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In a reciprocating piston engine, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio, is a term to describe the ratio between cylinder bore diameter and piston stroke. This can be used for either an internal combustion engine, where the fuel is burned within the cylinders of the engine, or external combustion engine, such as a steam engine, where the combustion of the fuel takes place outside the working cylinders of the engine.
A fairly comprehensive yet understandable study of stroke/bore effects was published in Horseless Age, 1916.
- 1 Conventions
- 2 Square, oversquare and undersquare engines
- 3 Notes
- 4 References
Stroke/bore ratio is less common than bore/stroke ratio, but is a term mostly preferred in German text (Hubverhältnis in German). It is used in some other countries too, such as Finland, likely due to German influence.
Square, oversquare and undersquare engines
The following terms describe the naming conventions for the configurations of the various bore/stroke ratio:
A square engine has equal bore and stroke dimensions, giving a bore/stroke value of exactly 1:1.
Square engine examples
1991–2014 Ford 4.6 V8 OHC engine with 90.2 mm × 90.0 mm (3.552 in × 3.543 in) bore and stroke. It has been the backbone of Ford V8-powered cars and trucks in different power levels and head designs for two decades.
1967–72 FIAT 125, 124Sport engine 125A000-90 hp, 125B000-100 hp, 125BC000-110 hp, 1608 ccm, DOHC, 80.0 mm × 80.0 mm (3.15 in × 3.15 in) bore and stroke.
The 1970s Ford 400M with a 101.6 mm × 101.6 mm (4.00 in × 4.00 in) bore and stroke.
The Opel/Vauxhaul 2.0 L GM Family II engines are square at 86.0 mm × 86.0 mm (3.39 in × 3.39 in) bore and stroke; example as C20XE C20NE C20LET X20A X20XEV X20XER Z20LET Z20LEH Z20LER A20NHT A20NFT.
Honda's J30A engine has an 86.0 mm × 86.0 mm (3.39 in × 3.39 in) bore and stroke.
Oversquare or short-stroke engine
An engine is described as oversquare or short-stroke if its cylinders have a greater bore diameter than its stroke length, giving a bore/stroke ratio greater than 1:1.
An oversquare engine allows for more and larger valves in the head of the cylinder, higher possible RPM by lowering maximum piston ring speed and lower crank stress due to the lower peak piston acceleration for the same engine speed. Due to the increased piston and head surface area, the heat loss increases as the bore/stroke ratio is increased. Thus an excessively high ratio can lead to a decreased thermal efficiency compared to other engine geometries. Because these characteristics favor higher engine speeds, oversquare engines are often tuned to develop peak torque at a relatively high speed. The large size/width of the combustion chamber at ignition can cause increased inhomogeneity in the air/fuel mixture during combustion, resulting in higher emissions.
The reduced stroke length allows for a shorter cylinder and sometimes a shorter connecting rod, generally making oversquare engines less tall but wider than undersquare engines of similar engine displacement.
Oversquare engine examples
Oversquare engines (a.k.a. "short stroke engines") are very common, as they allow higher rpm (and thus more power), without excessive piston speed.
Examples include both Chevrolet and Ford small-block V8s. The BMW N45 gasoline engine has a bore/stroke ratio of 1.167.
Horizontally opposed, also known as "Boxer" or "flat", engines typically feature oversquare designs since any increase in stroke length would result in twice the increase in overall engine width. This is particularly so in Subaru's front-engine layout, where the steering angle of the front wheels is constrained by the width of the engine. Although oversquare engines have a reputation for being high-strung, low-torque machines, the Subaru EJ engine EJ181 develops peak torque at speeds as low as 3200 rpm.
Nissan's SR16VE engine found in Nissan Pulsar VZ-R and VZ-R N1 is an oversquare engine with 86 mm bore and 68.7 mm stroke, giving it an impressive 175-200 hp but relatively small torque of 119 lb.ft-134 lb.ft
Extreme oversquare engines are found in Formula One racing cars, where strict rules limit displacement, thereby necessitating that power be achieved through high engine speeds. Stroke ratios approaching 2.5:1 are allowed, enabling engine speeds of 18,000 RPM while remaining reliable for multiple races.
Early Mercedes-Benz M116 engines have a 92 mm bore and a 65.6 mm stroke for a 3.5 L V8.
Undersquare or long-stroke engine
An engine is described as undersquare or long-stroke if its cylinders have a smaller bore (width, diameter) than its stroke (length of piston travel) - giving a ratio value of less than 1:1.
At a given engine speed, a longer stroke increases engine friction (since the piston travels a greater distance per stroke) and increases stress on the crankshaft due to the higher peak piston acceleration. The smaller bore also reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number. Because these factors favor lower engine speeds, undersquare engines are most often tuned to develop peak torque at relatively low speeds.
Undersquare engines are typically tuned to exhibit peak torque at lower RPM than an oversquare engine due to their longer crank throw and high piston speed at high RPM. While the surface area of the cylinder in an undersquare engine may be higher than that of a square engine cylinder, heat exchange through the cylinder wall is lessened compared to heat exchange through the cylinder head and piston since the cylinder wall is not in constant contact with the expanding gasses. Since each unit of area of cylinder wall permits less heat loss to the engine than through the cylinder head or piston, undersquare engines generally exhibit improved thermal efficiency compared to oversquare engines. For these reasons, diesel truck engines which require low RPM torque and high thermal efficiency more than high power density most commonly adopt an undersquare engine geometry.
An undersquare engine will typically be more compact in the directions perpendicular to piston travel but larger in the direction parallel to piston travel.
Undersquare engine examples
Many inline engines, particularly those mounted transversely in front-wheel-drive cars, utilize an undersquare design. The smaller bore allows for a shorter engine that increases room available for the front wheels to steer. Examples of this include many Volkswagen, Nissan, Honda, and Mazda engines. The 1KR-FE-engine used in the Toyota Aygo, Citroën C1 and Peugeot 107 amongst others is an example of a modern long-stroke engine widely used in FF layout cars. This engine has a 71 mm bore and 84 mm stroke giving it a bore/stroke ratio of 0.845:1. Some rear-wheel-drive cars that borrow engines from front-wheel-drive cars (such as the Mazda MX-5) use an undersquare design.
Despite their reputation as low-speed torque machines, some undersquare engines are designed for quite high speeds. The Honda Integra Type R's B18C5 engine has one of the highest redlines (8,400 rpm) of any production engine, yet features an undersquare design. The 2011 Ford Coyote engine is a modern undersquare engine with a 7,000 rpm redline.
BMW's acclaimed S54B32 engine was undersquare (91 millimetres (3.58 in) vs 87 millimetres (3.43 in) bore), offering a world record torque-per-litre figure (84.1 lb·ft per litre) for normally-aspirated production engines at the time; this record stood until Ferrari unveiled the 458 Italia.
Many British automobile companies used undersquare designs until the 1950s, largely because of a motor tax system that taxed cars by their cylinder bore. This includes the BMC A-Series engine, and many Nissan derivatives. The Trojan Car used an undersquare, split piston, two stroke, two cylinder in line engine; this was partly for this tax advantage and partly because its proportions allowed flexing V-shaped connecting rods for the two pistons of each U-shaped cylinder, which was cheaper and simpler than two connecting rods joined with an additional bearing.
The Ford 5.4L Modular Engine features a cylinder bore of 90.2mm (3.552 in) and a stroke of 105.8mm (4.165 in), which makes a bore/stroke ratio of 0.852:1. Since the stroke is significantly longer than the bore, the SOHC 16V (2-valve per cylinder) version of this engine is able to generate a peak torque of 350 lb·ft as low as 2500rpm.
The Dodge Power Wagon used a straight-six Chrysler Flathead engine of 230 cu in (3.8 litre) with a bore of 83 millimetres (3.27 in) and a stroke of 117 millimetres (4.61 in), yielding a substantially under-square bore/stroke ratio of 0.709:1.
The 4 litre Barra Inline 6 engine from the Ford Falcon, uses a 92.26mm bore and 99.31mm stroke, which equates to a 0.929:1 bore-stroke ratio. Although considered a performance engine, its proportionally weak valve springs, pistons and conrods cause the engine to redline at 6000 rpm.
The 292 Chevrolet I6 is also under square, with a bore of 3.875 in and a stroke of 4.125 in (bore/stroke ratio = 0.939:1).
Mitsubishi's 4G63T engine found primarily in many generations of Mitsubishi Lancer Evolution is an undersquare engine at 85 mm bore x 88 mm stroke.
Virtually all piston engines used in military aircraft were long-stroke engines. The PW R-2800, Wright R-3350, Pratt & Whitney R-4360 Wasp Major, Rolls-Royce Merlin (1650), Allison V-1710, and Hispano-Suiza 12Y-Z are only a few of more than a hundred examples.
All diesel-powered ships have massively undersquare marine engines. A Wärtsilä two-stroke marine diesel engine has a cylinder bore of 960 mm (37.8 in) and stroke of 2500 mm (98.4 in), (bore/stroke ratio = 0.384:1).
While most modern motorcycle engines are square or oversquare, some are undersquare. The Kawasaki Z1300's straight-six engine was made undersquare to minimise engine width, more recently, a new straight-twin engine for the Honda NC700 series used an undersquare design to achieve better combustion efficiency in order to reduce fuel consumption.
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Journalists and riders called it the king of motorcycles, and it gained a reputation as a super sport model all over the world. The Super Four boasted high performance and quality. This best-selling motorcycle won many prizes around the world within only six months of its release.
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