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 360° crankshafts, while some larger Japanese twins of the 1960s adopted the 180° crankshaft. In the 1990s, new engines appeared with a 270° crankshaft.
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", "vertical-twin" and "inline-twin" are used. Particularly in the UK, where many motorcycles had such engines, the term "parallel-twin" has been used to mean that the crank has an angle of 360 degrees, causing the pistons to run "parallel" in their respective bores. The "opposite" of this, when one piston travels up while the other travels down due to the 180 degree crankshaft angle, was called a "vertical twin". "Inline-twin" goes for both engine types, but sometimes 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 origin of the terms now seem to be partially forgotten, as people and even manufacturers wrongly use "parallel-twin", "inline-twin" and the other variants interchangeably and treat them as equivalent.
The first production motorcycle using a straight-twin motor was the Hildebrand & Wolfmuller of 1894, which was also the first motorcycle to be serially produced. Its cylinders lay flat and forward-facing, its pistons connected directly to the rear wheel with a locomotive-style connecting rod. Approximately 2000 were produced through 1897.
The second production motorcycle to use a straight twin design, this time a 'parallel twin' with vertical cylinders akin to the later British type, was the Werner of 1903. It used cast-iron cylinders with integral heads, and side valves, with a capacity of 500cc.
The most famous example of the straight-twin engine was produced by Triumph from 1937 onwards; the 5T Speed Twin. Experiments with this engine type began as designer Edward 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 an 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 360° 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 World Championship, the AJS E-90 Porcupine of 1949, which had nearly horizontal cylinders.
Comparison of twins in motorcycle use
|Characteristic||Flat-twin (long.)||Flat-twin (trans.)||Straight-twin (long.)||Straight-twin (trans.)||V-twin (long.)||V-twin (trans.)|
|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°. What follows below will mostly be concerned with four-stroke engines.
- In a 360° engine, both pistons rise and fall together. The dynamic imbalance is identical to that of a single-cylinder engine of equivalent reciprocating mass. Originally, only crank counter balances were used, but balance shafts and even a separate weighted conrod have been used to balance the crank-speed free force - again, much like singles. The cylinders fire evenly and sequentially, i.e. 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. With twice the number of ignition pulses per cycle it would feel much smoother than a single, even if it shares the same imbalance.
- In a 180° engine, one piston rises as the other falls. This balances the primary free force, but instead produces a rocking couple which is normally canceled by a balance shaft; it also shares the buzzy secondary imbalance of inline fours, normally left unchecked due to lower total reciprocating mass. The 180 degree separation means 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. The irregular ignition pulses change the harmonic contribution of the vibration due to combustion events that is transmitted into the drivetrain, frame / chassis and any persons there attached. Most importantly, it reintroduces the fundamental harmonic of a single cylinder, and other odd harmonics with it, making for a more lumpy delivery.
- In a 270° engine, one piston follows three quarters of a rotation behind the other. This results in a cancellation of the secondary free force imbalance, but not the rocking couple. The primary imbalance is a combination of free force and rocking couple and is once again mitigated by use of a balance shaft. It yields firing intervals identical to a 90° V-twin, namely: 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. The harmonic contribution of these intervals are different again, still irregular, but having a pulsing feel more like a V-Twin rather than the 360's thrum or the 180's throb; the same is reflected in their respective sounds.
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 because of uneven pulsing), 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 higher rpm and thus more power. For example, the 1966 Honda 450 cc dohc 180° "Black Bomber" could challenge contemporary British 650 cc 360° parallel-twins.
Many small motorcycles of less than 250 cc use a 360° crankshaft as the vibration issue was less significant; examples include 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 have 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 had 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 270° design of the new 700cc engine that powers the Honda NC700 series.
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. The use of a flywheel on the crankshaft makes the oscillating momentum non detectable when driving, but it still creates an oscillating force within the drivetrain which must be accounted for during design, e.g. in respect of longevity or its effect on the driven tyre(s).
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 was thought to be found when one piston is travelling fastest at the same time the other has stopped, but this discounts the other half of the cycle where the pistons' roles are reversed, but not exactly mirrored - meaning they don't cancel a second time. The minimisation of speed deviation over a complete rotation is actually achieved with something much closer to (but still not normally exactly) the 90° separation, confirmation of which would have required a lot of hand calculation and / or graphing in Irving's day. 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.
The first production parallel-twin motorcycles with a 270° engine were the 1996 Yamaha TRX850 and the Yamaha TDM, both now discontinued. Modern examples of 270° motorcycles in production in 2018 include the Donington Norton Commando, Triumph Thunderbird, Honda Africa Twin, Honda NC700 series, Triumph Thruxton 1200, and Royal Enfield Interceptor 650. KTM's 790 Duke has a similar crankshaft, varyingly quoted as 285 or 435 degrees (the two firing intervals), mimicking their 75 degree V-Twins in sound and feel.
A feature of the 270° parallel twin is that it provides most of the feel of the popular V-twin layout, along with further advantages:
- it is simpler and cheaper both to produce and to maintain;
- it needs only a single cylinder block and head;
- air filtration, fuel induction, exhaust and cooling are simpler;
- the engine is lighter and has better potential for an optimum CG position;
- battery location is easier;
- rear suspension design is simpler; and
- it allows a shorter wheelbase for sharper handling.
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 had a 360° crankshaft. The Yankee's configuration, which had 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 Fiat TwinAir engine - normally aspirated 974 cc and turbocharged 875 cc - 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, having 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
Inline twins are common in marine use both inboard and outboard, such as the Suzuki 15 models, and as jet pump motors.
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 and Lancashire Nobbys would sometimes retrofit an inboard engine, such as the Lister or the Kelvin E2 inline-twin. (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.
Inline-twin engines are sometimes also used in large scale flying model aircraft.
Such engines have commonly been used in snowmobiles and ATV.
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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
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Underneath the bodywork, you'll find a 597cc overhead-valve parallel twin with two valves per cylinder, fed by a 34mm carb.
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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.
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The important power plants will now be 399 to 440cc parallel twins.
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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.
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