Postby StewartD » Mon Apr 21, 2014 6:49 am
Bob,
Why do you think the oil supply is poor? I don’t think the oil pump changed on the singles, from the 1964 through to the 1974 models. If there was any problem, I think the factory would have got round to it after all this time.
As I mentioned earlier in this thread, as it is a gear pump, driven by gears at a fixed gear ratio to the crankshaft, it delivers a set amount of oil to the engine per rev; at high revs and at low revs. If it were a centrifugal pump there would be inefficiency at low revs, but a gear pump is ‘positive displacement’ in the jargon of hydraulics. It does not suffer reduced performance at low revs that a centrifugal pump, (‘non-positive displacement’), suffers.
Jordan,
The torque is averaged out. The piston exerts a large force on the conrod and onto the bigend over part of the power stroke which is a small proportion of the 4 stroke cycle. Flywheel effect damps this force out so the Dynamometer only sees a small proportion of the effect.
Firstly the crankshaft flywheel and the alternator flywheel store kinetic energy as a slightly increased r.p.m. This is according to the equation:
1/2 (Moment of Inertia) * ( rotational speed squared)
The moment of Inertia of a flywheel is basically the mass of the flywheel by the square of the radius. It needs integration to calculate. I’m sorry that I can’t show the equations a bit more conventionally but I’m having trouble with special characters in the program.
The kinetic energy is returned from the flywheel as it slightly slows down before the next power stroke; part of this Kinetic energy being returned from the flywheel is used to pump the exhaust out and next intake charge into the cylinder.
Secondly, the rear wheel, though it is geared down considerably, will store kinetic energy as a slightly increased r.p.m.
Thirdly, if the dyno is a rolling road type, the rollers will store more kinetic energy as a slightly increased r.p.m.
The rear wheel and the dyno rollers will both be slowed down by the torque required to drive the Dyno and doing so, return the kinetic energy that they stored.
The Dyno load cell will experience much reduced fluctuations in force, than what the piston exerted on the bigend. I think the Dyno readout would be electronically averaged out at this stage. I’m no electronics expert so I’ll leave it there.
The way you are thinking of load being distributed over a larger area, due to lower revs, and therefore the peak combustion pressure occurring over a greater angle of
crank rotation is quite appealing, but is wrong in mechanical engineering design.
The designer must find when the greatest force occurs and design the big end for that. This will involve analysing the highest combustion pressures and the crank angle it occurs at. By geometry the amount of force that acts on the big end can be determined, some will be wasted as piston side thrust, but some, when the connecting rod is not at 90 degrees to the instantaneous big end radius arm, will be wasted as force the crank will exert through the main bearings to the crankcase.
Consider one big end roller at a moment in the period of peak combustion pressure. It, with a few neighbors shares the load of the conrod thrust. Consider it stopped in its motion for an instant. At this instant, it has a crushing load from the conrod on one side and it transmits the same load to the crank pin on its opposite side.
It distorts slightly and the contact area, on both sides, is a narrow rectangular patch*. The pressure on this patch is what the big end designer must allow for. If the pressure is over the steel’s elastic limit, then the bearing, (at this spot), will fail, if not the bearing will survive. A millisecond later, the roller has turned around its own axis and the axis of the crank pin. At this point, the pin has no memory of what happened a millisecond ago.
Its new contact patches must either survive the pressure or fail, and the action continues. There is no knowledge or memory in the steel about when the combustion started or finished. The mechanical designer only considers forces acting at discrete moments in time; if the area presented by the rollers is large enough and the steel is strong enough, the design is adequate
*Theoretically, for a perfectly round roller, in a perfectly round track, there will only be a line of contact that is parallel to the roller axis. If any load is applied through this line contact then the pressure is infinite because a line has zero area and pressure equals force divided by area. (P = F / A; F / 0 = ∞).
In practice the roller is distorted by the load, within the steel’s elastic limit, and an area is formed to resist the force.
Where I wrote: ‘load is low at low revs’, this is in the context of a Dyno test where the load can be adjusted to match whatever the motor is producing at that time.
Jari,
Welcome to the Forum and thanks for posting that graph which is pretty scary. Turn off the motor straight away!
For this thread I only wanted to consider a motor running properly, and whether it can be slogged or not. Slogging a motor won’t cause detonation if things are in good order.
All:
I stuffed up a bit on the assumption of r.p.m.s on the graph I posted on a few days back. The 350 Sebring’s peak output is at 6250 r.p.m.
The 250 Mk 3’s peak output is at 8000 r.p.m. according to the Clymer manual. I will redo the graph shortly. The shape of the curves will stay the same though, and so, as I only wanted to compare the maximum torques to the minimum measured torques of the machines, it is not of great importance. If someone has Dynamometer graphs from any Ducati single, with r.p.m. marked or torque already plotted, it would be good to have them posted on this thread.
Cheers,
Stewart D