The Operating Temperatures of Cast Iron and Aluminium Pistons in a 12-Inch Bore Oil Engine

Abstract

The engine used for these tests is of the single-cylinder vertical Diesel type, having a 12-inch bore and 18¼-inch stroke and running at 250 r.p.m. Temperatures have been measured at nineteen points on a ribbed “jam pot” cast iron piston, at twenty-three points on a “mushroom” type aluminium piston, and at fourteen points on a cast iron piston with a conical head support. Temperatures have also been measured at eight points on the cylinder liner. A description is given of the pistons and special testing apparatus. The influence of subsidiary junctions in the thermocouple leads, which led to some discussion following Dr. Mucklow's recent paper on piston temperatures,† has been studied. Errors, varying with engine output and amounting to as much as 29 deg. C., have been measured where no compensator was used. The effects of the temperature of the liner and of the jacket water outlet on the pistons have been measured, and are discussed. The conclusions are in general agreement with those of Dr. Mucklow except such as apply to the central portions of the heads, where the great differences of area exposed to flame lead to remarkably divergent results. The temperature distributions throughout the pistons are considered, and a quantitative estimate is made of the value of the neck, ribs, etc., when these are considered as heat channels. The radial temperature differences recorded for the aluminium and cast iron pistons and for the piston tested by Dr. Mucklow, are roughly in the ratio of 1/5½/19. The effect of varying the engine output has been studied, also the time required to warm up the pistons, the light alloy reaching its maximum temperature in approximately half the time required by the cast iron. A decrease in the number of rings from five to three is shown to have very little effect except in reducing the fuel consumption, while rings of forged “Y” metal slightly reduce the temperature of the piston. The effect of totally enclosing the piston shows that only some 2 per cent of the heat received is normally lost to the air circulating within it.