Thanks for the comments.
I hope to be able to judge for myself the increase in horsepower on a
dyno. Mike Moore of AvWorks has extended to me the
offer of being able to run my engine (under close supervision) on a dyno
after it is built. I am looking forward to this for many reasons,
not
the least of which is being able to see exactly how much horsepower I am
getting.
The other benefits you mention are nice as well and you forgot to mention
how nice and clean the belly of my plane will look.
D. Brunner
----- Original Message -----
Sent: Monday, May 15, 2006 5:06 PM
Subject: [LML] Re: Positive Crankcase
Vacuum
Doug,
I'm not sure how much benefit in reduced windage losses you would get
from applying a vacuum to the crank case. The biggest areas for gain here are
in oil pan design, windage trays and crank scrapers that work to screen the
oil droplets from the air down into the sump and protect the oil surface from
the 'wind' in the crankcase. Yes, this can make a substantial difference in a
race engine, but as Paul said at 2700 rpm it's questionable just how much
power could be recovered from windage reduction.
There are two benefits from crank case vacuum that you haven't mentioned.
The first is reduced dilution of the fuel air charge in the cylinder and
its effect on power. During the intake stroke, cylinder pressure is well below
atmospheric (naturally aspirated) so ring leakage flows into the cylinder.
With crank case vacuum, the ring leakage flow is reduced and at times
even reversed. There are three effects as a result: First, the area between
the top ring land and the cylinder is full of exhaust products, partially
burned fuel and raw fuel all of which are detrimental to the power
stroke. During the intake and compression stroke, some of these products are
swept into the combustion chamber where they reduce power. Crank case
vacuum helps to remove them from the cylinder. Second, the air leaking
from the crankase is fuel lean and its addition to the cylinder is also
detrimental to power. Third
ring leakage into the cylinder carries with it a small amoint of oil.
This also displaces fuel air mixture and reduces power, but more importantly
oil has a very low octane rating and contributes to detonation.
The second benefit is improved oil life. Unburned fuel and exhaust gasses
are continually leaking from the cylinder during the combustion, exhaust and
compression strokes and contaminating the oil. The oil contains various
additives to prevent damage to the engine as a result of contamination with
these chemicals. As contamination increases, the additives are expended and
protection is lost. Crankase vacuum helps to volitize some of the compounds
and draw them out of the oil and into the exhaust. This was a beneifcial
consequence of the positive crankcase ventilation (PCV) system on cars in
the early '70's. While the vacuum generated in the crankcase was slight, the
system scavenged vapors from the crankcase and greatly reduced contamination
of the oil. This resulted in extended oil changes intervals from 3
thousand miles in the '60's to 4 or 5 thousand in the '70's. Of
course today, thousands of people throw perfectly good oil away every 3000
miles anyway. Old habits die hard.
Rob