DIY

Diesel Engine 201 – Choosing the Right Engine Oil: Let’s Talk Viscosity

Aug 15, 2018

There are many different viscosity grades available, and thinner motor oils are becoming more common. The first thing to consider is the OEM recommended viscosity grades. If your engine is stock, stick to what the engineers were using when they designed and developed your engine. Most engines will allow various viscosity grades, and that allows you to make the selection that best fits the application of your equipment. If your engine has been modified, then understanding this can help you make the right choice. Let’s dig in so this is not just a coin flip selection.

Starting with the basics, there are two types of viscosity, kinematic and dynamic. Kinematic viscosity is reported in the numbers we are all familiar with on a container of oil. A kinematic viscosity grade typically consists of either a single number (mono viscosity grade) or two numbers with a dash between them (multi viscosity grade). Since most heavy-duty oil is multi viscosity, we will concentrate on the two-number system. In 15W-40, the first number on the left (15 here) represents the cold temperature viscosity and contains the letter “W” (which stands for winter, engineers are so creative). The second number to the right (40 in this example) represents the kinematic viscosity at a normal engine operating temperature, usually 100 degrees C. The lower the kinematic viscosity number, the thinner the oil. For example, a 5W-40 oil will be thinner in cold temperatures than a 15W-40, but at normal operating temperatures both oils will flow the same. However, when you compare a 10W-30 vs a 15W-40, at both low and high temperatures the 10W-30 will be thinner, with less resistance to flow. Just remember, corn syrup is high, corn liquor is low.

So that’s kinematic viscosity. Now let’s talk about the newest API rating.  How is it that CK oils are different viscosity than FA oils, even if they are both 10W-30? Viscosity is affected by three things, temperature, pressure, and the speed that you shear it (which relates to engine speed). Mostly we are working at atmospheric pressure or a bit above so we can consider that relatively constant. The temperature variation goes from ambient to normal engine operating temperature which is usually around 100 degrees C. That means the biggest variable is speed. The kinetic viscosity is measured at a very slow speed, under the influence of only gravity. There are three dynamic viscosity measurements that define the SAE grade.  Two at low temperature, Cold Cranking Viscosity, CCS, done at high speed (shear) and Mini Rotary Viscosity, MRV, one at low speed (shear).  The common dynamic viscosity used for fuel economy improvements is referred to as the High Temperature High Shear (HTHS) viscosity. We have the technology in oil today to modify the viscosity such that the viscosity acts differently as speed changes, to help us meet the requirements of today’s modern engines.

From theory to practice

The tradeoff is choosing between lower viscosity to decrease pumping and shearing losses, versus keeping the crankshaft from touching the bushings by floating on a wedge of oil (the technical term is hydrodynamic lubrication.) Lower viscosity oil reduces parasitic losses, improving fuel economy and power. Parasitic losses are things that tax the engine’s power, think parasite, like a leach. Nice mental image now? Lower viscosity is easier to pump and to shear. This is why many heavy-duty oils are moving from the traditional 15W-40 to 10W-30. If the engine is designed and manufactured to use a 10W-30, it can improve fuel economy and power over using a 15W-40. The pumping difference is pretty straightforward, but what about shearing oil? That’s the hydrodynamic lubrication created when the crank slides across the wedge of oil provided by your oil pump inside the bearings. Thicker makes it consume more energy just to move. Think about throwing a baseball under water. If you go through the motion very slowly, the amount of work isn’t much different than doing it in air (kinematic). But as you try to throw it faster, the force necessary goes up at a higher rate than the change in speed of your arm (dynamic). Also think how much harder it would be in corn syrup than it would be in water. That is the same issue that we see in parasitic losses in engines, crankshafts and other moving engine parts must shear the oil.  And the slower it is sheared, or the thinner the oil is, the less energy we lose. This is why we have seen a constant decrease in the cruising speed of heavy duty engines, so that the parasitic losses are lowered, and fuel economy improves.

The same factors that influence fuel economy also influence power. However, if we continue to lower viscosity, we will reach a point where the oil is not thick enough to prevent the crank from rubbing the bushings during operation. Then we start losing engine life and durability. To understand the thickness needed, we need to learn about what engine designers call the Lambda ratio. Let’s start with the hardware. The distance between the journal bearings and the crankshaft depends primarily on three major factors; the viscosity of the oil, the area of the bushing contacting the crankshaft, and the force pushing the connecting rod. The loading of the piston through the connecting rod leads to the upper bushing on the connecting rod and on the lower bushing in the main bearings being the first to show wear. The combustion force results in the connecting rod trying to push the crankshaft out of the bottom of the block.

To keep this simple, let’s just consider Lambda as a ratio of oil thickness, force on the bushing, and asperity height. We’ve defined all of those factors except asperity height, so let’s start there. When parts are machined, even very smooth surfaces are not so smooth when you look close enough. It makes sense that the rougher the surfaces, the more oil thickness required to keep them from contacting each other. The smoother we can make the crankshaft, the less oil film we need to prevent rubbing. Improvements in manufacturing processes by engine manufacturers like burnishing, polishing, and super finishing of crankshafts can allow the use of thinner oils.

However, at the same time engine designers seeking increased fuel economy are increasing cylinder pressure, which increases the force on the connecting rod, pressing oil film thinner. Increased fuel economy and decreased carbon emissions are being achieved by lowering the speed of the engine, so the engine can shear the oil slower (Remember throwing the baseball underwater). At the same time the engine is using thinner oil for better fuel economy, it is seeing increased force on the connecting rod which would in the past require thicker oil. Advanced manufacturing techniques make this possible, and we work closely with engine manufacturers to create just the right balance of economy and durability.

Now a quick note to all of you who have chips or high-performance updates to your diesel engines. These increase cylinder pressures often well above the manufacturers design limits, so we would not recommend a lower viscosity oil. If the changes you have made have not included increased surface finishing techniques, you at least need to show your bearings some 15W-40 love.

In a commercial fleet with good fuel economy, going to a 10W-30 CK oil is a good move for fuel economy. If you’re more concerned about your engine durability, and you run engines to the very end of their useful life, and a couple percentage points of fuel economy is not that attractive, then you may choose 15W-40. Pick the right viscosity and your engine will thank you for it, by continuing to do its job.

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