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Kawasaki’s KR250 road race two-stroke most certainly developed more power
after some break-in time, but the energy argument disposes of the idea that
it could be a large amount. If much power was going into making metal particles
you’d see the metal – lots of it.
after some break-in time, but the energy argument disposes of the idea that
it could be a large amount. If much power was going into making metal particles
you’d see the metal – lots of it.
In four-strokes, the largest component of friction comes from the ring
stack and secondarily, from the piston, the reason being that ring-to-wall oil
films are very thin so the rate of shear is large. Because engines don’t fill
their oil with wear particles we can see that metal-to-metal friction is very
small. This is especially so in late-model stuff, which has all lapped
rings. I rented a Mitsubishi something-or-other at the Atlanta airport once with
7 miles on it, and was interested to pull the dipstick after 200 additional
miles of rush-hour 85-mph driving – the oil was still clear amber. Back in the
old days of cross-hatch honed cylinder walls that acted as mill bastard files to
shave rings into shape, the first oil change was dark with wear particles.
stack and secondarily, from the piston, the reason being that ring-to-wall oil
films are very thin so the rate of shear is large. Because engines don’t fill
their oil with wear particles we can see that metal-to-metal friction is very
small. This is especially so in late-model stuff, which has all lapped
rings. I rented a Mitsubishi something-or-other at the Atlanta airport once with
7 miles on it, and was interested to pull the dipstick after 200 additional
miles of rush-hour 85-mph driving – the oil was still clear amber. Back in the
old days of cross-hatch honed cylinder walls that acted as mill bastard files to
shave rings into shape, the first oil change was dark with wear particles.
At lower throttle angles a four-stroke also has a lot of pumping loss,
usually lumped in with friction. This is the energy used to pull a partial
vacuum in the cylinder on the intake stroke. Diesels, being unthrottled, have
little pumping loss, as do crankcase-scavenged two-strokes.
usually lumped in with friction. This is the energy used to pull a partial
vacuum in the cylinder on the intake stroke. Diesels, being unthrottled, have
little pumping loss, as do crankcase-scavenged two-strokes.
At idle or similarly low rpm there is an increase of friction loss from the
valve train as the heavy pressure between tappet and cam lobe increasingly
crushes through the dynamic oil film, leaving an increasing share of the load to
be carried by the oil additive layers on the parts. As this additive-carried
friction is typically 10-100X the friction of the oil film (usually of the order
of .001) this can be a fair amount, and is responsible for much of the rise in
friction at very low revs. During break-in this is probably a source of some
excess friction. Some anti-friction agents such as tricresyl phosphate (the TCP
of story & song) act as polishing compounds. As they react with the metal
surface they form friable phosphides, and concentrated friction may rub some of
this off, taking some metal with it. I have often seen power gears that were
fairly rough-looking when new assume a much more polished, finished appearance
after some hours of operation.
valve train as the heavy pressure between tappet and cam lobe increasingly
crushes through the dynamic oil film, leaving an increasing share of the load to
be carried by the oil additive layers on the parts. As this additive-carried
friction is typically 10-100X the friction of the oil film (usually of the order
of .001) this can be a fair amount, and is responsible for much of the rise in
friction at very low revs. During break-in this is probably a source of some
excess friction. Some anti-friction agents such as tricresyl phosphate (the TCP
of story & song) act as polishing compounds. As they react with the metal
surface they form friable phosphides, and concentrated friction may rub some of
this off, taking some metal with it. I have often seen power gears that were
fairly rough-looking when new assume a much more polished, finished appearance
after some hours of operation.
OIl film friction rises steeply with rpm, so the friction loss curve is a
“bucket” – high at the extremes and low in the middle.
“bucket” – high at the extremes and low in the middle.
KC