Astraight-four engine (also referred to as aninline-four engine) is a four-cylinderpiston engine where cylinders are arranged in a line along a common crankshaft.
The majority of automotive four-cylinder engines use a straight-four layout[1]: pp. 13–16 (with the exceptions of theflat-four engines produced by Subaru and Porsche)[2] and the layout is also very common in motorcycles and other machinery. Therefore the term "four-cylinder engine" is usually synonymous with straight-four engines. When a straight-four engine is installed at an inclined angle (instead of with the cylinders oriented vertically), it is sometimes called aslant-four.
Between 2005 and 2008, the proportion of new vehicles sold in the United States with four-cylinder engines rose from 30% to 47%.[3][4] By the 2020 model year, the share for light-duty vehicles had risen to 59%.[5]
A four-stroke straight-four engine always has a cylinder on its power stroke, unlike engines with fewer cylinders where there is no power stroke occurring at certain times. Compared with aV4 engine or aflat-four engine, a straight-four engine only has onecylinder head, which reduces complexity and production cost.
Petrol straight-four engines used in modern production cars typically have a displacement of 1.3–2.5 L (79–153 cu in), but larger engines have been used in the past, for example the 1927–1931Bentley 4½ Litre.
Diesel engines have been produced in larger displacements, such as a 3.2 L turbocharged Mitsubishi engine (used the Pajero/Shogun/Montero SUV) and a 3.0 L Toyota engine. European and Asian trucks with agross vehicle weight rating between 7.5 and 18 tonnes commonly use inline four-cylinder diesel engines with displacements around 5 litres.[6][7][8][9][10][11][12] Larger displacements are found in locomotive, marine and stationary engines.
Displacement can also be very small, as found inkei cars sold in Japan. Several of these engines had four cylinders at a time when regulations dictated a maximum displacement of 550 cc; the maximum size is currently at 660 cc.
Straight-four engines with the preferred crankshaft configuration have perfectprimary balance.[1]: p. 12 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 asecondary 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 four-stroke 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 effectgrows quadratically with engine speed (rpm).
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 delivery of power results in more vibrations than engines with more than four cylinders.
Abalance 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 identicaleccentric weights that rotate in opposite directions at twice the crankshaft's speed.[1]: pp. 42–44 This system was patented by Mitsubishi Motors in the 1970s, introduced in theMitsubishi Astron engine with the "Silent Shaft" name, and has since been used under licence by several other companies.[13][14]
Not all large displacement straight-four engines have used balance shafts, however. Examples of relatively large engines without balance shafts include the 2.4 litreCitroën DS engine, the 2.6 litreAustin-Healey 100 engine, the 3.3 LFord Model A (1927) engine and the 2.5 LGM Iron Duke engine. Soviet/RussianGAZ Volga andUAZ 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.[1]: pp. 40–44
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 1963–1967Honda 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 1999–2019Mitsubishi 4M41 diesel engine which was used in the Mitsubishi Pajero and has a displacement of 3.2 L (195 cu in).[15][16]
Significant straight-four car engines include:
Many early racing cars used straight-four engines, however the Peugeot engine which won the1913 Indianapolis 500 was a highly influential engine. Designed byErnest Henry, this engine haddouble overhead camshafts (DOHC) with four valves per cylinder, a layout that would become the standard until today for racing inline-four engines.[19]: pp. 14–17
Amongst the engines inspired by the Peugeot design was theMiller engine, which was a successful racing engine through the 1920s and early 1930s. The Miller engine evolved into theOffenhauser engine which had a highly successful spanning from 1933 until 1981, including five straight victories at the Indianapolis 500 from 1971 to 1976.[19]: pp. 182–185
Many cars produced for the pre-WWIIvoituretteGrand Prix motor racing category used inline-four engine designs. 1.5 Lsupercharged engines found their way into cars such as theMaserati 4CL and variousEnglish Racing Automobiles (ERA) models. These were resurrected after the war, and formed the foundation of what was later to becomeFormula 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-fourFerrari engine designed byAurelio Lampredi. This engine was originally designed as a 2 LFormula 2 engine for the Ferrari 500, but evolved to 2.5 L to compete inFormula One in the Ferrari 625.[19]: pp. 78–81, 86–89 Forsports car racing, capacity was increased up to 3.4 L for the Ferrari 860 Monza.
TheCoventry 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 inCooper's chassis in 1959 and 1960.[19]: pp. 130–133
In Formula One, the 1980s were dominated by the 1,500 cc turbocharged cars. TheBMW 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 (950 kW) in qualifying trim, at 5.5 bar of turbo boost.[20]
Belgian arms manufacturerFN Herstal, which had been makingmotorcycles since 1901, began producing the first motorcycles with inline-fours in 1905.[21] TheFN Four had its engine mounted upright with the crankshaftlongitudinal. Other manufacturers that used this layout includedPierce,Henderson,Ace,Cleveland, andIndian in the United States,Nimbus in Denmark, Windhoff in Germany, andWilkinson in the United Kingdom.[22]
The first across-the-frame 4-cylinder motorcycle was the 1939 racerGilera 500 Rondine, it also had double-over-head camshafts, forced-inducting supercharger and wasliquid-cooled.[23] Modern inline-four motorcycle engines first became popular withHonda'sSOHCCB750 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 thecruiser category, the inline-four is the most common configuration because of its relatively high performance-to-cost ratio.[citation needed] All major Japanese motorcycle manufacturers offer motorcycles with inline-four engines, as doMV Agusta andBMW. BMW's earlier inline-four motorcycles were mounted horizontally along the frame, but all current four-cylinder BMW motorcycles havetransverse engines. The modernTriumph company has offered inline-four-powered motorcycles, though they were discontinued in favour oftriples.
The 2009Yamaha R1 has an inline-four engine that does not fire at even intervals of 180°. Instead, it uses acrossplane crankshaft that prevents the pistons from simultaneously reaching top dead centre. This results in bettersecondary 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 inMotoGP by theSuzuki (since2015) andYamaha (since2002) teams. In2010, 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 byHonda based on theCBR600RR with a maximum power output of 110 kW (150 hp). Starting in2019, the engines were replaced by aTriumph 765 cc (46.7 cu in)triple engine.
Inline-four engines are also used in light duty commercial vehicles such asKarsan Jest andMercedes-Benz Sprinter.
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