why can't we use water instead of lubricant ?
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in which case we use wated and not lubricant
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Technically it could be possible. There has been some research into using water as a hydraulic fluid instead of hydraulic oil, but there are some issues, which manifest in similar ways if water is to be used as lubrication.
Water has a much lower viscosity than oil does. That makes it easier to pump around, which would reduce pumping loses in an engine lubricated by water. Which would be a bonus. Environmental reasons would also be a bonus.
The issue lies above, with the lower viscosity. That means it is worse at coating surfaces than oil is. Stuff coated in oil is hard to clean thoroughly without a degreasing agent. Thus surfaces, where the lubricating agent has drained off, are not as protected when using water as opposed to oil.
The main factor, connected to viscosity, is that the higher the viscosity, the larger the gap in ‘sliding’ surfaces there can be, since one floats over the other on a layer of lubricating film. Manufacturing processes have improved GREATLY, meaning we have much more reliable engines, but they still need to use oil. Water doesn’t have the same properties when used as a lubricating film. Besides, oils these days have many additives that improve the load bearing properties of lubricating films. This all means the oil films have a high load carrying ability, which means the engines can take a lot of stress in a small area (perfect for modern engines).
So while it could THEORETICALLY be possible to make an engine using water as a lubricant, by using much larger surfaces in sliding interfaces with much smaller gaps to ensure a proper lubricating film is formed, in practice many other properties of water mean this is impossible to achieve. Water tends to evaporate, doesn’t wet the surfaces as well as oil does, etc.
Mentioning hydraulics, all hydraulic systems work on the principle of leakage, internal workings of pumps and pistons are never completely sealed. Using water as a hydraulic fluid would mean the gaps would have to be greatly reduced, to lower the leakages to a minimum and the lubricating properties of these gaps would also be reduced using water as opposed to hydraulic oil. Again, possible in theory using correct tolerances and metallurgy, more or less impossible in practice due to the harshness of these demands.
Water has a much lower viscosity than oil does. That makes it easier to pump around, which would reduce pumping loses in an engine lubricated by water. Which would be a bonus. Environmental reasons would also be a bonus.
The issue lies above, with the lower viscosity. That means it is worse at coating surfaces than oil is. Stuff coated in oil is hard to clean thoroughly without a degreasing agent. Thus surfaces, where the lubricating agent has drained off, are not as protected when using water as opposed to oil.
The main factor, connected to viscosity, is that the higher the viscosity, the larger the gap in ‘sliding’ surfaces there can be, since one floats over the other on a layer of lubricating film. Manufacturing processes have improved GREATLY, meaning we have much more reliable engines, but they still need to use oil. Water doesn’t have the same properties when used as a lubricating film. Besides, oils these days have many additives that improve the load bearing properties of lubricating films. This all means the oil films have a high load carrying ability, which means the engines can take a lot of stress in a small area (perfect for modern engines).
So while it could THEORETICALLY be possible to make an engine using water as a lubricant, by using much larger surfaces in sliding interfaces with much smaller gaps to ensure a proper lubricating film is formed, in practice many other properties of water mean this is impossible to achieve. Water tends to evaporate, doesn’t wet the surfaces as well as oil does, etc.
Mentioning hydraulics, all hydraulic systems work on the principle of leakage, internal workings of pumps and pistons are never completely sealed. Using water as a hydraulic fluid would mean the gaps would have to be greatly reduced, to lower the leakages to a minimum and the lubricating properties of these gaps would also be reduced using water as opposed to hydraulic oil. Again, possible in theory using correct tolerances and metallurgy, more or less impossible in practice due to the harshness of these demands.
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