The answer is they both contain Calcium. Now before you start considering whether it is best to go for a semi–skimmed or fully–skimmed on your next car service, or start pouring a 15W40 on your breakfast cereal I must remind you that is where the similarities end and this article will talk about the properties of Calcium as part of engine oil additive packages.
Calcium is often found in engine oils for specific purposes. These include:
- Total Base Number – the calcium sulphonate or if over based (carbonates (chalk)) acts to neutralise the acids formed by the combustion processes in the engine.
- Detergents – these are soap like compounds within the oil that have a polar head (water loving) and non-polar hydrophobic (oil loving) tails. Similar to your washing up bowl you can remove oily (non polar) substances from your pan and allow them to enter the water to go down the drain. In your oil it is the the opposite in that the polar I.e. Water and oxidation products that need removing and so the detergents act to surround these substances in a polar bubble inside with a non polar outside to allow them to be picked up by the oil and taken to filters to be removed.
For those interested a detergent and soap are different in that soaps are salts of fatty acids e.g. R-COO-Na+ or COO-Ca2+-OOC-R. These are relatively weekly ionic bonds and so are easily displaced forming insoluble precipitates often known as ‘Scum’. Whereas detergents use much more intense ionic bonds such as sulphonates E.g. calcium sulphonate E.g. R-SOOO-Ca2+-OOOS-R.
Note magnesium can also be used as an alternative to calcium. Calcium values in engine oil can range from ~2000 to over 20000 depending on the base number required with smaller automotive or gas engines burning relatively clean fuels requiring lower base numbers compared to large crosshead low speed marine engine burning bitumen like heavy fuel oils.
The question you may ask is if Calcium is so useful why dont all oils have high levels. Apart from the obvious reason is why be wasteful and put more than is needed, high calcium can actually be harmful to engines. This is because low level lubricant will get past piston rings and onto the piston crown. This under the very high temperatures will burn leaving behind all the metals within the oil. (Calcium is considered a metal in chemistry terms). This is an ash deposit – similar to how on a barbecue you are left with a white ash once the charcoal is all burnt. This ash can 1) build up and ‘coke’ the engine meaning shorter overhaul times and 2) lead the pre-ignition as the hot embers of ash ignite the fuel before the piston reaches top dead centre causing wear.
This is why regulating bodies such as ACEA as well as engine manufacturers now specify maximum sulphated ash levels and limits for certain metals within the new oils which have ash forming properties. These metals are usually oil additives and hence the technology has had to catch up to provide similar properties with less metals, termed SAPs.
So think when you next do an oil change on your car how many properties your oil has to perform to keep it running and hence why monitoring it’s condition is essential to a healthy engine