Proceedings of the Institution of Mechanical Engineers

The Admiralty cathode ray oscillograph indicator was developed for use on compression-ignition engines of medium- and high-speed types. The size of the engine has not necessitated the development of the smallest possible unit. It consists of a pressure-sensitive device based on the magnetophone principle. The pressure to be recorded deflects a steel diaphragm and changes an air gap in a magnetic circuit. The magnet is energized by a high-frequency (20,000 cycles per sec.) alternating current flowing in an encircling coil. The pressure on the diaphragm modulates the high-frequency current, and the modulations are observed on the screen of a cathode ray oscillograph. The advantages of a high-frequency current in this connexion are: (1) all frequencies below about 2,000 cycles per sec. are faithfully reproduced; (2) steady pressures are recorded and calibration is effected with an ordinary pressure gauge; (3) amplifier and circuits have to transmit only a comparatively narrow band of frequencies. A disadvantage is that the upper limit of frequency (2,000 cycles per sec.) is not sufficiently high for the study of very rapid variations. The precautions necessary to ensure accuracy are discussed, and some examples of typical records obtained with the instrument are reproduced.

Increases in the speed and rating of internal combustion engines have necessitated the replacement of conventional engine indicators by the recently evolved cathode ray oscillograph engine indicator. The devices for converting mechanical pressure into an electrical effect fall into three classes according to whether (1) an electrostatic, (2) a resistance, or (3) an electromagnetic effect is used. Only the first and last classes have been found practicable, the first for rapidly varying and transient effects, and the last for slower phenomena. The members of each class can be used with one of three types of electrical energizing circuits: (1) self-excitation, (2) direct-current excitation, or (3) alternating-current excitation. Success in dealing with a particular problem depends upon the choice of a suitable combination of the device sensitive to pressure, and the electrical energizing circuit. The three most useful combinations are the piezo-electric crystal, and the moving iron, or magnetophone, with either direct-current, or high-frequency alternating-current excitation. Magnetostriction and the condenser microphone, both with either direct- or alternating-current excitation, will prove useful when developed further. The essential features of design for the study of the most difficult problems are now understood. The present form of these engine indicators is useful, but there is need for greater accuracy and reliability.

If a ferromagnetic material, for example nickel, is mechanically stressed, the magnetic permeability changes; the effect is large. The paper shows how the effect can be used to study transient mechanical stresses. The stress is applied, by suitable mechanism, to the ferromagnetic core of a transformer or coil, and the resulting magnetic changes of the core are observed, by the help of appropriate electrical circuits, on the screen of a cathode ray oscillograph, or otherwise. The conditions to be observed for success are set out, and numerical data are given as a guide to the design of magnetostriction stress-measuring devices.

The paper briefly reviews the problems arising in, and the system employed on, the lubrication of large modern steam-turbine electric generators. A description is given of the general layout, enumerating the various components and the reasons for their inclusion. The general qualitative relations governing the operation of film-lubricated bearings are briefly stated; published results of full-scale experiments on commercial types are compared; and a simple method is given for estimating the power absorbed, based on practical experience. Various types of oil pumps, and the principles adopted in fixing their capacity are described, the results on gear type pumps being shown by several curves.

The development of modern forms of oil-cooling apparatus is traced, attention being given to factors affecting the rate of heat transmission obtainable; a series of tests carried out on a modern type are described. Curves showing the relation between heat transmission rates and oil and water velocities obtained from these tests are reproduced. The disposition, design, and capacity of oil tanks are discussed, together with the practical aspects of piping layouts. The concluding section deals with the properties of lubricants, factors contributing to deterioration, and methods of purification commonly employed.

Based upon a hypothesis of surface anisotropy of an otherwise isotropic material, a criterion of the yielding of a ductile metal is suggested which embraces the several well known criteria as special cases. This criterion for a system of principal stresses X, Y, Z (= 0) takes the form = constant; or = constant, which for component shear and direct stresses, q and f respectively, becomes = constant, where f/q must fall in the range √2 to 2.

The proposed criterion is considered to be applicable in character to fatigue of a ductile metal, and it is applied to derive, from direct and shear stress fatigue test results, limiting fatigue stresses for any combination of shear stress and direct stress in phase, which is the most severe condition. Analysis indicates that the presence of static shear stress may increase the ability of a steel to withstand reversing direct stress. Apparently an opposite effect occurs when direct stress fatigue test results conform to the Goodman or modified Goodman diagrams.

The paper deals with an investigation of the fluctuations of pressure, due to piston motion on the exhaust stroke, which occur in the exhaust pipe of a single-cylinder four-stroke engine. Indicator diagrams of exhaust-port and of cylinder pressure, and measurements of air consumption were recorded, using exhaust pipes of three different diameters at three standard engine speeds; the exhaust pipe length was varied over a wide range in each case. In the light of the data thus obtained, the effects on air consumption of progressive alterations in valve timing were studied under known conditions of exhaust port pressure. Further trials were then carried out in which the valve timing which gave the maximum air consumption was determined for the full range of conditions of speed and exhaust pipe dimensions. The experimental results are discussed, and a method is derived by which the pressures in the exhaust port throughout the cycle may be obtained from theoretical considerations; the method is also directly applicable to induction pipe conditions.

Hydraulic power development may be divided into two groups, first, one in which mechanical and civil engineering developments combine to use natural forces for power production, and second, the phase of hydraulic power development which it is proposed to consider, namely that appertaining to mechanical production and automotive traction. Until recent times, no phase of engineering development has remained comparatively stationary for so long a period as that of power operation by hydraulic means. It is true that there has always been a field where the characteristics of hydraulic power were essential, but in the more recent drawing and stamping industries, hydraulic operation was unable to compete until developments in collateral fields removed all disadvantages.

Prominent amongst the developments to this end, has been the introduction of the variable-delivery pump to each machine, as against the central pumping station. These self-contained units are available with a normal maximum pressure of 1,000 lb. per sq. in., but in certain cases no less than 5,000 lb. per sq. in. can be obtained.

The authors stress the need for a sound theory of circulation in water-tube boilers, to enable weight and space to be saved and efficiency and reliability to be increased. They establish, as a standard of excellence with which other boilers may be compared, a simple boiler consisting of a single drum and a single U-tube.

The paper consists of three parts. Part I describes the nature of the change from water to steam; in part II the flow in the simple U-tube boiler is calculated; and in part III the features of various departures from the standard U-tube, met with in practice, are discussed.

An improved type of tube, approaching very nearly to the standard U-tube, is evolved, allowing much simplification of boiler design, with the advantages of excellent circulation, high gas speeds, and higher heat transmission rates. Other rational simplifications and improvements are also rendered possible, which enable substantial reductions to be made in the cost of manufacturing, installing, and maintaining boilers of any pressure and capacity, for any service, ashore or afloat.

The MS. of this paper was received at the Institution on 10th November 1938. The paper was presented for discussion at an Extra General Meeting of the Institution on 1st March 1940.

The dynamics of a pendulum damper is treated in a more general way than hitherto, all known types being included. A new method of considering the effects of a pendulum damper when fixed at any point in an engine system is developed, showing that its action results in an alteration or displacement of the critical speeds at which the harmonic orders occur.

A convenient method is outlined for arriving theoretically at the best design for a pendulum damper, whatever may be its location in the engine system. As a practical illustration, a brief account is given of experimental work which was undertaken to test the behaviour of a well-known engine when a certain pendulum damper was fitted to the flywheel. A further account of a similar application to a six-cylinder engine is also given. Special forms of pendulums, and a note on the estimation of angular amplitudes, are discussed in Appendix I, and a bibliography and a short list of the most important patent specifications relating to pendulum dampers are also included.

The paper describes experiments carried out on an ammonia-vapour compression refrigerator, with the object of investigating the conditions affecting the volumetric efficiency of the compressor. The most important part of the investigation deals with the effect of the condition of the vapour at the suction valve, over a range from about 0·75 dry to 45 deg. F. superheat, on both the actual and the indicated volumetric efficiencies. The actual volumetric efficiency drops about 20 per cent between the superheated and the wet conditions, but the indicated efficiency shows little variation. The results lead to the conclusions that the decrease in volumetric efficiency is due to condensation of ammonia on the cylinder walls during the compression and delivery stroke, and that the temperature of the cylinder wall is the critical factor determining the vapour condition at which the drop in efficiency begins. Other variables investigated were the condenser and evaporator pressures, and the compressor speed.

The paper records the results of a research into the comparative elastic and fatigue torsion strengths of bars of circular, square, and rectangular cross-sections.

The static torsion tests reveal that in the case of square and rectangular sections the use of enlarged ends leads to discrepancies between the experimental and theoretical values of the elastic torsion strengths.

The fatigue torsion tests disclose astonishing fatigue weaknesses in square and rectangular bars, the weaknesses being masked when the ends of the specimens are enlarged.

The fatigue torsion strengths of square and rectangular bars are compared with the fatigue torsion strengths of circular shafts with keyways and it is emphasized that outstanding fatigue weakness may be associated with certain forms of stress distributions even when high stress concentrations are absent.

Consideration is given to the value of the static properties of metals as safe criteria of fatigue strengths.

In an appendix the problem of the raising of the elastic stress in a material when the distribution of stress is uneven is discussed.

The problem was to correlate the tendency of a fuel to knock in a petrol engine, with the physical conditions in the cylinder which cause the knock, and with its ignition qualities in a bomb. The method of approach was to find a function of the physical conditions, the attainment of a critical value of which, during the stroke, is a criterion of knock. Theories of the mechanism of knock are reviewed, and suggest a rational form for this criterion. Engine tests of fuels were made at incipient and heavier knock under a variety of engine conditions, and indicator diagrams were obtained by an oscillograph indicator. Assumptions, based on the rational form, were made for the form of the criterion; and by analysis of the diagrams, etc., a reasonably satisfactory form was selected. Substantially the same critical value was then obtained for incipient knock under all running conditions, including change of compression ratio. The possible application to fuel rating is discussed. The ignition experiments indicated that the same critical value of the same function is a criterion of auto-ignition in a bomb, and they suggested a similarity between the mechanism of knock in an engine and auto-ignition in a bomb.

Charts are given in this paper from which may be found the indicated thermal efficiencies, the fuel consumption per indicated horse-power-hour, and the mean effective pressures which are practically attainable in a compact combustion-chambered petrol engine of any bore between 3 inches and 8 inches when running at any compression ratio between 4/1 and 9/1 and at any speed from 1,000 r.p.m. upwards on an octane, ethyl alcohol, or benzene-air mixture of any strength between 20 per cent weak and 20 per cent rich. From these charts similar information in regard to any mixed petrol may be deduced with fair accuracy.

The performance of actual engines is compared with the attainable performance in the light of views, given in some detail, relating to phenomena associated with the working fluid and the combustion of the charge. The conclusion is that improvement in charge mixing within each engine cylinder would lead to better performance. Characteristic merit and demerit curves are suggested which appear to be capable of yielding useful information in regard to the functioning of engines.

During experimental work on flexible belts running on pulleys of small diameter, the loss of speed of the follower arising from the elasticity of the belt was observed to be considerably greater than would be normally expected. It was also noticed that this loss of speed depended on the mean tension of the belt and the diameter of the pulley. The usual type of belt material does not follow a linear elastic law for strains in the direction of its length, the elastic modulus being relatively low at low tensions and when in compression. On the assumption that compressive stresses can be neglected, the resulting stress distribution is found to account partly for the experimental results. In addition the material has a relatively low transverse compression modulus, and the surface compression causes an additional slip loss which would be negligible with an isotropic material in all practical cases. The examination is applicable to V-belts when the effects producing radial movements are of greater importance because of the amplification by the groove angle. For both kinds of drive the relative influences of the component losses on efficiency are discussed, and conclusions can be drawn on the suitability of materials for small pulley drives.