I am trying to make picture more clear to understand how short and long stroke, long and short connecting rod make difference in piston velocity.

The short stroke engine has two obvious effects. The distance traveled by the piston per revolution is reduced and the load on the crankpin are increased for a given shaft torque (due to the reduced "throw" ). Also the resulting engine is squatter, enabling its external dimensions to be reduced, with the possibility of an appreciable saving the weight. And for the very high revving engines the reduction in friction and wear resulting from a lower piston speed makes the short stroke design more to be favoured than the long stroke counterpart. This advantage is gained at the expense of higher loads on the crankpin and main bearing for the same torque and a greater leakage patch around the piston (due to the increased circumference).

The long stroke engine was best for high compression ratios and a essential for model diesels, in general terms, the improved perfomance of model diesels has largely been due to "tailoring" them for high speed operation by increasing the bore-stroke ratio. In the long stroke engine the piston has to be accelerated from zero at BDC up to a maximum one-quarter of a revolution later, then decelerated to zero again at TDC. The corresponding velocity gradient for a short stroke engine is appreciably flatter. This means that appart from the mid position and BDC and TDC, the piston is sweeping any other point on the cylinder faster with a long stroke than with a short stroke at a given RPM. If, therefore, the port depth is limited the gas flow will have to be correspondingly faster, a feature which may not be clear from a study of a timing diagram alone. For the same opening period, compared with a short stroke engine, depth would have to be increased to correspond to the same percentage length of stroke in each engine. thus the only way to compare port timing without taking the bore/stroke ratio into account is to express it in terms of percentage stroke.

The timing period is also modified by the length of the connecting rod, relative to the stroke. Lengthening the connecting rod (for a given stroke) will modify the "velocity gradient" of the piston so that it tends to dwell about TDC and accelerate more rapidly through BDC. Similary, shortening the connecting rod will have the opposite effect - the piston tending to dwell about the BDC and accelerate rapidly through TDC.

In the Desaxe engine will piston accelerate faster from BDC., promoting quicker opening and slower closing, giving in effect a larger opening for a size of port. Actually the timing feature of a Desaxe cylinder is not necessary the reason for its adoption. It may be employed for mechanical reasons in that it greatly reduces the side thrust of the piston during power stroke.

Invariably, however all high speed engines are of short stroke design: and long stroke engines where still made, designed for generating high torque at low or moderate speeds. The true "general purpose" engine, it has been suggested, should have a stroke slightly greater than the bore, this being what we would classify as a "sports" type engine with a maximum life.

Obviously, however, many other factors come into account in commercial productions - following proven practice established by earlier designs - designing for "reworking" to a different capacity later for a new model in a different class; and so on...