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).
Big end loading in Bentley engines, comparing standard and large bore units.
| Identifier | ExFiles\Box 82\3\ scan0344 | |
| Date | 1st June 1935 | |
| To E.{Mr Elliott - Chief Engineer} c. to Wor.{Arthur Wormald - General Works Manager} c. to By.{R.W. Bailey - Chief Engineer} Hs{Lord Ernest Hives - Chair}/Smth.2/KW.1.6.35. Big End Loading - Bentley. Hs.{Lord Ernest Hives - Chair}1993/7/8 show polar diagrams of the big end loading in a standard engine with narrow pin rods and Aerolite pistons. It will be seen that the maximum inertia loading and the maximum gas pressure loading are very similar, in which case one would expect the severest conditions for the big ends to occur when the engine is working at full torque and low speeds. Under such conditions it is difficult to maintain an oil film because of the low rubbing velocity. This is confirmed by our general experience that the top half of the big end bearings suffer worse than the lower, and in particular by the strip report on 5-B-IV after 5,000 miles in France. The max. gas pressure loading is 2950 lbs/sq.in. which is almost exactly that on the Kestrel VI assuming a max. cylinder pressure of 600 lbs/sq.in. It is doubtful whether this pressure could be safely exceeded to any appreciable extent in view of the fact that the Bentley oil temperature sometimes reaches 120°C. It appears that for the standard bore unit, the narrow pin rod gives just about the right balance between gas pressure and inertia loadings. Large Bore Unit. The piston area on the large bore unit is 16% greater than standard, which means that for the same compression ratio and crankpin length the maximum big end loading is 3,420 lbs/sq.in. This loading would be reduced by about 9% if the compression ratio was dropped to 6/1, and the power would be reduced by 3 1/2%. The inertia loading is the same as on the standard engine since the pistons are of equal weights. | ||