I don't get it. Surely the stroke must equal the "throw" of the crankshaft. Wouldn't any increase in effective stroke at TDC due to the offset be cancelled out by a corresponding decrease at BTC. Or is this an over-simplification of a complex issue?
If the offset does increase the stroke, is this taken into account when calculating the cubic capacity?
In referring to the diagram above it may help to open it in another window (right click, save picture as …, etc.)
An annotation is missing from the diagram: point a2 should be marked on line CD at the intersection of the arc from point a1 on AB.
The circle represents the path of the centre of the crankpin and is divided equally at points a, b, c, etc. The line AB represents the centreline of a standard cylinder. The line CD represents the centreline of the offset cylinder (offset exaggerated).
From each of the points a, b, c, etc., at a distance representing the connecting rod length on the same scale as the crankpin circle, the position of the standard gudgeon pin centre is marked at a1, b1, c1, etc. and of the offset gudgeon pin centre at a2, b2, c2, etc.
It should be noted that the positions of TDC and BDC of the piston in each engine are established when its gudgeon pin, crankpin and crankshaft centres are co-linear. In the standard engine those are at g and a respectively, diametrically opposite each other, so that the distance between them around the crankpin circle is the same in each direction and therefore the mean linear velocity of the piston is the same on upstrokes and downstrokes.
In the case of the offset cylinder the positions of TDC and BDC are at T and B respectively and it will be seen that the distance TB in the clockwise direction of rotation is greater than the distance BT clockwise. As the crankpin travels with uniform velocity around its circular path, the mean linear velocity of the piston on its upstrokes will be greater than that on its downstrokes, the variation being dependent upon the offset.
From the diagram it will be seen that the length of stroke of the offset piston a2g2 is slightly greater than that of the standard piston a1g1 although the perpendicular distance from the crankpin centreline EF to TDC of the offset piston is less than that to TDC of the standard piston.
Thus, all else being equal, the offset cylinder engine exhibits a slight increase in swept volume with reduced piston side thrust and reduced frictional losses. However, the variation in mean piston velocity between up- and downstrokes gives rise to variation in inertia forces which may be difficult to counteract if the offset is appreciable.
I hope that this helps.
Onwards and upwards,
Biggles.