Hello,
So I am a college student writing a paper on tug boat propulsion…only problem is I study business so I only have a basic understanding of what is at work here. I am trying to make a case for hybrid/diesel-electric propulsion over traditional diesel-mechanical. I’ve found that the vast majority of the lifespan of a tug’s engines are spent at under 30% load. Next, I came to the conclusion that these engines (2,000-3,000 horsepower two stroke turbocharged diesel engines) run at their most inefficient point under low load situations. I have three sources supporting this. Using my non-engineering background I have determined the following: Diesel-mechanical engines are inherently inefficient because they have the dimensions and capabilities to produce large amounts of horsepower, however most of the time these large engines are being used to transport the boat which does not require large amounts of horsepower. Secondly, a turbocharger at low loads is not as effective as at high loads because the exhaust gas spinning the compressor is cool and at a lower volume. As the load on the engine increases, the turbocharger is introduced to more hot air and spins faster creating a more efficient burn. Thus the gallons used per unit of power created is greater under low load conditions.
Is this a sound explanation? If not can someone point me in the right direction? I am trying to prove why electric power is a viable option to increase efficiency and reduce emissions.
“In a conventional diesel powered tug the main engines and the diesel driven auxiliary generators are rarely operating in their most efficient mode, which is heavily loaded. Studies have shown that during harbour towage, 100% of full load (maximum bollard pull) on the main engines is used for only 2% of their operating life, leading to very poor fuel consumption and excessive exhaust emissions.”
Boats moving barges, especially ATB’s have a very different load profile. Probably above %75 most of the time they’re running. Many are four strokes and electronically injected.
Diesel-electric is great but it takes up a lot of real estate in the hull of a vessel. That’s all fine and dandy on a 1000+ foot ship that has the space for it but a tug may very well not have the space for all that equipment and an adequate amount of fuel capacity. Not all progressive technologies are 1-size fits all. This might work well on a ship-assist tug that doesn’t go very far from a fuel source, but I don’t see this working well for off-shore towing, at least not until the package size shrinks.
Why are harbor tugs with engines described above inefficient at low load ranges?
I am familiar with the Foss hybrid’s but cannot find an explanation for my above question.
[QUOTE=OK Corral;181787]Sorry!!
To clarify I am talking ONLY about harbor ship assist tugs[/QUOTE]
I’m fairly certain it’s obvious to almost everyone that a diesel electric harbor tug is more efficient than a conventional tug, that’s why Foss built a bunch.
The biggest problem is that most harbor tugs are very old and it’s extremely cost prohibitive to build brand new boats for a little fuel savings.
[QUOTE=OK Corral;181790]Question:
Why are harbor tugs, with engines described above,inefficient under low load?[/QUOTE]
Because internal combustion engines are inefficient at low RPM and low load. That’s the way it is.
Some engines like MTU’s only burn a couple gallons an hour idling. Don’t have the exact figure but it’s minimal. They only run on 1/2 the cylinders at idle.
EMD’s are two strokes and were more or less to run as generators in locomotives at 900 RPM. Sticking them in a tug was a compromise, doing something they weren’t really designed to do efficiently (idle, run at low loads).
Perfect justification for running hooked up from place to place. “Honest dispatch, we are just trying to run the engines at peak efficiency! This guys paper even says so!”
What you need to look for to assist with proving your hypothesis is the BSFC (brake specific fuel consumption) chart.
Also, with fixed pitch propellers, it can get confusing because of the exponential nature of how a prop absorbs power vs RPM. Because of this, you can’t directly compare fuel consumption at various RPM (because the HP absorbed by the prop is not linear).
This factor is one reason hybrid or diesel electric tugs have the efficiency boost. It’s another reason variable pitch propellers can increase efficiency–albeit a slightly different discussion. Another possibility would be to put different gear ratios or a CVT (continuously variable transmission) which would align the prop curve with the engine curve at lower operating RPM–again, not exact same concept for hyrbrid.
The diesel electric works well because in general, there isn’t a huge loss of efficiency to run an electric motor at reduced power. A smaller genset can be used to run at it’s peak efficiency.
One of the problems with diesel-electric power plant is that unless you have the engines on standby (running at idle or low load), there will be a delay between applying power and getting it. This could be a problem during harbour operations where full power may be needed on short notice, but the alternative is lower fuel efficiency as the gensets are not able to run at optimal speed. Of course, a smart power management system would probably run the engines in such configuration that the idling ones would not be connected to the network until they are needed.
[QUOTE=Traitor Yankee;181795]Perfect justification for running hooked up from place to place. “Honest dispatch, we are just trying to run the engines at peak efficiency! This guys paper even says so!”[/QUOTE]
Could have used that all those times we got into San Juan a couple of days early. . . . wow, we had a hell of a current. . . too bad that stopped when they started recording RPM. . . .
[QUOTE=Tups;181820]One of the problems with diesel-electric power plant is that unless you have the engines on standby (running at idle or low load), there will be a delay between applying power and getting it. This could be a problem during harbour operations …[/QUOTE]
Huh? Would you mind explaining that? That statement is circular and directly counter to the reason there are DE tugboats
Diesel electric tugs have been common since before WW2. They were built specifically because they offered precision power control and instantaneous power availability that a direct reversing diesel, a steam engine, and most definitely a geared diesel could not match. They were modeled on or based on locomotive drives which allowed for max torque at zero rpm, a handy feature for accelerating, decelerating, and reversing the large fixed pitch propellers used on tugboats of the time.
They still offer the same benefits in addition to reduced emissions which is what is driving the latest move to hybrid designs.
[QUOTE=Steamer;181839]Huh? Would you mind explaining that? That statement is circular and directly counter to the reason there are DE tugboats
Diesel electric tugs have been common since before WW2. They were built specifically because they offered precision power control and instantaneous power availability that a direct reversing diesel, a steam engine, and most definitely a geared diesel could not match. They were modeled on or based on locomotive drives which allowed for max torque at zero rpm, a handy feature for accelerating, decelerating, and reversing the large fixed pitch propellers used on tugboats of the time.
They still offer the same benefits in addition to reduced emissions which is what is driving the latest move to hybrid designs.[/QUOTE]
Read what he said more closely:
“Unless you have the engines running [at low load] there will be a delay…”
This does away with VFCs as is now used to drive and control AC motors, or SCRs to drive DC motors, when generators are producing AC.
The fact is that there is little AC power required on board a modern DC Diesel/Electric OSV, thus to have DC generators and convert to AC for those items that requires AC make perfect sense.
[QUOTE=Capt. Phoenix;181842]Read what he said more closely:
“Unless you have the engines running [at low load] there will be a delay…”
That’s absolutely true.[/QUOTE]
That is like saying it is dark at night. Of course you have to have engines running or there will be a delay … the delay of starting the things. If no propulsion power is being utilized then of course the engines are running at low or no load … when you need power the load comes on instantly, or as fast as the governor can deliver it, that is what makes DE so damn good for precision power delivery and maneuvering. Have you ever run a DE vessel?
[QUOTE=Steamer;181851]that is what makes DE so damn good for precision power delivery and maneuvering.[/QUOTE]
Yes, they are great for maneuvering. Especially because you can use 10 RPM if you want to.
But, that wasn’t the point of his post. He was pointing out that the efficiency is still hurt by the fact that when you’re maneuvering you still have to have more engine running than the load you’re using.
[QUOTE=Capt. Phoenix;181858] But, that wasn’t the point of his post. He was pointing out that the efficiency is still hurt by the fact that when you’re maneuvering you still have to have more engine running than the load you’re using.[/QUOTE]
Someone is completely missing the point. Efficiency? You have to run an engine to determine its efficiency, if stopping it and burning no fuel is more efficient than running it how can it even be called efficient? If you are running it with no load then it could be said to be -100 percent inefficient.
There is more to an efficient tugboat operation than the thermal or mechanical efficiency of the powerplant.
