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).
Technical information sheet discussing the merits of by-pass versus non-by-pass motor car thermostats.
| Identifier | ExFiles\Box 156\2\ scan0170 | |
| Date | 13th July 1928 | |
| [Handwritten] Juli 1328 [Handwritten] T THE BRITISH THERMOSTAT COMPANY LTD. Windmill Road, Sunbury-on-Thames, Middlesex. TECHNICAL INFORMATION SHEET NO. 410. [Handwritten Signature] W Ryson MOTOR CAR THERMOSTATS. To By-pass or not to By-pass. For some years past, automobile design has tended to the use of higher mean effective pressures, lower weight and consequently smaller bulk of the power unit. This has greatly enhanced the importance of adequate cooling of the cylinder head more particularly with a view to avoiding the formation of steam pockets which might cause "pinking". It has long been established that correct temperature conditions can best be ensured by thermostatic control of the flow of cooling water. There are two schools of thought with regard to the manner in which control should be exercised. One advocates a thermostat which acts merely as a throttle valve to restrict the total flow of water in the circulating system; the other favours a thermostat which allows the maximum flow of cooling water at all times but divides this flow between the radiator and re-circulation round the cylinder block and head. Thermostats with no By-pass. This type of thermostat is simple in construction, opens progressively, and can be constructed to give a complete shut off when the temperature is below a predetermined value, or to allow a fixed minimum flow. It is a comparatively inexpensive piece of apparatus, but cheapness is its only claim to distinction; for from the technical standpoint it is open to very great objections. It is invariably necessary to provide a permanent leak past the head and greatly increases the time taken to warm up from cold, during which period the circulating pump simply churns the water. Before the desired temperature is reached there is a tendency for steam pockets to be formed in the hottest part of the head due to local boiling. When the valve does finally open it remains open until all the hot water has been forced out of the head and cylinder jackets into the radiator, and as the hot water can only be replaced by water from the coldest part of the radiator, the temperature of the engine is suddenly lowered very considerably. Apart altogether from the poor temperature control thus obtained, entirely unnecessary cooling stresses are set up which at the best contribute nothing to prolonging the circularity of the cylinder bores, and at the worst may cause real trouble. It is questionable whether it is worth while fitting a thermostat of this nature as no material benefit is obtained beyond the provision of a sales point. | ||