In May 1917, one month after the US had declared war on Germany, a Federal task force known as the Aircraft Production Board summoned top engine designers Jesse G. Vincent (of the Packard Motor Car Company of Detroit) and E.J. Hall (of the Hall-Scott Motor Co. of Berkeley, California,) to Washington D.C. They were given the task of designing as rapidly as possible an aircraft engine that would rival if not surpass those of Great Britain, France, and Germany. The Board specified that the engine would have a high power-to-weight ratio and be adaptable to mass production. The Board brought Vincent and Hall together on 29 May 1917 at the Willard Hotel in Washington, where the two were asked to stay until they produced a set of basic drawings. After just five days, Vincent and Hall left the Willard with a completed design for the new engine.

The Liberty L-12 was a modular design where four or six cylinders could be used in one or two banks. A single overhead camshaft for each cylinder bank operated two valves per cylinder, in an almost identical manner to the inline six-cylinder German Mercedes D.III and BMW III engines, and with each camshaft driven by a vertical driveshaft that was placed at the back of each cylinder bank, again identical to the Mercedes and BMW straight-six powerplants. Dry weight was 844 lb (383 kg). Fifty-two examples of a six-cylinder version, the Liberty L-6, which very closely resembled the Mercedes and BMW powerplants in overall appearance, were produced but not procured by the Army. A pair of the 52 engines produced were destroyed by Dr. William Christmas testing his so-called “Christmas Bullet” fighter.

In July 1917, an eight-cylinder prototype assembled by Packard’s Detroit plant arrived in Washington for testing, and in August, the 27-litre (1,649 cubic inch) water-cooled 45° V-12 12-cylinder version was tested and approved. That fall, the War Department placed an order for 22,500 Liberty engines, dividing the contract between the automobile and engine manufacturers Buick, Ford, Cadillac, Lincoln, Marmon, and Packard. Hall-Scott in California was considered too small to receive a production order. Manufacturing by multiple different factories was facilitated by its modular design. Cadillac was asked to produce Liberty engines but William Durant was a pacifist who did not want General Motors facilities to be used for producing war material. This led to Henry Leland leaving Cadillac to form the Lincoln company to make Liberty engines. He quickly gained a $10,000,000 government contract to build 6,000 engines. Subsequently the order was increased to 9000 units, with the option to produce 8000 more if the government needed them.

However, Durant later changed his mind and both Cadillac and Buick produced the engines.

Other manufacturers in the program included Packard, Ford and Marmon. Lincoln had delivered 6500 of the 400 hp, V-12, overhead camshaft engines when production ceased in 1919.

Ford was asked to supply cylinders for the new engine, and rapidly developed an improved technique for cutting and pressing steel which resulted in cylinder production rising from 151 per day to over 2,000, Ford eventually manufacturing all 433,826 cylinders produced, and 3,950 complete engines. Lincoln constructed a new plant in record time, devoted entirely to Liberty engine production, and assembled 2,000 engines in 12 months. By the time of the Armistice with Germany, the various companies had produced 13,574 Liberty engines, attaining a production rate of 150 engines per day. Production continued after the war, for a total of 20,478 engines built between July 4, 1917 and January 1919.

While the 12-cylinder prototype was completed only 28 days after the first drawings were made, more than 1000 modifications to the design had to be made between September 1917 and February 1918. Nevertheless, 1050 Liberties were delivered to Britain between March and July 1918. They were fitted in the DH.9A.

Although it is widely reported otherwise, a few Liberty engines did see action in France as power for the American version of the DeHaviland DH4.

The Polikarpov I-2 used the M-5 engine with 12 cylinders in V and 420 hp of power (copy under license of the North American Liberty L-12 engine). It stood out for the careful work on the engine hood, of which at least two types were used. The most remarkable symbol of the upper region of the hood was the pipe through which the air flow entered the two “Zenit” carburettors. Cooling was basically achieved by means of a Lamblin radiator that could vary its angle of incidence by around 70º. The control of this radiator was carried out from the cockpit. A number of I-2s were delivered with fixed radiators attached to the fuselage.

The Mikulin M-5 engine was used in the Grigorovich I-1, I-2 and MRL-1 fighters.

Variants:

V-1650
An inverted Liberty 12-A was referred to as the V-1650 and was produced up to 1926 by Packard — exactly the same designation was later applied, due to identical displacement, to the World War II Packard-built version of the Rolls-Royce Merlin.

Nuffield Liberty
The Nuffield Liberty tank engine was produced in World War II by the UK car manufacturer Nuffield. It was a 27 L (1,649 in3) engine with an output of 340 hp (250 kW), which became inadequate for the increasing vehicle weights as the war progressed, and it suffered numerous problems with cooling and reliability. It was replaced in later British tanks by the Rolls-Royce Meteor, based on their Merlin aero engine.

Allison VG-1410
The Allison VG-1410 was an air-cooled inverted Liberty L-12, with a geared super-charger and Allison epicyclic propeller reduction gear and reduced displacement. (45⁄8″x7″=1,411 cu.in.)

Liberty L-6
A 6-cylinder version of the Liberty L-12, and nicknamed the “Liberty Six”:- a single bank of cylinders, with the resulting engine bearing a strong external resemblance to both the Mercedes D.III and BMW III straight-six German aviation engines of World War I.

Liberty L-8
An 8-cylinder V engine using Liberty cylinders in banks of four at 90°.

Applications:

Airco DH.4
Airco DH.9
Caproni Ca.60
Curtiss NC
Curtiss Carrier Pigeon
Airco DH.10
Douglas C-1
Douglas DT
Douglas O-2
Fokker T.II
Handley Page H.P.20
Witteman-Lewis XNBL
RN-1 (Zodiac)
Babs (land speed record car)

Specifications:

Liberty L-12
Type: 12-cylinder liquid-cooled Vee piston aircraft engine
Bore: 5 in (127 mm)
Stroke: 7 in (178 mm)
Displacement: 1,649.3 cu in (27.03 l) in3 (27 L)
Length: 67.375 in (1,711 mm)
Width: 27 in (685.80 mm)
Height: 41.5 in (1,054.10 mm)
Dry weight: 845 lb (383.3 kg)
Valvetrain: One intake and one exhaust valves per cylinder operated via a single overhead camshaft per cylinder bank
Fuel system: Two duplex Zenith carburettors
Fuel type: Gasoline
Oil system: forced feed, rotary gear pressure and scavenge pumps, wet sump.
Cooling system: Water-cooled
Power output: 449 hp (334.8 kW) at 2,000 rpm (takeoff)
Specific power: 0.27 hp/cu in (12.4 kW/L)
Compression ratio: 5.4:1 (Army engines) 5:1 (navy engines)
Specific fuel consumption: 0.565 pt/hp/hour (0.43 l/kW/hour)
Oil consumption: 0.0199 pt/hp/hour (0.0152 l/kW/hour)
Power-to-weight ratio: 0.53 hp/lb (0.87 kW/kg)

Hall-Scott L-6 / Liberty 6
1919
6-cylinder
224hp@1700rpm to 244hp@1830rpm
825.67ci
dry wt: 546 lb