CLASSIFICATION OF ENGINE ON THE BASIS OF BORE STROKE RATIO

The engine capacity is measured as Bore x Stroke. Here, Bore is the diameter of the cylindrical block and Stroke is the depth of the cylindrical block, as shown in the image.

When stroke/bore value is over 1:1 the engine is long-stroke or undersquare and when the stroke/bore value is under 1:1 the engine is short-stroke or oversquare. The square engine has a value of 1:1 in both cases.

For example an engine with 110 millimetres (4.33 in) stroke and 80 millimetres (3.15 in) bore, stroke/bore value 1.375, is an undersquare or long-stroke engine. An engine that has 70 millimetres (2.76 in) stroke and 100 millimetres (3.94 in) bore, stroke/bore value 0.7, is oversquare or short-stroke.

The stroke ratios are generally based under 3 catagories depending on the different ratios,

• Short stroke/Over square engine
• Long stroke/Under square engine
• Square engine

Short Stroke/Over square Engine:

As the name signifies the Stroke is shorter than the Bore. Due to the relatively shorter stroke, the engine revs fast and is more suitable where a quick build of power is important, more importantly where the bikes make power at higher rpms.

An oversquare engine allows for more and larger valves in the head of the cylinder, lower friction losses (due to the reduced distance travelled during each engine rotation) and lower crank stress (due to the lower peak piston speed relative to engine speed). Because these characteristics favor higher engine speeds, oversquare engines are often tuned to develop peak torque at a relatively high speed.

Short stroke engines revs faster and hence produce more power. It makes the bike very responsive which is very essential in racing bikes. Also the short stroke engines have a very narrow power range and hence requires very short gear ratios to keep them in power range.

Some examples:

• Apache RTR 180 (62.5mm X 57.8 mm) Bore x Stroke.
• Pulsar 220 (67mm X 62.4 mm), 180(63.5mm X 56.4mm) Bore x Stroke.

Formula 1 engines use short stroke designs because it allows the engine to rev higher. Because of this, they are capable of generating greater horsepower. Sports bikes and their racing version use short stroke engines.

Long Stroke/ Under square Engine:

As the name suggests the Stroke is longer than the Bore. Due to the longer stroke, the engine makes good torque at relatively low rpms. It is important where the bikes’ pulling more (torque) at relatively low rpms is more important than the top speed.

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 speed). 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.

Long stroke engine are usually used in heavy vehicles and they wont rev fast. But they are very durable and have a much broad power range which requires few gears.

Some examples:

• Royal Enfield Classic 500 (84mm X 90mm), Classic 350 (70mm X 90 mm)
• Hero Honda Karizma (65.5mm X 68.2mm)

Square Engine: 

As a square has both the sides equal, here also the Bore and Stroke are of almost same size.The engine tries to strike a balance between torque and top speed.

An engine is described as a square engine when it has equal bore and stroke dimensions, giving a bore/stroke value of exactly 1:1.

For example an engine which has 95 millimetres (3.74 in) bore, and an identical 95 millimetres (3.74 in) stroke, has a bore/stroke value of:

95 mm / 95 mm = 1.00

Square engines combines both the worlds, perfect for daily commuting with great mileage figures.

Some Examples:

• Yamaha FZ-16 (58mm X 57.9mm)
• Honda Unicorn (57.3mm X 57.8mm)