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Trend Analysis Important For Oil Testing
Professional Skipper Magazine Issue 45 May/Jun 2005 (unedited)
A few years ago when the editor of this most informative magazine stepped back from his much loved passenger ferry business, he decided to buy an old Cray fishing boat for entertaining his and his clients' pleasures. This vessel had in it an old Detroit 671N diesel, commonly known as a tank engine because of its use in battle tanks. The engine had clocked up over 16100 hrs and was understood to have been rebuilt some 6000 hrs earlier.
Keith was concerned that the wrong grade of oil had been used in the engine. He suspected a synthetic diesel engine oil had been put in instead of an SAE 40 mineral oil, which is more typical for these types of 2 Stoke engines. I recommended to Keith that it should be changed immediately and some hours done before an oil sample was pulled.
When I finally got to draw the sample the engine had done 16277 hrs. At first glance it was hard to believe that the oil had done 139 hrs because it still looked like brand new oil. It certainly wasn't black like most other engines and if you rubbed some of it between your thumb and forefinger it left no sooty residues. The oil still even had its light brown colouring, something that Keith and I found most unusual as we all know that as soon as you put new oil into a diesel engine it turns black. This is because the detergents in the new oil get to work straight away cleaning the sludge from the engine. The suspension additives are also hard at work carrying the sediment, soot and the like off to the filter for disposal. Other additives help to keep the individual soot particles apart so that they cannot join up, amalgamate and cause blockages in the oil galleries.
When the sample came back from the lab the results showed the oil was as close to new oil as you can get. No Soot, no Fuel Dilution, no Water and very little if any wear-metals. In fact none of the wear-metal counts were even in double figures. Even the TBN (Total Base Number - additive package indicator) was still very high. Along with a Viscosity (oil thickness) level that was almost identical to that of new oil these results tended to show that the oil was still in very good condition and capable of further service. I recommended to Keith to leave the oil change for another 100 hrs or so.
By then the vessel was in the middle of a long campaign namely the Louis Vuitton Cup. With the vessel doing long days on weather reporting duties for the race committee it did not take long to run up the hours. By the end of the cup the oil service hours had reached 240 hrs so we resampled the oil. The results again showed little if any wear-metals while the oil's condition its self was still in good shape. Yes the Viscosity was slightly higher and the TBN slightly lower but both were still well within their acceptable limits. The oil was also still in very good condition visually i.e. it held its colour and its Soot content was still almost non existent. Again we had no problem in deciding to leave the oil change for another 100 hrs or so, however this time the oil filter was spun off and changed.
Finally a couple of weeks back I got a call from Keith to say that he was doing an oil change and would I come over and take a sample. However, when I checked what the engine hours were I was surprised to find that they were 16557 hrs. That meant that the oil had done around 419 hrs in service. Not good I said, but hey Keith said, the oil looks as good as it always has done, clean as a whistle. In the end the results would speak for themselves I said.
When the results finally came back from the lab I was not surprised to find that little had changed. The results were not that different from those of the sample we took way back during the Louis Vuitton Cup in December 2002.
What these results showed is the importance of trend analysis when it comes to extended use of the oil. The results and subsequent reports from this one charge of oil showed that the extended use was within reason and that both the oil and engine suffered no damage as a result. However one thing that we also had to take into consideration was the amount of top-up oil used. Over this period enough oil was used to fill the sump capacity twice over. While this is quite normal for these engines high use of top-up can result in lower wear-metal counts. This is because they are either lost as a result of leakage or burnt out of the system. This engine does not suffer from excessive smoke or pools of oil beneath it so the low wear-metal counts that we were seeing in the results were considered to be consistent with the expected wear from the engine.
Extended oil service intervals are great and should be used where possible as they offer lower operating costs, however they should not be undertaken without a good used oil analysis programme. These programmes can determine how well your engine is performing and if any remedial work is needed before any extended oil hours are done. If your maintenance programme does not include regular oil sampling then I would recommend that you stick to the manufactures oil change service intervals.
Detroit/D 671N Detroit/D 671N Detroit/D 671N Delvac 1240D Delvac 1240D Delvac 1240D 16557 16378 16277 - 6000 since rebuild 419 230 139 Unknown 25 19 16.1 16.1 15.9 <0.1 <0.1 <0.1 8.5 7.0 8.3 <1 <1 <1 0.1 0.1 0.1 16 N/A N/A <10 <10 <10 5 4 2 <1 1 <1 21 11 9 1 <1 3 <10 <10 <10 <10 <10 <10 3 <1 <1 1301 1334 1302
Good Sailing
Written By Kelvin Hieatt
Managing Director Marine Diagnostics Ltd
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