Thanks guys. Some very good advice presented already.
But I shall add my pennorth, FYI. (I worked with the chemist making the recipe for motor oil during part of my career).
A university Doctor advised that "The oxidising properties of ferrous materials used commonly in our modern lives can be summarised in 2 words. - Steel rusts."
That said, the combustion gases are a horrible mix of organic and inorganic materials. Water, CO2, CO, N2, and Nitrous oxides. Also from the spark, a very tiny amount of O3. (Ozone). That makes a corrosive mix that rusts steel quickly.
The temperature, and pressure in the combustion cause some of these gases to dissolve in the water as it condenses (cooling and pressure drop causes condensation). Typically, a gallon of fuel makes more than a gallon of condensate - allmost all of which goes out the exhaust pipe. But something like 0.1~1% goes past the piston rings as blow-by gas. Cars always take this down a hose to the intake manifold and pass it through the engine to clean it in the combustion chamber and throw it down the exhaust.
But the condensate from blow-by gases mixing with oil are very acidic - they really rapidly combine with easily corrodable metals. Motor oils, unlike most other oils, are heavily loaded with
- detergents (to hold moisture in suspension and prevent it puddling in the sump.... - which could cause a bubble of water to be pumped through bearings with disastrous results, etc. - and permits the emulsion to carry the water to hot parts of the engine (above the valve gear in the cylinder head) where the water can be boiled-off and vented through the blow-by valve and pipework).
- Anti-acid stuff - typicaly zinc compounds, etc. - that counteract the gases dissolved it condensate water that make it acidic.
- Lubricant enhancers - solids dissolved in the oil that are deposited on bearing surfaces under high stress or when oil drains from surfaces, that prevent scuffing if the oil film breaks down, or for cold starting. The durability of engines left standing is enhanced by the anti-scuff compounds, but theses can be washed away by WD40, etc. so the oil film must be replaced after cleaning.
- Waxes and surface tension improvers, which help maintain some oil at a surface which keeps air and condensation from the surface, prolonging the oil film protection.
But valves in combustion chambers are usually clean of oil.
I have a cast iron frying pan. After washing, cleaning scouring to the fresh metal surface, it will rust overnight on the kitchen shelf. A very thin film (wipe ) with oil will prevent rusting. But on the outside, it has developed a lacquer from flames from the gas cooker, burning oils and fats vaporised during cooking and drawn into the gas flame. The black lacquer is very good at preventing corrosion. Decades ago we used to have to de-coke engines regularly, as this lacquer was detrimental. A product of the dirty combustion and combustion of a tiny amount of engine oil off the bore. But the lacquer we de-coked, did prevent valves and cast iron heads and seats from rusting!
I hope some of this helps?
K2
