thumb|right|200px|Un motore BMW M20B25 con la testata cilindri rimossa, in mostra i pistoni nei sei cilindri del motore.

Il '''motore sei cilindri in linea''', è un motore in cui sei cilindri sono disposti su una singola linea. 

Oltre ad essere uno tra i motori ormai più utilizzati, offre alte prestazioni, grazie alla possibilità di ottenere una maggiore potenza rispetto ad altre configurazioni, limitando però i consumi.<br />Il peso del motore viene inoltre distribuito in maniera più efficace su tutto il veicolo al prezzo però di una maggiore lunghezza del vano motore. 

I motori a sei cilindri in linea per la loro configurazione risultano essere fra i propulsori piu bilanciati ed elastici.<br />Tale fluidità è dovuta al fatto che avviene uno scoppio ogni 120 gradi angolari (nel 4 cilindri in linea ogni 180°, mentre nel sei cilindri a V lo sfasamento fra gli scoppi, che si susseguono al ritmo di uno ogni rotazione angolare, è pari a 120°, con anticipo o ritardo dell'angolo compreso nella V della struttura a gruppi di 3 cilindri, a seconda se tali scoppi avvengano in una bancata o l'altra, perdendo così in "rotondità di marcia").

Oltretutto tale peculiarità riduce drasticamente le vibrazioni e aumenta l'elasticità di marcia rendendo molto piacevole la guida, caratteristica importante per certe categorie di veicoli.<br />Nello stesso tempo tale geometria permette di incrementare le potenze specifiche e, quindi le prestazioni, molto più di quanto possa fare un motore a sei cilindri con una differente configurazione, per esempio un 6 cilindri a V, senza compromettere l'affidabilita.<br />Tanto per fare un esempio, il motore sei cilindri in linea è perfettamente bilanciato per quanto riguarda le forze alterne e i momenti ai quali è sottoposto il telaio e non necessita di masse equilibranti o smorzatori che assorbono energia.

thumb|300px|right|A [[BMW M20B25 engine with the cylinder head removed, showing the pistons in the six cylinders of the engine.]]
The '''straight-6''' or '''inline-6''' engine (often abbreviated '''I6''' or '''L6''') is a six cylinder cylinders mounted in a straight line 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-6'''. The straight-6 layout is the simplest engine layout which is in both primary and secondary mechanical engine balance, resulting in relatively low manufacturing cost combined with much less vibration than engines with fewer cylinders.<ref>
{{cite book
  | last = Nunney
  | first = M J
  | title = Light and Heavy Vehicle Technology, Fourth Edition
  | publisher = Butterworth-Heinemann
  | date = 2006
  | pages = pp. 12-13
  | isbn = 0750680377}}
</ref>

==Displacement range==
Usually a straight-6 is used for CID). It is also sometimes used for smaller  engines but these, although very smooth running, tended to be rather expensive to manufacture and they are inevitably physically longer than alternative layouts. The smallest production straight-6 was found in the Scania diesel engine used in heavy vehicles.<ref>Nunney, ''op. cit.'', p. 4.</ref> The largest are used to power ships.  They use diesel fuel and have displacements as high as  per cylinder.<ref>[http://people.bath.ac.uk/ccsshb/12cyl/ The Most Powerful Diesel Engine in the World<!-- Bot generated title -->]</ref>

==Modern trends==
Historically, straight-6 engines were introduced much earlier than front-engine/rear-wheel drive vehicles, but the modern move to the more space-efficient front-engine/front-wheel drive and transverse engine ("east-west") configurations in smaller cars caused the much shorter length of the V6 to become a major advantage. As a result, in recent decades automobile manufacturers have replaced most of their straight-6 engines (and many of their V8s) with V6 engines.<ref>Nunney, ''op. cit.'',p. 14</ref> 

Exceptions to the shift to V6 engines include Ford Falcon, which still uses a straight-6 configuration. Straight-6s also continue to be commonly used in medium to large trucks, and Vortec 4200 as part of the modular straight-4, straight-5 and straight-6 GM Atlas engine line.

==Balance and smoothness==
Straight-6 balance and require no balance shafts. They are in primary balance because the two ends of the engines are mirror images of each other and the cylinders move up and down in pairs, one on the compression stroke and the other on the exhaust stroke. Piston #1 balances #6, #2 balances #5, and #3 balances #4, canceling the end-to-end rocking motion that would otherwise result. (This does not apply to two-stroke cycle engines.) Secondary imbalance occurs in straight-4 engines because the two pistons on the upper 180 degrees of the crankshaft rotation move faster than the two pistons on the lower 180 degrees, creating an unequal motion. Straight-6 engines have cranks at 120 degrees to each other, so the differences in speed on different parts of the crankshaft rotation are offset by the changing number of pistons on each portion. 

Straight-4 or V6 engines without balance shafts suffer from intrinsic secondary dynamic imbalances which can cause engine vibration and noise. As a general rule, the forces arising from any dynamic imbalance increase as the square of the engine speed - that is, if the speed doubles, the vibrations will become four times worse - and as a result, they will produce much more noise and vibration at extremely high rpm. By contrast, straight-6 engines have no primary or secondary imbalances, and with carefully designed crankshaft vibration dampers to absorb their torsional vibrations, they will seem to rev quietly and effortlessly to their rpm limits. Historically, this has made them popular in European sports-luxury cars where smooth high-speed performance combined with good fuel economy is desirable. In addition, engine imbalance forces increase as the cube of piston mass - meaning that if the size of the engine doubles, the vibrations will become eight times worse - which has made the fully balanced straight-6 a preferred configuration for large truck engines as well.<ref> Nunney, ''op. cit.'', pp. 40-47.</ref>

The straight-6 is smoother than engines with a fewer number of cylinders because the power strokes of pistons partially overlap. Since each power stroke lasts 180 degrees of crankshaft rotation, while a new piston starts its power stroke every 120 degrees, there are 60 degrees of overlap on each stroke in which one piston is finishing while the next is starting. This results in a smooth delivery of power, unlike a four cylinder engine in which each piston must come to a complete stop before the next piston commences its power stroke. This makes the straight-6 engine's delivery of power much smoother than a four-cylinder engine. The fact that they are basically two straight sixes on the same crankshaft is the reason why 60°, 120°, and 180° V12 engines are considered optimal for smooth power delivery; they allow for triple overlap of the power stroke between three cylinders at all times due to the overlap from their two component straight-6 banks. Eight cylinder engines have even more power stroke overlap than six cylinder ones, but the improvement in smoothness is not as pronounced as that of sixes over fours. 

Historically, straight-8s and V8s were preferred over straight sixes for sports/luxury cars in the United States due to that country's large oil reserves and resulting low fuel costs compared to Europe, while straight-6s were preferred for economy cars. In Europe, with its higher fuel costs, smooth-running straight-6s were used for sports/luxury cars while noisier and rougher four-cylinder engines dominated the economy car market.

==Crankshaft design==
Crankshafts on 6 cylinder engines generally have either 4 main bearings or 7 main bearings.  Larger engines and diesels tend to use the latter because of high loadings and to avoid crankshaft flex.  Because of the 6 cylinder engine's smooth characteristic, there is a tendency for a driver to load the engine at low rpm.  This can produce crankshaft flex in 4 main bearing designs where the crank spans the distance of 2 cylinders between main bearings.  This distance is longer than the distance between two adjacent main bearings on a V6 with 4 mains because the V6 has cylinder bores on opposite banks which overlap significantly. In addition, modern high-compression engines subject the crankshaft to greater bending loads from higher peak gas pressures, requiring the crankthrows to have greater support from adjacent bearings, so it is now customary to design straight-6s with 7 main bearings.<ref> {{cite book
  | last = Nunney
  | first = M.J.
  | title = Light & Heavy Vehicle Technology
  | edition = 4th ed.
  | publisher =  Buterworth-Heinemann
  | date = 2007
  | pages = p. 38
  | isbn = 0-7506-8037-7}}
</ref>

However many of the more sporty high performance engines use the 4 bearing design because of better torsional stiffness (eg BMW small straight 6's, Ford's Zephyr 6).  In a 7 main bearing design the crank has two throws between each cylinder.  The accumulated length of main bearing journals and 12 crank throws, gives a relatively torsionally flexible crankshaft.  The 4 main bearing design has only 6 crank throws and 4 main journals so is much stiffer in the torsional domain.  At high rpm the lack of torsional stiffness can make the 7 main bearing design susceptible to torsional flex and potential breakage.  Note that a V12 engine can be made with the same number of crank throws as the 7 main bearing straight 6.  Another factor affecting large straight 6 engines is the front mounted timing chain which connects the camshaft(s) to the crank.  The camshafts are also quite long and subject to torsional flex as they in turn operate valves alternately near the front of the engine and near the rear.  At high rpm the camshaft(s) can flex torsionally while the crank is doing likewise.  This results in valve timing for the rear most cylinders becoming inaccurate and erratic, losing power and in extreme cases resulting in mechanical interference between valve and piston with catastrophic results.  Some designers have experimented with installing the timing chain/gears in the middle of the engine (between cylinders 3 and 4) or adding a second timing chain at the rear of the engine.  Either method can solve the problem.  

Another factor affecting the ability of the large 6 cylinder engines to achieve high rpm is the simple geometric reality of a relatively long stroke (undersquare) design.  A straight 6 is a long engine and the designer is usually encouraged to make it as short as possible while height is not usually a problem.  Hence the tendency to use a longer stroke and smaller bore than in a V engine to achieve a given capacity. By contrast, a long-stroke V engine tends to become too wide, which encourages increasing the bore rather than the stroke to increase displacement.<ref> {{cite book
  | last = Daniels
  | first = Jeff
  | title = Driving Force - The Evolution of the Car Engine
  | publisher = Haynes Publishing
  | date = 2002
  | pages = p. 100
  | isbn = 1-85960-877-9}} </ref> The typically longer stroke of the straight-6 increases crank throw and piston speed and so tends to reduce the rpm rating of the engine.

== History of the Straight 6 ==
The first inline six was produced by S.F. Edge. By 1909, there were some eighty manufacturers using it, 62 in Britain alone, including Darracq, Delaunay-Bellville, Vertex, MMC, White, Poppe, Mutel, and Ford.<ref>Georgano, G.N. ''Cars: Early and Vintage, 1886-1930''. (London: Grange-Universal, 1985).</ref>

==Straight-6 engines in Europe==
M10 four-cylinder design. In 1968 it introduced a M06 straight-6 design with the same 30-degree slant, overhead camshaft layout, and 100&nbsp;mm bore spacing as the four. It originally intended to follow up with a V8 engine line in the early 1970s, but when the M21 straight-6 diesels. In 1986 BMW introduced the M70 V12 which was essentially two 2.5&nbsp;L straight-6s on the same crankshaft. Currently, a straight-six is used in the BMW 1 Series, BMW 3 Series, BMW X3, BMW X5, and among the engines offered in the BMW 5 Series.

Daimler and Benz in 1926, the combined company produced a variety of powerful straight-6 engines, culminating in a 7&nbsp;L supercharged unit producing up to . Mercedes-Benz began the post-war era by producing straight-4s, but resumed making straight-6s in 1951 with the M130, which were the beginning of the modern era of MB straight-6s. Following that introduction, the company produced two lines of gasoline (petrol) straight-6s at any one time, a small six and a larger six, in addition to its straight-4s, OM603 a 3.0&nbsp;L diesel in 1985. In 1996 the company replaced its gasoline straight-6s with a series of 90-degree M112 V6 engines, although it continued to produce diesel straight-6s.

Volvo produced straight-6s, the Volvo B30 engine (1969-1975) also the B6304 and the B6254 engines during late 90's. As Volvo developed front-wheel drive models, they mounted their inline-6 engine transversely by using a short transaxle package and relocated engine-driven accessories. The 3.2&nbsp;L straight-6 introduced in 2006 was only slightly longer than its straight-5, achieved by moving the camshaft drive to the back of the engine and sharing the same gear train with ancillaries mounted in otherwise unused space over top of the transmission. It was short for a straight-6 and also very narrow. Volvo claims a transversely mounted inline engine leaves more crush space to protect against frontal impacts than a (shorter) transverse V6 or a longitudinally mounted inline-6.<ref> {{cite web
  | title = New six-cylinder engine from Volvo
  | publisher = Fosfor Wheels
  | date = 2006
  | url = http://wheels.fosfor.se/new-six-cylinder-engine-from-volvo/
  | accessdate = 2008-01-11}}</ref>

Admiral, Kapitän, Monza, Senator, Omega, and Commodore.

In 1959, Saab had an experimental car with two transverse straight-3 engines bolted together &mdash; the Saab Monster.

G1 and G2 models (1921-1923), RL model (1922–1927) and between 1925–1954 in Alfa Romeo 6C series road and racing cars, the 1500 version had one of the smallest straight-6 engines (1487 cc). The last Alfa Romeo model using straight-6 was Alfa Romeo 2600 (1961–1969).

==Straight-6 engines in Britain==
The straight-6 was the archetypal British engine for sports and luxury cars for many years. Rolls-Royce used straight-6 engines until changes in their design made the shorter V8 layout more suitable. Jaguar and other manufacturers built straight-6 engines from 1935 until the 1990s.

The most prominent of these was the Jaguar XK6 engine, which reportedly was developed during long nights during World War II when Jaguar founder William Lyons and his staff were on fire watch duty in the Jaguar factory in Coventry and had nothing better to do than design a new engine.<ref name="XKhistory"> {{cite web
  | title = Fifty Years of XK History
  | publisher = AutoWeb
  | date = 1998-02-25
  | url = http://autoweb.com.au/cms/A_50342/title_Fifty-Years-of-XK-History/newsarticle.html
  | accessdate = 2008-01-09}}
</ref> The result was displayed in the Jaguar XK120 at the London Motor Show in 1948. The 3.4&nbsp;L twin overhead camshaft XK6 engine engine was highly advanced compared to previous British engines, most of which were side-valve units. The Jaguar XK120 and the XK-powered Jaguar C-Type and Jaguar D-type, went on to score victories in races and rallies in the UK, Europe and North America. They dominated the 24 Hours of Le Mans during the 1950s, where Jaguar C-Types won in 1951 and 1953, and the D-Types had three more wins in 1955, 1956 and 1957. The engine design, enlarged to 3.8&nbsp;L, reached its apogee in the Jaguar E-type introduced in 1961, which was capable of . In 1964, the XK engine was again enlarged to 4.2&nbsp;L, which was considered the most powerful and refined of the series. The last XK-engined Jaguar went out of production in 1986, but some XK engined cars such as the Daimler DS420 limousine were still available into 1990s.<ref name="XKhistory"> {{cite web
  | title = Fifty Years of XK History
  | publisher = AutoWeb
  | date = 1998-02-25
  | url = http://autoweb.com.au/cms/A_50342/title_Fifty-Years-of-XK-History/newsarticle.html
  | accessdate = 2008-01-09}}
</ref> The XK6 engine was followed by the AJ6 and AJ16 engines. After Jaguar was acquired by Ford, these engines were replaced these engines with the Ford Duratec-derived Jaguar AJ-V6 engine.

MG also used a straight-6 in their MGC.

Bristol produced a straight-6 until 1961, based on a BMW design, that was also used by many small automakers.

The compact Triumph straight-6 powered their high-end saloon and sports cars from the mid-1950s to the mid-1970s. It was available in 1.6, 2.0, and 2.5&nbsp;L capacities. Triumph claimed that their TR5 model was the first car in the UK to come with fuel injection as standard; the TR5 has a 2.5&nbsp;L Triumph straight-6.
Other Triumph vehicles that use the Triumph straight-6 are:
*GT6 1966-73
*Vitesse 1962–71
*The 2000 Range 1963-77
*TR6 1969-76

The Rover SD1 saloon used a Triumph designed straight-6 of 2.3 and 2.6&nbsp;L capacities.

British sports car company Speed Six, which powers its current range of cars.

Land Rover used a 2.6&nbsp;L, straight-6 from 1967 in certain series Land Rover models.

Ford UK produced a straight 6 engine for the Zephyr and Zodiac range of passenger cars from the Mk 1 of 1951 (2262 cc) through the Mk 2 (2553 cc) and Mk 3 until 1966. The straight-6 was a 4 main bearing 12 overhead valve design with a short stroke.  Rated output grew from just  in the Mk 1 to  in the Mk 3 Zodaic.

==Straight-6 engines in the United States==
Engines of this type were popular before World War II in mid-range cars. Most manufacturers started building straight-6 engines when cars grew too large for the straight-4.

After World War II, larger cars required larger engines, and buyers of larger cars tended to prefer V8s; performance sixes such as the Hudson Hornet  engine were exceptions to the rule, and were not often top sellers although it became one of the hottest cars on the road and dominated stock car racing (NASCAR) in the early Fifties.<ref>[http://classified.independent.co.uk/cars/article2667213.ece Roberts, Andrew. "Classic Cars: The Hornet", The Independent, 19 June 2007]. Retrieved on January 10 2008.</ref>. 