542 Moving componentspistons

While in the cooling of cylinder liners and cylinder heads the relative motion between the coolant and the metal surface is imposed solely by the coolant pump, in piston cooling there may be additional relative velocities due to the acceleration of the piston. Indeed, with a 'cocktail shaker' type of piston, these velocities are the dominant ones.

Investigations into piston cooling have to be carried out either on an engine or on a rig with reciprocating motion of the piston. Ricardo's rig studies have been carried out on the rig described in Reference 13. The piston is electrically heated and the leads from the piston thermocouples are carried away along a swinging link. Coolant is fed to and from the test piston by means of articulated pipes. The rig can operate at speeds of up 8.5 rev/s, but for higher speeds, tests on an actual engine have been carried out, as described in the same Reference.

Very much higher heat transfer rates are obtained with water cooling than with oil cooling. There would therefore be much to be said for employing water colling of the pistons in highly rated engines. Unfortunaely the problems of obtaining a completely liquid-tight system are such that it is not possible to do this in a trunk type piston engine since water leaks into the lubricating oil are inadmissible.

The large slow speed main propulsion marine diesel engines however are of the crosshead type, and they incorporate a diaphragm seal at the crosshead to prevent the lubricating oil from the cylinder bores and the blow-by materials from contaminating the crankcase lubricant. Under these circumstances water cooling of the piston is feasible and is in fact employed on a number of engines. An increase in heat transfer coefficient of 5-10 times is obtained on low speed engines by the use of water cooling.

For trunk piston engines, oil cooling of the piston has to be used. Two basic types of cooling are employed, although the actual design of piston may incorporate both types.

(i) 'Solid' ms 'flow systems In this type the cooling passages run full of oil at all times and there is no cocktail shaking. It is found that the heat transfer coefficient is unaffected by the speed of rotation of the engine but is a function of the normal fluid parameters which affect heat transfer under pipe flow conditions. Under typical engine conditions it is found that the heat transfer coefficient obtained is about 1150 W/m2 K.

(ii)'Cocktail shaking' This is strongly influenced by the engine speed but hardly at all by flow rate. Experimental work carried out by Bush14 has shown that it is rotational speed rather than piston speed or velocity that is the important parameter, and the author and his colleagues have obtained good correlations using similar non-dimensional functions to those developed by Bush but with somewhat different indices.

The general equation under cocktail shaking conditions for the rig described in Reference 13 with both oil and water coolants

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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