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Engine cooling is still a major problem
Page 75
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C INCE Sir Harry Ricardo publicized the results of laboratory tests some 40 years ago showing that the acid products of combustion caused rapid cylinder bore wear if the engine was overcooled, the urgency of the cold corrosion problem has, to some extent, been alleviated by the availability of detergent-type lubrication oils and the general use of pressurized cooling systems combined with thermostatic control and improved cylinder materials.
This alleviation of the problem has tended, however, to obscure the benefits that can be obtained by rapid warming up and operation at the optimum cooling temperature and, although the subject has been reviewed in technical detail in many papers of the learned societies, it has not been given the practical attention it deserves by vehicle manufacturers generally.
In agreement with this assertion, Mr. Graham J. Ibbitson, chief engineer, commercial vehicles, of the British Motor Corporation, emphasizes that stop-start running presents a notably difficult problem with regard to quickly attaining and maintaining an acceptable coolant temperature; he contrasts this type of operation with climbing a steep gradient at a high altitude in a tropical area overseas when the problem of accommodating the low boiling point of the coolant is normally exacerbated by a high ambient temperature and a total absence of ram effect.
Without a positive controllable means of ensuring a constant jacket-coolant temperature with wide variations of ambient temperature, wind force and altitude, it is impossible to provide a cooling system that is suitable for both extremes.
In practice, the problem may be divided into three categories, namely stop-start work in a cold or temperate climate and substantially continuous running in first a cold climate and secondly in a tropical climate. Changes in altitude can further aggravate the problem in the tropics.
Mr. Ibbitson affirms that in the stop-start category prevention of overcooling eliminates a condition that can reduce the life of the unit between major overhauls by as much as five to six times and that it may afford a material improvement in fuel consumption. When running continuously in a temperate climate, deterioration of engine condition that results directly from overcooling is, in a typical case, limited to periods of lightload running. In a normal working day it may be marginal, but it has been shown that the acids produced by combustion when operating at, say, 20°F. or more below the optimum temperature, can cause accelerated depletion of the detergent additive in the oil and the formation of cold sludge.
This can increase the rate of wear of the piston rings, cylinder bores and bearings (or cause failure) if the lubricant is not changed at reduced intervals. At the tropical end of the scale, boiling can be prevented by increasing the number and pitch of the fan blades and by close-cowling the fan. This, however, wastes power excessively at lower temperatures and may cause severe overcooling during night running, and so on, with a light load.
Although an efficient thermostat is a valuable aid to rapid warming-up and controlling jacket temperature, Mr. Ibbitson emphasizes that other control devices are necessary to prevent a large difference between the jacket inlet and outlet temperatures. Reducing the temperature gradient across the cylinder block to 10-20°F. is essential if an adequate cylinder block temperature is to be provided and if undue thermal distortion is to be avoided. To optimize engine life and performance, it is necessary to raise the radiator temperature (to give an engine outlet temperature of say 185°F.) which requires sufficient fan power to cope with the heated coolant in the radiator and the heat produced by the engine, notably if it is required to operate on full throttle with zero ram effect. To cater for this condition and to obviate wastage of power under other circumstances, a fan, the output of which is controlled thermostatically, is required; it may take the form of an "on-off" electrically-driven unit, a mechanically-operated fan with an on-off variable drive or an hydraulicallydriven progressively-variable unit.
For best results For the best results, it may be necessary to use the fan in conjunction with an automatically-controlled radiator shutter actuated by the same thermostat. A radiator shutter may be functionally and economically preferable alone, without the adjunct of a controlled fan, despite resultant power losses. No fan or fan control equipment currently exists, Mr. Ibbitson claims, that has a sufficient range to meet all conditions of operation, apart possibly from the SerckBehr variable-output hydrostatic type incorporating shutters, which is at present too costly for applications to commercial vehicle engines. Reducing the temperature difference between the jacket and radiator also facilitates temperature control by minimizing the effect of transient thermostat opening by coolant surge (in the case of the bellows type) and of non-sensitivity of the thermostat, the latter resulting in an excessive amount of coolant passing to the radiator during "borderline" conditions.
Parasitic heat losses through the hose connections and radiator tanks are undesirable, Mr. Ibbitson points out, because they are uncontrollable. Mr. Ibbitson considers that the sealed "no-loss" system that can operate for around two years without attention would offer special benefits in terms of maintenance and operational consistency, the use of an anti-freeze solution with appropriate inhibitors being essential to such a system. Pressurization to 10 p.s.i. is also desirable, the provision of hoses, gaskets and seals to withstand this pressure for the stipulated period being an obvious necessity.
The position of the radiator in relation to the engine block is not important, in Mr. Ibbitson's opinion, so long as the length of the hose connections is reduced to a minimum. A cross-flow type of radiator may be essential to suit installation requirements because a wide low-height core can be employed, the maximum possible height being preferable in the case of the verticaltube radiator.
Because air-entrainment can result in the formation of dangerous hot-spots, Mr. Ibbitson recommends de-aeration of a cross-flow system before the vehicle is employed in service. In his view, a high rate of coolant circulation is unnecessary and may be disadvantageous in that it can promote cavitation erosion and create turbulence of the coolant locally.