Is Your Main Engine Hydraulic Oil Clean?


We are sure that you would be surprised to read the title.  Please go ahead and read it one more time.

Yes.  The question is

“Is Your Main Engine Hydraulic Oil Clean?”

The newer engines are more of electronics than mechanical.  The camshaft has been replaced with hydraulics whereby the actuation of the exhaust valve and the fuel boosters is achieved without conventional cams mounted upon shafts.

Such hydraulic actuation is achieved by using the main engine system oil (crankcase lube oil).  The lube oil from the main lubrication system passes through a filter after which the oil is boosted to very high pressures.  Solenoid valves triggered by electronic signals from the main operating panels (MOP) control the hydraulic valve actuation.  Though there are some special cases where some engines are equipped with a separate, independent hydraulic circuit for the valve actuation, the centralised common lubrication system is more predominant.

Engine makers claim that the lifetime of the hydraulic control valves is purely dependent on the property of the oil and its cleanliness.  Thus, it is essential to test the lubricating oil for its property and for its cleanliness, such that the hydraulic control valves suffer no damage or failures.

We requested Viswa Lab to comment on the oil cleanliness requirements and share some feedback on the quality of lubricants that arrive for testing at their facility.

“Most engine makers recommend to strictly adhere to a cleanliness code – ISO XX/16/13 – however this may vary depending on engine make/model”, says Viswa Lab’s Technical Director Mr. Amitava Talukdar.

He added that the particles greater than 4 microns in size are usually not specified by the engine maker, however, a limit on a number of particles greater than 6 microns and 14 microns are imposed.  Particles greater than 6 and 14 microns are critical in size which has a severe impact on the working clearances of the hydraulic components.

The hydraulic oil cleanliness is usually expressed in terms of ISO code as per ISO 4406 Standards.  “It is recommended to test the oil cleanliness every three months or as per maker’s instructions.  The oil sample has to be representative and the sample bottles used must be Viswa Lab’s sampling bottle, which is free from dust/moisture”, says Mr. Tin Tun, Viswa Lab’s Director.

Definitions and Standards:

MAN Diesel & Turbo specifies the international ISO 4406 standard to be used when defining the quantity of solid particle in the fluid used in a given hydraulic power system.

ISO 4406:

The scale numbers are allocated according to the number of particles per 100 ml of the fluid sample.  A step ratio of generally two, as given between the upper and lower limits for the number of particles per 100 ml, has been adopted to keep the number of scale numbers within a reasonable limit and to ensure that each step is meaningful, see Table I.


Fluid Maintenance

All fluid stored in sealed containers or delivered from an oil company must be filled through a filter cartridge with a filtration ability of ß6 (beta) = 200.

Beta ratio: example of filtration ability, valid for particles > 6 microns

               8,000,000 particles > 6 microns at filter inlet

ß6 =     ——————————————————————————  =   200 → ß6    =   200

                40,000 particles > 6 microns at filter outlet

From ISO 4406 Code 23 to ISO 4406 Code 16 after the first pass.


Thus, the assumption that the new oil is clean is not valid unless it is tested. It is for this reason, Viswa Lab recommends to land a fresh oil sample to test its properties and cleanliness.  Further, Viswa Lab claims that the testing of fresh oil sample is done at no extra cost when tested with other used oil samples.

We thank Viswa Lab for sharing its experience and insight on the particle counts and ISO 4406.  With more electronic engines out in the market, it is even more essential to keep the engine system oil clean.


Water % that the main system oil can tolerate is about 0.3%, whereas most hydraulic circuits can tolerate water up to 0.08% or maximum 0.1%.

Source: Viswa Lab – MAN B&W.