
are using higher boost levels) that IF you maintain the same boost at
altitude that you do at sea level, that the ratio between manifold pressure
vs ambient pressure will increase. This can push the operating point of the
compressor into the "Surge" zone. Compressor Surge is an unstable operating
point that can destroy a turbo unit. Normally at the lower boost levels
this is not a problem. But, one that should probably be looked into before
committing to buy a turbo unit. That being said, its hard to beat copying
someone who has sucessfully selected a turbo with the right parameters
provided they are operating in the same engine regime that you will operate
with.
You probably do NOT want to select a turbo for a 20B based on one that has
proven successful for a 13B as your engine will have basically 50% more Mass
flow. This means using a compressor/turbin section selected for a 13B
would result in less efficiency (more heated air) as well as likely
overspeeding of the turbin due to the greatly increased mass flow through
the turbin. You will also encounter more flow restriction that a properly
sized turbo.
The first parameter to consider in selecting your compressor is Mass Flow.
Most compressor maps use Mass Flow as an entry parameter. You can
calculate the mass flow of your engine by figuring the engine air flow
(cubic Feet/ Min) at the rpm you intend to run the boost and using a 12:1
Air/Fuel ratio also figuring mass (weight is close enough) of the mass flow.
Mass flow = CFM X (0.0756) + (CFMx(0.0756)/12 will get you in the ball park
at sea level. The first factor is the mass of air (CFMx0.0756) flowing into
your engine. The (CFMx(0.0756)/12 factor is the mass of the fuel (at a 12:1
A/F ratio). So together these two factors will give you a fair
approximation of your mass flow (lbs/Min) which you can use to ball park
your compressor selection. Hope this helps.
Ed Anderson
FWIW
|
|