Table of contents:
- Hydraulic cushion, the real lubricant
- First step, the fat
- Oil, yes, but by splash
- At last the pressurized oil circuit
- Oil and bearings the final solution
- And the two-stroke engines?
At the end of December Albi told us about the three false myths about the oil in our engines. A very interesting article, essential I would say, to understand that part of our engine. Today we are going to analyze a little more in depth what does the oil do inside our engine. Because it doesn't sit still there in the crankcase waiting, but it has a pretty busy life reaching the last recesses of that engine. For this we are going to rely on an article published in Motorcycle Thailand, which takes us step by step through the history of lubrication systems that have been used since internal combustion engines became common in the world. And some mechanics notes that I have around here at home.
The first thing is define what the oil does there. For this we have to understand that the mechanical parts that move inside all engines never really touch each other. In the event that this happened we would be facing what is commonly called a seizure, produced as a consequence of the increase in temperature caused by the anomalous friction of the pieces with each other and that in cases it gets to weld them.
Hydraulic cushion, the real lubricant
The oil forms three layers between each piece, the two outermost layers being those in contact with the pieces and which are held there by capillarity, which is the property that liquids have of seeping through solids. A third layer remains, which is called hydraulic pad, and that it is the one that is really in charge of smoothing the friction between the pieces. This layer is more or less thick depending on the viscosity of the oil, but the thicker it is not necessarily better. To understand the importance of the thickness of this layer, we can imagine that the oil is the dust that we sweep in our house, that we can easily collect it. If that layer becomes too thick, it stops working and becomes a problem when sweeping it. Can you imagine sweeping the beach with a broom?
Hence the importance of the degree of viscosity and its proper use. A lubricant is not better for being thicker or more fluid. For that there are some gentlemen who have taken care to find out which is the appropriate grade for the perfect operation of the engine. We don't want to know more than they do. Let's first see a rudimentary lubrication system to understand how this all works a little better.
First step, the fat
Fat is not just made of oilIf not, it is also made up of a good part of soap that gives it consistency. The trick is that this lubricant remains in a semi-solid state at room temperature, but as soon as the parts begin to rub against each other, a heat is generated that liquefies the mixture and allows the oil to form the necessary protective film so that everything works properly..
The best example of this can be seen in the grease on the axle of a wheel. While the grease is stationary, it appears more or less solid, but as soon as it begins to rotate, if there is enough quantity, it keeps the shaft lubricated, facilitating its rotation. This, which at first glance seems so rudimentary, was used for many years in all engines, that it required reaching the furthest recesses of the engine with “globs” of grease for it to work. Of course we are talking about engines that did not offer great benefits, so the system remained more or less in force for a long time. We all have in mind the engine drivers with his oil can and brush lubricating the moving parts. Or if you are more curious and have seen the engine room of an old ship the system will also sound to you.
Oil, yes, but by splash
When internal combustion engines started spinning at slightly higher revs, the grease wasn't enough to ensure constant performance, so experimentation was started with different lubrication systems. One of the first consisted of simply pour a liter of oil into the crankcase so that a "spoon" connected directly to the crankshaft that was soaked in that oil, splashed towards the other internal moving parts of the engine. That system is not the best, but it was also used a good season. The problem is that this splash was difficult to control, so the next leap in the scale of performance of the engines forced invent something a little more reliable.
At last the pressurized oil circuit
We had agreed that oils lose viscosity with increasing temperature, making it easier for the hydraulic cushion to disappear. So with engines that increased their performance we need to ensure that lubrication. The solution came by incorporating a high pressure oil circuit that ran parallel to the engine and allowed it to reach such critical points as the connecting rod head or the tappets and tappets that open the valves in a four-stroke engine. This oil, once its function has been fulfilled, falls by gravity to the lower point of the engine where it is sent again to fulfill its mission by means of a submerged pump or to an auxiliary tank in the case of being the engine of the type called Dry Sump.
At this point we run into two problems, the first is that the oil coming between the pieces drags carbon and micro metal particles, which in case of increasing its concentration can end up spoiling the lubricating qualities of the fluid. For this we have a filter, which is responsible for retaining these metallic particles and dirt, prolonging the useful life of the oil.
The second problem is that that oil passing through the engine also heats up. Let's remember that the parts are there rotating and producing heat, and if we talk about the combustion chamber, the temperatures skyrocket even more. So we find ourselves with a hot fluid full of impurities. The filter does its job and cleans it, but hot oil, even if it is diverted to a reservoir outside the engine, has a lot of thermal inertia, so it does not cool easily. The solution is patented Benelli in 1934, and it is as simple as using an external radiator for the oil. More technically speaking it is known as an oil-air exchanger.
Oil and bearings the final solution
The latest advance in engine lubrication did not come from the oil or the lubrication systems, but rather they brought it the bearings that replaced the archaic bearings. These were a simple tube of relatively soft metal that allowed another piece to rotate (or slide) on it. This system, despite having good lubrication, generates a lot of heat and metal particles due to wear, which means that its life is limited. For its part, a bearing is a pair of bearings that rotate concentrically on a crown of balls or rollers, with which the friction surface is much smaller, its lubrication is easier by having more holes for the oil to pass through and they last vastly longer than their simple counterparts. Thus the oil could lower its temperature a little, but as Since the engines have less friction, what they achieve is better performance, the heat generated in the combustion chamber more than compensates for this cooling.
We still have a function of the oil in a modern engine. We have commented that thanks to the bearings we have the engine spinning freely, but the combustion chamber continues to generate a lot of heat. And in many cases liquid cooling is not enough when we talk about high performance, so the last job entrusted to oil is cool the bottom of the pistons. But as these move faster and faster, the oil pressure in the pipes may not be enough, you have to go another way. This is neither more nor less than spitting it out through injectors towards the underside of the pistons. Speaking of memory, I think the first to use this system were the Suzuki GSX-F with SACS engine. But as those engines relied only on oil for cooling, the result was quite poor. However, the system was not forgotten and today there are many engines that continue to use the system in conjunction with a more conventional liquid cooling.
And the two-stroke engines?
Well, the lubrication system in a two-stroke engine is even more delicate despite not having moving parts that drive the oil. In this case we expect the oil to mix well with the gasoline and when entering the cylinder in the intake period, it is in charge of lubricating all the moving parts that are found on the way to the combustion chamber. This system is classified as one of the most polluting, since the oil mixed with gasoline does not usually burn completely and ends up being expelled to the outside contaminating the environment.
This could be solved if the greasing of the moving parts of this type of engine was done in a independent way of mixing. But since it is a complicated system to set up and that requires a lot of investment to achieve results, it has meant that the brands have killed the two-stroke engines in favor of the four-stroke ones. That they are more complicated to manufacture and maintain because they have many more parts, but they have sold us as much more ecological because their polluting emissions can be more easily controlled.
Now you know a little more about what what the oil does inside the engine and how important it is to ensure that everything works properly.