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The inline-four engine or straight-four engine is a four-cylinder internal combustion engine in which the cylinders are mounted in a straight line or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft. Where it is inclined, it is sometimes called a slant-four. In a specification chart or when an abbreviation is used, an inline-four engine is listed either as I4 or L4 (for longitudinal, to avoid confusion between the digit 1 and the letter I). Sometimes an inline-four engine is referred to by its slang term, “four-banger”.
The inline-four layout is in perfect primary balance and confers a degree of mechanical simplicity that makes it popular for economy cars. However, it suffers from a secondary imbalance that causes minor vibrations in smaller engines, and more powerful vibrations as engine size and power increase. So, the more powerful engines used in larger cars are generally more complex designs with more than four cylinders.
Today almost all manufacturers of four-cylinder automobile engines produce the inline-four layout, with Subaru and Porsche 718 flat-four engines being notable exceptions, and so four-cylinder is usually synonymous with and a more widely used term than inline-four. The inline-four is the most common engine configuration in modern cars, with the V6 engine the second most popular. In the late 2000s (decade), due to stringent government regulations mandating reduced vehicle emissions and increased fuel efficiency, the proportion of new vehicles sold in the U.S. with four-cylinder engines (largely of the inline-four type) rose from 30 percent to 47 percent between 2005 and 2008, particularly in mid-size vehicles where a decreasing number of buyers have chosen the V6 performance option.
This configuration is most commonly used for petrol engine displacements up to 3.0 L. Porsche, for instance, used a 3.0 L four in its 944 S2 and 968 sports cars. Production cars with inline-fours larger than 3.0 L have included the 1927 Model A Ford (3.3 L (201 cu in)), the 1965-1980 International Harvester Scout (3.2 L (195 cu in)), the 1961-1963 Pontiac Tempest (3.2 L (195 cu in) Pontiac Trophy 4 engine) and, in smaller quantities, the 1927-1931 Bentley 4½ Litre. A few very early vehicles had inline-four engines with much larger displacements, including the 1910 Blitzen Benz (21.5 L (1,312 cu in)) and the 1911 Fiat S76 Record (28.4 L (1,733 cu in)).
Inline-four diesel engines, which are lower revving than gasoline engines, often exceed 3.0 L. Mitsubishi still employs a 3.2 L inline-four turbodiesel in its Pajero (called the Shogun or Montero in certain markets), and several manufacturers of light commercial vehicles and large four-wheel drive vehicles, such as Fiat Powertrain Technologies, Isuzu, Nissan, Tata Motors, and Toyota employ a 3.0 L inline-four diesel.
Larger inline-four engines are used in industrial applications, such as in small trucks and tractors, are often found with displacements up to about 5 L. Diesel engines for stationary, marine and locomotive use (which run at low speeds) are made in much larger sizes.
Generally, European and Asian manufacturers of trucks with a gross vehicle weight rating between 7.5 and 18 tonnes use inline four-cylinder diesel engines with displacements around 5 L. The MAN D0834 engine is a 4.6 L inline-4 with 220 hp (164 kW) and 627 lb⋅ft (850 N⋅m), which is available for the MAN TGL light-duty truck and VARIOmobil motorhomes. The Isuzu Forward is a medium-duty truck which is available with a 5.2 L inline-four engine that delivers 210 hp (157 kW) and 470 lb⋅ft (640 N⋅m). The Hino Ranger is a medium-duty truck which is available with a 5.1 L inline-four engine that delivers 175 hp (130 kW) and 465 lb⋅ft (630 N⋅m). The earlier Hino Ranger even had a 5.3 L inline-four engine.
Brunswick Marine built a 127 kW (170 bhp) 3.7 L 4-cylinder gasoline engine (designated as the "470") for their Mercruiser Inboard/outboard line. The block was formed from one half of a Ford 460 cubic inch V8 engine. This engine was produced in the 1970s and 1980s.
One of the largest inline-four engines is the MAN B&W 4K90 marine engine. This two-stroke turbo-diesel has a giant displacement of 6,489 L. This results from a massive 0.9 meter bore and 2.5 meter stroke. The 4K90 engine develops 18,280 kW (24,854 PS; 24,514 hp) at 94 rpm and weighs 787 tons.
The largest on-road inline-4 cylinder turbo-diesel engine is the Detroit Diesel Series 50, with a displacement of 8.5 L. It is widely used in various applications such as buses, trucks, and more. Power ratings varied from 250 hp to 350 hp. It was manufactured from 1994, until 2005. The Series 50 was also marketed as the Series 50G, for its CNG and LNG versions.
Displacement can also be very small, as found in kei cars sold in Japan, such as the Subaru EN series; engines that started out at 550 cc and are currently at 660 cc, with variable valve timing, DOHC and superchargers resulting in engines that often claim the legal maximum of 64 PS (47 kW; 63 bhp). The 1.2 L, turbocharged, direct-injected, Toyota 8NR-FTS engine has a maximum power output of 114 hp (85 kW) and a maximum torque of 190 Nm (140 lbft) at a low rpm (1500 rpm).
A 4-stroke straight-four engine always has a cylinder on its power stroke, unlike engines with fewer cylinders where there are is no power stroke occurring at certain times. Compared with a V4 engine or a flat-four engine, a straight-four engine only has one cylinder head, which reduces complexity and production cost.
Primary and secondary balanceEdit
Straight-four engines with the preferred crankshaft configuration have perfect primary balance. This is because the pistons are moving in pairs, and one pair of pistons is always moving up at the same time as the other pair is moving down.
However, straight-four engines have a secondary imbalance. This is caused by the acceleration/deceleration of the pistons during the top half of the crankshaft rotation being greater than that of the pistons in the bottom half of the crankshaft rotation (because the connecting rods are not infinitely long). As a result, two pistons are always accelerating faster in one direction, while the other two are accelerating more slowly in the other direction, which leads to a secondary dynamic imbalance that causes an up-and-down vibration at twice crankshaft speed. This imbalance is common among all piston engines, but the effect is particularly strong on inline-four because of the two pistons always moving together.
The strength of this imbalance is determined by the reciprocating mass, the ratio of connecting rod length to stroke, and the peak piston velocity. Therefore, small displacement engines with light pistons show little effect, and racing engines use long connecting rods. However, the effect grows exponentially with engine speed (RPM).
Pulsations in power deliveryEdit
Four-stroke engines with five or more cylinders are able to have at least one cylinder performing its power stroke at any given point in time. However, four-cylinder engines have gaps in the power delivery, since each cylinder completes its power stroke before the next piston starts a new power stroke. This pulsating pulsating delivery of power results in more vibrations than engines with more than four cylinders.
Usage of balance shaftsEdit
A balance shaft system is sometimes used to reduce the vibrations created by a straight-four engine, most often in engines with larger displacements. The balance shaft system was invented in 1911 and consists of two shafts carrying identical eccentric weights that rotate in opposite directions at twice the crankshaft's speed. This system was patented by Mitsubishi Motors in the 1970s and has since been used under licence by several other companies.
Not all large displacement straight-four engines have used balance shafts, however. Examples of relatively large engines without balance shafts include the 2.4 litre Citroën DS engine, the 2.6 litre Austin-Healey 100 engine, the 3.3 L Ford Model A (1927) engine and the 2.5 L GM Iron Duke engine. Soviet/Russian GAZ Volga and UAZ engines with displacements of up to 2.9 litres were produced without balance shafts from the 1950s to the 1990s, however these were relatively low-revving engines which reduces the need for a balance shaft system.
Usage in production carsEdit
Most modern straight-four engines used in cars have a displacement of 1.5–2.5 L (92–153 cu in). The smallest automotive straight-four engine was used in the 1963-1967 Honda T360 kei truck and has a displacement of 356 cc (21.7 cu in), while the largest mass-produced straight-four car engine is the 1999-2019 Mitsubishi 4M41 diesel engine which was used in the Mitsubishi Pajero and has a displacement of 3.2 L (195 cu in).
Significant straight-four car engines include:
- 1954-1994 Alfa Romeo Twin Cam engine - one of the first mass-produced twin-cam engines. In 1990, it became the first production engine with variable valve timing.
- 1908-1941 Ford Model T engine - one of the most widely produced engines in the world.
- 1951-2000 BMC A-Series engine - the first engine to be used in a mass-production transverse-engined front-wheel drive car.
- 1966-2000 Fiat Twin Cam engine - one of the first mass-produced twin-cam engines, produced from 1959.
- 1968-1981 Triumph Slant-4 engine - an early multi-valve engine which formed the basis of Saab's first turbocharged engines.
- 2000-2009 Honda F20C engine - produced the highest specific output for a naturally aspirated engine of its time.
Usage in racing carsEdit
Many early racing cars used straight-four engines, however the Peugeot engine which won the 1913 Indianapolis 500 was a highly influential engine. Designed by Ernest Henry, this engine had double overhead camshafts (DOHC) with four valves per cylinder, a layout that would become the standard until today for racing inline-four engines.
Amongst the engines inspired by the Peugeot design was the Miller engine, which was a successful racing engine through the 1920s and early 1930s. The Miller engine evolved into the Offenhauser engine which had a highly successful spanning from the 1933 until 1981, including five straight victories at the Indianapolis 500 from 1971 to 1976.
Many cars produced for the pre-WWII voiturette Grand Prix motor racing category used inline-four engine designs. 1.5 L supercharged engines found their way into cars such as the Maserati 4CL and various English Racing Automobiles (ERA) models. These were resurrected after the war, and formed the foundation of what was later to become Formula One, although the straight-eight supercharged Alfettas would dominate the early years of F1.
Another engine that played an important role in racing history is the straight-four Ferrari engine designed by Aurelio Lampredi. This engine was originally designed as a 2 L Formula 2 engine for the Ferrari 500, but evolved to 2.5 L to compete in Formula One in the Ferrari 625. For sports car racing, capacity was increased up to 3.4 L for the Ferrari 860 Monza.
The Coventry Climax straight-four engine was also a very successful racing engine, which began life as a 1.5 litre Formula 2 engine. Enlarged to 2.0 litres for Formula One in 1958, it evolved into the large 2,495 cc FPF that won the Formula One championship in Cooper's chassis in 1959 and 1960.
In Formula One, the 1980s were dominated by the 1,500 cc turbocharged cars. The BMW M12/13 engine was notable for the era for its high boost pressures and performance. The cast iron block was based on a standard road car block and powered the F1 cars of Brabham, Arrows and Benetton and won the world championship in 1983. The 1986 version of the engine was said to produce about 1,300 hp (969 kW) in qualifying trim.
Usage in motorcyclesEdit
Belgian arms manufacturer FN Herstal, which had been making motorcycles since 1901, began producing the first motorcycles with inline-fours in 1905. The FN Four had its engine mounted upright with the crankshaft longitudinal. Other manufacturers that used this layout included Pierce, Henderson, Ace, Cleveland, and Indian in the United States, Nimbus in Denmark, Windhoff in Germany, and Wilkinson in the United Kingdom.
The first across-the-frame 4-cylinder motorcycle was the 1939 racer Gilera 500 Rondine, it also had double-over-head camshafts, forced-inducting supercharger and was liquid-cooled. Modern inline-four motorcycle engines first became popular with Honda's SOHC CB750 introduced in 1969, and others followed in the 1970s. Since then, the inline-four has become one of the most common engine configurations in street bikes. Outside of the cruiser category, the inline-four is the most common configuration because of its relatively high performance-to-cost ratio. All major Japanese motorcycle manufacturers offer motorcycles with inline-four engines, as do MV Agusta and BMW. BMW's earlier inline-four motorcycles were mounted horizontally along the frame, but all current four-cylinder BMW motorcycles have transverse engines. The modern Triumph company has offered inline-four-powered motorcycles, though they were discontinued in favour of triples.
The 2009 Yamaha R1 has an inline-four engine that does not fire at even intervals of 180°. Instead, it uses a crossplane crankshaft that prevents the pistons from simultaneously reaching top dead centre. This results in better secondary balance, which is particularly beneficial in the higher rpm range, and "big-bang firing order" theory says the irregular delivery of torque to the rear tire makes sliding in the corners at racing speeds easier to control.
Inline-four engines are also used in MotoGP by the Suzuki (since 2015) and Yamaha (since 2002) teams. In 2010, when the four-stroke Moto2 class was introduced, the engines for the class were a 600 cc (36.6 cu in) inline-four engine made by Honda based on the CBR600RR with a maximum power output of 110 kW (150 hp). Starting in 2019, the engines were replaced by a Triumph 765 cc (46.7 cu in) triple engine.
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