From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Paper on aircraft engine lubrication, detailing test conditions, methodology, and results for oil performance.
| Identifier | ExFiles\Box 154a\2\ scan0013 | |
| Date | 1st January 1939 | |
| January, 1939 AIRCRAFT-ENGINE LUBRICATION 13 show in an engine in which the rate of oil consumption is capable of adjustment by control of the pressure and rate of circulation of the lubricant, that the rate of sludge production with a particular oil increases with the oil consumption. Experiments of this nature are best done without the addition of fresh “make-up” oil which naturally tends to mask the differences arising from changes in consumption. In view of the complexity of the problem it is not surprising that oxidation experiments performed on oils in the chemical laboratory are ineffective for judging the sludging properties of the oils in service. The conditions of the usual oxidation test carried out in the temperature range of 150 to 300 deg. cent. entirely fail to reproduce the changes which take place in the lubricant while it circulates between the crankcase and combustion space. The relative rates of formation of asphaltenes in a series of oils undergoing oxidation may be parallel to those which occur in a carefully controlled engine test, but the results of such experiments merely indicate the general resistance of oils to oxidation, a property which bears only a very obscure and indirect relation to behavior in service. Conclusion In the present state of knowledge it is without doubt essential to evaluate the performance of aircraft oils in every important property, except that of viscosity, by actual tests in engines. A great deal of the preliminary work may be carried out in suitable small units, but the final judgment remains with the full-scale engine. Finally, the authors desire to thank the directors of The Asiatic Petroleum Co., Ltd., for permission to publish this paper. Appendix 1 Single-Cylinder J.A.P. Air-Cooled Engines These engines are production units made for motorcycles. Their dimensions are as follows: Bore and stroke (cast-iron cylinder), 62.5 x 80 mm. Swept volume, 250 cc. (15 cu. in.) Aluminum-alloy piston with two cast-iron pressure rings. Compression ratio, 5.2:1 approximately. Each engine is direct coupled to an electric brake. Cooling is provided by a variable-speed, electrically driven fan which supplies air through a duct to the exhaust side of the cylinder. For temperature control, nichrome-constantan thermocouples are fixed in four pockets drilled tangentially in the cylinder barrel between the top and second cooling fins, (see Fig. 4). For regulating the speed of the cooling wind the reading of the thermometer on the “inlet side” is used. This thermometer is diametrically opposite to the “exhaust side” of the cylinder on which the cooling wind impinges. A thermocouple also is fixed in the exhaust pipe near to the exhaust port. The exhaust pipe is lagged with asbestos to prevent the cooling wind from reaching it. Temperature readings also are taken, by means of a movable thermocouple, on the “inlet side” of the cylinder-barrel at two points vertically below the fixed “inlet side” pyrometer. The lower point is in line with the lowest position of the ring travel, and the intermediate point is halfway between this point and the fixed thermocouple. The standard “Amal” carburetor is used with the addition of a variable jet for controlling fuel consumption. A fuel flowmeter is provided. Engine lubrication is on the “total-loss” system. Oil is supplied at a measured rate, by pump, to the timing-gear chamber, from which it flows to the internal flywheels and is thrown to the cylinder. The roller bearing big-end is lubricated by splash. A special magneto is used provided with a hand-timing adjustment of wide range. Spark-plugs of racing type are employed. Fig. 4—Relative positions of thermocouples in J.A.P. engine cylinder Test Conditions Engine speed, 3000 r.p.m. Load, approximately full throttle (B.m.e.p., 90-100 lb. per sq. in.). Fuel consumption, 4 pt. per hr. (3.6 pt. per hr. for tests at “10 per cent weak”). Lubricating-oil feed, 1.3 cc. per min. (for normal conditions). Cylinder-barrel temperature (at fixed thermocouple on inlet side), 250-310 deg. cent. (depending on the oil and conditions of test). To obtain consistent results it is essential that the diametral clearance of the piston skirt and the side clearance of the piston-rings in their grooves should be kept within narrow limits. The diametral clearance of a new piston in the cylinder is adjusted to the following values at various points on the piston: Bottom of skirt, 0.008 in. Top of skirt, 0.010 in. Second land, 0.014 in. Top land, 0.025 in. When the clearance at the bottom of the skirt, owing to wear, exceeds 0.012 in., it is necessary to fit a new piston in order to obtain constancy in the ring-sticking temperatures. The side clearances of the piston-rings in their grooves are adjusted to ±0.00025 in., and total clearances in the range 0.0015-0.003 in. are used generally. The life of a standard cast-iron cylinder is usually 600-1500 hr., but pistons generally require replacement after about 100-200 hr. For tests under detonating conditions a fuel of C.F.R. octane number of 55 (Motor Method) is employed. For “no detonation” a gasoline of octane number about 70 is required. The general method of comparing the ring-sticking performance of lubricating oils is to determine the cylinder tem- | ||