>>By measuring the bore diameter
down its length, you will find that it is slightly hour glass shaped
i.e.
bore biggest at the TDC and BDC with the smallest diameter at mid stroke
where piston speed is the highest. <<snip>>
FWIW, this phenomenon is probably due to the inability of the oil to
create an elastohydrodynamic (EHL) film at the lower piston speeds.
This results in boundary lubrication - metal to metal contact between
asperities. In the presence of adequate lubrication, both sliding and
rolling contact establish EHL at some given speed. This means there is
a layer of oil between the two surfaces which is thicker than the height
of the asperities (irregularities on the surfaces). As long as you have
EHL, you have virtually no wear with the exception of the occational
contamination particle. When you lose EHL, you generate wear. That's
why cold starts are the major factor in engine wear.
Friction must also be considered, though. From memory (faulty usually)
friction increases with the square of speed, so friction increases
exponentially with speed - unless you have EHL. The bearings are pretty
much assured of having EHL unless the oil supply is cut off or the
pressure is insufficient, but the rings are a different story. There
are generally 3-4 rings a piston. The lowest one is an oil ring which
minimizes the amount of oil that gets to the other rings. Therefore,
the top sets of rings (compression rings) run with minimal lubrication.
Put it all together and you have wear on the cylinder bore and rings
which occurs at all speeds, but as previously mentioned, higher rpms
mean more trips to the top and bottom of the cylinder and hence more
wear AND higher speed means more friction and wear. Luckily, the brunt
of the piston's loading is transmitted through the piston skirt and the
wrist pin/connecting rod, both of which are well lubricated, so the
only major force on the rings is their spring tension - small, but it
does lead to some wear.
Another consideration is seal wear. Engine crank seals also rely on the
EHL principle, but are much more susceptible (sp?) to heat aging.
Higher speed means more heat which means reduced seal life because the
elastomers will age quicker due to the higher localized temperatures.
As for fuel economy, most engines have a fairly flat brake specific fuel
consumption (bsfc) curve through the power band, so you will get little
change in fuel consumption for a give output with one exception.
Friction. As previously mentioned, friction increases exponentially
with speed, so frictional losses can contribute to lower fuel economy at
higher engine speeds. This seems to be insignificant in our fine autos,
though.
One last thought (thank God!). Overdrive transmission durability has
very little to do with operating speed and everything to do with
complexity and design. ie, more moving parts and a less than optimal
design would account for the OD transmissions higher failure rate than
the 4-speed unit.
This is all from memory and follows a few pints of Bass, so it is
subject to revision/denial. I'll entertain/research any specific
questions regarding this subject during the week when I have access to
my technical references.
The ramblings of a mechanical engineer who happens to work for a bearing
company. . . .
Cheers,
Rich P.
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