TDReprint
MORE ABOUT OIL
 (Issue 28, pages 110 – 112)

The question of aftermarket oil additives keeps coming up (Steed, Prolong, STP, Microlon, world without end), and it always will. When a person has laid out big money for a shiny, wonderful new Turbo Diesel, that person intends to do more than just drive around in it. That person wants to have a relationship with that truck.

In the old days, the relationship was easy. You changed your own oil every thousand miles, you ground your own valves, and you rotated your own tires. In fact, there was more relationship between man and vehicle than most people wanted. That’s why today’s cars and trucks have become such turnkey operations, with extended oil drain intervals and no tune-ups. Just get in and drive.

One way to have a relationship with the new vehicle is to buy and mount a cast aluminum “Lone Wolf – No Club” license plate frame and some white rubber mudflaps with jeweled reflectors. Oh, and blue dots for your taillight lenses, to give them that distinctive purple look at night.

Okay, all that went out with the end of the 1950s. This is the 21st century here, a time when people are concerned over things like dietary fat and bad cholesterol. Because we are what we eat, and we want to be good, we have to eat carefully. This applies by analogy to new trucks that have cost us $32,000. Just as we are eating vitamin-C, DHEA, and no-flavor lean beef, so we are also tempted to pour expensive additives into the lubricating oil of our trucks, in hopes that performance will improve and that useful life will be extended.

I read a wonderful line somewhere, which went like this; “Vitamins were discovered in 1911. Before that time, people just ate food and died like flies.”  Something like this idea seems to drive people today to use additives – ordinary pump Diesel fuel and manufacturer-recommended oils can’t be enough. Aftermarket additives are, therefore, the “vitamins” we are tempted to give our vehicles. Never mind the fact that using “Nosmo King” anti-smoke additive adds seven cents a gallon to the already high price of Diesel. Never mind the fact that some highly-advertised “super” oils cost more per quart than most of us pay for a case.

The ads are wonderfully persuasive. One I saw recently features regular guys strolling in a junkyard. They approach a rusty clunker, start the engine, and listen to its assortment of clatters – collapsed tappets, rod knocks, loose wristpins. “Sounds pretty bad, Bob”, remarks one of the strollers. “That’s right, Bill,” returns another. “We’ll try a bottle of Noo-Life,” Bill confides to the viewer. They pour it into the oil filler and instantly the clattering goes away (or the technician at the audio mixer cuts the treble way down – it’s hard to tell exactly which it is). “Sounds pretty good now, Bill,” says the pourer, turning to the viewer and holding up the now-empty Noo-Life bottle for our inspection of the label graphics. “Why don’t you try a bottle today?”

In our minds, we know how it’s done, but in our soft hearts, we’re vulnerable, tempted to try a bottle. Yes, we know that unscrupulous used car dealers have, in the unregulated past, used sawdust to quiet timed-out transmissions, and we know that thick oil or a dose of motor honey (viscosity-index improver additive) will calm the high-frequency rattling of a worn-out engine. But, having laid out those thirty-two thousand ones end-to-end for that beautiful new truck (that’s more than three miles of money), it just doesn’t make sense to pass up products that might work, right? After all, they wouldn’t let ‘em say it on TV if it didn’t work as advertised, would they? Would they?

How and why does oil work as a lubricant, anyway? I’ve touched on this topic before in these pages (Editor’s note: Specifically, Issue 22, page 96, and Issue 26, page 128, are great reread material), but a deeper look always gives some fresh insight. As noted in a previous article, there are three regimes of lubrication:

(1) Full-film, or hydrodynamic lubrication – most of the parts in your engine are lubricated in this regime most of the time. Viscosity and the rapid motion of the parts drags oil between sliding parts, forming a full oil film that supports the load. There is no contact at all between the moving parts, as revealed by electrical conduction experiments.

(2) Contact, or boundary lubrication – in the absence of an oil film, there is either actual metal-to-metal contact between parts, or the parts are in some degree protected by chemical films of oil additive used for the purpose. Such films not only protect parts from damage, they reduce contact friction to 1/10 or less of what it would be in actual metal-to-metal friction.

(3) Mixed lubrication – some of the load is supported by an oil film, some by contact. This kind of friction occurs during start-up, after oil has largely drained from engine parts. It also occurs wherever parts motion is too slow to generate a full lubricant film – at low idle speed between cam lobes and tappets, or near TDC between piston rings and cylinder walls, when the piston is moving very slowly and combustion pressure is high.

In what follows, I want to describe in more detail how full-film lubrication works, and what affects it. In a later issue I’ll talk about multi-grade oils, oil additives, and their relation to snake oils.

THE PRESSURE IN BEARINGS: Our intuition tells us that crank and rod journal bearings must work because the oil pump forces oil into the bearings, and that oil pressure then supports the load. Simple arithmetic tells us this is false. A four-inch piston with 1000 psi of combustion pressure over it pushes down on the connecting rod and rod journal with a force of roughly 12,000 pounds. If it was the 60 psi from the oil pump that supports this load, the rod journal would need 12,000 ¸ 60 = 200 square inches of bearing area. Since the actual projected area of the rod bearing is more like two or three square inches, we can see this idea is ‘way off. In fact, this bit of figuring reveals what actual bearing pressures are like – namely the 12,000 pounds divided by, say, three square inches of actual bearing area, giving us 4000 psi as the peak pressure exerted on the oil film in actual con-rod bearings. Where does all this pressure come from, if it doesn’t come from the oil pump?

VISCOSITY: It comes from the rotation of the parts, acting with viscosity to drag oil from regions of low pressure, into the high-pressure region under the load. Viscosity is the internal friction of a fluid, such as oil, air, water, etc. If one solid surface slides over another, separated by a fluid, the layers of the fluid must slide past each other. The resistance to this sliding is called viscosity. It is easy to understand why this resistance occurs. As the molecules in one layer collide with those in the next, kinetic energy is exchanged. Because the collisions of these molecules are anything but orderly, this kinetic energy exchange produces random molecular motion, which is heat. Thus, the process of sliding one surface over another with a fluid between them converts orderly motion into heat. This produces a viscous drag force, tending to oppose the motion.

We know that the fluids known as oils have more viscosity than, say, air or water. Why should this be? Oils consist of molecules that are long chains, while the molecules of low-viscosity fluids like water are small and resemble balls more than they do chains. As one layer of fluid slides over another, long molecules transfer kinetic energy to more potential partner molecules because they are so long, surrounded by many other molecules. This produces a higher fluid friction, or viscosity. The small, ball-like molecules of water, because each of them contacts fewer other molecules, transfer kinetic energy less widely, and so display lower fluid friction, or viscosity.

THE BOUNDARY LAYER: Near a solid surface, the situation is a little different. Molecules – even those having the form of long chains – are very small. At any temperature above absolute zero, they are in constant motion. The molecules of a fluid are especially so, since in order for the fluid state to exist, the average molecule must have enough energy of motion to overcome any forces tending to bond it permanently to another. Therefore these molecules vibrate, rotate, wiggle, and slither over one another constantly. At any solid surface, these molecules collide with it steadily. Because, on the molecular scale, even the most finely polished surface is rough, these collisions result in rebounds at all angles, favoring no particular direction. For this reason, therefore, the fluid near a solid surface has no net motion along that surface. This relatively immobile layer near a solid surface is called, reasonably, the boundary layer.

This means that in the situation discussed above, in which one surface slides over another with a fluid between, the fluid cannot simply slide along the surface. The relative motion has to take place in the fluid, at some distance from the surfaces. This means that there is no escape from the effect of the fluid’s internal friction, or viscosity. No coating we could put on the solid surfaces would be smooth on a molecular scale, and so prevent the formation of a boundary layer there, permitting the fluid to simply slide along the surfaces. The relative motion always takes place in the lubricant itself, so power must be used to overcome the slight viscous drag involved in sliding the layers of lubricant past each other. No snake oil can change this!

PUTTING THE OIL UNDER THE LOAD: Now, why does oil remain between the sliding surfaces, rather than being immediately squeezed out by an applied load? Imagine a situation in which a loaded slider moves over another surface, with a viscous fluid – oil – between. The load, by exerting pressure on the film of oil between, tends to squeeze the oil out. If more oil does not somehow enter the space between the surfaces, this loss of oil will soon result in contact and possible surface damage. What can put oil into the space between surfaces?

FORMING AN OIL WEDGE: That something is viscosity. As the moving slider glides along, it assumes a slightly tilted position because the oil at its rear edge has been under pressure the longest, so the most has been squeezed out from that region. The oil film between therefore takes the form of a wedge, thicker at the leading edge, thinner at the trailing edge. As the slider advances, oil ahead of it does not immediately flow away because its viscosity prevents it from doing so. Once oil enters the wedge, the only way it can escape is to be squeezed out to the sides, or for the slider to pass completely over it. It’s hard to squeeze the oil out because forcing viscous oil out through such a narrow space requires very great pressure. Some does escape, naturally, but it escapes slowly.

As the slider advances, a steady state is soon reached, in which the rate at which oil enters the wedge at the front equals the rate of loss through being squeezed out at the sides and at the trailing edge. Oil enters the wedge at essentially zero pressure, but the advance of the slider, coupled with viscosity, carries it into regions of higher pressure – high enough to carry the load on the slider. The slider rises up on the wedge of oil thus produced. The more viscous the oil, the thicker the wedge.

Our “slider” could be the skirt of a piston, sliding on a lubricated cylinder wall, or it could be the lobe of a cam, rotating against a tappet. It could even be the rotating journal of a crankshaft, turning inside a sleeve bearing. In all cases, the load is carried by the same naturally-forming oil wedge. Oil enters the wedge at essentially zero pressure, and once the oil is between the surfaces, viscosity makes it easier for it to carry the applied load than for it to be squeezed out. In the process, the frictional drag force in the bearing is typically about one or two thousandths of the applied load. This is why engine friction is as low as it is.

The above argument shows that viscosity is necessary if loads are to be carried by sliding parts – it is what keeps the lubricant from escaping out from under the load so fast that the lubricant wedge collapses. Yet at the same time, the friction loss inherent in lubrication is produced by this same viscosity. It is therefore obvious that a compromise is necessary here. We must have enough viscosity to carry the loads on sliding parts, but much more than that simply increases the friction loss in our machine.

THE VISCOSITY COMPROMISE: When engineers specify oils for Diesel truck engines, they are obliged to make this compromise. They cannot allow the moving parts to touch each other, because that causes accelerated wear and parts damage. Therefore they must specify enough viscosity to keep parts separated as much of the time as possible. On the other hand, they also know that the more viscous the oil, the greater the force it takes to make lubricated parts slide over each other. Too little viscosity means wear and damage. Too much viscosity means power loss and increased fuel consumption. For example, a change from a 30 oil to a 50 oil increases friction-loss approximately 20%. Because well-designed engines typically lose about 15% of their power to friction, this means 20% of 15%, or a power loss of 3%.

Shall we play with this compromise ourselves, in hopes of either reducing friction and getting more power, or concentrate on extending engine life? It’s obviously true that we can cut the friction of well-lubricated parts significantly by reducing oil viscosity. This is a ploy constantly used in auto racing. Shall we run out and get cases of watery 0W-5 oil and reap the benefits of lower friction? We don’t do this because we know the factory chose a heavier oil to cover the full range of operating conditions that their product will meet in use. Yes, we might be able to get away with a lighter oil if we didn’t work our engines very hard – this is an old trick from the Mobil economy runs of years ago. But we bought these trucks to do heavy work, so when we’re towing that big stock trailer up the Rockies on a hot day, we’re going to need the viscosity of the factory-specified oil.

Okay, but what about engine life? Can’t I make my engine last longer by using thicker oil? Won’t that keep the parts separated by thicker oil films?

Enough is enough. If factory-recommended viscosity keeps the moving parts separated, making good oil films thicker with added viscosity gets us nothing but added fuel consumption. Also, some of us live in Thief River Falls, MN, where engines have to cold-start at minus forty. With that thick stuff in the crankcase, the starter won’t turn the engine. Even if it does, the heavy oil moves so slowly at that temperature that it will take long minutes for flow to reach all the way to the rocker arms and other parts most distant from the oil pump

The factory, based upon its thousands of hours of testing, and on the years of warranty and service experience, arrives at an oil specification for its engines. Can we get better information by watching the motor honey ads on late-night TV?  

Click on any link below for more TDReprints

MIXING GASOLINE AND DIESEL
(Issue 26, pages 14 – 15)

TIRES AND THE MARKETING OF AMERICA 
(Issue 31, pages 126 – 129)  

MORE ABOUT OIL
(Issue 28, pages 110 – 112)