Low r.p.m. urban myth

[Jordan]

I still maintain the load is low at low revs. The graph I have attached is from the thread ‘Horsepower claims, Top speed and Accuracy of Dynamometer’. It was posted by Stan Lipert on the 14th of June 2013. I have calculated some torque figures and graphed them. I have matched the colours of the torque curves to the corresponding horsepower curves.

I don't fully understand what torque values mean. Do they represent what is happening at a particular instant, or are they averaged out for a whole revolution?
In my unscientific way, I imagine a harmful effect of a short duration thrust on the big end. Could it be that a peak of maximum thrust, even at a moderate torque figure, could have a big effect on bearing life at low revs? Spinning the engine faster means the peak would act upon a larger bearing area, as it moves quicker and covers more distance, thus spreading the work.
I don't understand also "load is low at low revs". Isn't load independent of revs? I'm thinking of load as external, as like a hill, a high gear or a dyno brake.

[oh2jxg]

About a detonation

pressure_camdeg.jpg

Here is a picture of the book burning and combustion
The original name of the book is Poltto ja palaminen
the image is captured from page 596

The y-axis is the pressure in the cylinder and The x-axis is the crank angle
knocking sound is produced by the pressure peaks
which can be seen in the upper curve

I think that this will to help you
when you calculate the forces acting on the connecting rod

Please keep in mind the wear and tear of the engine as you study the image

Regards Jari

PS. I joined the forum at yesterday

-74 Condor A350

[Jordan]

Hello Jari,

Your graph shows 2 curves. One seems to be showing detonation, the other normal combustion.
But, we don't need to consider abnormal events like detonation. Lugging, or labouring of the engine can be happening with normal combustion.

The graph doesn't show engine speed, just pressure and angle.
It would be good to know how a change in engine speed affects pressure.

[StewartD]

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

[Bevel bob]

Hi Stewart, You can theorise as much as you like and are entitled to your opinion, I know what i know from riding a 175 tuned to the max with an 11 to one CR that was unmercifully flogged at max rpm and regularly pitted against 650's , no problems. A new Mach 1 ridden by my mate who thought it clever to give it WOT from low rpm wrecked a big end in 3K. When i rebuilt my 250 I thought it prudent to kick it over with the plug out to get the oil up to the cam,I wore myself out to no avail. Held at 3000 it took a while .

[double diamond]

Kevin Cameron's article on detonation: http://www.klemmvintage.com/camerondeto.htm
Although not specifically addressing the question of low RPM running on big end bearing life, there are some pertinent observations.
Matt

[Harvey]

by StewartD

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.

Well Stewart I think we are getting to the same point with this statement as I pointed out here;

“. It is that concentrated pressure on the area that causes it to distort, fracture the hardening flake off.”.

Not talking about the rollers, but the area of the rod track, and pin, that take this same high pressure distorting the surface at the same spot every firing.
The combustion pressure that builds to its maximum at about 15* ATDC acts on the rod and pin, over a short angle of about 20*, the pressure dropping as the piston descends, increases the volume. This is the area of the rod and pin that takes this same distorting pressure every firing, that does fatigue the hardening, to crack and flake off, to destroy the surface. The slower the rpm the higher the forces acting on this shorter area.

[StewartD]

Bob,

I've thrown out a fair bit of theory but I have personal experience of slogging a motor for extended periods with no ill effects. My 450 Desmo was my only transport for about 10 years. In this time I would go to many rallies, where as often as not, there would be muddy roads that forced one to stay at low revs. I had no big end problem, and I estimate I did 200,000 km in those years.
Do you think your friend was causing detonation to occur by extreme abuse or that it was a simply a rubbish bearing? Ducati quality control isn’t the best, anyone who has had Ducatis for a while knows this. Maybe my idea of slogging a motor is different from your friend’s.

Matt,

That's an interesting article. The part touching on low r.p.m. is in point 9 of 'Stopping the Show':

(9) Higher engine rpm This simply shortens the time during which the mixture is held at high temp. In Honda experiments in the 1960's, they found that an engine's octane requirements began to decrease steadily over 12,000 rpm, and were under 60 octane up near 20,000. In a more accessible example, note that engines knock when they are "lugged" run at low rpm, wide open throttle and stop knocking promptly when you shift down a gear and let the engine rev up more. This stops deto by not allowing enough time for the reactions that cause it.

He describes it in the 5th paragraph:

When it hits parts, it hits hard. If we hear it al all, it is as a high, dry, irregular clicking, not unlike the reverberating sound of rocks struck under water. Detonation's pressure front can damage bearings by its hammering shock, but the real problem is what it does to an engine's natural, internal insulation.

The wording suggests that the rider may not hear the detonation. I have had a motor suffer pre-ignition and the sound was quite audible. Detonation is a much more destructive event than pre-ignition so I am quite surprised for Kevin Cameron to say this. It is not clear from this article if he talking about a standard engine or a modified engine with a loud exhaust.

Harvey,

I used the example of the roller to help the understanding of the forces at play. It is correct that the rod and pin always have the peak load act on the same spot if the peak pressure occurs at the same crank angle.

I disagree with your statement, -
“The slower the rpm the higher the forces acting on this shorter area.”

This seems to be going over what we discussed before. My interpretation of your statement is:

‘since the time of combustion is constant, then at slower r.p.m., the force of combustion acts on a shorter proportion of the circumference big end pin the conrod’.

Can you confirm if my interpretation is accurate, before I continue with the discussion. We might be at cross purposes otherwise.

[Bevel bob]

Hi Stewart. Something to bear in mind,You are comparing apples to pears.In road tests stock 250 valve spring out performed the 450 widecase and desmo, they were carrying a higher state of tune. They were also running an inferior big end, and pulling very high gearing, the narrow case 250's in Mach 1 tune were a very raceworthy reliable motor providing they were used as such .

[graeme]

When the term "low revs" is used, what does that mean?
Low revs for a 250 is higher than for a 450?
Low revs I read as below 3k?
How much throttle at below 3k? A little won't hurt but wide open or 3/4 won't do the engine any good. (in my opinion)
I've made a habit of not using full throttle under 4k on any of my 450s.
If you listen and feel the engine it will let you know when it's not happy.

My tiny brain thinks that with a wide open throttle the slower the revs the more time there is for the cylinder to be filled, air fuel mix moves slowly and therefore the bigger the push or bang. = more load.

To me a free spinning engine (4k and over for a 450) is better off than a slogging engine, with a wide open throttle.

I'll keep my singles spinning.

Graeme