Not nor ever claimed to be an engineer, but do get it. That post makes a bit of sense. Well said. We are not golf partners by any means, but respect your experience.
Is this like the big ship version of ramming the throttles forward on a pure-mechanical turbocharged old Detroit Diesel and dumping a lot more fuel in than there was air to burn it with at that point?
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Yes, and very much like the old gas turbines where you could corncob the thing by a simple flick of the wrist.
Mostly sailed with 16 and 20 cylinder EMD diesels. Quite alright with them in any weather with a good engineer. Shook up crap in the fuel tanks, they stayed on it with filters. Rarely turned down a supply request from that department.They got me home for crew change. Helped change packs on the rare occasion it was needed while the other engine kept us rolling underway. It ainât all about the wheelhouse fellas.
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I think the âdumpingâ of fuel is what is known as a âtorque rich conditionâ. On a low-speed diesel the governor must reduce the amount of fuel when the load forces the rpm to decrease.
I assume thatâs the case anyway. In strong head winds I could observer the fuel rack position from a monitor in the wheelhouse and could see the rack position would decrease when ordered rpms had not been reduced.
Quite the opposite. The only way to maintain rpm with an increasing load is to add fuel. Once rpm setpoint is reached fuel flow is reduced to that required to maintain setpoint and then varies up or down in response to small changes in load.
If you are saying that you saw this after an order for higher rpm was set, it was because a greater amount of fuel is required to accelerate from a lower rpm and lower load. Remember a fixed pitch propeller imposes a higher load as rpm increases and more fuel is required to accelerate than to maintain a constant speed. Itâs just like driving your car, you need to step on the gas to speed up or go up a hill but let off to maintain the same speed on a flat road or when going downhill.
Depending on the vintage of engine and controls, there are fuel and torque limiters that may come into play to keep fuel flow and load within limits. Those âcontrol lawsâ are there to prevent the governorâs lizard brain from destroying its host.
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Say for example 100 rpm is ordered. In nice weather the tach will indicate 100 rpm and the fuel rack will be near but not at full.
If weather from ahead worsens the tack will show, say 95 rpm with the rack now at full. But if rpm drops further say to 90 rpm then the rack will no longer show full, that is IIRC. *
Presumably the engine controls reduce rack to avoid being in a torque rich condition.
- I should add that normally in this situation the ordered rpm would be reduced to more closely match actual to avoid wasting fuel and in some cases risk of over-speed.
Part of the equation includes boost pressure (turbocharger outlet air pressure) and mass flow. There are design limitations (control laws) imposed to reduce smoke and protect against high exhaust temperatures as well as torque limits. Modern variable geometry turbochargers add another factor to the mix.
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and then you have variable pitch props, âŚ
We had a diagram that looks similar to this, I had a copy in my office.
The red area was to be avoided, - Line 4 Torque speed limit. As long as everything was functioning properly the engine controls did this automatically - is the way I understood it.
Smaller feeder ships under 1000TEU are generally fitted with medium speed diesels and CP props. The engine RPM is set at 600 rpm with a shaft generator and gearbox output of 150 RPM.
In heavy weather fluctuations in load are controlled by adjusting the pitch using the engine management system.
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Did you have a day tank? I always liked the idea of having the contents of the tank to burn through while screwing with filters.
- down in WAFI land - see boat moving - getting the fuel polishing company out with their truck was never a bad idea before setting off on a new-to-you boat that maybe had 100 pounds of crap in the tanks waiting for the first big waves the boat had seen in years.
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Settling tanks and day tanks are pretty standard.
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Gets a tad more complicated when using a CPP in combinator mode and then in constant speed mode. Thanks for sharing, my students never could get their heads around this!
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The explanation I saw with regards to torque rich was is that it was similar to climbing a steep hill with a car in too high of a gear. If the throttle is mashed down in high gear on little 4 cylinder not much happens. If you shift down however the increase in RPM means the engine is able to pump more air which allows more fuel to be burned. Of course with a fixed prop thereâs no downshifting.
The is what Steamer has pointed out on this thread.
Any tug I ran had duplex filters so one could be changed without any issue. Oh, and day/settling tanks, too. Additionally we would have either centrifuges or Racor filters in addition to the engine mounted duplex units.
The car analogy isnât always correct. In a gasoline powered car, there is never going to be extra fuel, the fuel injection senses the amount of air being sucked into the engine and injects the appropriate amount of fuel.
My ancient Mercedes diesel on the other hand had no controls over fuel other than my foot. It very much could get caught out at low RPM, no boost, my foot on the floor, and all that extra diesel with no air to burn it with generated a smokescreen behind me that reduced visibility to zero for anyone too close.
Iâm racking my brain because itâs been years since I was down in the plant.
I think reset turns on a CVN are a bit similar to what KC is asking. A 180 degree reset turn will have you go from 20 knts forward, to 2/3 back, to 2/3 forward in a matter of about 10 minutes. The only parameter we had to stay within (out of concern for the propulsion plant) was being below 10 or 12 kts before ordering a backing bell during a turn. The other constraints were what Iâd call habitability (e.g. 3 degree heel limit to keep planes on deck). On a heavy flight op day in a restricted area, reset turns happen about every hour.
Over speeding the steam plant was not a big concern. We did have governors on turbines that would trip on overspeed, but it rarely happened. I recall it happening on the electric plant, but not the propulsion side.
Itâs a little different on a CVN because of the nuke plant, but not much. Main issue is not increasing steam demand too quickly; most of that has been âfixedâ with electronic throttles. Back when Noah was a deck seaman, a bad throttleman could shut you down. That was not due to steam plant issues that was RX plant concerns.
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It doesnât matter, if it is heavy weather or fouling of hull or propeller that slows you down, a fixed pitch propeller is only designed to be optimal at a certain water inflow to it. If you then try to increase power to or revolutions of the prop to maintain speed of the ship, nothing happens apart from increase in fuel consumption and that you may overload your engine (steam or diesel). The optimal speed of your ship is of course also a function of freight earned and cost of fuel, and if you are in doubt, easiest way is to slow down and re-do your calculations.
One key factor is that a steam plant has significant stored energy, so that the turbine can be brought to maximum torque as fast as the governor can move the throttle valves. This has been mentioned above, but would be fun to quantify.
Could someone with steam experience post boiler volume / temperature and power output?
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