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).
Engine cooling system thermostat and by-pass operation, including water flow timings and temperature control.
| Identifier | ExFiles\Box 156\1\ scan0101 | |
| Date | 20th May 1935 guessed | |
| - 4 - Messrs. Rolls-Royce Ltd. water entering the suction side of the pump takes 10 seconds to travel through the engine and to reach the thermostat mounted in the outlet pipe to the radiator, you will agree that when the thermostat opens discharge of heated water to the radiator will occur for 10 seconds, that is to say, until such time as the colder water entering the pump suction has travelled round the engine and made contact with the temperature sensitive element of the thermostat. During this period of 10 seconds when the thermostat is open, water is entering the pump suction both from the by-pass and from the radiator. If the by-pass is small, the ratio of cold to hot water entering the engine will be large, and when the thermostat closes once more the final temperature within the engine after mixing, will be appreciably reduced below the temperature at which the thermostat opened. As the area of the by-pass is increased, the ratio of cold to hot water introduced into the engine while the thermostat is open is reduced, and consequently when the thermostat closes, as a result of the passage of the cooler water round the engine, the quantity of cold water introduced is lessened, and the final temperature after mixing is nearer to the temperature at which the thermostat opened than was the case in the first example. You will notice from the prints which we sent to you that a number of our customers use a by-pass no less than 15/16" diameter, the principal object of this procedure being to decrease the ratio of cold to hot water entering the engine when the thermostat opens. We hope that this point is clear to you and that it answers your question satisfactorily. Your suggestion that the accuracy of control corresponds to the difference between the temperature at which the thermostat commences to open and that at which it is fully open, is clearly fallacious, since the thermostat cannot possibly prevent the water temperature from rising above the temperature at which it is fully open should the engine be sufficiently loaded and the capacity for radiation be suitably reduced. | ||