Poppet Valves and Cooling Systems
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Investigation of Valve Design and Materials Covers Most Aspects of Engine Efficiency By P. A. C. Brockington,
A.M.I.Mech.E. AT one time it was generally considered that the limiting factor in the development of petrol engines was the hot exhaust valve. The most important advantage of nonpoppet-valve designs was invariably given as the elimination of this bugbear to progress. It created the hot spot that was the chief cause of detonation, it had to be regularly ground in, and it was liable to early failure. Later, the oil engine was regarded as the unfortunate inheritor of an archaic valve systspg,
Improvements in ta,..oetane rating of fuels, and in combustion chamber form, removed most of the existing objections to the poppet valve for petrol engines with regard to performance, whilst improved valve steels greatly benefited the reliability and working life of both types of power unit. Today, grades of valve steel of the most advanced type are employed on only a limited scale because of their high first cost or production difficulties: they are not required for the majority of applications because cheaper, easily worked steels give a satisfactory operating life.
Wide Considerations
An investigation of valve design and materials involves considerations of engine features covering practically every aspect of efficiency, and the conclusion may be justified that many engine makers have ceased to appreciate the full importance of related factors in power-unit development. Valve problems that beset designers in earlier days have been satisfactorily overcome. With increases in output, however, a reappraisal of poppetvalve characteristics and allied cylinder-head features has become necessary, particularly in connection with turbocharged engines. It is (218 appropriate to associa e this reappraisal with a short reference to the challenge of the rotary engine.
Like the piston, the poppet valve has a reciprocating motion that involves very high mechanical stresses. In the case of a rotary valve applied to a piston engine and the valve ports of a rotary engine, no stresses are created, and when the port is fully open less resistance is offered to air or gas flow. The shape of the port during the opening and closing phases is not, however,, as favourable to free air flow as the annular passage provided by the poppet valve, and in practice it is probable that using any type of ported-sleeve principle will give inferior results operationally and economically to those offered by the poppet valve. Looking ahead, it may later be said of the poppet valve that it received full recognition as an efficient unit only when the piston engine was seriously challenged by a rotary type.
A problem common to the evolution of all types of improved heat engine is the necessity to combat severe thermal stresses caused by steep temperature gradients. While the complicated shape of a poppet valve assembly is not conducive to easy heat flow from the head to the water passages, the fact that the valve is a small component and is produced as a separate unit enables a high-grade material to be employed in conjunction with lower-grade cylinder-head and block materials. This is an allimportant consideration in production economy.
The fact that heat is not dissipated over a wide area to a surface at a much lower temperature may appear to be a fundamental disadvantage of the type. In practice, however, the temporary contact -between a very hot surface and one which is cooled to a much lower temperature in another part of the cycle can create exessive
thermal stresses. This is a feature of typical rotary valves and the valve ports of a rotary engine. It could well be, therefore, that the problems of thermal stressing, associated with the poppet valve, are far less critical than those that will be encountered in the development of a rotary engine. This has been demonstrated in some of the attempts to apply rotary or sleeve valves to piston engines.
Severe thermal stressing is normally caused by a "hot spot" rather than by a highly heated area, the heat flow from which represents a fairly gentle temperature gradient. In both cases, however, the effect of the hot area may be to reduce the volumetric efficiency, and therefore the performance of the engine, by overheating the incoming air. The fuel charge of a petrol engine is normally heated to aid vaporization of the fuel droplets when the engine is warming up and this lowers the volumetric efficiency. Because the combustion air of an oil engine is aspirated without fuel, a large reduction in air temperature has no attendant disadvantage.
Gains that could be derived from increasing volumetric efficiency of a normally aspirated engine by air-temperature reduction and the use of a free-flow or ram type of induction system are much more substantial than may be generally appreciated by engine manufacturers. It is possible that without any external cooling aid, applying known principles to standard types of oil engine could enable the brake mean effective pressure to be increased from a figure of around 105-110 p.s.i., to say, 130 p.s.i. Increasing the density of the air and reducing its temperature improves combustion characteristics, and a higher torque could be obtained without a corresponding increase in bearing pressures.
Higher Output Possible
Limiting factor in the output of the typical oil engine is its inability to burn more than about 65 to 70 per cent. of the air in the combustion chamber when running on full load. At peak r.p.m. air utilization is even more difficult and any increase in the weight of air aspired could enable the unit to operate at a higher speed as well as a higher b.m.e.p. An overall increase in output of some 35 to 40 per cent, is not impossible and the engine would be running under more favourable conditions than a unit that was turbocharged to give a comparable output.
The shape of a valve can be varied within fairly wide limits to improve air flow and to aid turbulence in the combustion chamber. A notable example may he quoted of petrol engine practice in which the metal under the head of the inlet valve is of convex form to match the shape of the hemis
phericai combustion chamber when the valve is open. Matching valve shape to that of the combustion chamber might also be applied to oil engines with conventional combustion chambers as a means of promoting air movement in the chamber without a mask.
Since it was discovered that cylinder wear could be greatly accelerated by condensation of the acidic products of combustion, accurate control of the cooling system to give a minimum operating temperature has been considered a necessary feature of up-todate designs. It is in practice necessary, however, to consider the cooling of the cylinder bores and of the cylinder head separately.
Rapid Deterioration
Because piston friction represents a high proportion of the total mechanical losses when an engine is running, a low bore temperature is a wasteful factor with regard to output and fuel costs. Moreover, oil sludge is produced below a critical temperature, and this results in rapid deterioration of the sump lubricant. Maintaining a minimum coolant temperature is, therefore, imperative to avoid unnecessary friction losses and contamination of the oil, despite the fact that advances in oil blending have enabled friction to be reduced at low temperatures and the formation of cold sludge to be partially inhibited.
When dealing with head cooling. volumetric efficiency is the main operational consideration. It is now considered by many leading authorities that the cylinder head of petrol engines and oil engines should run as cool as possible, any reduction in the average temperature being beneficial. Although (in contrast to the petrol engine) a high air temperature in the combustion chamber of an oil engine at the start of the combustion process is normally an advantage, a cool-running head does not, in effect, extract useful heat from the combustion chamber in any part of the outputr.p.m. range. Separate cooling of the cylinder head and cylinder block is. therefore, to be recommended. Because of the large difference in temperatures internally between the cylinder head and cylinder bore, a reduction in cylinder-head temperature would be advantageous with regard to thermal stressing.
A particular aspect of cooling system problems of more immediate concern to vehicle operators is the variety of problems resulting from air entrainment in the system, which may be considered in conjunction with the formation of steam pockets. The latter
are generally the result of hot-spot formation by inefficient coolant flow but the engine is more prone to this fault if air is entrained in the coolant. Air entrainment can be caused by a high rate of water circulation through a top tank of insufficient volume, or by an air leak on the intake side of the pump. Such an air leak can exist 'despite the absence of a water leak , when the engine has stopped running.
Impaired Circulation
When air is circulated through the cooling system with the water it can impair circulation in a section of the engine by virtue of its high expansion with a rise in temperature. This may also cause trouble by displacing a large volume of coolant and causing wastage of water through the overflow. In a critical case pressurizing the system will not eliminate aeration. Although it may not cause damage to the engine. it may necessitate daily or frequent topping-up of the system, and in some cases wastage of anti-freeze solution.
These comments may be of more than academic interest to some
operators in that they highlight the possibility of substantial power loss from faults in the cooling system as well as the damage they can cause. They also show that cooling the head and cooling the aspirated air should be considered .together. Feeding cool air to the intake and maintaining the cooling system in good order can. result in a valuable gain in output and fuel economy.
It may be pertinent to add as a footnote that a number of engine makers has carried out extensive research work on cooling problems in recent years. In no case, however. can full technical data be obtained for publication. The information is too valuable to pass on to other manufacturers.