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).
Conclusions on car engine performance, focusing on valve timing, cam design, and lift.
| Identifier | WestWitteringFiles\V\March1931-September1931\ Scan444 | |
| Date | 28th September 1931 | |
| HS.{Lord Ernest Hives - Chair} ) FROM R.{Sir Henry Royce} BY.{R.W. Bailey - Chief Engineer} ) C to SC. WOR.{Arthur Wormald - General Works Manager} E.{Mr Elliott - Chief Engineer} CAR ENGINE PERFORMANCE. x5090. x7090. At the moment the following conclusions have been agreed to: (1) That large inlet valves with ample clearance round the head are advisable, but that smaller ones can be made to do the work by using cams of long duration with a maximum amount of lap by increasing the lift and also by increasing the acceleration of the maximum accelerating part of the cam form. We have in the past been somewhat deceived by taking note of the long silencing toe of our cam design. This has been removed by timing with an increased clearance, namely, .013 for car work, and .020 for aero work. (2) We have also in our anxiety for silence kept the lift and acceleration in the cam design on the low side. This acceleration figure has been continually increased until it now lies between .0045 and .005. The extra lift of the valve suggested by RM.{William Robotham - Chief Engineer} certainly is useful and we think to a moderate extent can be adopted with the high rating thick wire valve springs and other improvements now available. The amount suggested is a change of .25 to .3 of the valve diameter. We are not entirely convinced of the safety of increasing the overlap, but it appears we may be able to commence the opening of the inlet valve a few degrees before T.D.C. and not closing the exhaust valve until 12 to 15° after T.D.C. The total lap may be from 15° to 20°, which seems to be obtained by our latest cams when the cam angle is in the neighbourhood of 112°. Referring again to the height of lift it would appear that we must not alter the centres of the rockers, but must make the cams actually higher in the form. This also has been arranged. The cam timing appears to be ideal when the exhaust commences to open about 50° before B.D.C., and the inlet closes at 55° after B.D.C. More than this loses too much power at low speed and would also result in difficulty in silencing the exhaust, which should not be lost sight of. Late exhaust opening naturally favours this. We might here remark that salt cooled valves have been introduced in aero work with great advantage, though we think the cost may prohibit the use of these in car work. At the moment we are somewhat anxious as there has been stretching of exhaust valve stems in France, but the design of these particular valves was greatly improved during the 1929. Schneider engine tests. These improved valves should be adopted in our production work at the first possible moment, and we should not consider other changes until these have proved imperfect. (Valve design sent from WW. some weeks past.) R.{Sir Henry Royce} | ||