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).
The function of engine cooling by-pass systems and general observations on bellows thermostats.
| Identifier | ExFiles\Box 156\2\ scan0171 | |
| Date | 28th December 1938 guessed | |
| - 2 - By-Pass Systems. When a by-pass is provided from some position below the valve head back to the suction side of the pump, circulation through the cylinder block and head takes place irrespective of whether the thermostat is open to the radiator or closed. As the engine warms up the thermostat opens progressively and a proportion of the water is allowed to pass through the radiator. A by-pass of small cross sectional area is of little value as although it does permit a permanent flow round the engine it is incapable of passing sufficient water to produce that high velocity in the head which is necessary to prevent the formation of steam pockets. The control also improves as the by-pass size increases, because the quantity of cold water sucked in during a given period decreases as the by-pass flow increases, and hunting is thus reduced. The by-pass should therefore be of sufficient size to ensure that adequate circulation does take place at all times, but some provision must be made for closing it off when the temperature has risen above a predetermined level. It is not sufficient merely to provide a large diameter by-pass without making any provision for closing it off as otherwise the overall efficiency of the cooling system is reduced due to the return to the pump of hot water which has by-passed the radiator. It is possible to increase the size of the radiator to overcome this loss, but as the resulting increase in cost of the radiator would greatly exceed the cost of making provision for closing off the by-pass, this is obviously not the solutionl General Observations. There is no secret about the effect of water pressure under the head of a bellows thermostat. If the area of the head is greater than the effective area of the bellows, the water pressure will tend to force the valve open; and if the area of the head is smaller than the effective area of the bellows the water pressure tends to lock the valve on its seat. It is usual to make the valve slightly out of balance so that it tends to open with increase in pressure, because bellows dimensions do vary very slightly, and the effect of out-of-balance in the wrong direction might be disastrous. Finally, the differential of a bellows-operated thermostat, that is to say, the difference between the temperature at which opening commences and is complete, is normally about 10°C. and there is little to be gained by reducing this differential since such a reduction tends to produce a "hunting" control due to the escape to the radiator of more hot water and the entry to the engine of more cold water than is necessary to maintain the desired temperature level. = = = = = = = = | ||