Diesel Oil Engine for Motorbuses.
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A Striking Development of a Highly-efficient Internal-combustion System.
Of late years the Diesel oil engine has come very much to the front for all purposes; it is now rapidly replacing steam engines, and in many cases gas engines, being by far tho most economical prime mover now manufactured, and probably one of the most reliable. Many large works, which at one time were concerned only with the -construction of steam engines, are now solely devoted to the manufacture of Diesel engines, as, for instance, the famous Augsburg works of -the Machinfabrik-AugsburgNurnberg in Germany, and, in a less marked degree, the factory of Sulzer Brothers, at Zurich, in Switzerland.
In this country, John I. Thornycroft and Co., Ltd., and Willans and Robinson, Ltd., are now building the stationary type of engine, while some half-a-dozen other prominent companies are engaged upon its manufacture. Its application for marine propulsion has lately become of extreme importance, several vessels up to 9,000 tons burthen and equipped with Diesel engines being already afloat or on the stocks.
The engine has long been employed for submarines in the French Navy, and for smaller boats and launches, and this fact at once points to its adaptability for automobiles. A large amount of experimental work has been carried out to make the engine thoroughly suitable for this purpose, and the results have been so far satisfactory that the largest engineering firm in Germany, the Allgemeine Electricitats Gesellschaft, of Berlin, have, it is stated, now taken up the licence to build Diesel engines for automobiles on a large scale. The Diesel oil engine—the invention of Dr. Rudolph Diesel, of Munich—differs essentially from practically all other oil engines in that it is purely an internal-combustion engine, as distinguished from an internal-explosion engine. It is ordinarily constructed of the four-stroke type, though a two-stroke engine, and, even, a two-stroke, double-acting engine for very large powers are now being built. In the first stroke, air is drawn into the cylinder from the atmosphere, being compressed to a very high pressure in the next stroke, whilst, at the commencement of the third stroke, oil is injected into the cylinder in a very fine spray. • by means of compressed air, at a slightly higher pressure than that of the air in the cylinder. Combustion occurs regularly during the early portion of the stroke, owing to the very high temperature (over 1,000 degrees Fehr.) of the compressed air in the engine cylinder, and, at the right moment, the supply of fuel is cut off and expansion takes place in the cylinder during the rest of the stroke, this being, of course, the working stroke. In the fourth and last stroke the products of combustion are exhausted and the cycle of operations is then recommenced. Large engines are started up by compressed air, which is stored in cast-steel bottles, whilst a further vessel is required as an air reservoir for the high-pressure air needed for the fuel injection. A compressor is fitted on the engine itself for maintaining a supply of injection air, and this is either of the single-cylinder, reciprocating type, worked from an eccentric off the crankshaft, or a fourcylinder compressor on the end of the crankshaft of the Reeve11, or similar type. It will readily be seen that an engine working on the cycle of operations described above must have a. better efficiency than other oil engines, since the fuel is not mixed with the air in the cylinder until after compression has taken place, and hence more useful work is obtained out of the heat of compression, and it is interesting to note that the engine gives an indicator card almost exactly resembling that of a good steam engine_ As mentioned above, the chief feature of the Diesel oil engine is its remarkable economy, and the great regularity of its fuel consumption for all sizes, and it may be reckoned that, for engines of moderate power, 0.45 lb. to 0.50 lb. of crude oil is required per brake-horse-power hour ; that is, oil which can be bought almost anywhere in England for about 45s per ton; and it will be found, upon examination, that the actual saving in running cast with a Diesel engine is very large when compared with any other type of engine. In Germany, before the licence for the construction of this engine for automobiles was taken up, the question of fuel cost was investigated very carefully, by ascertaining the actual cost with a large number of existing motor-omnibuses, and it was found that, reckoned on the basis of a ear with a 30 h.p. motor, running 10 hours per day, the cost of running showed remarkable economy over that of a petrol engine. This, evidently, opens out. a very wide field of economy for London omnibuses. The illustration shows the first Diesel motor which was constructed for use in an automobile, arranged for test ing. It has four cylinders, in two pairs, the air-inlet pipes being seen in front, these having slits for the admission of air. The air compressor is the extra cylinder at. the end, the crank chamber being totally enclosed and provided with inspection covers. The air inlet, fuel inlet, and exhaust valves are all placed overhead in a vertical position, being operated by a camshaft carried over the cylinder heads and actuated from a vertical shaft that is driven by worm gearing from the crankshaft. The fuel-inlet valve is of the needle type, so that the oil is injected in the form of a spray, allowing combustion to take place regularly and continuously. As is seen from the illustration, the motor is of very substantial construction. All the moving parts run in oil, including -the camshaft. In its present early stage of development the dimensions and weight seem unduly large. This drawback will probably be overcome in time. There is, of course, an absence of sparking plugs and of the usual ignition apparatus, and the running is comparatively silent ; moreover, owing to the perfect combustion, the exhaust is quite clear.
[The foregoing article was contributed to our sister journal, " The Motor." The writer had just returned from Germany after an interview with Dr. Diesel, and he drew attention to these important developments. We agree with the views expressed by the Editors of " The Motor" that the new engine must he regarded strictly in the light of an experiment, and that no one is at present in a. position to state anything definite as to the
effect which -this development may have as a rival to the existing types of petrol engine. As to the Diesel principle's being highly efficient, we have ample proof from our experience with a 100 b.h.p. electric generating set which has for years been used in our works.—En.]