From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Extract from 'The Autocar' magazine discussing engine forces, pressure diagrams, and the debate between long and short stroke engines.
| Identifier | ExFiles\Box 33\4\ Scan002 | |
| Date | 4th February 1915 | |
| R.R. 935A (300 H) (S.A. 853. 4-2-15) G.{Mr Griffiths - Chief Accountant / Mr Gnapp} 9970. Handwritten: C.C.1766 THE AUTOCAR, December 21st, 1912. Total pressure of gases on piston + weight of piston and connecting rod = acceleration forces. Exhaust stroke bottom centre. Pressure on part of pin next to shaft. = Acceleration forces + weight of piston and connecting rod. Induction stroke top centre pressure on part of pin next shaft. = Acceleration forces + weight of piston and connecting rod. Compression stroke bottom centre. Pressure on part of pin next shaft. = Acceleration forces + weight of piston and connecting rod. The acceleration forces at top centre are in pounds (w v² / r g) x (1 + 1/n), where w is weight of piston and weight of connecting rod, v is the circumferential speed of the centre of the crank pin in feet per second, r is the radius of the crank pin also in feet, n is the ratio of the length of the connecting rod to radius of crank pin taking the latter as 1. At the bottom centre the expression becomes (w v² / r g) x (1 - 1/n). It follows from the above that with a connecting rod four cranks long between centres the acceleration forces at top and bottom centres are to each other in the ratio of 1.25 to .75. [Diagram showing two circular paths labelled EXPLOSION, INDUCTION, EXHAUST, COMPRESSION] The pressure diagram referred to in the accompanying letter. The accompanying diagram shows the character of the pressure coming on the crank pin during one complete cycle. The rotation is taken as right-handed, and the lines between the two diagrams link them up. In this diagram no account is taken of the sideway acceleration and deceleration of connecting rod, and the arrows are omitted in the latter half of the compression stroke, as they may vary in sense in different cases, though no doubt when nearing the end of the stroke the pressure is on the underside of the pin if the speed of revolution is at all high. R.{Sir Henry Royce} DUMAS, Whit. Ex., M.I.M.E. LONG V.{VIENNA} SHORT STROKE ENGINES. [19015.]—Referring to Mr. Pomeroy’s article in your issue of the 14th inst. on the above subject, there is another reason why the bulk of the wear on the crank pin is on the side nearest the crankshaft, which no doubt Mr. Pomeroy is familiar with, but which he has not referred to in his letter. This is the effect of the acceleration of the piston and connecting rods during the first part of the upward stroke. Practically the whole of the effort required for this purpose comes on the half of the pin towards the crankshaft. This effect is present both during the exhaust and the compression strokes, being naturally more pronounced during the latter. The deceleration of the parts during the latter part of the upward stroke also brings pressure on the same part of the crank pin. This pressure has to be entirely taken up by the crank pin during the latter half of the exhaust stroke, but is largely neutralised by the pressure in the cylinder during the latter half of the compression stroke. A further reason for wear taking place in the same part is the pressure due to the sideways acceleration and deceleration of the bottom half of the connecting rod. The whole of this comes on the half of the pin nearest the crankshaft. The following expressions give the pressures on the pin at the dead centres for two revolutions: Explosion stroke top centre. Pressure on part of pin remote from shaft. [19016.]—I was under the impression that the Sunbeam car won the Three Litre Class Race in France last June through sheer merit, finest material, and perfect workmanship. Mr. Coatalen informs us, however, that it only won owing to its long stroke, and had it been 90 x 120 mm. instead of 80 x 150 mm. it would “have met with disaster.” When referring to vibration Mr. Coatalen seems to overlook the fact that the long stroke engine gives more angularity to the connecting rod, and hence more side thrust and wear on the piston, than the short stroke, and he brings forth no proof that vibration is less on the long stroke, but merely states it as a fact—and a fact which can very easily be disputed. I am surprised that Mr. Coatalen cannot follow Mr. Pomeroy’s clear argument as to the extra cooling effect, which means loss of power on the long stroke engine (see top of page 1043). The area of the cylinder wall for 80 x 150 mm. is 37,688 square mm., and the area for 90 x 120 mm. is only 33,929, or just 3,769 square mm. less. The petrol engine is a heat engine, and heat lost means power lost. At the same revolutions it is obvious that the area exposed in the long stroke engine is 3,769 x revolutions per minute greater than that in the short stroke engine. Mr. Coatalen tells us his engines never ran less than 2,400 revolutions per minute, hence the additional area exposed to cooling is no less than 3,769 x 2,400 x 4 ÷ 4 for a four-cylinder four-cycle engine, which equals 18,091,200 extra square mm. per minute. This figure only takes the firing stroke into account. I have no interest in long or short strokes, but am only interested in mathematical accuracy. UNBIASED. | ||