Oversquare and undersquare is a common term in internal combustion engine. what does this term relate to? What is the differences between the two in terms of performance?
Hi Eiman,
Oversquare and Undersquare are terms that describe the ratio of it’s bore and stroke. Bore is the cylinder’s diameter, and Stroke is the piston’s travel from the very top, to the very bottom:
- undersquare is when the stroke length is longer than the bore
- oversquare is when the bore is larger than the stroke length
undersquare engine is used for high torque vehicle such as truck, SUV and MPV
oversquare engine is used for high speed vehicle such as racing car and superbike
Have a nice day
Best Regards,
Tuan Rudi
Hello Eiman,
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.
In an over-square engine, 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.
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.
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.
In terms of performance, piston speed is typically lower for over-square engine designs. The larger bore in over-square designs can may also allow more room for increased valve size (and improved volumetric efficiency) and may offer more options for injector placement for direct-injection engines. There is a recent trend towards under-square designs with newer, high-efficiency SI gasoline engines, particularly engines using Atkinson or Miller Cycle and/or cooled/external EGR. Some manufacturers are targeting a S/B of approximately 1.2, in part to increase low and part low intake port velocities and generate increased charge motion (typically tumble in SI engines). An example includes the new Atkinson Cycle engine in the 2018 Toyota Camry (1). Honda investigated S/B of 1.2 and 1.5 for a Miller Cycle concept (2).
Diesel engines are typically under-square designs and slow-speed Category 3 marine diesel engines using cross-heads are significantly under-square.
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