I'm trying to understand why Ducati seems to prioritize bore over stroke.
Read somewhere that it means lower average piston speed... True? How?
you need to think about how far a piston travels
think of it 2 ways
2 inch bore 1 inch stroke
or
1 inch bore 2 inch stroke
now add 10,000 rpm
the 2nd scenario has to travel twice as far hence twice the speed
by having larger bore with shorter stroke you can get more rpms generally speaking
Quote from: J5 on October 06, 2010, 12:46:23 AM....by having larger bore with shorter stroke you can get more rpms generally speaking
.. and have room to fit in more or bigger valves.
And makes the engine shorter - handy for big L-twins.
you kind of have it backwards though IMO
Ducati prioritizes the stroke more than the bore.
the strength of the rod limits your RPMs.
Piston speed = STROKE (in inches) times RPM divided by 6
this gives you feet per minute.
you have to determine the rod strength or it's maximum feet per minute it can handle.
this will help you determine you max RPM
here's a minor example
the 696 has a stroke of 57.2mm or 2.251968504 inches
if say the theoretical max f/min of the rod is 5000 then the max rpm for the 696 would be about 13375 rpm
that's not to say it's breathing at that rpm, just the stroke helps push up its max rpm
but take the new (or old) 800 (803) with a stroke of 66mm or 2.598425197 inches
with the same theoretical max for the rod the max rpm for the 803 is approx. 11591 rpm
quite a change for 8.8mm in stroke length!
Ducati wants more power from their engines.
In some arenas, they are constrained by a maximum displacement rule.
So, the straightforward path to more HP is higher RPM.
The straightforward path to more RPM is shorter stroke, larger bore, especially when you are constrained by a maximum displacement limit.
Shorter stroke results in lower average piston speed for the same RPM.
AFAIK, average piston speed isn't really the issue, it's just an easy-to-calculate number that makes it easy to compare with.
Piston acceleration is the issue.
There are difficulties that must be overcome to make use of this RPM:
A) Piston is larger diameter, so it either gets heavier (which is a problem for the piston pin/rod/crank), or gets thinner everywhere (piston durability problem).
B) Need to be able to flow more air, but not a huge problem, as the bigger bore makes it easy to fit larger valves.
B1) But with the larger valves comes a larger area combustion chamber.
B2) Larger combustion chamber area means the chamber gets thinner to avoid losing compression.
B3) This negatively affects valve timing (avoiding valves hitting pistons) and reduces combustion efficiency (flame front moving a longer distance in a thinner chamber).
Ducati appears to be 'prioritizing' bore over stroke in their effort to get more power out of the same displacement engine.
Next question:
why would a four cylinder bike rev higher - smaller mass to move/accelerate?
Quote from: stopintime on October 06, 2010, 02:49:30 PM
Next question:
why would a four cylinder bike rev higher - smaller mass to move/accelerate?
I think you still have the stroke formula to deal with.
piston speed/acceleration are the same forces no matter the number of pistons.
Quote from: stopintime on October 06, 2010, 02:49:30 PM
Next question:
why would a four cylinder bike rev higher - smaller mass to move/accelerate?
For the same displacement, 4 cyl will rev higher than a 2 cyl, as both bore and stroke are smaller, so parts are lighter and don't have to move as far.
Quote from: Speeddog on October 06, 2010, 06:01:55 PM
For the same displacement, 4 cyl will rev higher than a 2 cyl, as both bore and stroke are smaller, so parts are lighter and don't have to move as far.
well the honda cbr600 has a longer stroke than the 696, but i expect their rods can handle more feet per minute due to like you said, lighter/less weight to move.
so using the same calculations i had before instead of say 5000fpm as a max they can have 7000fpm as max (arbitrary numbers of course)idiot search on my part. but was trying to make a point... with the correct stroke of 42.5mm using the same fpm max (which from what I heard has a lot to due with construction and material) theoretically your max is @ 18,000 rpm
but as Brad says in later posts, there are other factors.
Quote from: Raux on October 06, 2010, 10:09:27 PM
well the honda cbr600 has a longer stroke than the 696,
~~~SNIP~~~
What?
current cbr is 42.5mm. rods break on tdc overlap when there's no gas force to help slow the piston, so all the decel and accel of the piston is handled by the rod.
eh sorry. did a search and hit the bore not the stroke... my bad... but the point is the same.. i corrected my previous post
Another thing to consider when thinking about high reving multis vs Ducati twins is reciprocating weight. The smaller the valves, the less reciprocating weight a valve spring has to over come.
I think it right to say that Ducatis get more RPMs for a given reciprocating valve weight because they have no valve springs to worry about, thus no valve float problems.
Oversquare bore/stroke + desmo = higher reving big twins.
I don't know of any other big twins that rev as high, though I guess there are some.
Anyone care to clue me in on that?
Quote from: NorDog on October 08, 2010, 03:44:35 PM
I don't know of any other big twins that rev as high, though I guess there are some.
Anyone care to clue me in on that?
Aprilia? KTM? I've not checked but I think they are pretty close.
AFAIK, piston acceleration is still the rev limiter, bringing us back to the original question.
I believe the big plus for the desmo system at present is it allows a steeper cam profile, given the max rpm.
I used to shift my '64 250 cc Diana at 10,000. Never reved till it floated, but if I missed a shift it would float. I know that because I was running the slightly shorter Monza cyl, and every time I missed a shift [before I shimmed the cyl .010 ] it would bend the intake valve.
Quote from: 64duc on October 09, 2010, 02:49:21 PM
<snip>and every time I missed a shift [before I shimmed the cyl .010 ] it would bend the intake valve.
:-X
the last of the 999F motors were going to 13,500 or so i think, and the 749R motors were going to 14+. They ran tapered style collets and ti valves and still had fairly hideous cylinder head replacement intervals. of course, some of the corse cams are actually concave on one side, and look like a parallel sided oval which doesn't help. so desmo heads are not unlimited in rpm - there's still a point at which it all goes bad.
but the valves are bigger than 4 cyl valves - 11198 are 44 or so i think, most 1000cc 4 are 33 or 34mm, and that makes a big difference in comparative weights.
there's two ways to make power - capacity or rpm. if one is limited, you go the other.
Because air's elastic the piston in a short stoke engine is more effective at pulling air, if the stroke is half the distance it means only half as much elastic air medium has to be stretched through to pull the charge in to the cylinder.
for a given capacity the reduced stroke is offset by the larger bore, so wouldn't the actual inflow rate in volumetric terms be otherwise the same? the stroke/rod length ratio does affect the rate of piston movement relative to crank rotation over the movement from tdc to bdc, but that's another variable.
More than you will ever want to know about piston speed, exhaust tuning etc. :P
http://www.engineersedge.com/engine_formula_automotive.htm (http://www.engineersedge.com/engine_formula_automotive.htm)
Quote from: 64duc on October 11, 2010, 08:37:43 AM
http://www.engineersedge.com/engine_formula_automotive.htm (http://www.engineersedge.com/engine_formula_automotive.htm)
Holy formulas Batman!
its all about piston speed, acceleration (and derivatives thereof: jerk/snap), and flame-front size constraints
i got to do quite a bit of research on these principles as an undergrad. I had a lot of fun doing that kind of stuff
Quote from: 64duc on October 11, 2010, 08:37:43 AM
http://www.engineersedge.com/engine_formula_automotive.htm (http://www.engineersedge.com/engine_formula_automotive.htm)
Okay, I'm stuck in a perpetual loop.
The HP formula requires a value for "Brake Mean Effective Pressure".
But the BMEP formula requires a value for HP.
Quote from: NorDog on October 11, 2010, 03:45:28 PM
Okay, I'm stuck in a perpetual loop.
The HP formula requires a value for "Brake Mean Effective Pressure".
But the BMEP formula requires a value for HP.
Do like every engineer...
make shit up. :P
Without being familiar with the equations, I'm guessing it is an iterative solution. Guess at a HP to get a BMEP, then use that to calculate BHP. Keep tweaking until the HP # from both equations match...then you have the true answer.
Something I am familiar with --> A similar process takes place when we calculate the external buckling pressure of a large diameter steel pipe that is restrained by backfill (usually concrete in a tunnel). Not all values can be calculated simply. ;)
Quote from: Triple J on October 11, 2010, 04:50:54 PM
Without being familiar with the equations, I'm guessing it is an iterative solution. Guess at a HP to get a BMEP, then use that to calculate BHP. Keep tweaking until the HP # from both equations match...then you have the true answer.
Something I am familiar with --> A similar process takes place when we calculate the external buckling pressure of a large diameter steel pipe that is restrained by backfill (usually concrete in a tunnel). Not all values can be calculated simply. ;)
without looking up all the big words...
I'm guessing you're right.
HP is a target and you use the other variables to determine if you can reach it.