Low r.p.m. urban myth

[motoital]

I think the issue that isn't being discussed is filtration. It's no secret that filtration is marginal on the singles. I was told the reason to keep the revs up was to take advantage of the crank plug and I've opened a few to know the trap works. When under load I try not to stay under 3000 rpm. I'm no expert but I trust the advice I was given.

[Jordan]

Hi Stewart,
You remain skeptical about low revs + Wide Open Throttle causing undue stress to marginal big end bearings.
I take your point about maximum torque occurring at high revs, but I don't know why you avoid addressing the WOT situation.
As soon as the throttle is opened from idle, there's more force applied to the big end - that's what makes the engine speed up.
If WOT is applied at idle speed and the motorcycle is in gear and clutch engaged, both the force and the load are momentarily high.

There are reports of 860 twins that suffered premature big end wear, much worse than the 750. I think early 860s used the same size bearing at first, but were increased in size and maybe quality in later models.
They say the 860s are very torquey engines and a pleasure to experience it. But, it might have been a case of "strong muscles but weak bones".
Slogging a smaller engine at low revs isn't much fun, so keeping it at higher revs could be a blessing in disguise.

[Harvey]

Jordan,

Harvey,

Please note my comments about the bearing table load ratings; the tables only give static and dynamic load ratings. There is nothing about duration of force.

It is the force that a bearing is rated on. At low revs the force is very low compared the forces at high revs. A low force occurring for a long time is irrelevant to a designer who has a high force occurring for a short time. The designer knows that the highest force is the one that is going to damage an under sized bearing.

Yes the bearing tables don’t show that, but I guess that they weren’t considering the application of them being used in a con rod big end.
The combustion is a long build of pressure, that takes about 25* at 4000 rpm to burn 10% to 90% of the charge with max pressure of about 50 bar. occurring at about 10* to 18* ATDC. If the engine is doing 2000 rpm, this pressure build would occur over about 12.5*. So the rod and the pin are taking this pressure over a shorter length of track, where it would take the pressure over a longer 25* track length at higher rpms. It is that concentrated pressure on the area that causes it to distort, fracture the hardening flake off.

Harvey,

Dynamometer testing of any petrol engine shows that maximum torque occurs at elevated rpms; for a Ducati single, I think about 6000 rpm would be the maximum torque rpm. You nearly contradict yourself in saying - 'max torque is made at higher speeds, but if the throttle is open, the torque loading on the piston is almost as high at lower rpms.' Dynamometer (taken with throttle full open), readings give the maximum torque at all points in the rev range and do not lie. There would be no point to Dynamometer testing otherwise.

Cheers,Stewart D

Well I would not say that they “do not lie”, but they do give the best representation of what the cylinder pressures are.
I don’t have the figures for the 450, but a 1098 Ducati engine that develops max torque of 77 ft-lb at 8000 rpm, also produces 60ft-lbs at 3500 rpm. So to say “ At low revs the force is very low compared the forces at high revs.”. This is just not the case.

Got to say thanks for starting an interesting thread. There are things that you know to be right, but don't really think about too much. It is only when some body asks the question, that you have to ask " why is it so".

[Rocla]

Hello,

I didn't follow the whole conversation but let me remind something: "torque" is different from "power". A motorbike thermic engine gets its maximum power at very high rpm but the torque, especially for monocylinders (it is different for multiple japonese cylinders for instance), is obtained at a lower speed. For instance, classical figures for a 350 mono cylinder : 24HP at 6000 rpm, max torque at 4000 rpm.

[double diamond]

The comment that “All the information I have seen about detonation and pre-ignition does not suggest there is any danger of dangerously high loads on the big end bearing.” Is grossly inaccurate. The terms are often (mis) used interchangeably but a distinction is in order. Pre-ignition is usually used to describe the situation where a “hot spot” in the combustion chamber ignites the charge rather than the timed spark from the spark plug. Hence, the charge ignites before the timed spark, which effectively advances the timing. Detonation is the condition where, due to inadequate octane rating of the fuel for the compression the engine is running, the charge spontaneously ignites due to the compression being generated in the combustion chamber, and “explodes” rather than “burns”. A correct combustion event initiates the burn of the charge at the spark plug and the charge releases its energy in a controlled burn, with the flame front reaching the piston crown just as the piston passes TDC. Detonation is the uncontrolled explosion of the charge all at once (relative to the time interval a controlled burn would take). The effect is basically the gradual release of energy as the charge burn pushes down gradually on the piston, vs. the instantaneous release of the charge energy, long before TDC. Detonation is basically like hitting the piston with a big hammer, a sudden impact rather than the gradual push of a controlled burn, and is extremely destructive to small and big end bearings as well as the connecting rod and piston.

By the way, although most Ducati singles employ a mechanical advance mechanism, the Diana Mk III and perhaps some early Mark 3’s had a solid point cam with no advance mechanism. This was typical “speed tuning” thinking, even represented in the Herb Hitch/Jim Hayes “Blueprint for Power” (I’ve encountered several Scramblers with the stock mechanical advance welded in the full advance position). The Daytona Triumphs may or may not have been running an advance mechanism, but the engine speeds they were running likely were beyond the point of full advance. The point is, detonation destroyed the engines and “lugging” seems to exacerbate the situation when the conditions that lead to detonation are present.

Matt

[Nick]

What our bikes have, and what the old Daytona Triumph triples had, are centrifugal advance units. With these primitive systems, the timing will always be at full advance at any rpm above 2,000 or so. Modern bikes and cars have electronic advance systems which continually adjust the timing across the rev range to ensure ideal timing under all load / rpm conditions. Think about it, when was the last time you heard the tinking sound of detonation on a modern car? On modern bikes/cars, their electronic ignitions (with all their sensors) detect the onset of detonation and retard the timing as needed. A crank angle sensor is one of the most important sensors in these systems. Our old bikes can't do that.

In other words, aside from starting rpm and slightly above, the timing on our bikes is fixed. As I mentioned above, because peak combustion pressure occurs with the piston further away from TDC (on the up stroke) the load imposed on the big end at low rpm is much higher than the load imposed at high rpm. This is why detonation occurs under those load conditions. In short, while the timing is fixed on our bikes, the point at which peak combustion pressure is reached relative to piston movement changes depending on rpm.

It is, of course, great fun to try and dispel 'old wives' tales, problem is, a lot of those grannies were smarter than we thought.

[Harvey]

by Nick » Wed Apr 16, 2014 7:05 pm

In other words, aside from starting rpm and slightly above, the timing on our bikes is fixed. As I mentioned above, because peak combustion pressure occurs with the piston further away from TDC (on the up stroke) the load imposed on the big end at low rpm is much higher than the load imposed at high rpm. This is why detonation occurs under those load conditions. In short, while the timing is fixed on our bikes, the point at which peak combustion pressure is reached relative to piston movement changes depending on rpm.

This is not quite correct.
The reason that we set the ignition timing of the engine is have the max pressure developed when the piston, and the con rod are at the right angle to the crank shaft so that the thrust of the rod will turn the crank, and not try to push it out the bottom of the cases. This angle is reached at about 18* ATDC. So regardless of the speed of the engine, this angle has to be maintained. Too early and the force is wasted in trying to push the shaft out the bottom, too late and the crank angle is past the point of mechanical advantage.

Once the engine has reached about 2500 the ignition timing remains fixed at about 30* BTDC, and does not have to be advanced any more, due to the turbulence which reduces the burn time, as the rpm increases. So although the spark is held at the same point, the maximum pressure is maintained at 18* ATDC, through out the rest of the rpm rise, it does not change.

[Nick]

If peak pressure always occurs at the same crank angle why does detonation occur at low rpm and not high rpm? Surely a lack of turbulence is not the only factor.

Here's something copied from the Net: "It should be noted that the faster the engine is turning, the shorter the time for the crankshaft angle to reach that 16 degrees ATDC position (PCP). The burn time of the gas is controlled by the chemical makeup of the fuel itself, the temperature of the fuel, and how well it is mixed with the required oxygen. Octane additives do not change the burn rate of the gas. Racing engine fuel has a different chemical design so that it will burn faster to keep up with high RPM engines. Octane rating is NOT involved in this fuel burn time, regardless of what rumors you may have heard or may have said yourself. Combustion chamber shape will also affect burn time, and that will be explained later.
As the engine RPM increases, the ignition spark must be advanced tens of crankshaft degrees to have the peak combustion pressure (PCP) occur at 16 degrees ATDC. When the spark-timing advance results in MBT, this is referred to as the MBT ignition timing point. As the engine RPM increases, the MBT ignition timing point must be advanced to keep PCP at 16 degrees. This is why you need a timing advance curve based upon engine RPM."

Let's not forget the original post! We were discussing whether, on bikes like ours (with fixed timing), excessive load at low rpm (lugging) was detrimental to big end components. I believe it is, and all the old wives (and men) out there agree with me! Since new cars / bikes retard the timing under those conditions, lugging probably effects them very little.

Now, how about a discussion on high-velocity intake ports for our Ducs.....?

[StewartD]

dyno chart 1.jpg

Motoital,

You are right in saying a centrifugal sludge trap works better at higher speed. This is a separate issue to what I set out to discuss.

I think, however that oil contaminants will be more destructive to aluminium pistons and bronze bushes, than big end bearings. The races and balls/rollers of big end bearings and other bearings are made of very hard steel; the aluminium piston and plain bearings of bronze are much softer and more vulnerable to contaminants.

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.

To do this I assumed that the speed of 80 mph was at an engine speed of 8000 r.p.m. This might seem a big assumption, but the inaccuracy will be consistent in one direction. The proportionality of the figures I get over the range will be accurate.

I converted to Newton.Metre of torque and used the same scale at the left.

Torque (N.m) = Power (Watt) divided by rotational speed (Radian/second)
= Power (Horsepower x 745.7) / Rotational speed (R.P.M. x 0.1047)

So use factor 745.7 / 0.1047 = 7122

Torque (N.m) = (P / n) x 7122

P = Power in horsepower
n = Rotational speed in Revs per minute

Example: For the 350 Sebring at 35 mph: I have assumed it is at 3500 rpm; the original graph shows 4.5 HP:

4.5 / 3500 x 7122 = 9.2 N.m

Compare the torque maximums to the torques at the low rev readings

350 Sebring:
4.5 N.m @ 3000 rpm; 20.2 N.m @ 6000 rpm: 4.48 times the torque

250 Mk 3:
10.9 N.m @ 3250 rpm; 26.3 N.m @ 5000 rpm: 2.41 times the torque

250 Chris Stevens
8.7 N.m @ 3250 rpm; 19.0 N.m @ 4500 rpm: 2.18 times the torque

These torque figures are to show I am not avoiding the WOT issue! Dyno tests are done with the throttle wide open. The torque is directly proportional to the force in the big end bearing: Torque = Force x radius
So the force in the big end of the 350 Sebring at 3000 r.p.m. is (1 / 4.48) 22% of what it is at 6000 rpm.

Harvey,

What you say about the change of angle of rotation that combustion takes place over, at 2000 rpm compared to higher rpm I go along with, but I think your words ‘concentrated pressure is incorrect in this context. Your argument is that a lower force acting over a shorter travel distance is more destructive than a higher force that travels over a longer distance.

Pressure equals Force divided by Area. To calculate the pressure that might destroy a component, the distance that the force travels over is irrelevant.

The piece of steel that is subject to the low or high pressure has no knowledge of the distance that the force is acting over. It either survives or is crushed, depending on whether the pressure has exceeded the steel’s elastic limit.

The figures you quote are quite impressive; that is a very flat torque curve. I did not have 3500 rpm in mind though, when I talked about slogging the motor. My 1974 450 Desmo was at a very comfortable cruising speed of 60 mph/100 kmh at about 4000 rpm.

Also other people (Yourself on 12th April & motoital on16th April) have nominated 3000 rpm as a ‘safe’ and I assume, therefore not a ‘slogging’ speed. I was thinking of speeds of just off idle. The graph I have attached shows a very steep decrease in torque below what I have assumed as 3500rpm. It would be interesting to get a dynamometer graph with the rpm and torque readings down to below 2000 rpm.

Matt,

I haven’t seen any evidence that ‘ ...there is any danger of dangerously high loads’ I don’t know what is grossly inaccurate about my statement.

You have given a good summary of Pre-ignition and Detonation, but no further evidence that they constitute dangerously high loads. There is only your statement that,

‘Detonation is basically like hitting the piston with a big hammer, a sudden impact rather than the gradual push of a controlled burn, and is extremely destructive to small and big end bearings as well as the connecting rod and piston.’

Where did this information come from and what is the level of force, compared to a normal combustion?

Nick,

I agree with most of your first post, (17th April), except the following:

‘peak combustion pressure occurs with the piston further away from TDC (on the up stroke) the load imposed on the big end at low rpm is much higher than the load imposed at high rpm.’

If the peak combustion pressure is occurring on the upstroke then the motor would run backwards. Can you clarify that bit please.

Harvey,

The information you gave about the auto advance etc. all seems to agree with what Phil Irving says in his book Motorcycle Engineering.

[Bevel bob]

The Ducati single also differs in that the low pressure system oil supply is poor at low rpm, but it is a system designed for a 125 racer running in endurance events,that morphed into a production roadster.