Happiness is...
"Bruce in Bangkok" wrote in message ... Yes there "seems" to be that but mention of "it is bad to run the engine at low poser" seems confined to the boating world. I've worked many construction jobs where diesel engines were started in the morning and shut down at the end of the day. During smoke breaks or lunch they just sat there and idled. Cranes are a perfect example, that spend most of their life at very low power settings. Generator sets - I've never seen a gen set operating manual that said "run this engine at high power settings". My guess is that idling during smoke or lunch breaks does not constitute running at low power for a long time. My understanding is that a diesel engine is happiest running at a constant RPM under a load that represents about 80 percent of the engines power rating ... either in HP or watts. Another issue that enters the picture is if a turbocharger is used or not. The operator's manual for the Volvo diesels in our larger boat recommends running at 200 RPM below WOT for maximum engine efficiency. Our other boat, (single engined 36' GB) has two engine speeds .... 1700 rpm or Off. Eisboch |
Happiness is...
Bruce in Bangkok wrote:
On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B wrote: On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott wrote: Diesels don't have this problem, and so are less wasteful at low revs. That's true but they have maintenance issues if run at low speed/low load for extended periods of time. Yes there "seems" to be that but mention of "it is bad to run the engine at low poser" seems confined to the boating world. I've worked many construction jobs where diesel engines were started in the morning and shut down at the end of the day. During smoke breaks or lunch they just sat there and idled. Cranes are a perfect example, that spend most of their life at very low power settings. Generator sets - I've never seen a gen set operating manual that said "run this engine at high power settings". Not that I'm advocating idling your diesel for days and days but I do wonder about the people who worry about letting the engine idle. I've seen people that would hardly let the poor old thing cool down before stop-cocking it, "because it is bad to let the engine run at low load". The Perkins I have in the sail boat has a continuous rating of 3,000 RPM and for years I ran it at 1500 - 1800. When I overhauled it I could see no evidence of abnormal wear or carbon or any other evidence that slow running harmed anything. I have the feeling that someone once said "it's not a good idea to idle a diesel for a long time" and as the message passed from dockie to dockie it became an urban legend and now everyone is worried about idling the engine. The issue with my motor, the Yanmar 2QM15, is that it runs cold at low RPM. I can verify this. It's amazing how long it stays cold with no load. This would be bad for it, right? Stephen |
Happiness is...
"Stephen Trapani" wrote in message ... The issue with my motor, the Yanmar 2QM15, is that it runs cold at low RPM. I can verify this. It's amazing how long it stays cold with no load. This would be bad for it, right? Yes. That is the main reason to not let a diesel sit at idle too long. Low temp AND lower oil pressure at idle. For years truck diesels have had a 'high idle' setting ( either by air or electric solenoid, manual cable, or lately via computer) so they can run them at night while parked. SBV |
Happiness is...
On Sun, 25 Nov 2007 07:20:49 -0500, "Roger Long"
wrote: It simply has to take more horsepower so push a twin screw boat with a single engine than with both. One prop is dragging and whether it is fixed or freewheeling, it is going to increase the overall resistance. The thrust Snipped money? A mechanic I talked to recently told me that he sees a significant difference in interior condition and time between overhauls on sailboats that live on moorings or have engine driven refrigeration compressors because owners of those boats tend to idle for long periods to charge and cool. You are confirming my own experiences regarding running on one engine to save fuel is something I have never been able to achieve. Assuming normally sized engines. And, I'm sure that essentially running a diesel engine at idle continuously is probably bad for it but I have never been able to detect the, so called, glazing of the cylinder walls that is a supposed result. Every diesel engine I have taken apart had cylinder walls that looked exactly the same as any other engine, a slick polished surface. But anyway, Thank you for the comprehensive comments. Bruce-in-Bangkok (Note:remove underscores from address for reply) |
Happiness is...
On Sun, 25 Nov 2007 07:20:49 -0500, "Roger Long"
wrote: Engines that are idled a lot simply get overhauled a bit sooner. If you don't have good reasons to idle, why throw away your engine time which = money? That's the way I understand it also. Instead of getting maybe 5,000 hours between overhauls, you end up with 3 or 4,000 instead. With a turbo it can be a lot worse than that. I have some hard numbers for single vs low speed twins on my own boat, based on a relatively small number of data points. Running both engines slowly to achieve about 1.0 x SQRT(LWL), the best fuel economy I've been able to achieve is 1.4 NMPG. Running single engine with the other one freewheeling I have been able to get 1.7 NMPG. The boat has sight guages on the tanks so that I can measure fuel burn to within 5 gallons accuracy. The boat is a heavily loaded, semi-displacement GB49 with total weight in the of range 60 to 70,000 lbs. The engines are 2 stroke DD 6-71s, naturally aspirated, rated at 280 hp each. They will hit their rated max of 2400 RPM at WOT. Props are 4 bladed 30 x 25, reduction gears are 2.5 to 1. With both engines the boat will reach 1.0SQRT(LWL) at 1200 RPM, burning about 5 gph total (about 85 actual hp). Single engine at 1500 RPM, same speed, burns about 4 GPH (68 actual hp) . Since it seems reasonable to assume that the overhead of running a large engine slowly is greater than a small engine, it is entirely possible that a different outcome would be obtained with smaller engines running closer to their rated power output. |
Happiness is...
On Sun, 25 Nov 2007 08:11:30 -0800, Stephen Trapani
wrote: Bruce in Bangkok wrote: On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B wrote: On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott wrote: snipped The Perkins I have in the sail boat has a continuous rating of 3,000 RPM and for years I ran it at 1500 - 1800. When I overhauled it I could see no evidence of abnormal wear or carbon or any other evidence that slow running harmed anything. I have the feeling that someone once said "it's not a good idea to idle a diesel for a long time" and as the message passed from dockie to dockie it became an urban legend and now everyone is worried about idling the engine. The issue with my motor, the Yanmar 2QM15, is that it runs cold at low RPM. I can verify this. It's amazing how long it stays cold with no load. This would be bad for it, right? Stephen If your engine runs cold put a thermostat in it. Your Yanmar should warm up like any engine and stay the same temperature from then on. Running an engine at a lower then designed temperature is not the way to go. Bruce-in-Bangkok (Note:remove underscores from address for reply) |
Happiness is...
On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B
wrote: On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott wrote: Diesels don't have this problem, and so are less wasteful at low revs. That's true but they have maintenance issues if run at low speed/low load for extended periods of time. Bruce said: Yes there "seems" to be that but mention of "it is bad to run the engine at low poser" seems confined to the boating world. I've worked many construction jobs where diesel engines were started in the morning and shut down at the end of the day. During smoke breaks or lunch they just sat there and idled. Cranes are a perfect example, that spend most of their life at very low power settings. Generator sets - I've never seen a gen set operating manual that said "run this engine at high power settings". Not that I'm advocating idling your diesel for days and days but I do wonder about the people who worry about letting the engine idle. I've seen people that would hardly let the poor old thing cool down before stop-cocking it, "because it is bad to let the engine run at low load". The Perkins I have in the sail boat has a continuous rating of 3,000 RPM and for years I ran it at 1500 - 1800. When I overhauled it I could see no evidence of abnormal wear or carbon or any other evidence that slow running harmed anything. have the feeling that someone once said "it's not a good idea to idle a diesel for a long time" and as the message passed from dockie to dockie it became an urban legend and now everyone is worried about idling the engine. But what do I know? Bruce-in-Bangkok Bruce, I suspect that all this came about as there are different types of diesels out there, and *some of them* surely cannot be safely idled for long periods. I knew a few tow truck operators that drove ford diesels that told me that if the engines were not equipped with high-idle switches the engines consistently did not last that long (tow trucks spend a lot of time at idle). On the other hand, guys that drove some other brands of tow trucks said they didn't need the fast idle, the engines lasted just fine. My Dodge/Cummins pickup truck owner's manual says to avoid idle any longer than 3-5 minutes - kind of a bitch since I spend half the day in traffic. In boats we have some (mostly older now) slow-turning diesels that can be run all year at low speeds without any harm. But I suspect that most of the newer, lighter, fast turning diesels of recent vintage are better off above idle. One more thing... *most* people I've observed around here driving their boats into the dockage area are idling anyway. By the time they are docked the engine is already sufficiently cooled and needs no more idling at the dock, yet they usually spend another five or ten minutes wasting fuel. Red |
Happiness is...
Roger Long wrote:
The bigger the diesel, the more of a factor this is which is why railroad engines are seldom shut down for maintenance. Roger, I am under the impression that railroad diesels are run at pretty high "idle". In fact when waiting for the various commuter trains to get out of the way around here I get the impression of a pretty fast running engine when they are stopped. They do not sound anywhere near idle. Is this the case just when they are dropping off and picking up and/or is there another time when they actually idle slower? Red |
Happiness is...
Bruce in Bangkok wrote:
On Sun, 25 Nov 2007 08:11:30 -0800, Stephen Trapani wrote: Bruce in Bangkok wrote: On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B wrote: On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott wrote: snipped The Perkins I have in the sail boat has a continuous rating of 3,000 RPM and for years I ran it at 1500 - 1800. When I overhauled it I could see no evidence of abnormal wear or carbon or any other evidence that slow running harmed anything. I have the feeling that someone once said "it's not a good idea to idle a diesel for a long time" and as the message passed from dockie to dockie it became an urban legend and now everyone is worried about idling the engine. The issue with my motor, the Yanmar 2QM15, is that it runs cold at low RPM. I can verify this. It's amazing how long it stays cold with no load. This would be bad for it, right? Stephen If your engine runs cold put a thermostat in it. Your Yanmar should warm up like any engine and stay the same temperature from then on. Running an engine at a lower then designed temperature is not the way to go. Why didn't Yanmar put a thermostat in this engine? Is there any disadvantage to having the thermostat? Stephen |
Happiness is...
On Sun, 25 Nov 2007 17:49:19 -0800, Stephen Trapani
wrote: Bruce in Bangkok wrote: On Sun, 25 Nov 2007 08:11:30 -0800, Stephen Trapani wrote: Bruce in Bangkok wrote: On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B wrote: On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott wrote: snipped The Perkins I have in the sail boat has a continuous rating of 3,000 RPM and for years I ran it at 1500 - 1800. When I overhauled it I could see no evidence of abnormal wear or carbon or any other evidence that slow running harmed anything. I have the feeling that someone once said "it's not a good idea to idle a diesel for a long time" and as the message passed from dockie to dockie it became an urban legend and now everyone is worried about idling the engine. The issue with my motor, the Yanmar 2QM15, is that it runs cold at low RPM. I can verify this. It's amazing how long it stays cold with no load. This would be bad for it, right? Stephen If your engine runs cold put a thermostat in it. Your Yanmar should warm up like any engine and stay the same temperature from then on. Running an engine at a lower then designed temperature is not the way to go. Why didn't Yanmar put a thermostat in this engine? Is there any disadvantage to having the thermostat? Stephen Well, I think that they did. At least the 2GM20 I owned had a thermostat and the parts book shows two, a normal temperature one for freshwater cooling and a low temperature one for salt water cooling. Bruce-in-Bangkok (Note:remove underscores from address for reply) |
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