g'day my name's Dave and welcome to my
youtube channel if you haven't been here
before we do advanced mechanics
tutorials and we've been doing a two
stroke series for a while and for the
last couple of months it's been winter
the ski season down in Australia and
I've been skiing as we do every year so
been sort of busy doing that but now the
snow is kind of melting because it's
getting warm and we're gonna get back to
it at the moment I'm still working on
the two-stroke expansion chamber
calculation software and I will get that
done in due course but what we're gonna
do is a couple of side videos in the
meantime now this is a question that you may
or may not pose before but has
been mentioned in videos on YouTube and
that is the question is is what is the
maximum engine rpm limitations that is
to is what is the thing that stops the
engine revving any higher or what
determines the maximum rpm of any engine
essentially now for all intents and
purposes in an engine a single cylinder
engine let's say a motorcycle engine the
we will assume that the crankshaft is of
sound construction and all other things
being equal no other parts of air engine
are going to fly off or fly apart or
break or you know I may go into some of
the various modes of failure that we can
account encounter with engines but
essentially the limitations of rpm on an
engine or how how how high can you rev
your engine comes down to the parts of
the engine that are reciprocating that
is then the parts of the engine that are
going up and down not the parts going
around and round like a crankshaft okay
now that comes down to in a single
cylinder two-stroke engine without
valves a reciprocating part is our
piston in a four-stroke that will be our
piston and it will also be our valves
which also
go up and down so in other words
reciprocating meaning they go up in one
direction they stop and then they go
back in another direction and stop and
back and forth okay so that's in
reciprocating motion and parts that move
in a reciprocating motion obviously
well maybe not obviously but the parts
that live in reciprocating motion are
the ones that have some weight or some
inertia behind and have a resistance to
deceleration or a resistance to stopping
and and of course a resistance to being
accelerated in one direction and then
stopping and being accelerated in the
opposite direction now with the engine
how we determine the maximum rpm of the
engine in the engine is the piston and
what we refer to as the mean piston
speed now for most engines that will be
a figure of roughly 25 to 30 meters per
second so we can work out the actual
mean piston speed of our engine with a
fairly simple equation and what we have
here is our equation which is two times
our stroke in meters so we have to take
our stroke in millimeters divided by a
thousand to get our stroke in meters two
times their stroke because their stroke
moves in our stroke moves down from the
top to the bottom and then back up again
so it moves two strokes multiplied by
rpm and we want to turn that into
revolutions per second because we want
to have meters per second okay so we
just say two times their stroke in
meters multiplied by our rpm or or
revolutions per second gives us our
figure so you can immediately see here
that a smaller engine of a hundred CC
with a 50 millimeter stroke 50
millimeter bore that is a square stroke
and bore the bore is the same as the
stroke at 15,000 RPM will have a mean
piston speed of 25 meters per second
you'll then be able to see if we've got
a bore of 100 millimetres and a stroke
of 100 millimetres and then we've got a
total capacity of 785 or 786 CC or there
abouts at 15,000 RPM for that engine and
mean piston speed is at 50 meters per
second now that goes beyond our 30
meters per second which is probably
absolute limit so a more realistic rpm
we would have for a single cylinder
engine make keep in mind that's for just
one cylinder a bore of 100 mil and a
stroke of 100 mil would be to achieve
the same piston speed is our 100 cc
engine of 25 meters per second at 15,000
RPM for this engine we can only River to
7,500 rpm okay now why is it limited by
the maximum piston speed okay well some
people may believe that that is down to
the weight of the piston but that is not
the case because the piston is securely
attached to the con rod via the bearing
and there's no FreePlay there but where
there is an amount of free play or a
matter of slack if you will all lash is
that the piston ring gap so what can
occur once we start accelerating the
engine to very high rpms is the maximum
piston speed will not come down to the
weight or the size of the piston so much
as it will come down to the actual
weight and size of the piston ring now
as you can see in a bore of 50
millimeters we'll have a piston ring
that's a certain size and weight
double the size bore will have double
the size piston ring which will be about
double the weight okay
now what occurs when we go beyond the
limitations of our piston ring because
our piston ring is the ultimate maximum
rpm limit is our piston will rise to the
top and this represents our piston ring
inside the the groove I've exaggerated
the clearance here because there is some
clearance but when the piston is moving
upwards the piston ring is against the
basic land okay which is the bottom of
the slot in the piston this is the
piston and this represents the cylinder
wall okay now as the piston rises up
towards top dead center the ignition
fires just before top dead center gas
pressure represented by red arrows gas
pressure rises rapidly as combustion
begins
okay the piston still moving upwards
near top dead center gas down the side
of the piston here pushes the piston
down against the piston piston ring gap
land and gas also pushes in from behind
the piston ring forcing the piston ring
against the cylinder wall with greater
force than just the actual spring
tension of the piston ring against the
wall so the force against the wall is
also from the gas behind it but when we
get to very high rpm what can happen is
that the once the piston stops or
decelerates to a stop due to the weight
of the piston ring itself if we've got a
very large and heavy piston ring the
piston ring can actually jump up off the
land so in other words a piston stops
but our piston ring keeps moving what
happens when a piston ring keeps moving
is now gases can now escape below the
piston ring okay so a gases can escape
here and our gases can also escape down
the front because we've no longer got a
pressure of gas pushing the ring out
against the wall
so our rings can unseat from the land
and from the wall okay at the piston
gets to the top all right as the piston
gets to the top obviously there's a
point where the the ring itself
decelerates to a stop and then we get
the next stage where the gas pressure
continues to rise with combustion now
our piston starts going down our piston
ring has now gone back down and landed
on the land at the bottom of the slot
gas pressure is sealed from escaping
down down the bottom and gas pressures
rising and it forces the ring downwards
and it forces the ring out and the ring
impacts the sidewall and the land okay
so it slams against the wall and it
slams against the piston land all right
this continual this continually happens
okay it at every cycle at very high
speed the piston gets to the top the
piston stops the ring keeps going jumps
off it the gas by blows past it loses
pressure off the wall the ring stops gas
pressure is still rising combustion is
not complete yet the ring then slams
back down on the land and out against
the wall this continues and this cycle
is referred to as ring flutter and this
is the ultimate maximum rpm limit of the
engine is the piston ring not the piston
itself so eventually this continues and
eventually causes failure of the Rizk
piston ring and damage to the piston
ring gap as you can imagine because the
piston rings jumping up and then
slamming back down on the land and into
the wall piston ring cracks in pretty
quick order and in extreme cases it can
actually knock the whole crown of the
piston off separating at the piston ring
gap at the piston ring land it's in
extreme cases but otherwise it will just
basically crack the rings so that's
where we've reached the maximum rpm of
our engine so what can we do to
alleviate this well one of the things is
we could have thinner rings we can have
a smaller ring
so the problem with it was decreasing
the size of the of the piston ring to
make it lighter so that it doesn't it's
not as prone to actually jumping off the
top of the you know of continuing to
keep traveling with momentum of weight
is that will have a very thin contact
patch on the on the cylinder wall the
ring will do two things it will wear
very quickly but most importantly there
will be a very small surface area
between the cool cylinder wall and the
piston ring and less heat will be
transmitted away from the cylinder okay
a way into the cylinder wall from the
piston ring now the piston ring
transmits quite a bit of heat through
the ring away to the cool cylinder wall
okay the piston is not the piston has a
barrier of lubricant between it and the
wall whereas the piston ring is hard
against the wall suffice to stop gas
pressure so heats transmitted away from
there so if we have a very thin ring
we'll have a problem with the ring
getting hot we'll also have a problem
with wear so to overcome this we can use
what's called a dykes piston ring and
that is an l-shaped piston ring all
right now the benefit of the l-shaped
piston ring is as you can see it's got
still got quite a large face on the
actual piston ring which gives us good
thermal conductance to our cool cylinder
wall but the piston ring the rest of the
piston ring is is is still quite small
in comparison to a completely
rectangular ring okay so the l-shaped
piston ring gives us a light way to ring
and the light of the Ring is the less
prone of these two ring flutter jumping
off the land and losing pressure and
it's got a nice large area against the
cylinder wall so it transmits good
amount of heat and it doesn't wear out
prematurely and you'll find these are
very common in high reving two-stroke
spring cart engines
rotary disc valve intake engines and red
valve engines that Rev in the quarter in
the order of 16 beyond 20 20,000 rpm and
we'll do it all day long and obviously
these are single ring Pistons okay now
there is quite a bit of difference in
the amount of friction that very high
rpm with one ring and two rings so in a
racing environment you know we use a
single ring piston for longevity in
road-going engines the don't rivers hi
and we were just used to stand type
piston rings now that's for a two-stroke
engine on a two-stroke engine for this
reason because the piston ring is the
ultimate arbiter of maximum rpm two
strokes have the capability of revving
higher than four strokes okay now I just
qualify that in the past older engines
are older four strokes did generally
have an RPM limit that was quite a bit
lower than two-stroke engines but in
more modern engines four strokes now Rev
quite a bit higher than they did there's
a couple of reasons for that
one reason is is that the precision of
machining is is considerably better than
it was back then
so the tolerances are have improved and
therefore we can have higher revving
engines simply for that reason that the
secondary reason is is that modern
engines have multiple valves and as you
can see with the old old two valve
cylinder head we have one large valve
and obviously one large valve has more
weight or more inertia and will have a
propensity to bounce off the cam and
perhaps even clip the piston at a fairly
low ash rpm whereas when we put two
valves or more than two valves per
cylinder okay we have the inertia is
split up into two so the weight is half
so
two smaller valves so obviously the
there won't be as much inertia in a
smaller valve and you'll be able to rev
it higher without it actually bouncing
off the valve so as you can see a
multiple valve doesn't just increase the
amount of breathing space or area by
having multiple valves
that's one function that it does but it
also allows the four-stroke engine to
rev a lot higher because each individual
valve is much lighter than a single
cylinder engine so that's why a single
overhead valve engine double over double
overhead cam engine will rev lower than
a multi-valve engine and that is the
reason why is because each valve is
lighter and as I spoke to you before as
I mentioned before the the valves are a
reciprocating part so they also have
that same problem about accelerating
stopping and then accelerating one in
the direction in the other direction
stopping at very high speed nevertheless
the valves are still a limitation for
maximum rpm the next limitation with the
four-stroke engines is also the actual
crankshaft bearings themselves now most
four strokes have a shell type bearing
which is a good bearing last lasts a
long time which stands a lot of pressure
and is pressure fed via all galleries in
the crankshaft and lubricated whereas a
two-stroke is as you know is lubricated
not by pressure fed oil it's lubricated
by oil in the fuel which comes down into
the crankcase and lubricates the
bearings but nevertheless a two-stroke
will often have needle bearings and
needle bearings in a cage now needle
bearings will operate at higher speeds
than shell bearings that's the bottom
line
however needle bearings do have a
limitation and the limitation for speed
on needle bearings is not actually the
bearings themselves
it's the cage so the cage in ball
bearings and needle bearings is the weak
link okay
is it a manufacture easy to install but
for higher speeds what we have is a
crowded type roller that's where we
actually removed the actual cage we have
no cage at all the problem is is the
crowded rollers this is not a crankshaft
bearing this just demonstrates what I
mean by crowded rollers and I'm by that
I mean the rollers are actually next to
each other rollers you actually have to
pack the rollers in there and put the
thrust washers inside of it so it's a
little bit difficult to assemble whereas
the roller with the cage is easy to
assemble nevertheless the crowded roller
will operate at a higher speed than a
caged roller because the cage is a weak
link that will do it for now if
anybody's got any questions about that
put them in the comment section we could
do another video of this and cover any
questions that anyone's got I'll be
happy to do that if you haven't
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