Proceedings of the Institution of Mechanical Engineers

The experimental plant installed by the British Coal Utilisation Research Association for testing gas-producers, filters, and engines is described in detail.

The results of tests on the engine bench show that a petrol engine converted to use producer gas will only develop about one-half of the normal brake horse-power; moreover, the peak power with gas occurs at a lower speed than with petrol, due to the decrease in volumetric efficiency at high speeds. The effect of increasing the compression ratio was not investigated by the author, but tests by other workers have shown that the gain in power obtained by this means is proportional to the increase in the air standard efficiency.

Producer gas-driven engines are very sensitive to mixture strength, and maximum power is obtained with mixtures of gas and air in the proportions theoretically required for the combustion of the gas. Maximum efficiency is obtained with a mixture containing 10 per cent of excess air. Engine tests have also been made to investigate the effect of gas calorific value, of the pressure drop through the producer system, and of petrol addition. Special tests were made to determine the flexibility characteristics of the engine and producer, and the effect of running with various throttle openings, as in actual road operation. Fuel consumption under the latter conditions is about 35 per cent greater than that determined under full-throttle and constant-speed conditions.

The paper contains the results of experimental work on caustic embrittlement. The type of apparatus chosen was based on one used in recent American work carried out by Straub and Bradbury (1938). In this, the solution to be tested is contained in a hollow sealed tubular specimen, heated to any desired temperature in the steam boiler range, and subjected to a tensile load by means of a spring.

It has not been possible to reproduce the results claimed by Straub, but a number of examples of intergranular cracking have been produced. Some of these have been obtained when using dilute solutions containing amounts of NaOH and silicate such as would be found in boiler waters. There is evidence that embrittlement occurs more readily in poor-quality steel and under conditions of non-uniform stress distribution, but it has not been possible to reproduce results with any degree of certainty.

A few experiments with an entirely different type of specimen have been carried out; and in these, intergranular failures have also been produced.

The paper also contains a survey of the more important literature on caustic embrittlement published between 1935 and 1941.

The reduction of the number of boiler tube failures to a minimum is now a matter of national importance.

Revolutionary improvement is not expected in the nature of the tubes. Tubes fail rapidly by local, and less rapidly by general, corrosion.

The removal of scale by shot blasting before use will reduce the extent of losses due to pitting, and so will additions of 0·2–0·5 per cent of copper.

Metallurgical data regarding tubes, and chemical data regarding feed waters are summarized in the paper, from the author's extensive records. After discussing how corrosion occurs at cracks in oxide, scale, or paint, suggestions are made for preventing corrosion in locomotive, marine, and stationary boilers.

Recommendations include simplification and standardization of boiler water tests by a suitable British Standards Committee, and the suggested Specification is sketched in outline.

The paper is concluded by Appendixes comprising (i) a questionnaire (with answers) on the present-day practice as regards the manufacture of boiler tubes in Great Britain; (ii) a list of reports published by the American Railroad Engineering Association; and (iii) a bibliography of selected items on corrosion, for the index prepared and maintained at the Chemical Research Laboratory, Teddington.

The nature and extent of the deposits which occur on the heating surface of a boiler unit are variable in the extreme. The method of firing and the rate of combustion are open to choice. The completeness of combustion can be controlled to some degree, but the quantity and chemical constituents of the ash can seldom be forecast, and may vary widely.

The form of fouling, and the effect on the different portions of the heating surface of the boiler unit, are examined.

Development of the application of soot blowers from 1919 is discussed, particularly the use of the single-nozzle blower. The design of the boiler to permit full use of the different types of soot blowers available is referred to, and mention is made of the effect on furnace walls, boiler tubes, and superheaters, economizer, and air heater surfaces. The future design and rating of boilers depend to a great extent on the development of the means of cleaning the heating surface. Reference is made to cleaning by water lancing and other means for special cases.

The importance of the availability of plant is emphasized, and essential features of power-operated mechanical soot blowers, with special reference to automatic operation, are outlined. The scope of use of the three main types of soot blowers is set out, as well as the essential features of their design. Points for consideration in the periods between individual soot blowing operations and the extent of each blowing, are enumerated, and in section (3) of Appendix I (p. 96) the types of automatic soot blowers on the market are briefly described; a general indication of the trend of modern design of blowers, both electrically and hydraulically operated, is also given.

The Third Report was originally projected as the Second Report,† but the necessity of making use, under war conditions, of the most readily available indigenous ores, caused the Committee to concentrate immediate work on the influence of phosphorus upon mechanical properties. The Second Report has already dealt with the influence of this element on the properties of the plain cast irons, whilst the Addendum to the Second Report‡ covers the alloyed cast irons.

The Third Report contains in Part 1 further results on commercially made high-duty cast iron, i.e. those equivalent in properties to, or better than, grade 2 of British Standard Specification No. 786 (1938), and in Part 2 the results of the remainder of the experimental programme concerning alloy cast irons, showing the influence of additions of nickel, copper, chromium, and molybdenum, both severally and in various combinations, on certain selected compositions. Both parts of this Report refer to the machinable engineering cast irons, having a pearlitic structure with normal flake graphite, covered by class 11 of the classification§ given in the Reports on Special-Duty Cast Irons. The more recently developed so-called acicular cast irons, showing excellent mechanical properties, do not fall within this category, and are being made the subject of experimental work.

The problem of the accurate estimation of resonant amplitudes of torsional vibration in small marine Diesel-engined propulsion sets is considered from the viewpoint of the design engineer. A short discussion on torsiographs is followed by the basic theory of the energy method of calculation, with particular reference to the calculation of vibration amplitudes assuming linear damping proportional to the phase velocity of the vibration. Harmonic input torque diagrams are given for a small marine two-stroke engine.

The methods put forward by different investigators are considered by comparing the calculated results with those obtained by torsiograph from a number of marine Diesel units investigated by the author. Both the one- and the two-noded forms are considered. In the former, propeller damping is shown to be the main influence and a method of computation put forward by Dorey is shown to be satisfactory after slight modification. For the two-noded form the position is not so satisfactory. Hysteresis damping in the crankshaft, viscous damping at the bearings, overall velocity damping, etc., are considered in turn and rejected as unsatisfactory. The importance of apparent damping effects—particularly cyclic fluctuations of speed or cyclic irregularity—is stressed, and a method is evolved in which this consideration is utilized in conjunction with results published by Major Carter which the author has found fairly satisfactory to date. It is concluded that much has still to be learnt on the subject before empirical methods can be dispensed with.

During the year 1942 a series of informal discussions was arranged in Manchester, at meetings of the North Western Branch with the Manufacture Group. The full list of meetings held in 1942 was published in the Annual Report of the Council for 1942.* Below are given summarized accounts of the discussions, and it is regretted that, owing to the severity of present-day restrictions on space, it is only possible to publish these brief accounts. It is, however, felt that the necessary brevity of these records will bring out clearly the salient points under discussion, and help to provide a useful record of the way in which engineers are meeting to ventilate certain problems confronting them as a result of the present emergency.