I'm working on designing a boat and looking at various diesels for propulsion. My question is, how slow can you run a modern diesel engine? I need about 110-120 HP at the shaft, and I want the engine to run as slow as possible. one idea I'm looking at is to use a much higher rated engine and run it at a very slow speed. For example, 250 HP Volvo Pentas and even 450 truck engines can be had at reasonable prices used, and looking at the power curves they seem to produce about the tight HP at about 1000 odd RPM. Can I go lower on speed? Here's what I really want: http://www.youtube.com/watch?v=jBOJ779LtBM&feature=related http://www.youtube.com/watch?v=LYKqmAFhsLI
im assuming your talking primarily about car and truck engines? most diesel car/truck engines would have no problem with 1,000 rpm. modern vehicle diesel engines generally idle at 600-700 RPM. keep in mind that you wont be making anywhere near your peak torque at idle speed though. im assuming your going for low speed with direct drive with a large-ish prop such as in a tug application? why not just run the engine through a gearbox so it can be run at its peak torque rpm? this will net you more power and better efficiency vs lugging the motor at idle. another drawback to your "bigger than needed engine" is the fact that diesel engines weigh a LOT. if memory serves, a ford (international) 6.0L diesel tips in at around 1,000 lbs. the cummins 5.9 is even heavier. if you ran...say...a 1.9L VW motor through a gearbox you would get more than your needed 120HP at the shaft, and the whole mess (motor and gearbox) would weigh less than one of the big truck engines. BTW, those videos you poster are showing lister diesel engines. those are extremely good motors, but spendy. and they are also nowhere near modern (circa 1900's design?). they generally make 6HP per cylinder, so they really wouldnt work for your application. they do make gobs of torque however thanks to their MASSIVE flywheels (those motors are usually 600lbs or so IIRC?).
On our boat, we run two turbocharged 400 Hp Volvo Penta engines, and our maximum cruising rpm is around 1500-1600 rpm. They don't really like to be idled that much, they do it just fine, but they tend to build up soot if you idle them for hours on end, but they still run at that speed just fine. That would be at about 600-700 rpm. However, the reason our engines don't like to be idled, is because they are designed to run at the highest efficiency possible at their cruising speeds, so when they are idled the turbo doesn't spool up as much, so they engine runs a bit richer than normal so it leads to soot accumulation. If you want an engine to output their maximum power at low running speeds, consider going with an Atlas Imperial engine , they may be old, but they ran at about 600 rpm full throttle and 350 rpm half throttle. But you could get them to output all of their available HP at 600 rpm (which could be up to 1000hp in some cases ) But they will give you the same Chug.. chug... chug... in the videos you posted My skipper on the boat I worked on this summer used to run an old cargo ship with a ~1000hp Atlas engine. He said you could practically hear the individual cylinder firings when at speed, but the engine would be outputting so much power he would have a rooster tail and the front of the boat would raise up a few feet. Then when he got into the harbor, he would have to just chug the engine a few times as he approached the dock, then make sure he stopped the engine so the #1 cylinder was at the top of its stroke. Then re-start the engine in reverse so he could slow down going into the dock. If you didn't stop the engine exactly at the top of the stroke, you would have to turn it over until it was before you could attempt to start it again. This tended to cause a little bit of cursing when you didn't stop the engine right, while approaching the dock at a few knots. So you pretty much wont find many modern engines that are designed to run at low rpm's, as the modern trend is to go smaller, faster, lighter, more powerful/efficient. If you want a low rpm motor, they will tend to be a much larger displacement engine, than a similarly powerful high rpm motor. So if you could give a bit more details about your size and space requirements I'd be happy to help you scrounge the internet for a decent engine for your boat. Just whatever you do, don't go with a Detroit Diesel 6-71, we have two of them for our main generators. They may be cheap, and have lots of parts available for them. But they tend to spit oil constantly as well as they are the loudest engine known to man. If it was calm out you could hear the engine screaming away from several miles away... such a relief when we didn't have any reason to run them anymore and could just use our small axillary generator.
nothing beats those old motors for reliability. ive heard of old lister style diesels being run non-stop for years on end in stationary, industrial type applications. they will basically run until the world runs out of old dinosaurs to feed them. but from an efficiency standpoint, they are far from the best choice...particularly in a mobile application like a boat. a 110HP 6 cylinder atlas can weigh over 15,000lbs...the engine in my escort makes the same amount of power (albeit at a slightly higher RPM ) and it probably doesnt weight more than 400lbs.
large, slow props are good for low speeds and high pulling power. small, fast props are good for high speeds. a gearbox allows you to optimize engine and prop speed to get proper performance out of both.
centrifugal force. jack your car up and use the motor to spin the tires up to 100 mph then pop it in neutral. now try to stop the wheels with your hands flywheels act as energy storage.
Couldn't you just rev the engine up or is the whole point of a diesel to stay at a constant engine rotation speed and only change the gearing? BTW you would want a lighter flywheel for a more responsive engine
I won't disagree that it for energy storage, but this energy is conserved until the flywheel is decelerated (the role played by your hands in your experiment ). Surely torque output is related to chamber pressure and piston area more than size of flywheel?
trying to spin a huge prop at high RPM is going to waste a lot of energy in cavitation. every propeller works most efficiently at a certain speed. likewise every internal combustion engine works most efficiently at a certain speed. if the 2 speeds dont line up, a gearbox can be used to make up the difference. a lighter flywheel in a race car is great because you want quick revs. you dont really "rev" tug boats. at such low RPMs and without a large flywheel for storing energy, the engine would stall between cylinders firing while the engine was under load. EDIT, i got sniped your right. the displacement of the engine does create much of the torque in the system, but peak torque is created by the flywheel. if you could run one of these motors without the flywheel, it would easily stall once a load was applied.
A mate had a old Perkins 4108 in his, Used to work great and got to be one of easiest engines to work on I've come across could rebuild it in a couple of hours, It had cylinder liners so you didn't have to take the block out for rebores. They also was fitted to the old ford Transits.
Although keep in mind, even though the atlas engine may only output 120 Hp, it has an ungodly amount of torque. Since horsepower is the measure of (torque*distance)/time, the slower rpm of diesel engines makes them appear like they are less powerful, but I could guarantee if you took even a high output race engine, and an old slow turning atlas engine, and attempted to do a tug-of-war between them, the atlas engine would absolutely annihilate the race engine, albeit the annihilation would take place at 600 rpm The use of a flywheel tends to smooth out the power-band of an engine, but at the cost of acceleration. Enduro motorcycles generally have larger flywheels then their standard motocross counterparts, since the flywheel prevents the engine from stalling at low speeds, and smooths out the power-band a bit, since it takes more time for the engine to spin up. There are a few boat engines that do not need a gearbox since they turn slowly enough, such as the engines used in super-freighters with pistons the size of pick-up trucks just have the pistons connect direct to the drive shaft. Some of the older Atlas style engines didn't need a gearbox either since they turned at low enough rpm. But unless your engine is turning at only 500 rpm, you will need a gearbox otherwise you will lose too much energy to friction and cavitation. Wicked_Sludge is right on the money The problem is, most props require a speed that is significantly lower than the turning speed of the engine, and yes the idea of using a diesel engine is to have a relatively constant power output for very long periods of time. Running an engine 24/7 is a relatively common occurrence when you are traveling, since you will be running at the same engine rpm for most of that time, having a responsive engine isn't very necessary. edit: Most big diesel engines have cylinder liners, which does make rebuilding very handy. I personally like Volvo Penta engines, or Isuzu, as they are extremely reliable and relatively cheap to work on, and are very efficient. We had a small 4 cylinder 40 Kw Isuzu gen-set, and that thing ran nearly 24/7 for about 12 years. It had something like 100,000 hours on it before the cylinder walls got ate through by the coolant.
The other reason for a gearbox is so that you can reverse the prop to go the other direction. If you look at the first video I posted you can see how many times the helmsman had to reverse to get turned around in that harbor. Some of the old diesels, including some of the Listers I think, didn't have the reducing gears. They just ran slowly and to go backwards you stopped the engine and then started it in the other direction. Called Direct Reversing engines. I plan to use a controllable pitch prop so instead of a reversing gear. It adds simplicity because the shaft never has to go in a different direction, just the blades change their "bite" on the water. The engine can also run at a constant RPM and if that speed is slow enough there is also no need for a reversing gear. I would dearly love to find an old Atlas or Sabb or any of those other cool old engines, but as you said they are rare and ungodly heavy, plus hard to find in the power range I need. Interestingly enough, some of the Lister designs are now being made in India and China and are referred to as a class as "Listeroids". Sabb made a single cylinder diesel into the early 90s, but I don't think it was large enough, either. The boat it's intended to go into (I'm hoping to start construction next year) will be a 34' power cruiser. Design displacement is 21,000 lbs and the plans specify a 107 hp JD 4045 diesel. I need to talk to the designer and see what he allowed for engine weight in the design as a starting point for what is feasible. The most promising idea right now is one of the Chinese made Deutz designs. The whole powertrain budget is going to be about $6000 and the Sino-Deutz is about the only new engine that will fit in that range. I take it that a non-turbocharged diesel would be better in a slow running application? I hear you on the Detroits. They're famous for how much oil they leak. I don't know if I can pull it off, but I'd love to have a Listeroid generator
Here are the original C-Trawler 34 plans by Karl Stambaugh at Chesapeake Marine Designs: And here are the plans after some modifications I've made: I did look at the diesel electric concept, but it pays an effiency penalty by having two electrical - mechanical changes.
Diesel engines take to turbo chargers so well, I personally feel its a crime not to turbo them. It's all about what rpm your engine is designed to run at. If you take a engine that is designed to run at xxxx rpm and then attempt to only run it at a slower rpm then what it is designed for, It will not run as efficently. Just choose an engine that runs at the speed you want it to. 6000 dollars is a pretty skimpy budget to power a boat with. Granted you only want around 120 hp, you would be better off going with a higher quality used engine, rather the a cheap new engine. As in the end you might end up paying a lot more in maintenance costs going with an off brand engine, which has hard to find parts, or poor quality so it breaks down all the time. A boat is already a constant maintenance effort, try to reduce the amount of maitenece as much as possible. Our current axillary gen set we are using is a 40 kw Deere, and the thing is great. That 4045 engine would be an extremely good engine to use, they are simple, easy to work on, reliable, and are very quiet. So finding a used Deere would be a much better option in my opinion than buying a cheap off-brand engine. And I think I just found your engine. http://manenginetrader.com/engines/engine_details.cfm?id=7913 Fairbanks Morse: 450rpm 120hp although it weighs 7258 kg... minor details I say minor details... I'll do some more searching later.
how about a cummins 4BT? 130HP, turbo, and under 750lbs (according to wiki). its the same engine used in fullsize dodge pickups, but with 2 cylinders shaved off. commonly found in cube vans (UPS trucks, bread vans, stuff like that)...so shouldnt be difficult to find second hand.
