BETTER COOLING OR LESS?
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OF all the claims made by leading research engineers in ' recent years giving promise of a breakthrough in diesel engine design, a statement by Dr T. Priede of the Institute of Sound and Vibration Research, Automotive Group, at a symposium on pollution in Southampton in January of last year is probably the most significant.
In a brief reference during a discussion of the work of the Institute on minimal-loss cooling systems (CM Jan 17 1975) he said that currently 30 per cent of the energy input of conventional diesels was wasted by heat loss in the coolant and reducing this loss was the only means of improving diesel • characteristics.
Insulation of the combustion chamber, he claimed, had reduced coolant heat loss to 10 per cent enabling a lighter, stiffer engine structure to be employed—because the cross section of the water passages could be reduced correspondingly.
While the declared objective of the Institute in continuing research was to save fan power and to achieve a lower fan noise level (a stiffer structure would also help to damp mechanical noise) the most significant promise of the development in the author's opinion might well be a reduction in thermal gradients and the facility this would give to uprate engine performance by turbocharging. Heat loss to the exhaust would be increased by an amount approximately equal to the reduction of heat dissipated to the coolant and this would raise the potential blowing power of the turbocharger.
A very considerable reduction in the temperature gradient across the piston is an advantage in a fixed-head engine which has had scant publicity —but it is one which could account for the success of the Leyland 500 series diesel after a bad start in service due in part to the lack of head deck stiffness of the earlier engines.
Combining structural stiffness in the head area with a pattern of cooling-water flow giving adequate temperature control with appropriate regard to casting irregularities is a problem which would appear to be far more complex than than providing a robust crankcase structure.
A very high rate of heat dissipation from the hottest parts of a head can be obtained by controlling water flow in such a way that steam is formed on the surfaces, which is then imniediately replaced by liquid coolant condensing before it reaches the outlet of the block, thus making use of the high latent heat of vaporisation of water. But if a pocket of steam is formed which resists the inflow of coolant severe local overheating is inevitable. In a so-called steam-cooled petrol engine tried out in an application to a light aircraft 50 years or more ago, wet steam from the cylinder block was directed into the head where the water droplets were converted into dry steam, and this was later condensed in a high-mounted radiator.
A very high rate of heat dissipation was claimed, and it might be supposed that a modern well designed steamcooled system would have low temperature gradients, but a lot of development work would he required.
In any case, a high rate of dissipation is obtainable very simply: it is the provision of uniform dissipation from the hottest areas which should be the target. Could it be done the ISVR way by inhibiting dissipation from some sections of the static and working parts? What oonfused thinking overall I Cooling, thermal loading, combustion efficiency, durability and engine weight get all mixed up.
Unlike a petrol engine, there is no combustion advantage in preventing the combustible charge of a diesel from exceeding a critical maximum —which is necessary in the case of a spark-ignition engine to obviate detonation, although cool incoming air gives better filling.
Dr Priede has mentioned the use of insulation to inhibit heat dissipation to the cooling water and from the crown of the piston to the body of the piston. There was hope at one time that a heat-reflective combustion-chamber surface would enable more heat to be made available for useful work and the engine to run, cooler. Insulation might give some extra heat for conversion into crankshaft torque.
It is notable that Gardner plays safe about cooling, so the company's diesels operate at a relatively low temperature in defiance of the pundits' warning that overcooling causes condensation and that the condensate can produce rapid corrosion of metal parts and hasten deterioration of the lubricating oil. Gardner has successfully defined the pundits for a long time now.
Citing some of the variables in cooling technology has been deliberately haphazard, in an attempt to show the contrariness of the variables. But whether efficiency and durability depend on less cooling, more cooling, redirected cooling or low-gradient cooling, there is obviously more scope for improving the diesel by attention to temperature control than by any other line of research.
But a breakthrough technically is not necessarily a breakthrough commercially; if, for example, it was announced that the ISVR concept could be applied to standard types of diesel without much bother the news would create worldwide interest. But if application of the concept necessitated redesign of the block and head and retooling for production it could be shrugged off as another " ideal " which was unlikely to get off the ground for 20 or more years.
It took British Leyland a long time to get rid of the bugs in the fixed-head 500. And there were fixed-head petrol engines on the go before the first world war.