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).
Piston performance, discussing friction, expansion, clearances, and test results for semi-split and Aerolite pistons.
| Identifier | ExFiles\Box 86\2\ scan0200 | |
| Date | 14th May 1934 | |
| -2- It was very noticeable when fitting the semi-split pistons that the friction was considerably less than with standard pistons (E.M.1209), so that at first we expected a considerable increase in power over standard pistons. The power is certainly considerably better than standard at low speeds, no doubt due to the reduced friction and possibly to some extent to the Gudgeon Pin Boss Struts being carried right across the crown. This will have the effect of carrying the heat well away from the crown, and preventing the skirt from expanding on the thrust diameter. At high speeds, however, with the piston crown much hotter, it is quite probable that the piston expands considerably across the thrust diameter. This of course will be in one direction, towards the pressure side, since the horizontal slot at the top of the skirt on the non-pressure side of the skirt will act as a heat trap, and prevent the heat from the crown travelling directly to this side of the skirt. Assuming this to be correct, we may reduce the friction at high speeds by cutting a horizontal slot at the top of the pressure side of the skirt, so that the skirt cannot then expand much on the thrust diameter, and providing the skirt is relieved well on the diameter measured across the gudgeon pin bosses, we should not be troubled with seizures. At the moment this clearance is .014 at the top of the skirt and .005 at the bottom. We were not troubled with knocks with these pistons but we shall rely on the endurance run to show these up. It will be remembered that the first set of Aerolite pistons seized after a short run with a clearance of .012, measured on the minor axis (i.e. across the gudgeon pin bosses). These pistons were ground with this Co's. No. 10 Cam. All later pistons were ground with their No. 14 Cam, which allows a clearance of .016 on the minor axis and none of these pistons have ever seized. The clearance was exactly the same measured on the thrust diameter on both types of pistons, namely,..003. In the light of this experience we may have to relieve our pistons a little more if we make another slot on the pressure side. We have since our previous report carried out further tests determining the I H.P. and I M.E.P. at 4500 r.p.m. and it will be noticed on the attached curve (B.V.21) that the I H.P. reaches a peak at slightly below 4500 r.p.m. the peak of the I M.E.P. curve is at approximately 3700 r.p.m. and above this speed the I M.E.P. falls rather rapidly. | ||