Beginning
In 1901, 20 year-old William S. Harley drew up plans for a small engine with a displacement of 7.07 cubic inches (116 cc) and four-inch (102 mm) flywheels. The engine was designed for use in a regular pedal-bicycle frame. Over the next two years, Harley and his childhood friend Arthur Davidson worked on their motor-bicycle using the northside Milwaukeemachine shop at the home of their friend, Henry Melk. It was finished in 1903 with the help of Arthur's brother, Walter Davidson. Upon testing their power-cycle, Harley and the Davidson brothers found it unable to climb the hills around Milwaukee without pedal assistance. They quickly wrote off their first motor-bicycle as a valuable learning experiment.
Work immediately began on a new and improved second-generation machine. This first "real" Harley-Davidson motorcycle had a bigger engine of 24.74 cubic inches (405 cc) with 9.75 inches (25 cm) flywheels weighing 28 lb (13 kg). The machine's advanced loop-frame pattern was similar to the 1903 Milwaukee Merkel motorcycle (designed by Joseph Merkel, later of Flying Merkel fame). The bigger engine and loop-frame design took it out of the motorized bicycle category and marked the path to future motorcycle designs. The boys also received help with their bigger engine from outboard motor pioneer Ole Evinrude, who was then building gas engines of his own design for automotive use on Milwaukee's Lake Street.
The classic Harley-Davidson engines are V-twin engines, each with a 45° angle between the cylinders. The crankshaft has a single pin, and both pistons are connected to this pin through their connecting rods.
This 45° angle is covered under several United States patents and is an engineering tradeoff that allows a large, high-torque engine in a relatively small space. It causes the cylinders to fire at uneven intervals and produces the choppy "potato-potato" sound so strongly linked to the Harley-Davidson brand.
To simplify the engine and reduce costs, the V-twin ignition was designed to operate with a single set of points and no distributor. This is known as a dual fire ignition system, causing both spark plugs to fire regardless of which cylinder was on its compression stroke, with the other spark plug firing on its cylinder's exhaust stroke, effectively "wasting a spark". The exhaust note is basically a throaty growling sound with some popping. The 45° design of the engine thus creates a plug firing sequencing as such: The first cylinder fires, the second (rear) cylinder fires 315° later, then there is a 405° gap until the first cylinder fires again, giving the engine its unique sound.
Harley-Davidson has used various ignition systems throughout its history – be it the early points and condenser system, (Big Twin up to 1978 and Sportsters up to 1978), magneto ignition system used on some 1958 to 1969 Sportsters, early electronic with centrifugal mechanical advance weights, (all models 1978 and a half to 1979), or the late electronic with transistorized ignition control module, more familiarly known as the black box or the brain, (all models 1980 to present).
Starting in 1995, the company introduced Electronic Fuel Injection (EFI) as an option for the 30th anniversary edition Electra Glide. EFI became standard on all Harley-Davidson motorcycles, including Sportsters, upon the introduction of the 2007 product line.
In 1991, Harley-Davidson began to participate in the Sound Quality Working Group, founded by Orfield Labs, Bruel and Kjaer, TEAC, Yamaha, Sennheiser, SMS and Cortex. This was the nation's first group to share research on psychological acoustics. Later that year, Harley-Davidson participated in a series of sound quality studies at Orfield Labs, based on recordings taken at the Talladega Superspeedway, with the objective to lower the sound level for EU standards while analytically capturing the "Harley Sound." This research resulted in the bikes that were introduced in compliance with EU standards for 1998.
On February 1, 1994, the company filed a sound trademark application for the distinctive sound of the Harley-Davidson motorcycle engine: "The mark consists of the exhaust sound of applicant's motorcycles, produced by V-twin, common crankpin motorcycle engines when the goods are in use". Nine of Harley-Davidson's competitors filed comments opposing the application, arguing that cruiser-style motorcycles of various brands use a single-crankpin V-twin engine which produce a similar sound. These objections were followed by litigation. In June 2000, the company dropped efforts to federally register its trademark
Small V-twins
- D Model, 1929–1931, 750 cc
- R Model, 1932–1936, 750 cc
- W Model, 1937–1952, 750 cc, solo (2 wheel, frame only)
- G (Servi-Car) Model, 1932–1973, 750 cc
- K Model, 1952–1953, 750 cc
- KH Model, 1954–1956, 900 cc
- Ironhead, 1957–1971, 883 cc; 1971–1985, 1,000 cc
- Evolution, since 1986, 883 cc, 1,100 cc and 1,200 cc