Why is it necessary to change vehicle oil regularly, and not just change the filter and add new oil as needed?
In: Engineering
Lubricants break down with heat. Each time the engine runs the oil gets a little less effective at lubricating the parts. In extreme cases of failing to change the oil, it can even bind up on itself to form a kind of plastic-like material that is very likely to destroy the engine.
Motor oil consists of a base oil and specific additives, such as detergents, dispersants, anti-wear viscosity improvers, corrosion inhibitors, etc.
The base oil lasts significantly longer than the additives do, seeing as it doesn’t really get used up at the temperatures and pressures in most modern engines.
The dispersants deal with sludge which comes from burnt and unburnt fuel slipping past the piston rings and the detergents “wash” the engine, cleaning up varnish and other residuals.
These additives get used over time (due to corrosion and water ingress) as well as ‘miles’ (due to fuel and temperature)
The oil also fills with sludge and soot which can be filtered out when they combine and become big enough, but if you make the filters better so that less soot swirls around, causing wear and varnish, you end up removing the additives also – so there’s a very fine balance to be maintained.
It is definitely possible to sample the oil during a filter change and only add what is needed, but this is more expensive than just changing all the oil at the recommended intervals.
Someone told me jokingly that “the first law of organic chemistry is that if you mix 1 kg of cherry jam with 1 kg of shit, you get 2 kg of shit”.
The oil filter can only catch stuff like larger metal shavings, it passes contaminated / burnt oil through. At some moment it should be discarded completely.
Because the filter really only is there for when things have already gone wrong.
It can only filter actual particles. Like metal shavings or stuff that fell in while you put in oil.
But metal shavings only come by when the oil didn’t properly lubricating things.
Thing is, it’s the oil itself that breaks down when it gets heated and put under pressure. This causes parts of it to become sticky. Which prevents it from properly lubricating the engine.
That’s why the oil needs to be fully replaced.
For many chemical reactions the reaction also happens more quickly the more of the breakdown product is already there, so removing as much oil as possible also makes the new oil last longer.
That’s why synthetic motor oils are made.
They are extremely pure and specific molecules, rather than the ‘normal’ dark motor oil which was partially purified crude oil.
It’s the same with cooking oils btw: virgin olive oil can’t really be used for frying. But refined olive oil can be used.
The rifining process removes most of the non oil contaminants that would get burned during frying, allowing you to use the oil at hotter temperatures without burned taste.
But for actual industrial frying you use further chemically treated and purified oils, to allow them to reach even higher temperature without charring.
So the filter won’t do anything with the parts of the motor oil breaking down, but the oil won’t libricate well anymore.
If you engine is in good order, and you put in oil under hermetic conditions, the filter would barely be necessary
ELI5 answer:
Firstly, engine oil is more than just oil. It has a bunch of other stuff (additives) to help clean the engine, protect from corrosion and neutralize any bad stuff that forms from the heat and byproducts of combustion.
Now to answer your question, the oil as well as the additives actually degrade and break down on a molecular level as they do their job. Oil thins out and can’t lubricate as well, and the additives all break down or get used up as they do their job protecting the engine.
That means you have to replace the old oil with fresh new oil periodically. That’s why you see pictures of 20 year old engines that look basically new inside because the owners changed the oil religiously.
Oil breaks down over time and essentially turns into paste.
If your oil turns to paste, not only does it not lubricate anything (because it can’t flow through the engine anymore) it gums everything up. This is very bad for the engine.
Technically with sufficient filtering, you can do what you suggested with fully-synthetic oil. But it would require engines to be designed for it, which they aren’t.
There are some particles that pass through the filter. Rather than filtering, we get rid of those fine particles with regular oil changes. Without changing the oil periodically, these smaller particles would build up and gunk things up. Simply adding a finer filter would mess up the designed oil pressure and flow rates. To address this, there are better filters that try to stay within the engine spec for flow rates and pressures, and some oil plugs are magnetized to try to capture these small particles without messing with the flow rates, but that doesn’t capture everything.
But in theory, it would be possible to design an engine to work with a filter fine enough to remove all particles, and then all you would need to do is change the filter and top off the oil. But car engines just aren’t designed for that.
you wouldn’t eat at a restaurant whose deep fryer policy is changing the oil filter and topping up new oil as needed.
no. a filter can handle burnt bits, but the oil itself is degrading with each heating cycle and also over time just sitting there.
for engine oil policy it is some number of thousand miles driven, recommended by the auto manufacturer, or some time period since the last oil change, recommended by the oil manufacturer, whichever comes first. old oil needs to be drained.
US engines seem to need new oil every month, judging by comments on here and the ‘lube shops’ everywhere. I suspect big oil, the driving everywhere culture, and the larger brute engines all play a part.
The average EU / Japanese engine seems just fine with an annual oil change.
Because heart and oxygen change the molecular structure of the oil. Especially if some has burnt off, the non-combustables will be left behind and start to gum up parts.
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