Bking:
Buckstones:
Somewhat confusing comment above:
I thought high pressure fuel pumps rely on the lubricating properties of diesel fuel (so fail when cars are mis-fuelled with petrol).
How is it that the same fuel manages to become an abrasive in cylinders? Also, polished rings and cylinder walls should improve compression shouldn’t they? That’s how it works in air-rifles, hydraulic cylinders etc isn’t it?
The diesel washes the oil off the cylinder walls so it acts as an abrasive and have you ever heard of a cylinder hone?
Its also called a glaze buster.That might give you a clue.
I think on the proper analysis the washing off of lubricating oil leads to wear between unlubricated parts - it’s not the diesel itself that becomes abrasive, it’s the unlubricated parts which are abrasive.
Also the reason why hydraulic rams are polished while piston bores are honed is because each approach solves a different problem.
With hydraulic rams, it’s important that the hydraulic oil is retained under huge pressure (not present in an engine to the same degree), excess oiling of the exposed part of the ram would lead to soiling and retention of abrasives from the open air, the lateral forces are more modest, and the number of reciprocations far lower. I think rubber is the normal choice for hydraulic seals - this has good sealing capability, but a relatively short lifetime against the number of reciprocations of the ram, and would wear even more rapidly against an unpolished surface (without the honed surface adding any extra useful lubrication). The oil retained on the exposed side of the hydraulic seal is sufficient to re-lubricate the exposed side of the ram as it returns home through the seal (and cleaning and reoiling the exposed side of the seal manually from time to time, is a feasible servicing task, although worn seals would allow a certain amount of seepage anyway due to the oil pressure behind them).
With engines, rubber is not an acceptable kind of seal in the first place, and modest passage of oil into the combustion chamber is not a problem (and air in the combustion chamber is pre-filtered).
The more important factor is the retention of combustion gases, which is not a static pressure but varies through the combustion cycle, but again modest leakage of gases across the seal is acceptable (much moreso than for a hydraulic piston, where leaked oil could not be captured without additional equipment, and would cause the piston to gradually move of its own accord, which is undesirable in that application, and where the ingress of air into the hydraulic side is completely unacceptable).
Given that piston rings are not as sacrificial as rubber and are capable of wearing the bore, and neither the seals nor the bores are expected to be serviced as frequently as seals on hydraulic rams are (relative to the number of reciprocations completed), and considering that the combustion chamber is a hostile environment that will consume oil residue on exposed surfaces, honing the bores so that they retain extra oil to help lubricate the rings is considered a desirable and acceptable compromise.
The rings don’t wear against the honed surface as quickly as rubber would, because they have a harder surface and are not sufficiently flexible to fill into the troughs in the way that rubber would.
Correspondingly, polishing the bores in an engine would tighten the seal and save oil, but would also lead to inadequate lubrication and therefore rampant wear of both rings and bore (which, if not serviced, would eventually cause the seal to degrade to a point far worse than that caused by honing), and the loss of lubrication also leads to the forfeit of the sealing effects of the oil itself, which when present frustrates the passage of high-pressure combustion gases across the seal and into the crankcase (similar to how a stuffing box on a boat uses thick grease and reinforcing fabric to prevent the ingress of seawater along the propellor shaft and into the boat).
This gets me thinking, nowadays mechanical engineering is seen as like baking a cake, but engines incorporate enormously complex physical processes. This is a prime example, where if you’d asked me what the role of oil was in an engine, I’d have said lubrication, without ever considering its (more minor) function as a gas seal - as well as cooling, cleaning, noise reduction, and probably other desirable side effects.
While if it takes load to warm up the cylinders why do we need bleedin great big fans to keep an idling engine cool while it’s stationary.Let alone when the thermostats are closed with no circulation at all to the radiator/oil cooler. 