thumb|upright=1.36|[[Delage straight-eight racing engine]]
thumb|Straight-eight engine with firing order 1-4-7-3-8-5-2-6
The straight-eight engine (also known as a inline-eight engine; abbreviated as I8) is an eight-cylinder internal combustion engine with all eight cylinders mounted in a straight line along the crankcase. The type has been produced in side-valve, IOE, overhead-valve, sleeve-valve, and overhead-cam configurations.
A straight-eight can be timed for inherent primary and secondary balance, with no unbalanced primary or secondary forces or moments. However, crankshaft torsional vibration, present to some degree in all engines, is sufficient to require the use of a harmonic damper at the accessory end of the crankshaft. Without such damping, fatigue cracking near the rear main bearing journal may occur, leading to engine failure.
Although an inline six-cylinder engine can also be timed for inherent primary and secondary balance, a straight-eight develops more power strokes per revolution and, as a result, will run more smoothly under load than an inline six. Also, due to the even number of power strokes per revolution, a straight-eight does not produce unpleasant odd-order harmonic vibration in the vehicle's driveline at low engine speeds.
The smooth running characteristics of the straight-eight made it popular in luxury and racing cars of the past. However, the engine's length demanded the use of a long engine compartment, making the basic design unacceptable in modern vehicles. Also, due to the length of the engine, torsional vibration in both crankshaft and camshaft can adversely affect reliability and performance at high speeds. In particular, a phenomenon referred to as "crankshaft whip," caused by the effects of centrifugal force on the crank throws at high engine rpm, can cause physical contact between the connecting rods and crankcase walls, leading to the engine's destruction. As a result, the design has been displaced almost completely by the shorter V8 engine configuration.
Early period (1903–1918)
The first straight-eight was conceived by Charron, Girardot et Voigt (CGV) in 1903, but never built. Great strides were made during World War I, as Mercedes made straight-eight aircraft engines like the Mercedes D.IV. Advantages of the straight-eight engine for aircraft applications included the aerodynamic efficiency of the long, narrow configuration, and the inherent balance of the engine making counterweights on the crankshaft unnecessary. The disadvantages of crank and camshaft twisting were not considered at this time, since aircraft engines of the time ran at low speeds to keep propeller tip speed below the speed of sound.
Unlike the V8 engine configuration, examples of which were used in De Dion-Bouton, Scripps-Booth, and Cadillac automobiles by 1914, no straight-eight engines were used in production cars before 1920.
Inter-war period (1919–1941)
Luxury automobiles
thumb|upright=1.14|[[Dual overhead camshaft Duesenberg Model J engine]]
Italy's Isotta Fraschini introduced the first production automobile straight-eight in their Tipo 8 at the Paris Salon in 1919 Leyland Motors introduced their OHC straight-eight powered Leyland Eight luxury car at the International Motor Exhibition at Olympia, London in 1920. The Duesenberg brothers introduced their first production straight-eight in 1921.
Straight-eight engines were used in expensive luxury and performance vehicles until after World War II. Bugattis and Duesenbergs commonly used double overhead cam straight-eight engines. Other notable straight-eight-powered automobiles were built by Daimler, Mercedes-Benz, Isotta Fraschini, Alfa Romeo, Stutz, Stearns-Knight and Packard. One marketing feature of these engines was their impressive length — some of the Duesenberg engines were over long, resulting in the long hoods (bonnets) found on these automobiles.
Premium automobiles in the United States
In the United States in the 1920s, automobile manufacturers, including Hupmobile (1925), Chandler (1926), Marmon (1927), Gardner (1925), Kissel (1925), Locomobile (1925) and Auburn (1925) began using straight-eight engines in cars targeted at the middle class. Engine manufacturer Lycoming built straight-eight engines for sale to automobile manufacturers, including Gardner, Auburn, Kissel, and Locomobile. Hupmobile built their own engine. Lycoming was purchased by Auburn owner Errett Lobban Cord, who used a Lycoming straight-eight in his front-drive Cord L-29 automobile, and had Lycoming build the straight-eight engine for the Duesenberg Model J, which had been designed by the Duesenberg brothers for the Cord-owned Duesenberg Inc. The automobile manufacturers within the Cord Corporation, comprising Auburn, Cord, and Duesenberg, were shut down in 1937. Lycoming continues to this day as an aircraft engine manufacturer.
thumb|[[REO Motor Car Company|REO Straight 8 from 1931-1934]]In the late 1920s, volume sellers Hudson and Studebaker introduced straight-eight engines for the premium vehicles in their respective lines. They were followed in the early 1930s by Nash (with a dual-ignition unit), REO, and the Buick, Oldsmobile, and Pontiac divisions of General Motors.
The Buick straight-eight was an overhead valve design, while the Oldsmobile straight-8 and Pontiac straight-8 straight-eights were flathead engines. Chevrolet, as an entry-level marque, did not have a straight-eight. Cadillac, the luxury brand of General Motors, stayed with their traditional V8 engines. In order to have engines as smooth as the straight-eights of its competitors, Cadillac introduced the crossplane crankshaft for its V8, and added V12 and V16 engines to the top of its lineup.
Ford never adopted the straight-eight; their entry-level Ford cars used flathead V8 engines until the 1950s while their Lincoln luxury cars used V8 from the 1930s to the 1980s and V12 engines in the 1930s and 1940s. Chrysler used flathead straight-eights in its premium Chrysler cars, including the Imperial luxury model.
Airships
The British R101 rigid airship was fitted with five Beardmore Tornado Mk I inline eight-cylinder diesel engines. These engines were intended to give an output of at 1,000 rpm but in practice had a continuous output rating of only at 900 rpm.
Post-war
After World War II, changes in the automobile market resulted in the decline and eventual extinction of the straight-eight as an automobile engine. The primary users of the straight-eight were American luxury and premium cars that were carried over from before the war. A Flxible inter-city bus used the Buick straight-eight.
During World War II, improvements in the refinery technology used to produce aviation gasoline resulted in the availability of large amounts of inexpensive high octane gasoline. Engines could be designed with higher compression ratios to take advantage of high-octane gasoline. This led to more highly stressed engines which amplified the limitations of the long crankshaft and camshaft in the straight-eight engines.
thumb|upright=1.14|1940s [[flathead engine|flathead Oldsmobile straight-8 engine]]
Oldsmobile replaced their straight-eight flathead engine with its famous overhead valve Rocket V8 in 1949. Chrysler replaced its straight-eight with its famous Hemi V-8 for 1951. Hudson retired its straight-eight at the end of the 1952 model year. Buick introduced a V8 in 1953, with similar displacement as their straight-8, which was produced until the end of the 1953 model year. Pontiac maintained production on their straight-eight, as well as a L-head inline six, through the end of the 1954 model year, after which a V8 became standard. Packard ended production of their signature straight-eight at the end of 1954, replacing it with an overhead valve V8.
By the end of the 1970s overhead valve V8s powered 80% of automobiles built in the US, and most of the rest had six-cylinder engines.
Bugatti experimented with straight-eight engines from 1922, and in 1924, he introduced the 2 L Bugatti Type 35, one of the most successful racing cars of all time, which eventually won over 1000 races. Like the Duesenbergs, Bugatti got his ideas from building aircraft engines during World War I, and like them, his engine was a high-revving overhead camshaft unit with three valves per cylinder. It produced at 5,000 rpm and could be revved to over 6,000 rpm. Nearly 400 of the Type 35 and its derivatives were produced, an all-time record for Grand Prix motor racing.