What is Viscosity Index?

While researching lubricants, there are many factors to consider in selecting a lubricant: viscosity, flash point, pour point, and oxidation stability. Viscosity is the most important parameter since the viscosity grade can be the difference between optimal performance and machine breakdown. However, the ISO Viscosity Grade (VG) is determined at 40⁰C and will fluctuate depending on operating temperature. Viscosity index is a measure of how much the viscosity will change as temperature rises or falls.

Viscosity Index Explained

Viscosity requirements are based on things such as: component design, loads, and speed. Machine recommendations do not account for operating temperatures and temperature ranges. Therefore, it is imperative to take into account average operating temperature when selecting a viscosity. To account for changing temperatures, the viscosity index was developed to measure viscosity stability as temperatures change. Viscosity index is a unit-less number that is derived by measuring a fluid’s viscosity from 40⁰C to 100⁰C.

The higher the viscosity index, the greater the stability of the lubricants viscosity. As shown in the chart below, the difference in viscosity index could greatly affect lubricant viscosity and performance:

Source: Machinery Lubrication, Noria

As temperatures move towards extreme highs and lows, the difference in Oil A and Oil B is magnified. Oil B, which has a VI of 150, maintains a viscosity closer to its ISO VG of 150 as temperatures rise and fall. On the other hand, Oil A fluctuates much more and could adversely affect performance at extreme temperatures. If your operation will have fluctuating loads, speeds, temperatures, etc., it is imperative to select a lubricant with a higher viscosity index.

Viscosity indexes, which can be found on most product data sheets, typically range from 90 to 160, but can exceed 400 and be as low as -60. The viscosity index can also give insight into the type of base oil and its quality. More refined mineral oils and synthetics will have higher VIs than lower quality base oils. Some products may include viscosity-index improver additives to help stabilize the lubricant in extreme conditions. VI-improver additive molecules adopt a coil shape in cold temperatures and have little effect on viscosity. In higher temperatures, the molecules uncoil and thicken the oil to stabilize viscosity. However, it is important to note that oils with VI-improvers will see permanent loss of VI and viscosity over time.

When Should You Opt for Higher VI

If your operations are going to have variable loads, variable temperatures, variable speeds, and other environmental variables, it is important to select a lubricant with a higher viscosity index. As these variables change, so will the lubricants viscosity. Therefore, it is crucial to invest in a lubricant that will maintain an optimal viscosity across different operating conditions. Conversely, if your operation is fairly consistent, it may suit you to select a lubricant with a lower viscosity index in order to save money.

Some machines may not possess data to identify the optimum viscosity, which could be problematic as ISO viscosity grades are separated by 50% increments between grades (e.g. 46 → 68, 100 → 150). With such large increments, finding the precise optimal viscosity becomes even more difficult. This problem is magnified at lower temperatures, where differences in lubricant viscosity are much larger (as shown in the chart above).

Calculating VI

If you are unsure of a lubricants viscosity or viscosity index, there are online calculators available to help you. If you are unsure of a viscosity index, simply enter the viscosity at two different temperatures and it will return the viscosity index. If you are unsure of a viscosity at a given temperature, enter a known viscosity, known temperature and viscosity index to find the desired temperature to find the new viscosity.

Key Takeaways

In conclusion some of the key reasons to have a lubricant with a higher viscosity index include:

  • Optimal operating viscosity is unknown
  • Varying operating temperatures and/or extreme operating temperatures
  • Other operating variables such as speed and load
  • You want to increase energy efficiency
  • You want to extend oil service and machine service life

Most of these involve improving performance that may be adversely affected by operating uncertainties. In these instances, it is ideal to opt for lubricants with higher VI. In the following instances, using cost-effective lower VI lubricants may prove beneficial to your bottom line:

  • Constant speeds and loads
  • Operating temperature remains the same
  • Optimal viscosity is known and can be consistently reached

If there is more certainty with your operating process, it may not be necessary to invest in a lubricant with a higher VI. It is important to evaluate your operating processes and consult machine manuals to understand your operating conditions. If you are faced with uncertainty and variance in your operations, a higher viscosity index will help smooth operations and increase performance across different loads, speeds, and temperatures.

What are Lubricant Detergents?

Detergents Defined

Detergent additives perform two key functions. Like household detergents, the additives keep metal components clean and free of deposits. Additionally, detergents neutralize acids that form in the oil. This is key for systems where component cleanliness is essential. Originally developed for engine oils, detergents addressed carburetor deposits that could hamper performance. Detergent additives were also found effective in fuel injectors. The detergents reduced deposits that affected fuel spray patterns.

How do Detergents Work?

Detergent additives are basic in nature, thus serve as a neutralizer for acidic contaminants that may arise in your lubricant. In the past, these detergents were barium-based, however modern chemistry has allowed manufacturers to move to different formulations. Today, most additives use either calcium-based chemistry or magnesium-based chemistry. As an oil is subjected to oxidation, it will start to collect acids. As these acids build up, the oil’s Total Acid Number (TAN) will increase. The basic and alkaline detergent will neutralize the acids and reduce the TAN. However, as the detergent is used, the Total Base Number (TBN) will decrease to point where the oil will need to be replaced. Therefore, measuring TBN is crucial to engine performance and lubricant effectiveness.

In high-temperature applications, metal compounds leave an ash deposit when burned. This residue buildup requires many OEMs to require low-ash oils. Detergent additives are used to clean these deposits. However, dispersants are included as well to help clean the engine. Dispersants are used to keep engine soot particles suspended and prevent agglomeration (forming larger soot deposits). The dispersant and detergent work together to suspend contaminants and neutralize acids. Eventually, the additive capacity will exceed its limit and require users to change the oil and replenish the additives.

Detergent v. Non-Detergent Oil

How do you know if you need a lubricant with detergent additives? Usually, an OEM will specify whether the equipment needs a detergent oil or non-detergent oil. Applications that could face high levels of water and contamination are good fits for detergent oil. Some examples include: off-road equipment, marine equipment, trucks & fleets, and many more. The high levels of contamination need to be neutralized with dispersants in order to keep pumps and valves clean and running.

Sometimes, OEMs require oils to not have detergent additives. Some manufacturers will produce special Non-Detergent oil to meet these specifications since, most oils now have detergent additives for better performance. Non-detergent oils are used in bearings and chains in non-critical once-through systems. It is also recommended for gas-powered appliances such as lawnmowers and tractors. Some non-detergent oils are not recommended for automotive gasoline engines (detergent oils are recommended).

Detergent Oil Today

With the developments in detergent and dispersant technology, most oils now have some sort of detergent additive to help combat high TANs and prevent sludge build-up. Even though non-detergent oil is still marketed today, it is only required for a few specific applications and not recommended by many OEMs. When selecting your lubricant, detergency is important to consider because high detergency will protect your parts, keep your system clean, and maximize performance. If you are using non-detergent oil, consider making the switch to an oil that has detergent additives.

Twin Specialties offers both detergent and non-detergent oils to meet your specifications and OEM requirements. We also offer a variety of motor oils and heavy duty engine oils with high-quality detergent additives to meet your specifications and budget. Contact us today for more information.