A straight-twin engine, also known as straight-two, inline-twin, vertical-twin, or parallel-twin is a two-cylinder piston engine which has its cylinders arranged side by side and its pistons connected to a common crankshaft. Compared to V-twins and flat-twins, straight-twins are more compact, simpler, and usually cheaper to make, but may generate more vibration during operation.
Straight-twin engines have been primarily used in motorcycles, but are also used in automobiles and in powersports applications. Automobiles with straight-twin engines are usually very small and include city cars and kei cars. Recent examples of cars with straight-twin engines include the Tata Nano and Fiat Group automobiles using the TwinAir engine. Powersports applications include use in outboard motors, personal water craft, all-terrain vehicles, snowmobiles, and ultralight aircraft.
Different crankshaft angles are used in four-stroke straight-twins to achieve different characteristics of firing intervals and engine balance, affecting vibrations and power delivery. The traditional British parallel twin (1937 onwards) had 360º crankshafts, while some larger Japanese twins of the 1960s adopted the 180º crankshaft. In the 1990s, new engines appeared with a 270º crankshaft.
- 1 Advantages and disadvantages
- 2 Construction
- 3 Motorcycle use
- 4 Automobile use
- 5 Marine engine use
- 6 Aviation use
- 7 Other uses
- 8 Notes
- 9 References
- 10 External links
Advantages and disadvantages
Straight-twins have the advantage of being more compact, relatively simple, and cheaper to make in comparison to V- or flat-twins. They may be prone to vibration, either because of the irregular firing interval present in 180° crank engines or the large uncountered reciprocating mass in 360° crank engines. Inline-twins also suffer further from torsional torque reactions and vibration.
Unlike V-twins, straight-twin engines do not use a common crank pin for both connecting rods, each cylinder has its own crank pin. Most vintage British straight-twin motorcycle engines, such as Triumph, BSA, Norton and Royal Enfield, had two main bearings, the exception being AJS/Matchless, which used a third, center main bearing.
Honda straight-twin engines, which began appearing in the late 1950s, had four main bearings. Subsequent engines had four or occasionally three main bearings, ball bearings being better than shell bearings for this engine configuration.
In motorcycles, as with cars and other vehicles, the terms "parallel-twin", "inline-twin", "vertical-twin" and "straight-two" are used. The term "parallel-twin" has been used to mean that the crankshaft is transverse across the frame, while "inline-twin" meant that the cylinders are arranged front to rear, in line with the direction of travel. This special meaning for "inline" has been used for motorcycles with a longitudinal crankshaft, such as the Sunbeam S7, and for tandem twins, with a transverse pair of crankshafts, but the cylinders arranged longitudinally, one in front of the other. The term "parallel twin" has also been used to refer specifically to a four-stroke straight-two engine with 360º crankshaft causing the pistons to travel parallel to each other. Other times, "parallel-twin", "inline-twin" and the other variants have been used interchangeably and treated as equivalent.
In conducting experiments with a single bank of Edward Turner's square four engine design, Turner and his supervisor, Ariel chief engineer Val Page both saw potential in the straight-twin as a motorcycle engine. From the experiments, it was shown that a 360° crank angle was better suited to the use of a single carburettor than a 180° crank angle. After leaving Ariel for the Triumph Motor Company, Page designed the Triumph 6/1 with a 650 cc 360° twin as a sidecar hauler. A 6/1 hitched to a Triumph sidecar won a International Six Days Trial silver medal and the 1933 Maudes Trophy. The decline in the sidecar market caused the 6/1 to be discontinued in 1935.
Edward Turner's 1937 Triumph Speed Twin started a trend, and up to the mid-1970s four-stroke 360º parallel-twins were the most common type of British motorcycles, being produced by Triumph, BSA, Norton, Ariel, Matchless and AJS. Italian and German manufacturers have also made parallel-twins, as had American manufacturer Indian, whose parallel-twins included the 1949 440 cc Indian Scout and the 1950 500 cc Indian Warrior. BMW and Japanese manufacturers still made them as of 2010, particularly for middleweight bikes.
In four-stroke designs, the parallel twin is usually vertical or near vertical. One exception is the only parallel-twin to win a 500cc Grand Prix, the AJS E-90 Porcupine of 1949, which had nearly horizontal cylinders.
Comparison of twins in motorcycle use
|Characteristic||Flat-twin (long.)[t_note 1]||Flat-twin (trans.)[t_note 2]||Straight-twin (long.)[t_note 1][t_note 3]||Straight-twin (trans.)[t_note 2][t_note 4]||V-twin (long.)[t_note 1]||V-twin (trans.)[t_note 2]|
|Example||Typical BMW boxer||Pre-war and inter-war Douglas||Sunbeam S7||Typical post-war big British bike, e.g. Triumph Bonneville, Norton Commando||Moto Guzzi V-twin (e.g. Moto Guzzi Le Mans), Honda CX series||Typical V-twin cruiser (e.g. Harley-Davidson, Honda Shadow) or sportbike (e.g. Ducati, Suzuki SV650)|
|Air cooling||Ideal; cylinders sticking out into the airstream, exhaust can exit to well-cooled front of cylinder||Rear cylinder out of airstream, not as well cooled as front cylinder||Rear cylinder out of airstream, not as well cooled as front cylinder||Good; both cylinders upright in the airstream, exhaust can exit to well-cooled front of cylinder||Near ideal; cylinders sticking out into the airstream, exhaust can exit to well-cooled front of cylinder||Rear cylinder out of airstream, not as well cooled as front cylinder|
|Centre of gravity||Constrained by need for cornering clearance for wide engine||Can be as low as the frame allows||Can be as low as the frame allows||Can be as low as the frame allows||Can be as low as the frame and the V angle allow||Can be as low as the frame allows|
|Wheelbase length||Short engine gives short wheelbase||Extremely wide engine gives long wheelbase||Long engine gives long wheelbase||Narrow engine gives short wheelbase||Short engine gives short wheelbase||Wide engine gives long wheelbase|
- Table notes
- For the purposes of this table, long. means crankshaft in line with the frame
- For the purposes of this table, trans. means crankshaft across the line of the frame
- This configuration has also been referred to as an "inline twin"; see Terminology section for more details on the usage.
- This configuration is one of several interpretations of the term "parallel twin"; see Terminology section for more details on the usage.
Although the rise in popularity of the large V-twin motorcycle has seen the across-the-frame parallel-twin fall out of favour, the latter retains these advantages over the former: Compared to a conventional layout V-twin:
- Siting of ancillaries (air-filter, carburettors, ignition, etc.) is simpler.
- This simpler layout can potentially make maintenance access easier.
- Provided a 270° crank is used, a four-stroke parallel twin can simulate the slightly "lumpy" feel of a four-stroke V-twin.
There are three main crankshaft configurations for this engine: 360°, 180°, and 270°: There are minor differences in the applications for four stroke and for two stroke engines, largely pertaining to ignition intervals. For example, the 360 twin is the natural configuration for a two cylinder four-stroke engine, since four piston strokes add up to 720°. The below will mostly be concerned with four-stroke engines.
- In a 360° engine, both pistons rise and fall together. The dynamic balance is identical to that of a single-cylinder engine, but with twice the number of ignition pulses. The firing order is offset, so that cylinder 2 fires 360 degrees after cylinder 1, and 360 degrees later cylinder 1 fires again at 720 total degrees, the beginning of another four-stroke cycle.
- In a 180° engine, one piston rises as the other falls. This gives good primary balance, albeit with a rocking couple; but results in irregular ignition pulses. This is because cylinder 2 fires 180 degrees after cylinder 1, and cylinder 1 does not fire again for another 540 degrees - always adding up to the 720 degrees of rotation for a four-stroke cycle.
- In a 270° engine, one piston follows three quarters of a rotation behind the other. This results in a mixture of the imbalances in the first two types and yields firing intervals identical to a 90° V-twin. Firing order here is that cylinder 2 fires 270 degrees (3/4 of a rotation) after cylinder 1, and cylinder 1 fires again 450 degrees (one and a quarter rotations) after cylinder two, again at a total 720 degrees and the beginning of the next cycle.
360° and 180°
From the 1930s, following the work of Val Page, most British four-stroke parallel-twin motorcycles used a crank angle of 360°, which allowed the use of a single carburettor (180° and 270° twins need twin carburettors), as did an early Meguro which was a copy of the 360° British BSA A7. However, in the 1960s, Japanese manufacturers favoured the 180° whose smoothness allowed more revolutions per minute and thus more power. For example, the 1966 Honda 450 cc dohc 180° parallel-twin "Black Bomber" could challenge contemporary British 650 cc 360° twins.
Many small motorcycles of less than 250 cc use a 360° crankshaft as the vibration issue was less significant; examples includes Honda's CB92, CB160, and CM185. Larger twins over 500 cc, such as the Yamaha's XS650 and TX750, have used 360° crankshafts, but such parallel twins tend to feature balance shafts. The Honda CB-series in the 250 to 500 cc range used 180° crankshafts. Both the 1973 Yamaha TX500 and the 1977 Suzuki GS400 featured a 180° crankshaft and a balance shaft, while the 1974 Kawasaki KZ400 used a 360° crankshaft and a balance shaft. The 1978 to 1984 Honda CB 250 N and CB 400 N are 360° designs, too; later Honda straight twins from 1993 onward until today are, again, 180° designs, with the exception of the new 700cc engine that power the CTX700.
A 180° crankshaft engine suffers fewer pumping losses than a 360° twin, as displacement in the crankcase stays roughly constant. However, a 180° engine requires a separate ignition system, points or otherwise, for each cylinder. The 360° twins can have a single ignition system for both cylinders, with a wasted spark on each cylinder's exhaust stroke. The BMW F800 parallel twin motorcycle is a 360° design. Inherent vibration in the BMW F800 means its engine is limited to 9,000 rpm. BMW reduced the vibration using a third "vestigial" connecting rod to act as a counterbalance.
A modern development of the straight-two engine, pioneered by the Yamaha TRX850, is the 270° crank, which imitates the sound and feel of a 90° V-twin, but requires a balance shaft to reduce vibration. Effectively, the 270° crank is a compromise which allows a more regular firing pattern than a 180° crank and less vibration than a 360° crank. As with a 90° V-twin, the pistons in a 270° inline twin engine are never both stationary at the same time, thereby reducing the net momentum exchange between the crank and pistons during a full rotation. The oscillating momentum manifests itself as an oscillating crank rotation speed, which, when paired with a driven-wheel rotating at the more steady road speed, will introduce an oscillating torque in the drivetrain and at the tyre contact patch. Examples of 270° parallel-twin motorcycles in production in 2014 include the Donnington Norton Commando, Triumph Thunderbird, and Honda CTX700.
Phil Irving undertook to minimise this oscillating torque, and for one particular connecting rod to stroke ratio, arrived at an optimal separation of 76° (294°), instead of the 90° (270°) described above. The optimum for two pistons is found when one piston is travelling fastest at the same time the other has stopped; maximum piston speed occurs when the connecting rod and crank throw are at right angles, not when the crank throw is at 90° to the cylinder bore. This minimisation of so-called inertial torque was also one of the goals Yamaha achieved with its "cross-plane" R1 engine. Note that in neither case was the oscillation completely eliminated, only reduced significantly.
In two-stroke engines, the crank angle is generally 180°, which gives two power strokes in each revolution. This configuration vibrates at twice the frequency but half the amplitude of a single cylinder engine of the same capacity.
An exception is the Yankee, which featured a 360° crankshaft. The Yankee's configuration, which featured separate combustion chambers for the two cylinders, should not be confused with that of a split-single. Another example with a 360° crankshaft is the military edition of the Jawa 350.
Engine in line with frame
The inline-twin engine design has been used often during the history of motorcycling for both two-stroke and four-stroke engines. Examples include the Dresch 500 cc Monobloc and the Sunbeam S7 and S8.
Although mounting the engine in line with the frame allows for a motorcycle as narrow as a single cylinder engine, they also create a longer engine. A significant disadvantage for air-cooled engines is that the rear cylinder runs hotter than the front cylinder. For motorcycle racing purposes, they minimise the front area of the engine and chassis allowing for a more aerodynamic and narrower front profile equivalent to a single cylinder vehicle.
Gottlieb Daimler introduced his Phoenix inline-twin engine in 1895; these engines were used in Panhard motor cars that year. Another early automotive inline-twin was used in the 1898 Decauville Voiturelle which used a pair of cylinders taken from a de Dion model mounted fore and aft and positioned below the seat.
In 1955, engineer Aurelio Lampredi designed an experimental straight-twin cylinder Formula One engine on the theory that it would provide high levels of torque for tight race circuits. The result was the 2.5-liter Type 116 prototype. Upon testing, it vibrated so much that it broke the test bench. The engine was never used in a racing car.
Straight-twin engines have been used in very small cars, e.g. microcars, kei cars, and city cars such as the Fiat 500 and 126, NSU Prinz, VAZ Oka, Dacia Lăstun, Daihatsu Cuore, and Mitsubishi Minica. From 1967 to 1972, Honda produced the N360 and its successors N400 and N600 with straight-two engines in 360 cc, 400 cc, and 600 cc sizes. The Z600 was produced from 1970 to 1972. From 1958 to 1971, Subaru produced the 360 with a rear-mounted, rear-drive 358 cc air-cooled engine.
Straight-twin petrol engines currently used in production cars include the 623 cc engine used in the Tata Nano, and the TwinAir turbocharged 875 cc engine used in the Fiat 500, the Fiat Panda, the Fiat Punto, the Lancia Ypsilon, and the Alfa Romeo MiTo.
Straight-twin diesel engines currently used in production cars include the turbocharged 1,201 cc engine, featuring common rail injection, used in the Piaggio Porter. Another straight-twin diesel engine has been under development for the Tata Nano, and according to media reports it could be launched at the beginning of 2014. Its specifications have not been disclosed, although unofficial reports claim it will have a displacement of 624 cc.
Marine engine use
From the 1950s, manufacturers of outboard motors had settled on the use of the basic inline engine design, cylinders stacked on top of each other with the crankshaft driving the propellor shaft. An experimental engine used two inline engine blocks joined in order to make a square-four engine.
In the early 20th century, gaff-rigged fishing boats such as Morecambe Bay Prawners[note 1] and Lancashire Nobbys would sometimes retrofit an inboard engine, such as the Lister or the Kelvin E2 inline-twin.[note 2] (It was often found easier to arrange the propeller shaft asymmetrically to exit the hull abeam the centreline, even though this gave a steerage imbalance).
Although modern narrowboats on the English canal system now tend to have 4-cylinder marinized automotive engines, traditionalists prefer to install the older 2-cylinder "thumper" diesels such as the air-cooled Lister Petter (which also had a 3-cylinder version).
Inline-twin engines, such as the Hirth 2704 are commonly used in Ultralight, single seat gyrocopters and small homebuilt aircraft  originally sourced from snowmobiles. Another popular inline-twin two-stroke engine was the Cuyuna 430-D, also a modified snowmobile unit which produced 30 hp (22 kW). More modern ultralights tend to use engines such as the Rotax 503 or Rotax 582 which are designed for the purpose.
- eg. the Ramsey-registered RY51 "Campania"
- Kelvin E2 3-litre 2-cylinder petrol-paraffin engine
- Taylor, Charles Fayette (19 Mar 1985). The Internal-combustion Engine in Theory and Practice: Combustion, fuels, materials, design. 2 (2 ed.). MIT Press. ISBN 0-262-70027-1.
- Nunney, M. J. J. (31 May 2012). Light and Heavy Vehicle Technology (4, illustrated, revised ed.). Taylor & Francis. ISBN 978-0-75068-037-0.
- Barger, Sonny; Holmstrom, Darwin (Jun 8, 2010). Let's Ride: Sonny Barger's Guide to Motorcycling. HarperCollins. ISBN 978-0-06196-426-8.
- Mitchel, Doug (Oct 18, 2005). "1959". Honda Motorcycles: Everything You Need to Know About Every Honda Motorcycle Ever Built. Krause Publications. pp. 12–15. ISBN 978-0-87349-966-8.
- Frank, Aaron (Jul 12, 2003). "Chapter Three: Honda Who?". In Holmstrom, Darwin. Honda Motorcycles. MotorBooks International. p. 34. ISBN 978-0-76031-077-9.
- Vandenheuvel, Cornelis (18 July 1997). Pictorial History of Japanese Motorcycles. Devon U.K.: Bay View Books. ISBN 1-8709-7997-4.
- Hunt, Phil; McKay, Malcolm; Wilson, Hugo; Robinson, James (2012), Duckworth, Mick, ed., Motorcycle: The Definitive Visual History, DK Publishing, Penguin Group, pp. 126, 210, ISBN 978-1-46540-088-8
- Wilson, Hugo (1995). "Glossary". The Encyclopedia of the Motorcycle. London: Dorling Kindersley. pp. 309–310. ISBN 0-7513-0206-6. "in-line Engine layout in which the cylinders are arranged in a row, and in-line with the wheels of the machine... parallel twin A two-cylinder engine layout in which both cylinders are side by side and mounted across the frame."
- Henshaw, Peter (Jun 15, 2008). "Super Bantam". The BSA Bantam Bible: All Models 1948 to 1971. Veloce Publishing. p. 42. ISBN 978-1-84584-159-1. Retrieved July 13, 2012. "...and the Sunbeam S7, that civilised but low-powered shaft-drive in-line twin that BSA hoped would fill a niche as a gentleman's machine..."
- Walker, Mick (2000), Mick Walker's European Racing Motorcycles, Redline Books, ISBN 978-0-95313-113-6
- Setright, L.J.K. (1976). Motorcycles. London: Weidenfeld & Nicholson. p. 48. ISBN 0-297-77234-1.
- Tuttle, Mark, Jr. (December 2005), BMW F800S, Rider: 15
- Davis, Simon (May–June 2013). "Triumph's First Twin: Triumph 6/1". Motorcycle Classics. Topeka, Kansas, USA: Ogden Publications. p. 1. Retrieved 2014-04-26.
- Davis, Simon (May–June 2013). "Triumph's First Twin: Triumph 6/1". Motorcycle Classics. Topeka, Kansas, USA: Ogden Publications. p. 2. Retrieved 2014-04-26.
- Wilson, Hugo (1995). "The A-Z of Motorcycles". The Encyclopedia of the Motorcycle. London: Dorling Kindersley. p. 181. ISBN 0-7513-0206-6.
- Brooke, Lindsay (5 January 2003). "Chapter Two: Speed Twin and Tiger 100". Triumph: A Century of Passion and Power. MotorBooks International. pp. 36–37. ISBN 978-0-76030-456-3. Retrieved 2012-09-08. "Stylish, light, responsive, smooth, and refined, the Speed Twin changed the direction of motorcycle design and dominated it for thirty years."
- Wilson 1995, pp. 53, 120, 122
- Bacon, Roy (1990). Norton Dominator Twins. p. 7. ISBN 1-85648-306-1.
- Oliver, Lance (December 2004). Bill, Wood, ed. "Parallel Universe". American Motorcyclist (Pickerington, OH USA: American Motorcyclist Association) 58 (12): 13. ISSN 0277-9358. Retrieved 2012-09-08.
- Gingerelli, Dain; Everitt, Charles; Michels, James Manning (10 January 2011). 365 Motorcycles You Must Ride. MBI Publishing. p. 8. ISBN 978-0-76033-474-4. Retrieved 2012-09-08.
- Wilson 1995, pp. 108–109
- Clarke, Massimo (17 April 2010) . Modern Motorcycle Technology: How Every Part of Your Motorcycle Works. Motorbooks Workshop (English ed.). MotorBooks International. ISBN 978-0-76033-819-3. Retrieved 2012-09-08.
- "Yamaha T-Max (2001-2011)". Motorcycle News UK. Bauer Media. Archived from the original on 2010-09-30. Retrieved 2012-09-08. "Engine specification 4 stroke, parallel twin, auto"
- "2008 Honda Silver Wing ABS Review". Motorcycle.com. Verticalscope. Jun 12, 2008. Archived from the original on 2010-07-27. Retrieved 2012-07-11. "The engine is no longer an oversized, vibration-prone single but a smooth and reasonably powerful in-line twin."
- Westlake, Andy (15 November 2008). "Chapter 17: Wasp - A Sting in the Tail". Off-Road Giants!: Heroes of 1960s Motorcycle Sport. Dorchester, UK: Veloce Publishing. pp. 117–122. ISBN 978-1-84584-190-4. Retrieved 2013-02-01.
- Wilson, Hugo (1995). "The A-Z of Motorcycles". The Encyclopedia of the Motorcycle (in UK English). London: Dorling Kindersley. pp. 26–27. ISBN 0-7513-0206-6.
- Willoughby, Vic (1977) . "Douglas". Classic Motorcycles (Third impression ed.). The Hamlyn Publishing Group. p. 23. ISBN 0-600-31870-2.
- Coombs, Matthew; Haynes, John; Shoemark, Pete (2002), Motorcycle Basics (2nd ed.), Haynes, p. 1•31, ISBN 978-1-85960-515-8
- Dean, Paul (May 2005). Edwards, David, ed. "Service: Oil miser". Cycle World (Newport Beach, CA USA: Hachette Filipacchi Media) 44 (5): 160. ISSN 0011-4286. Retrieved 2013-05-26. "An air-cooled big-bore V-twin in particular can get very hot, especially the rear cylinder, which is not exposed to as much cooling air as the front."
- Cocco, Gaetano (2004). "Chapter 11: The Engine". Motorcycle Design and Technology (English ed.). St. Paul, MN USA: Motorbooks International. pp. 117–118. ISBN 0-7603-1990-1. Retrieved 2013-09-09. "In some construction layouts the tranverse width is the same as a single-cylinder engine, which allows very narrow frames and bodywork with small frontal areas."
- Fast Bike magazine August 1995 page 21[full citation needed]
- "Fast Bike" magazine August 1995 page 20
- American bicyclist and motorcyclist, Volume 6. Cycling Press, 1911
- Henshaw, Peter (Sep 15, 2012). "Two–Engine". How Your Motorcycle Works: Your Guide to the Components & Systems of Modern Motorcycles. Veloce Publishing. p. 19. ISBN 978-1-84584-494-3. "Unmachined forging of a 270-degree crankshaft for parallel-twin engine. (Honda)"
- Ried, Rick (December 2001). "British Heritage, American Style". American Motorcyclist (Pickerington, OH USA: American Motorcyclist Association) 55 (12): 18–20. ISSN 0277-9358. "...the America utilizes a 270-degree crank. That gives the America a lumpier cadence at idle that's a fair replica of a V-twin beat, although updated balancer shafts keep the vibrations under control."
- JAE: the journal of automotive engineering, Volume 2. Institution of Mechanical Engineers (Great Britain). Automobile Division. Automobile Division, Institution of Mechanical Engineers., 1971
- Cutts, John; Scott, Michael (1 August 1991). World's Fastest Motorcycles. Book Sales. ISBN 1-5552-1708-7.
- Clew, Jeff (1 February 2007). Edward Turner: The Man Behind the Motorcycles (illustrated, revised ed.). Dorchester, UK: Veloce Publishing. ISBN 978-1-84584-065-5.
- McDiarmid, Mac (1 January 1995). "Honda CB450 'Black Bomber'". Classic superbikes from around the world. Parragon. pp. 52–53. ISBN 0-7525-1017-7.
- Holmstrom, Darwin (4 December 2009). BMW Motorcycles. Brian J. Nelson, photographer. MotorBooks International. ISBN 978-0-76033-748-6. Retrieved 2012-09-10.
- Cycle World, Volume 46. CBS Publications, 2007
- Classic Bike, April 1994
- Phil Irving, 1962, as reprinted in Classic Motor Cycle, February 1992
- Holcolmb, Hank (October 1964). Juettner, Walter R., ed. "Inside Today's Outboards". MotorBoating (New York, NY USA: Hearst) 114 (4): 34–35. ISSN 1531-2623. Retrieved 2013-05-18.
- 487 cc Sunbeam. Walneck's Classic Cycle Trader, Dominion Enterprises, Dec 1996 reprinted from Classic Bike Magazine, 5 January 1950.
- Watanabe, Laurie; Mitchel, Doug (November 1996). "That magnificent man on his riding machines". Dealernews (Trade Publication) 32 (12). "If you press him, though, Jim is likely to expound on his 1947 S7 Sunbeam - "twin-cylinder, rubber-mounted engine. They were an inline two-cylinder, not crosswise in the frame, like conventional twins are.""
- Brown, Roland (1 September 2003). Ultimate History of Fast Bikes. Parragon. ISBN 978-1-40541-592-7.
- Brazendale (1983). Classic Cars: 50 Years of the World's Finest Automotive Design. Bookthrift. pp. 49, 57. ISBN 0-6710-5103-2. Retrieved 26 April 2014.
- Antique cars Michael Sedgwick, 1980 - 93 pages "The engine, an inline twin, lived in a box like a meat-safe at the rear."
- St. Antoine, Arthur; Stone, Matt (October 2007). "The Secret History of Ferrari: Sixty years of cars, stars, and stories you've never heard". Motor Trend. Source Interlink Media. They Can't All Be GTOs. Archived from the original on 2012-11-04. Retrieved 2012-07-06. "In 1955, for instance, engineer Aurelio Lampredi—working with Enzo and son Dino—creates an experimental inline-two-cylinder F1 engine (the theory being that two large cylinders would provide enormous torque for tight circuits like Monaco)."
- Norbye, Jan P. (1981). The complete handbook of automotive power trains. Tab Books. ISBN 0-8306-2069-9. Retrieved 2013-05-18.
- Norbye, Jan P. (February 1975). Luckett, Hubert P., ed. "The new logic in small-car engineering". Popular Science (New York, NY USA: Times Mirror Magazines) 206 (2): 56–59. ISSN 0161-7370. Retrieved 2013-05-18.
- Caunter, Cyril Francis (1958). The light car: a technical history of cars with engines of less than 1600 c.c. capacity. Science Museum (Great Britain) (Second illustrated ed.). H.M.S.O. p. 128.
- "Noutăţi. VAZ 111". Autoturism (1/1989). p. 8.
- Teodor, Pompiliu. "Dacia 500 Lăstun". Autoturism (2/1989). p. 5.
- Mastrostefano, Raffaele, ed. (1985). Quattroruote: Tutte le Auto del Mondo 1985 (in Italian). Milano: Editoriale Domus S.p.A. pp. 233–234. ISBN 88-7212-012-8.
- "World's cheapest car launched: Tata Nano". Autocar (Haymarket Consumer Media). Retrieved 2012-07-12. "The twin-cylinder, 623cc petrol engine powering it drives the rear wheels, and sits directly above them to the right of the rear of the car, with a four-speed manual gearbox immediately to its left."
- "Fiat's TwinAir 2-cylinder engine makes the 500 "pure fun"". autoblog.com. Retrieved 2010-08-01.
- "Piaggio Porter Maxxi". Piaggio Veicoli Commericali (in Italian). Retrieved 21 January 2014.
- Ingram, Antony (3 January 2014). "World's Cheapest New Car, Tata Nano, Gets Diesel Option". High Gear Media. Retrieved 21 January 2014.
- Sharma, Amit (2013-12-26). "Tata Nano diesel to unveil at 2014 Indian Auto Expo". Girnar Soft Automobiles. Retrieved 2014-01-21.
- "Tata Nano Diesel". Girnar Soft Automobiles. Retrieved 2014-01-21.
- Popular Mechanics - Feb 1989 - Page 109. Vol. 166, No. 2 - 136 pages - Magazine "Also new from Suzuki are the inline twin 15 and the inline 3-cylinder 25. Both are equipped with loop-charging, oil injection and external-mount tilt and trim. The 25 also has pre-atomized oil injection and an overrev limiter."
- 2 Stroke International Marine Engines & Jet Pumps
- Windsor, Henry Haven (Jr.), ed. (September 1957). "Tomorrow's Outboard – What Shape Will It Be?". Popular Mechanics (Chicago, IL USA: Popular Mechanics) 108 (3): 150–152. ISSN 0032-4558. Retrieved 2012-07-14. "The required gearing tends to be both noisy and expensive. For this reason the square type probably will never see service as an outboard motor"
- Cliche, Andre (2001). Ultralight Aircraft Shopper's Guide (8th ed.). Cybair Limited Publishing. pp. G–3 and G–4. ISBN 0-9680628-1-4.
- Markowski, Michael A. (May 1984). Gilmore, C. P.; Ortner, Everett H., eds. "Build your own ultralight? –tips from an expert". Popular Science (New York, NY USA: Times Mirror Magazines) 224 (5): 103–107,158. ISSN 0161-7370. Retrieved 2012-09-13.
- Machine Design: Volume 54, Issues 1-6; Volume 54, Issues 1-6 1982 "A modified snowmobile power plant, the 30-hp inline twin has been detuned by lowering the compression ratio and modifying the carburetion"
- Berger-Burr's ultralight and microlight aircraft of the world, Volume 16. P.185. Alain-Yves Berger, Norman Burr. Haynes Publishing Group, 1 Jan 1983
- Ultralight aircraft Michael A. Markowski - 1982 - 320 pages - "The Cuyuna 430-D is an inline twin cylinder of 30 hp."
- "Operators Manual for Engine Types 447, 503, 582" (pdf) (operators manual). BRP-Powertrain. 2010. Retrieved 2012-10-05.
- 3W-150iR2 Inline Twin, Specifications Sheet
- Janson, David R. "Model Engine Designer and Manufacturing Profiles: Taplin Twin". Retrieved 2012-10-05.
- Parsons, Grant (February 2003). Wood, Bill, ed. "Top Gear: Polaris 600 Sportsman Works Hard and Plays Hard". American Motorcyclist (Pickerington, OH USA: American Motorcyclist Association) 57 (2): 16. ISSN 0277-9358. "Underneath the bodywork, you'll find a 597cc overhead-valve parallel twin with two valves per cylinder, fed by a 34mm carb."
- Taylor, Rich (May 2003). Oldham, Joe, ed. "Fighter Jets". Popular Mechanics (Hearst Magazines) 180 (5): 114–117. ISSN 0032-4558. "Powered by a turbocharged 4-stroke Parallel Twin that has half the displacement of the Sea-Doo's supercharged GTX, the lightweight Polaris MSX 150 is almost as fast and $2200 less expensive."
- Samson, Jack, ed. (October 1972). "Quieter & Safer: the '73 machines". Field & Stream (New York, NY USA: CBS Publications) 77 (6): S10 – S14. ISSN 8755-8599. "The important power plants will now be 399 to 440cc parallel twins."
- Klancher, Lee (13 May 2011). The Art of the John Deere Tractor: Featuring Tractors from the Walter and Bruce Keller Collection. Voyageur Press. ISBN 9780760339497.
- Snowmobile Service Manual, p. 297, at Google Books Intertec Publishing - 1986 - 488 pages "Engines consist of two types; the axial fan, inline twin and the simultaneous firing opposed twin.
- "WHICH BIG BORE ENGINE LAYOUT IS BEST?". Dirt Trax Magazine. 3 August 2011. Archived from the original on 17 December 2013. Retrieved 6 August 2012.
- "BEST IN CLASS: BEST 2-UP ATV". Dirt Trax Magazine. 9 March 2012. Archived from the original on 17 December 2013. Retrieved 6 August 2012.
- "Emission-compliant diesel engines cover 83 to 1,350 bhp in 10 models." Oil, Gas, & Petrochem Equipment May 2006: 18. General OneFile. Web. 29 June 2012. "Cat petroleum diesels line includes eight inline two, three, and four-cylinder engines
- "PSA fires up two-cylinder turbo". Automotive Engineer 33 (9): 5. October 2008. "Taking an 1,124cc naturally-aspirated inline-four as a baseline, it selected the inline-twin configuration as offering the lowest friction and thermal losses."
- Media related to Straight-two engines at Wikimedia Commons