Does a turbo-electric or double reduction gear plant consume less fuel?

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Does a turbo-electric or double reduction gear plant consume less fuel?

Joined: March 25th, 2018, 4:08 pm

March 26th, 2018, 1:47 am #1

I know that turbo-electric drives were the 1920's solution to the turbine speed/prop speed problem for greater fuel efficiency, but by 1940 double reduction gears from the turbines had much the same effect, at considerably less weight and volume.What I'm wondering about is (in the WWII era) whether a turbo-electric drive is still has a lower specific fuel consumption compared to double reduction gears. If both use the same boilers (and temperature/pressure), all else being equal would the turbo-electric plant still consume less fuel? I haven't even been able to find fuel consumption data for the 1920's US plants.

Thanks.
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Joined: January 5th, 2009, 5:56 am

March 26th, 2018, 8:08 pm #2

BraselC5048:

Welcome to the forum.

The biggest bang for buck from turbo-electric drives is it's cruise efficiency.  e.g for USS Lexington transiting in peace time for San Diego to Pearl, you run 4 boilers at their maximum efficiency to power one turbo generator at its most efficient which drives the electric motor in their slow speed setting and get most efficient power.

Double reduction gears might allow turbines/propellers to operate more efficiently at their top end or cruise, but not both.
     RN optimized top end, then added cruising turbine; USN optimized cruise, then added sections for top end.

If you look at QE class CVs, they use gas turbines/diesels to drive generators which power electric motors.  In my books, that says turbo-electric is still best 100 years later*

*dirt burning; if you are splitting atoms, cruise inefficiency really doesn't matter and gears are more compact.  And if you are volume limited CV, take the extra space.
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Joined: February 10th, 2010, 3:58 pm

March 31st, 2018, 1:32 am #3

BraselC5048 wrote: I know that turbo-electric drives were the 1920's solution to the turbine speed/prop speed problem for greater fuel efficiency, but by 1940 double reduction gears from the turbines had much the same effect, at considerably less weight and volume.What I'm wondering about is (in the WWII era) whether a turbo-electric drive is still has a lower specific fuel consumption compared to double reduction gears. If both use the same boilers (and temperature/pressure), all else being equal would the turbo-electric plant still consume less fuel? I haven't even been able to find fuel consumption data for the 1920's US plants.

Thanks.
Sister ships.....
New Mexico (electric) used 2055 tons to cruise 7000 miles @ 10 knots
Mississippi (turbine) used 2790 tons to cruise 7000 miles @ 10 knots
Idaho (turbine) used 2850 tons to cruise 7000 miles @ 10 knots.
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Joined: February 13th, 2013, 12:56 pm

April 3rd, 2018, 8:31 am #4

Of turbo-electric or diesel, which would be most fuel efficient in 1940? In other words, which was best of the German and the US solution?
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Joined: May 27th, 2009, 10:43 am

April 3rd, 2018, 10:56 am #5

If you just look at fuel efficiency - diesel! No doubt about that. A US DE with a turbo electric drive needed 335 tons of fuel for 3870 miles at 14 knots. A diesel DE needed just 197 tons for 4670 nm.

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Joined: March 25th, 2018, 4:08 pm

April 3rd, 2018, 8:29 pm #6

I was looking for the difference between turbo-electric and double reduction gear plants (1930's onward) for the same boilers and temperature/pressure;; the Mississippi used a single reduction gear plant, since double reduction gears hadn't been made yet. Or not put it another way, if you were designing a warship for WWII and could choose between a double reduction gear and turbo-electric, which would offer better fuel economy? Or would they both be about the same?
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Joined: January 5th, 2009, 5:56 am

April 5th, 2018, 12:20 am #7

3 sister ships: USN DEs

Data from Friedman.
Class Endurance (war)  Top Speed   Std Disp.  Power
Edsall - Diesel Electric - 9,100nm/12kn   21kn   1,281tons    6k hp (4 x 1,500hp)
Rudderow - Turbo Electric - 5,050nm*/12kn   24kn   1,435tons    12k hp
John C. Butler** - Double reduction gears - 4,650nm/12kn   24.15kn   1,367tons    12k hp

Cousin:
Black Swan - Double reduction gears -7,500nm/12kn   20kn  1,350tons  4.3k hp

*Edsall &Rudderow classes endurance improves ~10% when operating outside "war" conditions - i.e. power plants not needed cold. But you would be asking for "HMS Glorious situation" to do that in wartime.
**John C. Butler class doesn't have cruising turbines.  Theoretically, it could make 25% difference in range (Andy01's figures for Mahan versus Farragut DDs), but it would hurt top speed - weight of the cruising turbine and its gearbox.

Diesel with reduction gears might allow for 6 engines (then supercharge them and power would very closely match the steam ships).  Probably requires 3 shafts ala Z-51 versus the twin shafts of all the above.  10-20% reduction in range for weight of extra engines/supercharger power requirements versus Edsall class.
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Joined: August 30th, 2005, 9:15 am

April 5th, 2018, 9:04 am #8

A reasonable estimate is that an efficient gearbox loses about 2% per gear pair, or perhaps 3% for an epicyclic stage. There's also a significant static friction effect which means that at low torques they are less efficient than that. The precise details of that low torque friction depend on the detailed design of the gearbox, and to some extent you can trade off high torque efficiency for low torque efficiency.

You can't go too far in that direction, as that implies your gearbox oil will burn during high torque operation. As an example automotive diffs are designed so they are most efficient at high torques typically.

I know nothing about the details of designing non electronic electric drives, so i can't help much there.
"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)
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emc
Joined: May 12th, 2004, 9:51 am

July 11th, 2018, 1:17 am #9

Turbomachinery is designed for specific values of steam flow and rpm;  in off-design conditions, some rows may be operating far, far away from their optimum condition;  it's possible that some stages may even absorb power.  See, for example, https://www.academia.edu/26438081/Perfo ... Conditions

Since a turbo-electric ship enables the turbines to be kept nearer the design conditions at low power levels, the turbines are operating more efficiently, and this may be sufficient to overcome the inherently lower efficiency of the motor-generator system when compared with reduction gears.
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