Straight-six engine
A straight-six engine is a piston engine with six cylinders arranged in a straight line along the crankshaft. A straight-six engine has perfect primary and secondary engine balance, resulting in fewer vibrations than other designs of six or fewer cylinders.
Until the mid-20th century, the straight-six layout was the most common design for engines with six cylinders. However, V6 engines gradually became more common in the 1970s and by the 2000s, V6 engines had replaced straight-six engines in most light automotive applications.
Due to their high and smooth torque, simplicity and reliability, weight and space, and balanced power delivery, straight-six engines are a common power source for trucks and buses.
Characteristics
In terms of packaging, straight-six engines are almost always narrower than a V6 engine or V8 engine, but longer than straight-four engines, V6s, and most V8s. Compared to V-configuration engines with similar power and displacement, the straight configuration has fewer injectors, a single head, and a single exhaust manifold, all contributing to better reliability and performance.Straight-six engines are typically produced in displacements ranging from, however engines range in size from the Benelli 750 Sei motorcycle engine to the Wärtsilä-Sulzer RTA96-C two-stroke marine diesel engine. Due to its well-balanced configuration, the straight-six can be scaled up to substantial sizes for heavy trucks, locomotives, industrial and marine use.. When two straight-six engines are mated with a common crankshaft it forms a V12 engine.
Engine balance and vibration
If an appropriate firing order is used, a straight-six engine has perfect primary and secondary engine balance. The primary balance is due to the front and rear trio of cylinders moving in pairs, thus canceling out the rocking motion present in a straight-three engine. The secondary balance is due to the crank throws being arranged in three planes offset at 120°, resulting in the non-sinusoidal forces summing to zero for all free forces until the sixth order.The engine balance characteristics of a straight-six engine compare favorably with the more common straight-four engines, V6 engines, and V8 engines which experience significant secondary dynamic imbalance, resulting in engine vibration. As engine reciprocating forces increase with the cube of piston bore, the straight-six is a preferred configuration for large truck and industrial engines.
Two-stroke engines
An even-firing six-cylinder two-stroke engine requires ignitions at 60° intervals, or else it would run with simultaneous ignitions and be no smoother than a triple in power delivery. As such, it also requires crank throws at 60°. Such designs appear to have been limited to diesel engines such as the Detroit 71 series, marine engines, and outboard motors.Some of the 120 possible crankshaft configurations have useful properties. Still, all of them have a rocking imbalance that may or may not require a balance shaft, depending on the application. The six pistons with six unique phases cannot be "paired" as in the four-stroke case. The Detroit engines used a configuration that, once the primary rocking couple was balanced out, was also perfectly balanced at all other rocking couples until the 6th order. Mercury came to use a configuration that canceled only the primary rocking couple and was run without a balancer.
The reciprocating masses of all configurations are still imbalanced at only 6th-order and up in their plane of motion. Still, the balance of kinetic energy exchange between pistons has improved to a residual 6th-order-and-up inertial torque oscillation compared with the four-stroke design being imbalanced at 3rd-order and up.
Crankshaft
s for straight-six engines usually have either four main bearings or seven main bearings :- Large displacement and diesel engines typically use seven bearings to minimize crankshaft flex. When an engine is subject to high loads at low speeds, the greater distance between main bearings causes increased crankshaft flex. Modern high-compression engines subject the crankshaft to greater bending loads from higher peak gas pressures, requiring the crank throws to have more significant support from adjacent bearings. So, it is common for straight-six engines to use seven main bearings.
- Smaller and high-performance engines typically use four bearings since having fewer main journals decreases friction and increases the torsional stiffness of the crankshaft. The lack of torsional stiffness can make the seven main bearing designs susceptible to torsional flex and potential breakage at high engine speeds. Any torsional flex in the crankshaft is compounded by the torsional flex of the camshafts for the rear cylinders since the camshafts are rather long and subject to torsional flex. At high engine speeds, the combination of camshaft and crankshaft flex results in inaccurate timing of the valve openings, which, in the worst case, can cause the valves and pistons to collide with catastrophic results.
Usage in cars
Since 2017, the trend of switching to V6 engines has reversed due to the ability to create a modular engine family of straight engines sharing many components. Examples include the 2017–present Mercedes-Benz M256 engine, the 2019–present Jaguar Land Rover Ingenium engines, the 2021–present Stellantis Hurricane engine and the 2022–present Mazda Skyactiv-X and Skyactiv-D engines.
Europe
Alfa Romeo's first production straight-six engine – 6.3 L flathead petrol engine – was introduced in 1921 in the Alfa Romeo G1 luxury car. An overhead valve design was introduced in the 1922 Alfa Romeo RL sports car, and an overhead camshaft design was used in the 1927 Alfa Romeo 6C sports car and various racing cars from 1927 until 1954. The last Alfa Romeo model using a straight-six was the 1961–1969 Alfa Romeo 2600 executive car before the company switched to V6 engines.Mercedes-Benz's history of straight-six engines began with the 1913 Mercedes D.I aircraft engine. The first automotive straight-six engine was the 1924–1929 Daimler M836 3.9 L petrol engine. Following World War 2, Mercedes resumed production of straight-six engines with the 1951 introduction of the Mercedes-Benz M180 overhead camshaft engine. In 1985, the Mercedes-Benz OM603 3.0 L diesel straight-six engine was introduced. In 1996, the company replaced its petrol straight-sixes with a series of V6 engines, although it continued producing diesel straight-six engines. Production of petrol straight-six engines resumed in 2017 with the introduction of the Mercedes-Benz M256 turbocharged DOHC engine.
Opel began production of straight-six engines in 1927 with a 1.8 L flathead petrol engine used by the Opel 8/40 PS. The displacement of this engine was expanded as it was used in later models such as the Opel Kapitän and Opel Admiral, with later versions switching to an overhead valve design. In 1968, the straight-six versions of the Opel CIH engine were introduced, initially using a single overhead camshaft with some later versions using double overhead camshafts. Production of the Opel CIH engine continued until 1993, when a V6 engine replaced it.
Volvo began production of straight-six engines with the 1929–1958 Penta DB flathead petrol engines. The company resumed production in 1969 with the Volvo B30 overhead valve petrol engine, followed by the straight-six versions of the Volvo Modular Engine introduced in 1995 and then the Volvo SI6 engine introduced in 2006. Several models used the uncommon design of a transversely-mounted straight-six engine. Production of Volvo straight-six engines ceased in 2015.
BMW's first product was the 1917 BMW IIIa straight-six aircraft engine. The company began production of automotive straight-six engines in 1933 with the BMW M78 petrol engine, a 1.2 L overhead valve design that evolved over the years into the BMW M337. Production of straight-six engines resumed in 1968 with the BMW M30 single overhead camshaft engine, built for 27 years and used in various models. The 1978–1989 BMW M88 engine was a double overhead camshaft design that was introduced in the BMW M1 mid-engine sport car. BMW's introduction of turbocharged straight-six engines was in 2006 BMW N54 and the production of naturally aspirated engines ceased in 2015., the BMW B58 turbocharged straight-six engine remains in production, along with its higher performance BMW S58 variants.
United Kingdom
Rolls-Royce's first straight-six engine was a 6.0 L IOE petrol engine, which was used in the 1905 Rolls-Royce 30 hp luxury car. This car was replaced by the 1906–1926 Rolls-Royce Silver Ghost, which switched to a flathead design for its straight-six engine.In 1906, the Standard Six luxury car was introduced, powered by a I6 petrol engine. Standard's engines were also used in several cars built by SS Cars and its successor Jaguar, such as the 1932 SS 1 sports car, the 1936 SS Jaguar 100 and the 1938 Jaguar 3½ Litre sports saloon/coupe and the 1948 Jaguar Mark V luxury car.
The 1927 Rover Two-litre luxury car introduced the company's IOE straight-six petrol engine. This engine was used in various Rover models until the Rover P5 was discontinued in 1973, and in various Land Rover models from the 1961 Land Rover Series IIA until 1980 Land Rover Series III.
The 1928 Austin 20/6 luxury car introduced Austin's flathead straight-six petrol engine. The 1938–1939 Austin Twenty-Eight used an enlarged version of this engine. This was replaced by the Austin D-Series engine, an overhead valve engine initially designed for trucks, which was used in passenger cars from 1947 until 1968. The overhead valve BMC C-Series was used by various BMC brands from 1954 to 1971, followed by the 2.2 L version of the BMC E-Series overhead camshaft engine, which was produced from 1970 until 1982.
The 1930–1936 Wolseley Hornet six lightweight car was powered by a 1.3–1.6 L overhead camshaft straight-six petrol engine.
The 1931–1932 MG F-type tourers, 1932–1934 MG K-type sports cars, and 1934–1936 MG N-type sports cars were powered by an overhead camshaft straight-six petrol engine.
During the mid-1930s, the Riley MPH sports car and Riley Kestrel 6 saloon were produced in small numbers and were powered by dual overhead camshaft straight-six petrol engine.
The 1947 Bristol 400 luxury car was powered by an overhead valve straight-six petrol engine based on the design of the BMW M328 engine. This engine remained in use until the Bristol 406 was discontinued in 1961.
The dual overhead camshaft Jaguar XK6 engine petrol engine was produced from 1948 to 1992 in the Jaguar XK120 sports car. Introduced as a 3.4 L, it was used in passenger and racing cars, produced in displacements of 2.4 to 4.2 L. The XK6 engine was followed by the AJ6 and AJ16 engines, produced from 1984 to 1996, before being replaced by a Ford-derived V6 engine.
The 1948–1959 Lagonda straight-6 dual overhead camshaft petrol engine was used in various Aston Martin and Lagonda cars. This engine's successor was the Tadek Marek-designed straight-six used in the DB4, DB5, DB6 and DBS.
The Ford Zephyr 6 overhead valve engine was used in the Ford Zephyr executive car and several other models from 1951 to 1966.
The Triumph I6 overhead valve straight-six petrol engine was produced from 1960 to 1977 and debuted in the Standard Vanguard Six sports saloon. The Leyland PE166 engine was loosely based on the Triumph design and was produced from 1977 to 1986.
The 1972–1977 TVR 2500M sports car was powered by the Triumph I6 engine. Then, from 1999 to 2007, TVR's own TVR Speed Six dual overhead camshaft engine was used in several of the company's sports cars.