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).
Cause of slight explosions in the silencer due to a customer's driving habits.
| Identifier | ExFiles\Box 47\5\ Scan186 | |
| Date | 7th August 1928 | |
| BJ. c. Hm. {Capt. W. Hallam - Head Repairs} c. Hs. {Lord Ernest Hives - Chair} c. Wor. {Arthur Wormald - General Works Manager} ---------------- X4118 BY7/G.7.8.28. CHASSIS NO. GRJ-23. COLVILLE. ------------------------------------ Hm. {Capt. W. Hallam - Head Repairs} has handed over to me your memo BJ1/E. 25.7.28. and copy of Mr. John M. {Mr Moon / Mr Moore} Colville's letter of July 23rd. suggesting that it would be better for me to reply on the technical issue involved directly to you. Mr. Colville's trouble, you will remember, consisted in the fact that he insists upon driving his car with the controls in such a position as to ensure the engine running at a slow speed when he de-clutches, evidently this customer is extremely afraid of stopping his engine in traffic. The result of this practice is a series of slight explosions in the silencer every time he luffs up. The facts of the case are that providing the carburation is correct every car shows this same peculiarity, and there is no real cure that one knows of for the complaint. Every Rolls Royce car we have made will show the same complaint if driven under similar conditions, that is with the throttle always slightly open on the hand controls. Certain American cars which appear to be free from the trouble, notably the Buick, have overcome it by using an abnormally rich mixture at slow speeds. Under these conditions the small amount of gas which is passing through the engine and the silencer unfired is so rich as to prevent the setting up of the explosion, in other word the speed of the flame is so low that practically no increase of pressure over atmosphere throughout the exhaust system is attained. When a car is driven at fair speed with the throttle set slightly open on the hand controls, on releasing the accelerator lever the mixture drawn in through the slightly open throttle is so small, relative to the burnt gas left in the compression space, as to make the mixture too weak to fire, the temperature of the mass has been reduced, previously exhausted unburnt mixture is drawn back through the exhaust valve into the cylinder as the engine runs, and gradually the strength of the mixture is increased in the cylinder until it is just rich enough to fire, but the flame passes so slowly through the still relatively weak mixture that it is still burning when the exhaust valve opens and allows the hot burning gas to communicate with the silencer which has been in the meantime filled with a burnable mixture, providing combustion can be started by a fairly large source of heat and this it finds in the flaming mixture projected into it from the exhaust port of the engine as it opens after reaching the stage described | ||