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Section 3 Cummins Diesel Generator Engine Oil Viscosity

Oct. 15, 2019

We have learned Cummins generator engine oil function in previous article. Well, let‘s continue to learn the engine oil viscosity.

Viscosity is a measure of the resistance to flow offered when one layer of oil molecules move relative to an adjacent layer. This resistance comes from the friction generated by the oil molecules as they move past each other. This shearing action occurs constantly in the oil films lubricating all moving parts of an engine.

The viscosity characteristics of all fluids are affected by temperature. Multi-grade oil viscosity tend to be less sensitive to temperature changes because of the addition of viscosity improvers in their formulation. The viscosity of multi-grade oils is also a function of their rate of shear, or the relative speed of moving parts. The lower the relative speed, the greater the apparent viscosity of most multi-grade oils.


Most of the wear an engine ordinarily experiences occurs at initial startup in some applications before oil has time to fully circulate. The correctly formulated multi-grade oil is the ideal engine lubricant for a Cummins engine. Relatively thin oil is available tor rapid lubrication and easy cranking while starting.

CCEC Cummins diesel engine


Oil Viscosity and Engine Performance


The selection of oil of the correct viscosity is extremely important for optimum performance and for maximum engine life. If the oil is too viscous, engine drag is increased with the following effects:


● Engine is difficult to start

● Engine power output is reduced

● Engine cooling is reduced

● Internal wear is increased

● Engine parts run hotter

● Fuel consumption is increased.


If the oil viscosity is too low, the engine experiences:


● Increased wear from metal-to-metal contact

● Increased oil consumption and leakage

● Increased engine noise


Some oil suppliers might claim better fuel economy for the lower viscosity oils, Lower viscosity results in tower oil film thickness. Therefore, Cummins Inc. has required that all multi-viscosity 30 weight (xW30) oils registered under CES 20078 (API CI-4) must meet the minimum High Temperature / High Shear viscosity of 3.5 cSt. These can be used over a wider temperature range than other 10W30 or 5W30 oils. As these oils will have directionally thinner oil films than 15W40 oils, top quality Fleetguard filters must be used above 20°C [70°FJ. See Figure l for viscosity recommendations.


As CCEC can neither approve nor disapprove any product not manufactured by CCEC,

these claims are between the customer and the oil supplier. Obtain the oil supplier's commitment that the oil will give satisfactory performance in Cummins engines, or do not use the oil.


Viscosity Recommendations


CCEC recommends the use of multi-grade lubricating oils with viscosity grades shown in Figure 1 for the ambient temperatures indicated. Only the preferred oil grades are shown in the figure.


If mono-grade oils are substituted for multi-grade oils in the areas where multi-grades are not currently available, shortened drain intervals might be required, as determined by close monitoring of the oil condition with scheduled oil sampling. Use of mono-grade oils can affect engine oil control.


When use of mono-grade oils, must insure oils working range within the ambient temperature listed in Table 2.

Table 2:

SAE Viscosity Grade

Ambient Temperature


-20 ~ -6


-10 ~ -20


4 ~ 32



Synthetic Oils

Use of "synthetic engine oils" (those made with API group 3 or group 4 base stocks) is permitted subject to the same performance and viscosity limitations of petroleum (mineral) based engine oils. The same oil change intervals must be applied to synthetic oils that are applied to petroleum (mineral) based engine oils.

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Cummins diesel generator

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