The straight-eight engine or inline-eight engine 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, overhead valve and overhead cam configurations.
A straight-8 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-8 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, the straight-8 does not produce unpleasant odd-order harmonic vibration in the vehicle's driveline at low engine speeds.
The smooth running characteristics of the straight-8 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, could 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 and sturdier V8 engine configuration.
Early period (1903-1914)
The first straight-eight was conceived by Charron, Girardot et Voigt (CGV) in 1903, but never built.<ref name=Georgano>Georgano</ref> Great strides were made during World War I, as Mercedes and BMW made straight-eight aircraft engines. Advantages of the straight-8 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.
Inter-war period (1919-1941)
Italy's Isotta-Fraschini delivered the first production automobile straight-eight in 1920, and the Duesenberg brothers introduced their first production straight-eight in 1921.<ref name=Daniels48>Daniels, p.48</ref> Leyland Motors introduced their OHC straight-8 powered Leyland Eight luxury car at the International Motor Exhibition at Olympia, London in 1920.<ref name=WelshMotorSport>Welsh Motor Sport - Cars</ref><ref name=HistomobileLeyland>Histomobile: Leyland - 1920s</ref>
Straight-8 engines were used in expensive luxury and performance vehicles until World War II. Bugattis and Duesenbergs commonly used double overhead cam straight-8 engines. Other notable straight-8-powered automobiles were built by Mercedes-Benz, Isotta-Fraschini, Alfa Romeo, Stutz, Auburn, Packard, and Cord. One marketing feature of these engines was their impressive length - some of the Duesenberg engines were over 4 feet (1.2 m) long, resulting in the long hood (bonnet) found on these automobiles.
During the early 1930s, the Buick, Oldsmobile, and Pontiac divisions of General Motors all introduced straight-8 engines for the premium vehicles in their respective lines. The Buick straight-8 engine was an overhead valve design, while the Oldsmobile and Pontiac straight-8s were flathead engines. Chevrolet, as an entry-level marque, did not have a straight-8, and Cadillac stayed with their traditional V8 engines. In order to have engines as smooth as the straight-8s 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-8, using V8 engines in their entry-level cars and V8 or V12 engines in their Lincoln luxury cars from the 1930s on. Chrysler used a flathead straight-8 in its premium cars, including the Imperial luxury model.
Several fast and expensive straight-8 automobile brands, including Auburn, Cord and Duesenberg eventually became part of the Cord Automobile empire, which collapsed in 1937, a victim of the Great Depression.
After World War II, changes in the automobile market resulted in the decline and final extinction of the straight-8 as an automobile engine. The primary users of the straight-8 were American luxury and premium cars that were carried over from before the war. A Flxible inter-city bus used the Buick straight-8.
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-8 engines.
Oldsmobile replaced their straight-8 with a V8 engine in 1949, at which time Cadillac also introduced their first overhead valve V8. Buick introduced a 322 cubic inch (5.3 liter) V8 in 1953, which coincidentally had the same displacement as their straight-8, the latter being produced until the end of the 1953 model year. Pontiac maintained production on their straight-8, 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-8 with its zeneth also at the end of 1954.<ref>Hemmings Classic Car Volume 6 issue 5, February 2010 page 39</ref>
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.<ref>Daniels, pp. 99-103, 116-117</ref>
In Europe, many automobile factories had been destroyed during World War II, and it took many years before war-devastated economies recovered enough to make large cars popular again. The change in the design of cars from a long engine compartment between separate fenders to the modern configuration with its shorter engine compartment quickly led to the demise of the straight-8 engine. As a result, four and six cylinder engines powered the majority of cars in Europe, and the few eight-cylinder cars produced were in the V8 configuration.<ref>Daniels, pp. 103-113, 119-135</ref>
Performance and racing cars
Despite the shortcomings of length, weight, bearing friction, and torsional vibrations that led to the straight-8's post-war demise, the straight-8 was the performance engine design of choice from the late 1920s to the late 1940s, and continued to excel in motorsport until the mid-1950s. Bugatti, Duesenberg, Alfa Romeo, Mercedes-Benz and Miller built successful racing cars with high-performance double overhead camshaft straight-8 engines in the 1920s and 1930s.
The Duesenberg brothers introduced the first successful straight-8 racing engine in 1920, when their 3 litre engine placed third, fourth and sixth at the Indianapolis 500. The following year one of their cars won the French Grand Prix, while two others placed fourth and sixth in the race. Based on work the company had done on 16 cylinder aircraft engines during World War I, the overhead camshaft, three-valve-per-cylinder engine produced 115 brake horsepower (86 kW) at 4,250 rpm, and was capable of revving to an astonishing (at the time) 5,000 rpm. No Grand Prix engine before the war had peaked at more than 3,000 rpm.<ref>Ludvigsen, pp.22–25</ref>
Bugatti experimented with straight-8 engines from 1922, but in 1924, he introduced the 2 litre 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 WWI, and like them, his engine was a high-revving overhead camshaft unit with three valves per cylinder. It produced 100 bhp (75 kW) 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.<ref>Ludvigsen, pp. 26-29.</ref>
Alfa Romeo were the first to react to the engineering problems of the straight-8: in their racing car engines for the P2 and P3 and in their Alfa Romeo 8C 2300/2600/2900 sports cars of Mille Miglia and Le Mans fame the camshaft drive had been moved to the engine centre, between cylinders # 4 and 5, thus reducing the aforementioned limitations. The straight-8 was actually built as a symmetrical pair of straight-4 engines joined in the middle at common gear trains for the camshafts and superchargers. It had two overhead camshafts, but only two valves per cylinder.<ref>Ludvigsen, pp. 34-37.</ref>
The Alfa Romeo straight-8 would return after World War II to dominate the first season of Formula One racing in 1950, and to win the second season against competition from Ferrari's V12-powered car in 1951. The Alfa Romeo 158/159 Alfetta was originally designed in 1937 and won 47 of 54 Grands Prix entered between 1938 and 1951 (with a six-year gap in the middle caused by the war). By 1951, their 1.5 litre supercharged engines could produce 425 bhp (317 kW) at 9,300 rpm, and could rev as high as 10,500 rpm. However, the engines were at the end of their potential, and rule changes for the 1952 season made the Alfettas obsolete.<ref>Ludvigsen, pp. 67-69.</ref>
Mercedes-Benz would create the last notable straight-8 racing cars in 1955, with the championship-winning W196 Formula One racing car and the 300SLR sports racing car. The 300SLR was famous for Stirling Moss and Denis Jenkinson's victory in the 1955 Mille Miglia, but notorious for Pierre Levegh's notorious accident at the 1955 24 Hours of Le Mans. The 300SLR was the final development of the Alfa Romeo design of the early 1930s as not only the camshaft, but now also the gearbox was driven from the engine's centre. Engineers calculated that torsional stresses would be too high if they took power from the end of the long crankshaft, so they put a central gear train in the middle (which also ran the dual camshafts, dual magnetos, and other accessories) and ran a drive shaft to the clutch housing at the rear.<ref>Ludvigsen, pp. 94-79.</ref>