Bore and Stroke of Engine

Understanding technical details about Bore and Stroke of an Engine

Stroke and Bore define the characteristics of any internal combustion engine. The dimension and features affect the performance of the engine.

Bore is the diameter of the cylinder where the fuel and air comes into contact to combust which in turn forces the piston to move. Larger the diameter, larger the power output with respect to the performance of the engine.

Stroke - when combustion takes place inside the cylinder, it pushes the piston to create a torque moment and comes back up again to continue the process. How far the piston travels back and forth, defines Stroke. Longer the stroke, longer the time and path.

Bore Stroke Ratio

By dividing the bore by the stroke, a ratio is obtained which characterizes the engine of the bike. On average, manufacturers tend to look at a ratio of 1.2 for their engines, generally balancing bore and stroke values. A value of more than 1 means that the engine is high-revving and a value of under 1 denotes a low-revving but torqy engine. Long stroke engines are not much in preference of late due to the friction developed by the moving parts and the operational temperature and subsequent maintenance involved.

High revving engines use shorter strokes and wider bores to maximize power output in the higher RPM ranges. However, making the pistons go faster (up and down) can stress the parts and eventually lead to failure. Long stroke engined bikes or bikes that are built for long distance trips tend to have lower revving engines that gives power in the lower ranges of the RPM.

Bore and Stroke dimensions define the overall capacity of the engine. The displacement of any engine is defined as the difference between the volume of the cylinders at Bottom Dead Centre (BDC) and Top Dead Centre (TDC). BDC is when the piston moves up from the bottom of the stroke to the top and TDC is when it moves down from the top. The formula to calculate the displacement of an engine is: ? x (bore/2)^2 x Stroke x No. Of cylinders = Displacement.

Examples

Examples of short stroke engine bikes: TVS Apache RTR 180, Bajaj Pulsar 220.
Examples of long stroke engine bikes: Royal Enfield Classic 500, Hero Honda Karizma.

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