Wear Mechanisms In a 4 Stroke Combustion Engine
Discussion
Hey guys and girls,
I was told a while ago that an engine at very low RPM's with a high load (i.e a lot of throttle... like having an engine at 20mph in fifth and with the right foot mashed into the carpet) will have significatinly more wear than an engine at high RPM with a high load. I'm trying to understand (in terms of physics) if this is correct and why this is.
My hyposthesis is this, and I would be greatful if anyone could guide me as to whether I am on the correct lines.
At higher RPM's with nice hot oil, the forces on the pistons will be high but because of the speed of the crankshaft, a film of oil sits between the bearing and the crankshaft (hydrodynamic effect), absorbing the mechanical work and coverting it to waste heat. Figure 1 below illustrates this:
Figure 1: Illustrating hydrodynamics between the shell bearing and crankshaft.
However, when an engine is at low RPM and under high load, there is less hydrodymanic effect between the crankshaft and bearing and thus the crankshaft is able to come into contact with the bearing due to the higher forces acting downwards on the piston, causing greater wear (esp with colder oil)
Can anyone advise me if I am thinking along the right lines? I'm just very interested to know!
Many thanks,
Matt
I was told a while ago that an engine at very low RPM's with a high load (i.e a lot of throttle... like having an engine at 20mph in fifth and with the right foot mashed into the carpet) will have significatinly more wear than an engine at high RPM with a high load. I'm trying to understand (in terms of physics) if this is correct and why this is.
My hyposthesis is this, and I would be greatful if anyone could guide me as to whether I am on the correct lines.
At higher RPM's with nice hot oil, the forces on the pistons will be high but because of the speed of the crankshaft, a film of oil sits between the bearing and the crankshaft (hydrodynamic effect), absorbing the mechanical work and coverting it to waste heat. Figure 1 below illustrates this:
Figure 1: Illustrating hydrodynamics between the shell bearing and crankshaft.
However, when an engine is at low RPM and under high load, there is less hydrodymanic effect between the crankshaft and bearing and thus the crankshaft is able to come into contact with the bearing due to the higher forces acting downwards on the piston, causing greater wear (esp with colder oil)
Can anyone advise me if I am thinking along the right lines? I'm just very interested to know!
Many thanks,
Matt
Seriously Matt
A the oil film isn't, it's a closed pressure system around the crank or cam kept up by the oil pump
B the bearing shouldn't contact the crank and won't unless the pump is knacked or the oil has leaked away (guess who drives an MG Midget)
The point I'm trying to get over is that with a two part bearing the "weight" of the metal will be held in check from all directions until wear wins
cue the real experts who will prove I should have gone down the pub this Saturday night
A the oil film isn't, it's a closed pressure system around the crank or cam kept up by the oil pump
B the bearing shouldn't contact the crank and won't unless the pump is knacked or the oil has leaked away (guess who drives an MG Midget)
The point I'm trying to get over is that with a two part bearing the "weight" of the metal will be held in check from all directions until wear wins
cue the real experts who will prove I should have gone down the pub this Saturday night
I think it's one of those old wives tales. I've heard it a few times, but it doesn't really make sense since as has been said, the bearing faces will never make contact unless there's a serious problem.
Most wear occurs at engine start after a long period spent off, so the oil pressure has dropped. There may be slight contact for a second or so while the oil pressure builds, but other than that it should be fine.
There is another issue, that of fatigue but that's cyclic and load related, so the more high load, high cycles applied to, say, a con-rod, the closer you get to its eventual failure point. However, that means high revs, not low (usually).
Most wear occurs at engine start after a long period spent off, so the oil pressure has dropped. There may be slight contact for a second or so while the oil pressure builds, but other than that it should be fine.
There is another issue, that of fatigue but that's cyclic and load related, so the more high load, high cycles applied to, say, a con-rod, the closer you get to its eventual failure point. However, that means high revs, not low (usually).
When an engine is fully loaded and at very low rpm the oil flow is reduced as the pump is not supplying as much oil as it would at higher rpm's, labouring an engine will cause more wear then keeping the revs up, it actually says in my Bmw manual not to drive the car under 1500 rpm in high gear change down and bring the revs up.
The oil is pressure fed into the crankcase main bearing saddles.
This then pressurises the oil galleries inside the crankshaft and exits via holes in the crankshaft journals around which the big end and main bearing shells run.
If the oil pump is not making enough pressure or your bearings are worn (thus not holding a seal against the journal, losing pressure), then the film of oil can be overcome.
This then pressurises the oil galleries inside the crankshaft and exits via holes in the crankshaft journals around which the big end and main bearing shells run.
If the oil pump is not making enough pressure or your bearings are worn (thus not holding a seal against the journal, losing pressure), then the film of oil can be overcome.
steve_bmw said:
When an engine is fully loaded and at very low rpm the oil flow is reduced as the pump is not supplying as much oil as it would at higher rpm's, labouring an engine will cause more wear then keeping the revs up, it actually says in my Bmw manual not to drive the car under 1500 rpm in high gear change down and bring the revs up.
Unless your oil pump is knackered, it will make no difference. The oil pressure is more than adequate at all running speeds to prevent the bearings and crankshaft journals from making contact, so no wear will occur. The advice in your manual is more to do with keeping the engine in the most efficient, and responsive, parts of the rev band. No doubt it also mentions not over-revving (despite it having a rev limiter).
The rotation speed of the crank also has an effect on the oil bearing.
As the engine starts the crank is in contact with the shells. The rotation of the crank (and the now flowing oil) causes a pressure build up in front of the contact point. As the rotation speed increases the crank is lifted from the shell and it rides on the pressure wave. at the same time the oil pressure is increasing, helping the crank and shell keep apart.
In short low rmp will have a negative effect on the oil bearing, but I've no idea if its enough to cause extra wear. I only know the theory of how it works
As the engine starts the crank is in contact with the shells. The rotation of the crank (and the now flowing oil) causes a pressure build up in front of the contact point. As the rotation speed increases the crank is lifted from the shell and it rides on the pressure wave. at the same time the oil pressure is increasing, helping the crank and shell keep apart.
In short low rmp will have a negative effect on the oil bearing, but I've no idea if its enough to cause extra wear. I only know the theory of how it works
Edited by 98elise on Tuesday 24th July 07:02
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