So, I had another lesson in tribology recently (the subject of friction and lubrication), and I can now tell you all that you should immediately drive off after starting a car, here's why:
First I'll start with some basic tribology:
To lube a contact is to separate 2 surfaces by means of a fluid. However we all now that if the surfaces are stationary relative to each other, you can squeeze the fluid out from under it, it may take a few seconds with a viscous substance such as oil, but it will happen.
Now, contact between the surfaces is something you want to avoid, and from aforementioned experiment you can conclude that when nothing is moving, you have contact.
The way lubrication works is that when your engine is turning, the main crank bearing is OFF CENTER in the journal bearing. Now imagine being a molecule of oil, swinging around in the bearing. What happens is that the cross sectional area (in rotating direction) changes area, from small on one side, to large on the other, the smallest cross section obviously being the side to which the crank is being pushed. As you know, pushing a fluid through a small hole creates a pressure that counteracts that (like in a shock absorber). This pressure pushes out in every direction, including against the main crank bearing. This pressure is so high that it counteracts the forces of the piston pushing down on it, preventing all contact between the bearing shell and the crank. This is called full film lubrication
Now the next step:
As I said, when an engine rotates the oil creates a pressure supporting the crank. What factors influence this pressure? Let's look at them one by one:
- Firstly: the rotational speed. I mentioned that the variation of the cross section creates the pressure. As you know from a shock absorber: the harder you push, the more resistance it offers. The same goes for a journal bearing (the type that supports your crankshaft): The faster it rotates, the faster the surface area changes: after all: after one revolution you are back at the same spot with the same cross section. So: a higher speed creates a larger pressure.
- Second: the relative change in the area of the cross section. Suppose the main bearing has an inner diameter A, and the bearing shell has diameter B. In practice these generally differ no more than a tenth of a millimeter (even that is a large difference!). Once again a shock absorber analogy, take a very simple shock absorber with a piston with holes that let the oil flow through. The smaller the hole, the more resistance (higher pressure) you will get. If we extend this to the bearing: the smaller the cross section, the more pressure. But, small is relative in this case! Compare 2 situations: 1: The smallest crosssection has a value of 1, and the largest a value of 2. Another situation: the smaller cross section is 2, and the larger one is 3. You can already conclude that the first situation will yield a higher pressure and hence load carrying capacity because the ratio of the 2 cross sections is 2, as opposed to 1.5 in the second situation.
In fact: if the ratio of the cross sections were 1, you would have no load bearing capacity because there is no change in cross section!
In conclusion: the ratio of smallest and largest cross section also influences load capacity. This ratio is in fact determined by the load which pushes the main bearing off center in the bearing, hence creating a variation in cross section.
- Lastly, the viscosity of the oil. We all can understand that water flows easier than honey. A thicker oil flows slower than a thin oil. When we try to force it through a hole the thicker oil will create a higher backpressure, quite simple.
Abovementioned variables have different influences in the load bearing capacity, one could vary quadratically, and the other with the square root, but I gave a quantitive description of each (if it increases or decreases the pressure and load carrying capacity).
The sharp readers may have noticed that a higher viscosity oil (cold vs warm) has a higher
load carrying capacity, and hence, I have contradicted myself. This is true if you only look at the bearing! But here's where the rest of the engine and the lubrication system comes in:
The oil pump is designed to deliver oil to all parts of the engine. A thicker oil flows slower through the passages and would require a lot of push from the pump. But because the pump speed is directly proportional to the crank speed, so are the pressure and capacity that the pump can supply (not directly, but a higher pump speed will give a higher capacity and pressure).
When an engine is cold, the oil is cold and hence it flows very hard through the passages.
When idling you are not helping this because you are effectively reducing pump output, and hence oil supply to all engine components. In extreme conditions this could cause the bearings to run dry in some spots reducing load carrying capacity and if you run out of luck you will get direct contact, and thus high wear, in your bearings! In most engines, the pump is designed to work right at operating conditions (high temp, and revs), so this is generally the case with a cold idling engine.
This problem could be solved by throwing in a larger oil pump, but the drawback is that the oil pressure at operating conditiions will rise even more. With starting conditions (cold engine) the system would be lubricated well, but you'd have higher pressures at operating conditions. Also: a larger pump draws more power from the crank and is larger! This of course puts more weight in the car and takes away (much needed!) space in the engine bay.
In passenger vehicles designers almost always choose for a smaller pump that operates satisfactory at operating conditions, and slightly less in cold engines and low revs.
As I said above a higher rotational speed of the components increases the load carrying capacity of the bearing, and increases oilpump speed and ultimately provides a larger supply of oil to the bearing, preventing dry contact.
So the best practice is to start your car and immediately
drive away! This way, you prevent the bearings from running dry due to oil starvation, minimizing wear and increasing engine life.
Hope y'all learned from this