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A different sort of hybrid powertrain

4000kW, four diesels and a huge battery pack - submarine technology over 70 years ago

by Julian Edgar

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When considering hybrid and electric powertrain technology in cars, it’s easy to forget that hybrid diesel-electric technology using battery energy storage is actually old technology – submarines have been using it for about 100 years. In fact, even in World War II, hybrid systems with current flows of over 2500 amps at 415 volts were being used, and there were traction motors rated at more than 1000kW.

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So how did those systems work? What batteries did they use? And how, well before high current solid state switching devices were invented, did they control these propulsion systems?

Let’s take a quick look, using as the guinea pig the WWII USS Bowfin, a Balao-class United Sates submarine, now on display at Pearl Harbor in Hawaii.

Propulsion system

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The long-range US fleet submarines built during WWII used four diesel engines. A ‘series hybrid’ approach was adopted: the diesels drove only the generators. (With this approach the diesels were not connected directly to the propeller shafts, as they had been in older submarines.)

The generators could be used to either power traction motors mounted on the propeller shafts, or to recharge the battery.

Each propeller shaft had two electric traction motors, powering the propellers through reduction gears.

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Each of the four diesels comprised a General Motors Model 16-278A lightweight engine. These were 16 cylinder, two stroke designs using a 40-degree V angle. They had an 8.75 inch bore and 10.5 inch stroke, and power was 1600hp at 750 rpm.

The engines used unit fuel injectors, where the functions of the fuel injection valve and fuel injection pump were combined. Starting of the diesels was by compressed air.

Each head was fitted with four exhaust valves, the unit injector, a rocker lever assembly, an engine over-speed injector lock, cylinder test and safety valves, and the air starter check valve.

The engine was cooled by freshwater that was in turn cooled by seawater.

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It’s difficult now to imagine it, but in the United States - if not the world - in the 1940s large and reliable diesel engines were quite a recent development.

It was in only 1933 that GM showcased a pair of 600hp diesel engines at Chicago’s World Fair. There they were seen by Ralph Budd, a director of the Burlington Rock Island Railroad, who decided then and there to use them in his new streamlined train, the Pioneer Zephyr.

The decision was quite risky: the engines still needed plenty of development, but GM obliged – not only in providing them, but making them reliable.

The train was a great success, not least because of its engines.

In the submarine each diesel engine direct-drove its own generator via a flexible coupling. The generators comprised two-wire, DC, shunt-wound designs using commutators. The shaft bearings used forced-lubrication, with an oil feed from the diesel engine. Cooling was by air and water.

The generators were rated at:

  • 2,650 amps at 415 volts (propulsion rating)

  • 3,600 amps at 296 volts (battery charging rating)

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The main traction motors were direct-current, compensated compound type with shunt, series, and commutating field windings. Separate excitation for the shunt field was provided by power from either battery. Cooling was by a fan which circulated air through cores cooled by water.

The traction motors were rated at 1,375hp each. Each of the two propeller shafts was driven by two of these traction motors, working through a single reduction, double helical gear. These gears comprised two pinions driven by the traction motors, with the main gear connected to the propeller shaft, giving a reduction in speed from 1300 rpm to 280 rpm.

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The submarine had two lead-acid Exide batteries, each comprising 126 cells. Each cell was about 137cm high, 38cm deep, and 53cm wide, and weighed about 750kg. The cells were each housed in separate acid-proof tanks. All cells were connected in series. Each battery was fitted with forced-air ventilation, using four fans per battery rated at 500 cfm.


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For surface operation, motor speed control was achieved by controlling the generator speed and shunt field, thus varying the voltage supplied to the traction motors.

When submerged, speed was controlled by varying the motor shunt field or by connecting the motors in differing combinations of series and parallel. Reverse operation was accomplished by reversing the direction of the flow of current in the motor armature circuit.

Control of the submarine propulsion system was achieved from one large console of interlocking levers, rheostats (variable resistors) and meters.

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The levers comprised:

- Reverser levers (2), used to change the direction of rotation of the main motors by reversing the current flow through the armatures. Each lever had three positions: AHEAD, OFF, and ASTERN.

- Starter levers (2). These levers each had five positions (SER. 1, SER. 2, etc) that varied the resistance placed in series with the armature, thus reducing starting current. As the motor increased in speed, the resistance was reduced, until the motor was spinning fast enough to be connected directly to full voltage.

- Generator levers (4). These switched the generators to battery charging or traction motor loads. An extra mechanical latch on each lever prevented accidental movement from the ‘off’ position.

- Battery selector lever. This allowed the aft, forward or both batteries to be connected to the battery bus. In the ‘both’ position, the batteries were connected in parallel. In addition, this lever allowed the generators to be connected to either or both batteries.

- Bus selector lever. This allowed connections between port and starboard motor buses, the battery bus to be connected with the motor bus, and the operation of all four motors in series.

One example of the control approach is shown below. This is for quick reversing of the submarine:

1. Turn motor field rheostat to maximum.

2. Reduce engine speed to minimum.

3. Turn generator field rheostat to minimum but do not open.

4. Move starter lever to STOP.

5. Move reverser lever to BACK.

6. Move starter lever to SER. 1 and then to SER. 2 and SER. 3.

7. Increase engine speed.

8. Increase generator speed.


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The propulsion system of the USS Bowfin is genuinely impressive – and especially impressive considering the complete lack of electronic computers, ‘smart’ control systems, high current power switching electronic devices, and so on. It’s easy to be blasé until you read the meters and see how they’re rated in kilo-amps – and kilo-amps at no less than 415 volts DC!

For an extraordinarily detailed look at US WWII submarine technology, go to


USS Bowfin (SS-287)

CLASS: Balao

CONSTRUCTED: Portsmouth Navy Yard, Portsmouth, New Hampshire (actually located in Kittery, Maine)

KEEL LAID: 23 July 1942


COMPLEMENT: Originally 70 men (7 officers & 63 enlisted), later increased to 80 men (8 officers & 72 enlisted)

DISPLACEMENT: 1,810 tons (diving trim); 2,415 tons (submerged)

LENGTH: 311 feet 9 13/16 inches

MAXIMUM BEAM: 27 feet 3 inches

MEAN DRAFT: 15 feet 3 inches (diving trim)

FREEBOARD: 12 feet 5 inches @ bow (diving trim), 3 feet 11 inches @ stern (diving trim)


TORPEDO TUBES: 6 bow and 4 stern (MK 39)

TORPEDOES: 24 total (14 in reload racks) -- MK 14-3A, MK 18, MK 23, and MK 27

GUNS: Originally, one 4 inch 50-caliber (MK 12) and two 20 mms (MK 5). Finally, one 5 inch 25-caliber (MK 40), one 40 mm (MK 3) and one 20 mm (MK 10).

MAIN ENGINES: Four General Motors Model 16-278A diesel; 16 cylinder V-type, 2-cycle, 8 3/4 bore x 10 1/2 stroke; 1,600 BHP developed @ 750 RPM (stbd. RH rotation, port LH rotation).

MAIN GENERATORS: Four General Electric 1,100 kw; 2,650 amps/415 volts (propulsion rating); 3,600 amps/296 volts (battery charging rating).

MAIN MOTORS: Four General Electric 1,375 HP; two-wire, direct-current, compensated compound/shunt, series and commutating field windings.

GEARS: Combining and reduction type/reverse by reversing input.

BATTERIES: Two 126-cell Exide type


MAXIMUM SPEED: 20.25 knots surface speed; 8.75 knots submerged speed

PROPELLERS: Two four-bladed, 8 foot diameter

RUDDER: One balanced streamlined type. Train limits 38 degrees to port and starboard.

FUEL OIL CAPACITY: 54,000 gallons normal; 116,000 gallons maximum

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