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#1
02-26-2006, 05:05 PM
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| Over Proping Apologies if this has been discussed, but I wasn't able to find it through search. Conventional wisdom says that you should size your prop so that the prop curve matches the engine power curve at max rpm/redline. This came up again recently during the discussion of the diesel-electric systems from OSSA, and is mentioned here http://ossapowerlite.com/tech_librar...efficiency.htm I can't think of any boat that I've owned or been on that we ran the engines consistently at redline. The OSSA article says that the downsides are the potential to overload the engine at high RPM and higher thrust at idle. But the upside is better engine loading, lower rpm at cruise, quieter, better fuel efficiency. Why would I not just put on a governor to keep the engine from being overloaded? (or just never run above the new redline based on load) I can handle the higher thrust at idle given the advantages. In these Yanmar charts: http://www.brownmarine.com/PDF/6LPA-TP.pdf you can get near full power out of the engine at 1000 rpm under redline. So I'm not really throwing away any horsepower. It seems most of the time you are running, you are neither at idle, nor redline, so why sacrifice the 95% for the 5%. I'm sure I'm missing some piece of the puzzle, but it doesn't seem that it's discussed much.  |
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#2
02-26-2006, 07:03 PM
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| If the engine can't reach its recomended operating range, it will never produce the rated HP. Also, it will run at cylinder temperature and pressure above the designed parameters.
__________________ Gonzo |
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#3
02-27-2006, 12:00 AM
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| Quote:
I'm using the graphs from http://ossapowerlite.com/tech_librar...efficiency.htm but they are the same as most of the Yanmar and outboard curves. In this chart  In this example of a slightly over-proped engine, the engine actually gets to rated horsepower at a lower RPM. And up to point 'A', you are well under the rated load so should be under the rated temp/pressure. So if you limit yourself to 3500 rpm, shouldn't you be ok? (and perhaps a little better of since you are getting a little more HP out of it) In the next example, they over did it a little:  but if you backed off just a little, the engine would be loaded more in line with its specs. I'd expect you'd get better thermodynamic efficiency and running at lower rpm will result in less viscous losses in the engine. I always thought gen sets ran at much lower RPM and higher load. I guess if you are saying that the curve represents the "max" hp at each rpm, but not the "recommended", I could buy that. I'd be surprised if the recommended load was that far below the max throughout the rpm range. But it still seems that the idea of trying to match the prop and engine at redline isn't really related to real world needs. |
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#4
02-27-2006, 05:34 AM
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| Most cruising sail boats overprop as a constant. The engine lives far longer producing the tiny power required at lower rpm where the power is a high percentage of what's avilable. The DANGER is should someone attempt to operate at max rpm, the engine would be overloaded. An EGT would be the best way to operate at higher than normal power, But a 10% cut back works as well. With WHATEVER prop is installed go to full power for 15 seconds (even if an overload this will do no harm , but may blow black smoke). Reduce the RPM by 10% or 300rpm and you can easily operate for indefinate time. Look at your graphs , with the "prop" graph and you will see a 10% rpm reduction always gets you far enough from max power that you can operate continusly. FAST FRED One cure is to |
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#5
02-27-2006, 09:18 AM
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| This is a good topic. One that I've struggled with as well, and I have finally convinced myself that I have come to some understanding. I can't promise its correct, but I offer the following with humility, as I am sure I could learn more on the subject. Lets say we are talking about a marine diesel engine, a very basic set up with a jerk style pump, not too fancy. Engine overloading kills an engine fast, and this occurs at any RPM, not just near the max RPM. Yes, you can develop full or rated horsepower at a lower RPM, but that is exactly what you don't want to do. Black smoke and high EGT are symptoms telling you to figure the problem out- and quickly- but they are not what kills you. Yes, the EGT might roast your turbo and push the exhaust past its limits, but the real engine damage comes from too much fuel injected into the cylinder too early in the cycle. Consider what happens on the compression stroke just above idle, say at 600 RPM. Thats 10 times around the crankshaft every second, or 5 powerstrokes per second on the 4 stroke engine. Since the time available for compression stroke is short, and gets even shorter with climbing RPM, fuel injection occurs before the piston reaches top dead center (BTDC). A typical normally aspirated engine might have the injection start 14 degrees BTDC. The brief time delay from the start of injection to start of fuel burn (ignition) allows the piston to reach TDC without encountering any force from the burning fuel fighting against the pistons forward progress. This is the critical part of understanding how bad overloading is - that there is fuel in the cylinder as the piston is climbing to TDC. Normally, not a problem, as this is really a small amount of fuel at lower RPMs, and the piston will not feel much of a fight against its forward progress from whatever small fraction of fuel ignites prior to reaching TDC. As RPM increases, more fuel is injected during each cycle. As a consequence of this, more fuel is injected BTDC. The time from the start of injection to TDC is also shortened, however, so the engine does not "feel" the combustion of this fuel as the piston advances very rapidly to TDC. This greater amount of fuel injected continues up to the rated WOT RPM, always correlated however with a shorter and shorter duration of time elapsing from the start of injection to TDC. So take an engine chugging happily along at 1500 RPMs, and now suddenly put a prop on it thats too big or overpitched. The resistance will drop the RPM, say 1300. This is a problem, because the governor will also react by injecting more fuel. It will try to get back to 1500 RPM. It may succeed in doing so, but now has alot more fuel injected BTDC than it was supposed to have for that RPM. There will be more combustion taking place prior to the piston reaching TDC, and so the piston, wrist pins, connecting rods and crankshaft will "feel" the resistance to its foward progress, really straining these parts. You can begin to see the loss of efficiency to forward progress, stress on components, and decrease in RPM are related. No problem for the governor, just dump more fuel into the cylinder. And thats just what the governor tries to do to compensate, keeps adding more fuel. At some point, there is more fuel than available air. This results in incomplete combustion or the famous "black smoke" of the diesel. Also causes high exhaust gas temps (EGT). Just to unify the theory a bit...notice that over advancement of the timing (injection timing) on a diesel engine will cause black smoke. Its for the same reasons discussed above. The early ignition is felt at the crankshaft in stress forces against the direction of rotation, lowering RPM. Compensation is by adding more fuel to the powerstroke, and when it gets bad enough, black smoke will be present in the exhaust. Anyway, I hope that makes some sense. You can develop full horsepower below rated RPM, its a bad idea though. And not just near the rated WOT RPM, but anywhere. Just because you don't yet see black smoke doesn't mean you are not stressing your engine out very badly. Mike |
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#6
02-27-2006, 09:17 PM
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| Thanks Fast Fred, you've confirmed my thinking. And the EGT monitor makes sense as well. I remember from some of the Dashew info on one of their sailboats they used a CPP and would dial in the pitch until they hit a target EGT. Quote:
Thanks all. Mark |
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#7
03-02-2006, 05:30 PM
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Thanks for your post. I am like you in that I'm not entirely sure that what I know is correct so I want to get a cross check here. Set up: Boat. Diesel engine. EGT gauge. Controllable variable pitch propeller (Hundested, Helseth or similar) Out cruising on the ocean sea (or lake or bay or...) and you want maximum efficiency and minimum fuel consumption. Choose an RPM that is (more or less) 1/2 way between idle and WOT - say 1500-1700 RPM. While watching the EGT adjust the VPP until the EGT starts to go into the "too hot" zone and the RPM starts to drop as you describe, then back off a bit until the EGT is back in the green and the RPM is pretty close to the original setting - maybe minus 50 RPM or something close. Cruise to your hearts content - at whatever the resulting speed is - without worry of over-fueling your engine and the resulting ugliness. This is based on my understanding that a diesel "likes" to have a load and runs best and longest with a (debate continues on the exact %) 60% to 80% load. By using a VPP one can achieve the 'load' without running the engine at high RPM's. Does your understanding extend to this sort of situation? Do you believe that this would result in too much fuel injected? Thoughts? Yea or nay? Thanks, Leo |
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#8
03-02-2006, 06:09 PM
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If I'm reading the graph right, if you have the flow rate constant by the throttle, then RPM and load go hand in hand. For the above article:  Say you throttled up to 2200 rpm, you'd be at about 8 L/hr. If you kept the throttle constant and adjusted the VPP, the EGT should start going into the red zone as you approach 1400 rpm. As you backed up the VPP, the EGT should come back into normal range, say at 1500-1600 rpm. But if you kept backing off till you were back at 2200, you'd essentially be back at the same underloaded condition where you started. Am I reading that right? I'd also be curious if this same thing applies to gas (inboards and outboards) or just diesel. |
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#10
03-02-2006, 07:24 PM
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| This continues to be an interesting thread... I like the idea of using your EGT to set prop pitch. That strikes me as pretty much the ideal scenario. If you've got a VPP and EGT sensors...I've got neither on my diesels! Mark asked about the "original RPM" in this scenario, and I wanted to offer that it shouldn't mean much of anything at all. I think this is where you were headed with that Mark, and stated this very clearly at the end of your post. You don't really know (or care) what the original pitch was when you set the throttle, but you put the throttle somewhere around the speed that you thought you might like to cruise, and then adjusted the prop to match ideal EGT's. Doesn't matter what the RPM is as long as you are going as fast or slow as you want to be going. If the EGT's are good (warm, but not hot), your diesel is happy. Thanks for posting that Leo, very interesting, and makes great sense at my level of understanding. I just wanted to make a point for Mark about "setting the throttle." I can't quite tell how you mean this. But when you say, "if you have the flow rate constant by the throttle" I take it to mean "flow of fuel". If I am taking you correctly, I believe that's not quite right, and there lies the problem. You can only set the RPM with a throttle on a diesel. The governor is constantly adjusting the amount of fuel to meet that RPM. Not guaranteed to get there, but he does try hard. There is no way to keep fuel flow constant. You are helpless in this regard, and downright at the mercy of the governor! Thats why, with too large a prop, even with the RPM well below WOT, the governor is capable of dumping so much fuel into the cylinder trying to bring the engine up to your RPM set point that this fuel will not be completely burned in the cylinder, but continue to burn in the manifold, turbo, etc, making for very high EGTs. When more severely overloaded, you will see the black smoke exiting the boat. If you can drop the prop pitch with a VPP, the RPMs climb and the amount of injected fuel is less - EGT drops. Again, this is just where I am at in my understanding. It is great to discuss this kind of thing, though. Mike |
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#11
03-02-2006, 08:04 PM
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That goes back to my original concept. Imagine you had a VPP and had to freeze it at some setting. Rather than freezing it the setting where the engine was properly loaded at redline, you could freeze the VPP at somewhere south of redline, that point being dictated by how much top speed you were willing to give up for better economy at cruise. The catch would be, you'd have to some how either by adjusting the governor cable, using a rev limiter, or being well disciplined, never ask the governor to push the engine to a point were it was overloaded. In essence the VPP allows you to "overprop" on command. I've had so many people quote the rule that the prop load must match the engine HP at engine red line, it's nice to finally understand the real story. I'm not a speed demon, I like my boats quiet and efficient, with just a bit of pep. Now, I just need to understand Leo's comment about this appling to diesels and not gas engines. Thanks so much to all. Mark |
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#12
03-03-2006, 10:52 AM
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Perhaps someone that has a chemical engineering degree can jump in here and clarify this issue for us, but I’ll try to provide my understanding as long as you (and everyone) understand that I don’t have a degree and in-depth knowledge of this subject. And please recognize that I may not use the exact proper terms, but that is more a function of my chemical engineering ignorance than any willful desire to deceive or mislead. The following is based on my layman’s understanding. And I want to add that I've treated this from a high level and the details get really convoluted. That disclaimer said, my understanding is that this all revolves around energy content (based on equal volumes) of each fuel and the volatility for each combined with the way that each type internal combustion engine uses their respective fuels. Diesel – per equal volume – has ~18% more energy content than gasoline. Diesel’s volatility is also less than gasoline, i.e., a higher boiling point and lower vapor pressure, thus it is harder to ignite than gasoline. OTOH, gasoline has a significantly lower boiling point and therefore a higher vapor pressure thus it is more volatile and easier to ignite. ********************************************** Besides the energy content of the fuel, a significant difference between a diesel and gasoline engine is air flow. Naturally aspirated diesels flow a constant volume of air through the engine (at any given RPM) whereas gasoline engines control the volume of air to maintain the optimal fuel/air ratio of ~14.7:1. Gas engines (generally) need that 14.7:1 A/F ratio to run efficiently at any RPM. However diesels at idle may have an air/fuel ratio of 50:1 or more. As more power is needed – the throttle is advanced – the gasoline engine allows more air into the engine and the carburetor or fuel injection attempts to optimize the F/A ratio at the ideal 14.7:1. With a diesel, the advancing of the throttle causes more fuel to enter the cylinder and thus there is more “energy” available for combustion and either the RPM increases (relatively) rapidly with a light load or the amount of torque produced increases as the RPM increases at a slower rate than under the lightly loaded condition. It is my understanding that (generally) diesels rely on torque to perform work whereas gas engines (generally) rely on RPM to perform an equal amount of work. Take two engines of equal horsepower rating and (generally) you’ll see that the diesel develops that HP rating at a significantly lower RPM than the gasoline engine. In other words, a diesel can do 20HP worth of work at (say) 2000 rpm. A gasoline engine may need to rev to 3000 RPM to produce the same 20HP. So back to the continuously variable pitch prop topic. It appears to me that a CVPP would not be applicable to a gas engine because of the required operating conditions, i.e. the 14.7:1 F/A mix and the increased volatility and lower energy content of gasoline. We must remember that a gas engine has a relatively narrow optimal operating band with regards to the F/A mixture. Run the engine lean and the combustion temps increase and that provokes pinging and increased NOX emissions. Run the engine rich and the combustion temps cool and the result is wasted fuel and increased emissions via the unburned portion of the gasoline. If we attempted to increase the pitch of the CVPP the amount of torque required from the engine would climb. Since the engine needs a 14.7:1 F/A mix the only way to deliver the demanded torque will be by increasing the RPM, thus requiring more fuel and air to produce the required amount of torque to turn the CVPP at the desired pitch setting. Self defeating. Can’t get there from here – no road leads to this Shangri-La. On the other hand, the operating characteristics of the diesel engine lend itself to operating with a variable torque load at (more or less) a constant RPM. Because the diesel runs with a constant air volume (at any given RPM) and the only variable is fuel, and the amount of work produced is directly dependent on the quantity of fuel injected. If we attempted to increase the pitch of the CVPP the amount of torque required from the engine would climb and the governor (or computer on more modern engines) would detect the RPM drop as the torque load increased and add more fuel to the engine in an attempt to maintain the ‘set’ RPM. Continue increasing the demand for torque and the governor will continue to dump fuel into the engine as it tries to maintain the RPM. Too much fuel raises the exhaust gas temperature, causes the black smoke we see and even worse is that there is unburned fuel left behind in the cylinder and that washes off the lubricating oil film on the cylinder walls. Moreover it also moves past the piston rings and dilutes the lube oil with the resulting long-term damage there. So we go back to my earlier post about utilizing the EGT gauge to find that optimal CVPP pitch setting for any given RPM. *********************************************** Again, my understanding… Without a CVPP one would want the pitch and diameter of the prop to allow the engine to reach its RPM limits so that it won’t be overloaded at any lesser speed. But with this compromise we lose some amount of efficiency at those lower RPM’s. And choosing a fixed pitch prop is a whole ‘nuther subject that I ain’t gonna get mixed up in. This treatise just about exhausts (no pun intended) my understanding of diesel operation versus gas operation. I hope that this has been clear, but if there is some confusion I’d be happy to attempt to clarify any point that I’ve made. Best, Leo |
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#13
03-06-2006, 05:48 AM
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| Most gas engines don't bother with such sophistication, mostly as its complex & expensive and not really suitable for day boats. I have used this system, years ago in aircraft where the mixture and prop loading (pitch) could be changed. 3350 , 27 cylinder radials For diesel boats the "loading" will change little as the pitch is adjustable , BUT the diameter is not. Since its the diameter that most absorbes power , varing the pitch will load the prop better for cruise at lower fuel burn. The charts of power/Rpm are always at full throttle for that rpm. The ideal is a wide open throttle at the cruise rpm desires , although this is seldom possible as the rpm climbs with open throttle, due to too small prop diameter Best that can be done is to find a "sweet Spot"{ (near the torque peak) where the engine is quieter and smooth , and load her up with the pitch , readjust the throttle to hold rpm , untill you get to the long term EGT you can live with. FAST FRED |
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#14
03-06-2006, 06:47 AM
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Has anyone here seen this site before? http://www.atlashovercraft.com/index.html Diesel / Electric hovercraft. The site should be updated soon, of interest is in the specfication section: http://www.atlashovercraft.com/Speci...s_AH-100-M.htm Propulsion System (Diesel / Electric) Standard Metric 4 Cummins Diesels (4000 Hp / 2984 bkW Total) 1000 hp @ 1800 rpm 746 bkW @ 1800 rpm |
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