ACCURATE control of engine coolant temperature is one thing. Making
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provision for good cooling of the cylinder block is a much bigger thing. To maintain the coolant in the cylinder block at a more or less constant temperature with changes of load and ambient temperature is important. And welltried means are available that give automatic control, the only snag being the extra cost and some added complexity. But if everything is done that can be done to prom ote accurate control—including the use of thermostatically controlled radiator shutters and fan drive, the best type of wax thermostat, and a source of external heat to warm the coolant before the engine is started—an engine may still be badly cooled.
There's no such thing as a perfectly cooled engine. The water has to flow in a tortuous way through passages having large variations of section over surfaces that give out a lot of heat in one area and absorb heat in another, and it frequently has to change direction through a sharp angle. In a conventional removable-head engine there is a mass of metal at the join of the head and the block that impedes cooling at the hottest part of the bore (and the piston at td.c.) and the space above the combustion chamber is cluttered up with valves and passages for the air or gas. Moreover, the water passages have a rough surface which can increase flow turbulence and the likelihood of hot spots. Entrapped air can make matters worse and if the block liners are of the wet type, cavitation erosion of the liners may be severe. Reducing the temperature differences or gradient across an engine or parts of the unit is typically of greater importance than maintaining the optimum temperature of the coolant in the block.
Better to overcool?
It may be better to overcool an engine if this induces the temperature gradient than to run it at the nominal optimum temperature. The temperature gradient across the engine (that is between the inlet and outlet of the block) gives some indication of the efficiency of the cooling system, but even if the gradient is commendably low (say 10 deg C) the gradients across the bore or through sections of the head may be high.
A high temperature difference causes distortion, and allowing for distortion is often the designer's biggest headache. Making allowance for expansion and contraction with variations of engine temperature is a relatively simple exercise, given that tem
perature gradients are small at all temperatures and are predictable. Two basically similar blocks may produce different coolant flow patterns because of casting variations. In service the pattern may change because of local deposits, which may also impair heat dissipation.
It is axiomatic that the clearances of all working parts should be as small as possible compatible with reliable operation with changes of load, speed and running conditions generally. A steep temperature gradient necessitates the use of relatively large clearances to cater for distortion.
Valves are highly functional components of an engine and their number and layout are normally determined by the "filling" requirements of the engine, with due regard to ease of operation and production cost. Inclined valves are almost universally employed in racing car engines in conjunction with a hemispherical head, or the equivalent, to give good combustion characteristics. They also enable the temperature gradient across the valves to be reduced and a higher heat flow to be accommodated without undue distortion, which can cause cracking of the head. If due consideration were given to cooling efficiency, it might be concluded that changing the valve layout at considerable cost would be a worthwhile modification as an aid to cooling.
Local hotspots
From time to time the cooling characteristics of the cylinder block of a new engine are publicized with claims that the direction of coolant flow gives improved cooling of the hottest parts of the head or a reduced overall temperature gradient. But a better system of cooling will have little practical advantage if local hotspots can develop as the result of turbulent flow or pockets of inert fluid.
How many makers investigate the coolant flow and heat gradient of a prototype engine with the aid of thermocouples? The answer to this question is not known, but the exercise can be a prolonged and costly one, improvements taking the form of small changes in many places. To incorporate the necessary modifications, would not, however, add to the production cost in a typical case.
I recall mention of a prototype petrol engine produced by a well-known maker, one cylinder of which produced some 25 per cent less power than its rated output. Modifications had been made to the exhaust and inlet manifold without improving the output and checks made of gas leakage and so on. An engine re-design was contemplated but before such a serious step was decided upon a consultant was called in, and he traced the fault to a hotspot caused by an inert pocket of coolant in a passage between the combustion chamber and the intake. It was found that the water in the pocket boiled but was not replaced by a continuous supply of water from other parts of the head (which would have increased the rate of cooling) and dry steam formed in the pockets. The water that continued to boil on the outside of the pocket condensed before it reached the outlet and no tell-tale bubbles were produced. Re-shaping the water passage cured the trouble.
Reducing the temperature gradient across the engine may be achieved by increasing the rate of water circulation. And a steep gradient locally may be dealt with in the same way by the use of a water manifold or by appropriate shaping of the passages. Increasing the rate of flow across the engine or locally is, however, a risky business because it can produce eddy currents and create local hotspots.
The moral of these observations is that good cooling starts in the cylinder block and with painstaking attention to flow patterns in every area. Improvements in casting techniques could play an important part in promoting better cooling. Although the use of an aluminium block to enable improved coolantpassage shapes and contours to be provided might be regarded as a counsel of perfection (a pressure die-cast block might well be the ideal) it could be the deciding factor in its adoption as an added advantage to reduced weight. If an engine designer was confident that the maximum temperature across the engine or locally would not exceed say 7/8 deg C, it could well be that he could evolve a unit having a much longer life than an average unit and that he could increase its power output very considerably without reducing its reliability. The potential gains of "perfecting" the cooling system combined with accurate control of temperature are multiple.
In addition to improved life and an increased power potential, they include a lower fuel consumption, a longer period between oil changes, and a reduced tendency to produce diesel smoke when the engine is first started or is running under light load. If full advantage of the system were taken by the designer it might well be possible to provide a quieter engine at a lower cost.