RUNNING ON LOW Running an engine at low temperature can cause problems, and this is the first report on CaptnWil's investigation into this issue. Understanding the cooling circuits in connection with our engines is where it will begin. The lubricating system is one of the cooling circuits, and we'll begin there. Isn't it strange that once you learn something you can't understand why it wasn't obvious all along? So it is with CaptnWil and engine oil temperatures. The instrumentation on most recreational trawlers provides coolant temperature and oil pressure, and we come to associate the coolant temperature with the engine temperature and assume that the oil temperature follows the coolant temperature. It turns out that oil temperature can be more directly estimated by relating it to the oil pressure than to the coolant temperature. In addition to coolant temperature, my Cummins 8.3C reports engine oil and transmission fluid temperatures. The oil pressure decreases in almost inverse proportion to the oil temperature. It has been interesting to note that except in near full load operation, the oil temperature never reaches 180-degrees F. The concern for boaters, if there is one, is that most recreational trawler engines seldom operate at anywhere near full load, and CaptnWil suspects that oil temperatures remain low most of the time. THE OIL COOLING CIRCUIT Most diesels on medium and heavy-duty over-the-road vehicles and vessels are fitted with oil coolers. They all have some items in common, but differ in some of the details. They all are connected to the fresh water-cooling system in some way. My Cummins 8.3C oil cooler is connected to the coolant system in parallel with the engine. The water leaving the oil cooler goes back to the radiator. The cooling water is delivered to the oil cooler from the water pump (fresh water). The coolant thermostat on the 8.3C starts to open at 179 degrees F. and is fully open at 204 degrees F. The manual allows the maximum coolant temperature to be 212 degrees F. In addition, the oil cooler is fitted with a thermostat that determines when and how much oil is allowed to pass through the oil cooler. The thermostat is set to begin opening at 220 degrees F. - it took a call to Cummins to find that little bit of information. This all means that no oil goes through the oil cooler until its temperature reaches 220 degrees F. That temperature has never been observed. I have seen it get a little above 210 degrees F. on steep mountain grades at full load. The circuit in my Lehman-Ford in AfterSail was typical of what I have seen in the marine world. The raw-water pump delivered water, in series, to the coolant heat exchanger, the oil cooler, and the transmission cooler. Note that this is raw water and begins its journey through the heat exchangers at a much lower temperature than the coolant in a land-based engine. Also note that in normal operations, the raw water entering the oil cooler has already been heated some because of extracting heat from the coolant system of the engine. There was no thermostat in the oil side of the oil cooler. That means that all the oil in that engine went through the oil cooler at all times. The only thermostat in the whole system was on the fresh-water side of the water coolant system that operated in the expected fashion. It modulated the engine fresh-water coolant through the coolant heat exchanger. It is CaptnWil's opinion (he always tries to separate fact from opinion) that a system with a thermostat in the oil side of the oil cooler is superior to a system without one. We'll trace the operation in this system to justify that opinion - dissenters must defend their own opinions. Considering the Lehman-Ford and such marine engines: At engine start-up, the fresh-water side of the engine is cold. Its thermostat is set to circulate all, or most, of the water through the engine and bypass the heat exchangers. The raw water enters the oil heat exchanger at about the same temperature it entered the pump. That temperature will seldom be greater than 80-degrees F. in summer and much colder in other seasons. In any case, it is very likely that the seawater temperature is less than the oil temperature in the engine at start up. That means that even at start up, when the engine oil temperature is lower than desired, the oil cooler will cool it further - not the desired result. It is easy to see how this condition gets worse as we operate our boats in cooler water. Light load operation is better, but maybe not much. As the engine warms up, the fresh-water thermostat begins to send some water to the coolant heat exchanger. It warms the raw water because of the heat exchange. At light loads, it warms the raw water very little. We have assumed that the engine oil temperature was at the coolant temperature, but that, it turns out, is seldom the case. A better guide about the oil temperature is to look to the oil pressure gauge. It was always a mystery to CaptnWil why the oil pressure stayed high in those "no wake" zones at low RPM. Student: But teacher, why does the oil pressure fluctuate during those long, slow speed, no wake zones. Teacher: Dummy! The oil pump stays the same. The oil passages stay the same. The temperature of the oil changed, and with a positive displacement pump, the flow stays the same and the pressure changes with the temperature When the load on the engine reaches the capacity for which the system was designed, all of the fresh water is circulated through the heat exchanger, and its temperature is increased to its design value. Consider that the leaving temperature of the raw water from the heat exchanger must be less than the operating temperature of the coolant system. In fact, it must be a good deal less to transfer the heat if the size of the heat exchanger and raw water pump are to be reasonable. If the fresh water system is designed to operate at 180 degrees F, the leaving raw water temperature is likely to be designed to be less than 160 degrees F. at full load. Consider the raw water in the oil cooler to be at 160 degrees F. at full load. That is what is going to cool the oil - it's 180 degrees F. on the 8.3C. Keep in mind that the engine manufacturer most likely wants the oil temperature to be at 212 degrees F. Whatever the design calls for, it is balanced for full load operation, and let's assume the system works like that. When the load is decreased, the devil steps in for engines that have oil coolers without an oil cooler thermostat. As the load is reduced, at the same RPM, the coolant (the raw water) flow rate is the same, and the oil flow through the oil cooler remains the same, and the temperature of the water and oil that enter the oil cooler decreases. The heat transfer from oil to water increases or stays the same, and the temperature of the oil decreases - not what we wanted. The lighter the load, the more pronounced the effect. That is all to say that an oil cooler could be a liability when it isn't needed and most certainly is a liability during long periods of light load and/or idling. Does low oil temperature matter? I don't know for sure, but it surely adds to the general problem of low temperature operation. This likelihood of low oil temperature does speak directly to the use of multi-viscosity oils in four-cycle diesels. The viscosities of these oils will change less under varying temperatures than single viscosity oils. It is of interest that Mobil Delvac 1 synthetic oil is the only 5W-40 diesel motor oil I know of. All of the other multi-viscosity diesel motor oils I know about are 15W-40. It is unfortunate that the API specification doesn't let us know how the oil viscosity (weight) changes at different temperatures, and it is distressing that the oil companies CaptnWil has contacted refuse to give that information. THE IDILING STORY For some reason, it has become common over the years for diesel engines to be idled for long periods of time. Truckers, in truck stops, idle their engines for many hours. This practice started when diesel engines were hard to start, and the truckers did not shut them down because of fear they wouldn't start again. Now, they tell me, they idle them all night because they sleep in the cabs and use the engines for heat in the winter and air conditioning in the summer - the oil companies love it. Engine manufacturers warn against long idling all the time, but that doesn't prevent truckers from idling a 600-hp engine all night long. Here are some statements from the 8.3C Operators Manual: STARTUP ... Move the throttle position to idle as soon as the engine starts. Engine oil pressure must be indicated on the gauge within 15 seconds after starting. When starting a cold engine, increase the engine speed (RPM) slowly to be sure adequate lubrication is available to the bearings and to allow the oil pressure to stabilize. CAUTION: Do not idle the engine for excessive long periods. Long periods of idling (more that 10 minutes) can damage an engine because combustion chamber temperatures drop so low the fuel will not burn completely. This will cause carbon to clog the injector spray holes and piston rings, and can cause the valves to stick. If coolant temperature becomes too low (60-degrees C [140-degrees F]), raw fuel will wash the lubricating oil off the cylinder walls and dilute the crankcase oil; therefore, all moving parts of the engine will not receive the correct amount of lubrication. Idle the engine 3 to 5 minutes before operating with a load. (CaptnWil Note: This is most likely a shock. Re-read it.) NORMAL OPERATION Do not operate the engine at full throttle below peak torque engine speed (RPM) for extended periods (more than a minute) of time. (CaptnWil has written on this subject before - see the archives.) SHUT DOWN Allow the engine to idle 3 to 5 minutes before shutting the engine off after a full load operation. (CaptnWil Note: Note that this is FULL LOAD OPERATION. We seldom, if ever, come near full load.) CAUTION: Continuous operation with low coolant temperature (below 60-degrees C [140-degrees F]) or high coolant temperature (above 100-degrees C [212-degrees F]) can damage the engine. (Second such command in the same book.) Engine Shutdown Procedure Allow the engine to idle 3 to 5 minutes after a full load operation before shutting the engine off. This allows the engine to cool gradually and uniformly. (CaptnWil Note: Second time for this command. They're trying to tell us something.) * * * In conversations with Cummins, they understand that truckers will idle the engines for very long periods, but when they put the rig on the road, it will operate at near full load for many hours, and that will heat the oil enough to evaporate the diesel in the oil and mitigate the other devils. They have concerns about condensation formation from long idling times, but believe that the water will be boiled off because of the long hours of near full load operation. That is not the typical operational cycle we find on our trawlers. We seldom, if ever, run many hours at near full load. If we did, most of us could not afford the fuel. Our normal operational cycle is to run the engine(s) at light loads for a few hours after idling for a long time. Then we idle them a long time and then shut them off. This is just the beginning stage of this investigation, so CaptnWil can't say many things for certain yet, but one fact does stand out for certain. DON'T IDLE FOR LONG PERIODS. This becomes difficult to do in some cases. Sometimes we must wait an hour or more for a bridge to open with nowhere to anchor, etc. But we can say with certainty, in normal circumstances, don't idle more than five minutes. You often see boats idle for 30 minutes before normal operation is started. By the time we can get the anchor up or throw off all the dock lines and secure the electric lines, the engine has almost certainly idled long enough to get underway and slowly come under load. It is instructive to consider how generators normally work. They start up and immediately run at operating RPM. My Onan 8K gen. set automatically delays connecting the load to the engine for 10 to 30 seconds depending on the temperature OF THE AIR - not coolant temperature and not oil pressure. It is impossible to make the generator engine idle under normal operation. It starts at operating RPM and shuts off at operating RPM. There is not one word in the owner's manual about operating the engine without load at startup or shutdown. CaptnWil does run it 3-minutes without load at shutdown. These little engines are common in many different gen. sets and they seem to run forever. The engine manufacturers say little about low oil temperatures. All of their admonitions that I have seen about low operating temperatures are concerned with the coolant temperature. But keep in mind that most instructions are given for land based engines, not our marine versions of those same engines. As we have seen, oil coolers operate differently in most marine and land-based engines. Most of all, there is a vast difference in the operation of a commercial trawler that runs at full load for many days at the time and our recreational trawlers that run with very light loads. It seems only natural that TWL will establish a foundation to get CaptnWil a new trawler so he can continue research into this situation - about 50-ft would be just right. ANALYSIS and FIRST AID It's pretty easy to keep track of the coolant temperature, and the rule is to keep the coolant temperature at the temperature specified by the manufacturer as much as possible. If adverse conditions - no wake zones, bridge openings, etc. - make long low temperature operations necessary, put the engine under substantial load as soon as possible for a considerable time to purge the devils that crept in. At bridges, etc. where long waits are necessary, anchor and shut the engine down if possible. Use the oil gauge to help judge the load. The oil pressure will be higher at light loads than at heavy loads. Determine what the pressure is at considerable load and use it to help you judge the load and oil temperature. Use good grade multi-viscosity oil in four-cycle engines. The evils of low temperature operation will show up in laboratory analysis and/or POA readings long before they cause a problem if those analyses are done on regular and frequent intervals. A depth-type bypass oil filter will capture any moisture or other contaminates that low temperature operation allows getting into the oil system. FINALLY Run it hard often and idle it little. CaptnWil PS The Admiral proofs CaptnWil's writings. She says, "I don't know why anyone would want to read that. All I want to do is turn the switch on and let it go." Maybe there are others who share her opinion.