Around and Around – Where the Oil Goes in Your Engine

Most people know that they should add oil to the top of their engines and that the oil drains out the bottom. Having worked in auto repair for 35 years, it’s no mystery to me what happens between a fill-up and an oil change. but I am amazed at how many people have no real idea of ​​the path the oil travels as it makes its way into the engine.

One of the most frequently asked questions I get is:

“how often should I change the oil in my car and what should I use?”

To answer that question, I would use the Socratic method and ask some questions of my own: What kind of car do you drive? What driving conditions do you find more? where do you live? How old is your car?

The answer to these questions will determine the best oil for your vehicle and how well it protects and lubricates your engine as it spins and spins inside.

Where does the oil travel, in what order, and what exactly does it do inside your engine?

First, the oil you pour into the top of the engine goes through many paths, eventually reaching the lower oil pan, often called a sump, where the drain plug is located. the oil goes through several different paths to return to the bottom, but only one path, under pressure, to do its job.

Figure 1 shows a tube with a loose metal mesh at the bottom of the tray. the screen is attached to a pickup tube, which leads directly to the oil pump. the tube and screen are immersed in the oil to a depth of about four inches. the screen prevents large pieces of debris, typically larger than 1/32-inch, from entering the oil pump.

Many people don’t realize that most oil pumps are just a set of special gears, which take the oil at low pressure and squeeze it out at high pressure, where it then passes through a chamber with a spring loaded valve. . the valve allows oil to escape only under a specific pressure, typically between 1 and 60 lbs./in.2 any pressure above this will be vented back to the sump because high oil pressure can damage bearings.

From the pump, it goes to the outside of the oil filter, where it is forced through the filter media to the center, where it exits into the oil passages inside the engine. The oil filter also has a bypass valve to prevent pressure from dropping too low if the filter becomes clogged. Motor oil’s first and most important job is to lubricate the rotating components of an engine, and it must be under good pressure to do its job.

Oil is forced into the space between the bearings making contact with the crankshaft journals and journals. Bearings are simple metal sleeves that surround the rotating components of the engine. the block has main bearings on the crankshaft and the connecting rod bearings are on the crankshaft throws.

This thin space, usually a thousandth of an inch on newer engines, contains a thin film of oil between the bearings and the moving surfaces of the crankshaft. Under pressure and within the correct operating temperature, the oil protects and prolongs the life of machined parts. metal should never touch other metal surfaces while moving.

It is important to note that some of the oil runs out the sides of the bearings and drips back into the sump. if the clearance is too much, say 0.004 of an inch or more, the pressure starts to drop at the top end of the engine. A flashing oil light or a slight knocking sound in the rocker arm area at the top of the engine is a good indication that not enough oil is getting to the top of the engine under pressure.

Looking aside for a minute, I’d like to see an automobile engine with roller or needle bearings that replaces the sleeve bearings that are much cheaper and durable enough. I know it would cost a fortune to build such an engine, but it would last forever. many larger motors have needle/roller bearings. they generally rotate at a lower rpm (speed) than gasoline car engines. rpm is not the limiting factor.

I’ve flown model aircraft for 40 years and many of my higher speed engines (over 25,000 rpm compared to 2,500 rpm for a car engine) are equipped with roller bearings to reduce friction and increase rpm. a car engine equipped with needle/roller bearings would have higher power and longer life, but at what production cost?

Most of the oil lubricates the crankshaft area, while the rest lubricates the camshaft and rocker arms. If your car has pushrods instead of an overhead camshaft, then the oil is forced under pressure into the valve lifters. these lifters also pump oil through the hollow pushrods to lubricate the rocker arm area. If your car has an overhead cam, the oil is drawn into the cam and spills over the contact points between the cam and the valve stems.

After lubricating the camshaft and related components, the oil flows by gravity through the channels in the head and engine block to the sump, ready to begin another journey.

In many of the connecting rod designs, there is a small hole that sprays oil into the cylinder to lubricate the piston ring contact area of ​​that cylinder. Special rings at the bottom of the piston ring set wipe off excess oil and return it to the sump.

Regarding oil consumption, you may need to add a quart of oil to your engine, at regular intervals of 3,000 miles. most newer cars will not use oil at the first few oil changes. after that, oil consumption will gradually increase with age. what is too much consumption? if I had to pick an ideal figure, I’d say a quarter every 5,000 miles. best car i ever owned let me know it was time for a change by regularly running a quart low at 4,000 miles. I saved by adding a room and changed the entire sump and filter.

why do I prefer a little oil consumption? In my opinion as a lifelong auto mechanic, those engines that consumed a little oil by allowing it to pass around the rings minimized upper cylinder and ring wear. Years ago, we used to add top oil to our gasoline for that very purpose.

External oil leaks can be messy, a potential fire hazard, and just plain ugly. Why do used car dealers go to great lengths to clean an engine before putting it on display for sale? our overall impression of the engine is formed around how clean it is and how smooth it runs. most people open the hood before starting it up. if the seller starts it up before opening the hood, it relies on the first impression of a well running engine negating what is likely to be a dirty engine under the hood.

If your dealer didn’t clean your engine, you most likely have an oil leak that you don’t want to fix. if you open the hood and it works fine, look where the car is parked while you test it. the oil in the lot will give you a bargaining tool. many types of leaks can be fixed for less than $100.

oil consumption

one of our readers wrote in asking three different questions about his car and its recently modified oil consumption. for 30,000 miles, his car had used no oil between changes and was suddenly using oil at a rate of one quart per 1,000 miles. Although the consumption rate is excessive and I think there is some leak or oil burning, he asked the following valid questions:

1. what is normal consumption? and why didn’t his car burn oil for 30,000 miles?

2. Why does oil consumption occur during highway miles and not during stop-start driving?

3. What caused the oil usage pattern to change after driving the car so long (30,000 miles)?

Cars consume more and more oil over time. normal consumption is a subjective call; I did mine to a quarter for 5,000 miles. I also said that a lot of cars won’t burn oil for a while, again a variable.

the fact that its consumption is caused by road conditions leads me to suspect an internal oil leak around the valve stem seals or some fault in the pcv system.

the fact that the pattern abruptly changed would reinforce my belief that a fault (either an undetected leak or abnormal consumption) is to blame.

I have a car with over 175,000 miles and it uses oil at the rate I like it to be: a quart every 4,000 miles. my newest car with only 70,000 miles also uses a quart every 4,000 miles, and always has.

Oil leaks are difficult to detect in a car. the engines are hermetically sealed and difficult to see from any angle. add a list of bolt-on accessories to the block and visibility approaches impossibility. however, in the next issue of machinery lubrication, I will introduce some of the latest leak detection techniques. phosphorescence, acrylic polymer, ultraviolet rays, smoke, and maybe even mirrors will be covered in the next issue.