Chapter Five – It’s more complicated than you thought.
Engine oil today is much more than just the viscosity grade. For an introduction to engine oil, learning the viscosity grades and how they relate to each other is an excellent place to start. That’s why it’s what we started with in Chapter 1-4. An engine oil has to perform a lot of functions in the engine and over time the formulating of engine oil has become more and more complex with chemical additives being put into the engine oils with every couple of years new specifications are brought out by the auto makers and new technologies produced by the oil companies.
We aren’t going to go into any long chemistry discussions here. But you could spend your entire life learning about, researching and working on different engine oil chemistry – in fact there are thousands of people who do exactly that to bring us the products we buy. Engine oil is no cheap commodity. It is a highly complex blend of synergistic components that come together to provide performance, protection and advance the cause of better fuel economy and more powerful engines everywhere.
What we are going to talk about here are some of the key functions that an engine oil must perform and how the engine oils formulas are adapted to those functions:
In previous chapters we talked about how engine oils are getting thinner in order to reduce energy consumption and improve fuel economy. An engine oil that is closer to the optimum operational viscosity (in our previous examples it was 10 cSt but it could be as low as 4cSt on some newer engines) at start up requires less energy going to heat the oil and make it thinner. This can also be done with viscosity modifiers that respond to the shear rate of engine components. In high shear environments (like cam shafts) the viscosity modifier can temporarily thin out and make it easier for the oil to flow. This lower HTHS keeps the oil film thickness at the right level. In high load, low rpm environments, the film may need to be thicker to provide the right level of protection. During engine development and testing, the engine manufacturer determines the HTHS requirements for the lubricant. In the SAE J300 there is a minimum HTHS requirement for each engine oil grade.
There are also other additives that help to reduce friction in places where it is not the viscosity but a microscopically thin layer of protective additives that keep metal parts moving past each other. Reducing the friction overall can also help to improve fuel economy.
Keeping the parts of an engine moving past each other is a big part of what an engine oil does. The viscosity or thickness of the oil film on these parts play a role in making this happen – but so does other additives that have proven effective when added to the engine oil. Oil makers use these to make the engine more robust and make claims about how advanced their durability is. Every surface no matter how smoothly polished at the microscopic level is made up of mountain peaks and valleys (these are called asperities). What these additives do is create a “snow cap” layer on the peaks that gets brushed off and replenished as the two surfaces rub past each other. This layer prevents the metal from wearing away and can greatly improve the life of engine components. Engine oil flows constantly through the engine fortified with these additives maintaining this layer over the duration of the oil drain interval. When all these additives are used up, it is time to change the oil.
Another thing that can stop an engine from working is the development of varnish, sludge and deposits. Oil makers add in special additives called detergents and dispersants which help to keep engine parts clean. These work by cleaning off deposits from surfaces and then keeping them trapped in the oil so they can get pulled out by the oil filter, or drained out with the oil at the oil change. Some oil makers focus their product claims on how well they clean or keep engine parts clean.
Remember how oxidation can help contribute to deposits and varnish – we said that synthetic oil was more resistant to oxidation. Well there are additives that can also help with this too. Anti-oxidants and metal deactivators are used in both synthetic and mineral engine oils to help keep oil going longer and engines running cleaner.
Compatibility with Engine Materials
There are lots of different materials in your engine. The block, pistons, rings, liners, bearings, shafts, and seals. Each of these parts may respond differently to synthetic oils, mineral oils and all the chemical additives that go in them. For this reason oil makers have to be careful about what products they use and make sure there are no bad interactions. When synthetic oils first came out there was some issues with engine seals leaking. That has been fixed – in part by changing some of the seal materials and also by adding ingredients to the oil blend that helps prevent the problem. Catalytic converters are sensitive to certain elements like Sulphur and phosphorus, so additives are developed to provide protection but to conserve the resources used these emission control devices.
We could go on and on about all the different complications in engine oils today. In fact there are many regular contributors to the Bob is the Oil Guy Forums who would be happy to expand your knowledge on some of these engine oil attributes. In addition there are also specifications that are focused on these key engine oil functions to provide some sense of understanding about how well an engine oil stands up to regular use in your car. The more you learn about engine oil the better you will be at maintaining your vehicle, getting longer life from your engines and saving money in fuel economy. Take the time to read and explore and we will try to help you as best we can.
Soon you will find yourself obsessing over minute details and starting to collect your favorite engine oils in your garage to try out different things in your car. Don’t worry that’s normal, it just means you are officially part of the BITOG community.Chapter Four