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1. 09:01 AM - Re: Lycoming Thunderbolt Engines (Gary Casey)
2. 09:47 PM - Re: Lycoming Thunderbolt Engines (Red Hamilton)
Message 1
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| Subject: | Re: Lycoming Thunderbolt Engines |
--> Engines-List message posted by: Gary Casey <glcasey@adelphia.net>
From the sound of some of the posts our friends at Lycoming and
Continental haven't been reading the technical literature or have
been asleep for the last 50 years or so. While they could perhaps be
more progressive than they have I'm not so sure that some of the
"easy" fixes make as much sense in aircraft engines as it sounds:
1. Roller-tipped rockers will reduce wear on the valve tip and will
reduce side loading on the valve. that is especially important for
engines with small-diameter stems, high spring loading and short-
radius rockers (such as when increasing the valve lift of an
automotive engine). Aircraft engines have relatively long rockers
and low spring loads, so I don't see any critical need. It would
reduce the wear and increase the life of the rocker arm (which now
usually last over 4,000 hours), but it would introduce other failure
modes. Aircraft engines are designed, as much as possible to have
soft failure modes, such as the wear-out of a component. A roller
tip, because of the higher contact forces, must be hardened and it
and its ancillary components could break, putting hardened steel
shrapnel into the engine. I don't think the increased horsepower
(which would be miniscule) would be worth the risk. The same is true
for roller follower, but these seem to be endorsed by Lycoming so
perhaps the potential failure modes are addressed; I don't know. I
would be reluctant to use needle-bearing rocker pivots for the same
reason.
2. One big advantage of roller followers is that they allow a
concave cam profile to be ground, increasing acceleration rates. If
the spring loads were increased this would have a positive impact on
the design of the cam profile, allowing a shorter overall duration
for a given breathing capability. This would provide measurable
improvements in BSFC, BMEP or both. Does anyone use concave profiles
with the roller followers? I doubt it - it requires much more
expensive cam grinding equipment. Is the power increase worth the
trouble of going to roller followers? One clue would be for someone
that changed to rollers to compare oil temperatures before and
after. Oil temperatures are a function of lots of variables, but any
friction savings that would result in a measurable power increase
would have to also result in substantially less heat going into the
oil. I'd be surprised if the gain were anywhere near 1 horsepower
per cylinder.
3. Possibly the reason Lycoming endorses roller followers is the
reduced probability of galling after extended periods of non-use.
This has been a chronic problem with the Lycoming high-mounted
camshaft and roller followers might be the only answer.
4. Aluminum rockers? Aluminum has a finite fatigue life, unlike
steel, so aluminum can be effectively used in drag engines and other
finite-life engines. I wouldn't use aluminum for this application in
an aircraft engine. This is especially true since the likely failure
mode is a fractured rocker, disabling one cylinder without warning.
4. One limitation to high valve accelerations is the length of the
pushrods and these apparently are small enough in diameter to be
prone to buckling. Very little that can be done without increasing
their diameter.
5. Broken crankshafts? Don't know the root cause, so I can't
comment intelligently. Certainly there isn't enough money in the
aircraft engine business to pay for exotic engineering tools that are
used in the automotive business, and weight sensitivity precludes
burying uncertainties in cast iron (the old saying is that doctors
bury their mistakes in the ground while engineers bury their mistakes
in cast iron). More careful control of process parameters is
probably the crutch that has to be used.
6. I can't comment on "machining tolerance", but aircraft engines
can swallow a lot of garbage. Often the first sign of cam wear is a
rough engine and inspection will show that the cam lobe is gone. For
some time the engine tolerated that much iron and didn't fail.
Fairly large clearances coupled with high-viscosity oil will allow
such things. Do we want to give up damage tolerance for being able
to use smaller crank journals or to increase the loading? Maybe not.
I really like the idea of using a high-tech automotive engine in an
aircraft and and did a lot of analysis. In the end I concluded that
I could get equivalent power and durability and almost equivalent
BSFC, but I couldn't avoid a 50-pound weight penalty. A big-bore air-
cooled direct drive engine is a pretty good match for most aircraft.
Gary Casey
>
> .....has been using a variety of these for about 50 years.
> They are available in extruded aluminum, forged aluminum, and
> stainless,
> fully rollerized, or bushed and roller tipped, and in any ratio
> desired.
> (After correcting the ratio on a number of aircraft rockers, it is
> obvious that
> the factory does not pay close attention to this)
> As far as strength, no contest here: Where an aircraft may have open
> pressure
> of approx. 300lbs, we run open pressures of around 1000 lbs.
> Another significant advantage to the roller rockers is reduced valve
> guide wear.
> Not sure of HP gains on a slow turning ac engine, but might be
> interesting to dyno each.
> Archie
Message 2
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| Subject: | Re: Lycoming Thunderbolt Engines |
Archie,
If you ever do that please let us know.
Thanks,
Red Hamilton
----- Original Message -----
From: Archie
To: engines-list@matronics.com
Sent: Saturday, June 24, 2006 6:45 AM
Subject: Re: Engines-List: Lycoming Thunderbolt Engines
Two companies tried that in aviation, and failed.
There are a series of parameters that do not directly apply to
aircraft use,
but for the most part, why re invent the wheel, when the racing
industry
has been using a variety of these for about 50 years.
They are available in extruded aluminum, forged aluminum, and
stainless,
fully rollerized, or bushed and roller tipped, and in any ratio
desired.
(After correcting the ratio on a number of aircraft rockers, it is
obvious that
the factory does not pay close attention to this)
As far as strength, no contest here: Where an aircraft may have open
pressure
of approx. 300lbs, we run open pressures of around 1000 lbs.
Another significant advantage to the roller rockers is reduced valve
guide wear.
Not sure of HP gains on a slow turning ac engine, but might be
interesting to dyno each.
Archie
----- Original Message -----
From: n801bh@netzero.com
To: engines-list@matronics.com
Sent: Saturday, June 24, 2006 2:12 AM
Subject: Re: Engines-List: Lycoming Thunderbolt Engines
Geez,,, Roller rockers are pretty easy to fabricate, test and
furnish.. Cheap HP gains from them too.. You would figure a company like
Lycoming, with 60+ years of supposable R&D would have addressed that
simple thing years ago. Now,,, if they can get their crankshafts from
breaking they might go somewhere. You can bet if one does break on this
new line of experimental engines the their response will be "_uckoff"
,,,,Their is something wrong with in installation.. They will not stand
behind their certified engines.!!!!!!!!!!!!!!!!
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