Engine oil temperature

[gunnarsilins]

I had this question out on the Motorboat forum...with no reply!

So I try here!

What is the prefered engine oil temperature on a marine diesel and suggested upper and lower limits?

 

[philip_stevens]

I've just looked through my Bukh workshop manual, and it gives everything except lubricating oil temperature. Oil pressure, lub oil viscosity type for air temp, and water temperature - but no lub oil temp.

Perhaps it is not too important as long as the water temp is between given limits.



regards,
Philip

 

[mickp]

If its any help my Thornycroft T95 ( 39HP diesel ) manual states that Max oil temp must not exceed 99 c ( 210 f ) and that engine should not be operated continuosly below 60 c ( 150 f ) Elsewhgere it states the alarm will operate at 99 c

 

[vyv_cox]

80 degrees Celsius seems to be about the optimum figure. As temperatures reach 100 degrees C you will begin to expreience varnish deposition on hotter surfaces and below about 65 - 70 C some components, especially top piston rings and cam/tappets, are not fully lubricated, causing high wear rates. Whether it is possible for a raw-water cooled engine in a yacht to achieve 80 C is another question altogether!

 

[Trevor_swfyc]

Interesting question, the engine manufacturer picks an oil that is ideal for the engine taking into account the running temperature of the engine. The temperature is in the most part controlled by the water cooling / thermostat salt water cooling operating 15 degrees lower than fresh. If the engine cooling starts to fail due to various reasons blocked cooling in the engine block, failed or blocked water pump the temperature will rapidly increases and the oil viscosity drops, eventually the oil film breaks down and metal-metal separation is lost. If the engine is continued to be run it will seize as friction causes parts to weld together. Fortunatly under normal running modern oils with viscosity index improvers can stand up to some overheating without breaking down. Certainly your water temperature alarm would warn you of a problem long before the oil dangerously overheated. Its more of a problem running under temperature as the oil being more viscous will not circulate as easily also sludge and moisture would contaminate the oil.
So the answer to your question is to control the water temperature 75-95 degrees centigrade 95 being preferable but only if you have fresh water cooling.
If you have a turbo engine in an offshore power racing boat then thats another thing altogether!
You may be able to get more info from the Castrol web site.
A final parting tip change your oil just before the winter haul out, then it sits in fresh oil during the lay up rather than the contaminated stuff.
All the best
Trevor.

 

[gunnarsilins]

Interesting answer!

Some details:
My friends boat is a semi-planing Fjord 34 powered with twin Perkins 6.354 HT. These are turbocharged.

Details below are valid for both engines, they behave more or less exactly identical.

The boat is quite heavy loaded because he´s cruising quite long distances. To keep up he´s cruising speed (14 knots) he is running the engines on 80-85 % of max RPM.
After a couple of hours of continous running the oil pressures begins to drop slowly, but soon gets back to normal values when slowing down a bit. This lead us to the conclusion that the oil was overheated and loosing its viscosity.
There is no oil temperature gauge but we made a temporary arrangement and read values of around 120 C after about 45 mins running (yet no pressure loss). We came to the conclusion that the oil temperature probably would rise even higher when running for hours and the oil pressure starts to drop.

The water temperature (fresh water cooled) stays at normal values (90 C) and the oil cooler is clean and nice with good waterflow.
Both engines had a recent total rebuild and have run approx 250 hours since then.

My suggestion to him is to add one extra oil cooler in series with the existent. But I´m a bit afraid that the oil might run too cool, especially in cold water. And I also suspect he´s pushing the engines a bit too hard.

 

[vyv_cox]

Interesting problem!

This is a very high temperature for sustained running. Assuming that the best quality synthetic lubricant is being used there is still a risk that bearing, ring and tappet lubrication is being compromised. A basic principle is that oil temperature should not exceed water temperature for extended periods. It doesn't sound as though low oil temperature is going to be a problem, as seawater temperature does not vary enormously, not much more than 10 degrees in temperate zones. It would seem that something needs to be done - check for full flow, is there a working thermostat?