Friday, August 5, 2011

Aeromodeller 1956-07

The timing diagram of an engine�expressed in terms of crankshaft rotation, as explained in an earlier article�gives us only part of the picture. The actual opening and closing time of the various ports�expressed in fractions of a second (or more truly milliseconds)� will be dependent on the bore/stroke (or stroke/bore) ratio for a given capacity, the length of the connecting rod relative to the stroke, whilst any asymmetry of the cylinder axis relative to the crankshaft centre line will alter the relative speeds of port opening and closing. To illustrate the effect of varying the sizes of the bore and stroke for a given capacity we can take the three different arrangements for an imaginary 0.1 cu. in. (1.6 c.c.) engine�one with a stroke appreciably longer than the bore; one with equal bore and stroke (usually referred to as a "square" layout); and one with the stroke much shorter than the bore, or an "over square" layout. These are shown diagrammatically in Fig. 1. Shortening the stroke (i.e., increasing the bore/stroke ratio or decreasing the stroke/bore ratio for a given capacity) has two obvious effects. The distance travelled by the piston per revolution is reduced; and the load on the crankpm is 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 in weight.