L.G.O.C. Experiences with Compression -ignition Engines
Page 63
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Satisfactory Service Obtained from Several Oil Power Units Which Hate Been Operating Experimentally in Buses and Lorries
THE experiences of the London General Omnibus Co., Ltd., with compression-ignition engines, as described in some notes of a discourse given at a luncheon of the Oil Industries Club by Mr. Arthur A. Durrant, A.M.I.A.E., A.M.Inst.T., assistant chief engineer to the company, are of great interest. The conclusions formed from these experiences have satisfied the L.G.O.C. engineers that there is a definite future for the high-speed compressionignition engine for bus operation.
This type of engine has been proved to possess certain definite advantages over the petrol unit. Of these, economy is first named, and even were the cost of fuels, together with any tax which might ultimately be applied, to reach the same level as that of petrol, an appreciable saving in fuel costs would still be attainable. .
Immunity from fire, both on the road and in the garage, comes next in importance, for besides safety on the road— which is clearly of great moment—the advantage of using a non-volatile fuel in the garages also has considerable value, as many restrictions would, without doubt, be removed.
Mameuvrability is also quoted among the advantages ; the enhanced accelerating capabilities of compression-ignition engines and their well-sustained torque facilitate driving in congested areas.
It has been found unnecessary to allow protracted time for warming up when starting from cold, a point being that the fume,s which collect in the garage when a large number of vehicles is being started up are reduced. Finally, there -is less carbon monoxide produced during combustion in this type of engine. A point stressed is that it is unwise on the part of sup pliers of lubricating oils to be dogmatic in regard to the type of lubricant best suited for compression-ignition engines. Experience has shown that two entirely different characteristics are exhibited in two different types of engine. .
Four Types Employed.
Pour types of unit have been in use—the Junkers SA9, the A.E.C. Acro, the A.E.C. Ricardo and the Gardner 5LW. Details of the engines and their performances are contained in the accompanying table.
The Junkers engines,-owing to their unusual shape and high torque at low speeds, were not suitable for buses, and were, therefore, incorporated in two A.E.C. 5-type lorries. They proved very reliable, the only real trouble experienced being a rather heavy loss of power in one ease. This was traced to slight scoring of one fuel pump caused by the presence in the gas oil originally used of suspended particles small enough to pass through the rather iradegtate filter fitted. This experience made it clear that it is essential to use as clean a fuel as possible and to provide an efficient filter. No trouble of a similar nature has since been encountered on this or any of the other vehicles.
Twelve A.E.C. high-speed oil engines of the Acre type were brought into service early in 1931, being installed in
four-wheeled buses. The most valuable feature of _these engines is probably their wide speed range. Their operation in the buses was satisfactory. When idling, "Diesel knock" was apparent, although when 10 m.p.h. was attained it was practically impossible to ,distinguish between this type of engine and a petrol unit.
With regard to smoke when operating under heavy injection, this was noticeable, and when the vehicles were first put into service complaints from the general public regarding the smell of the exhaust fumes were not infrequent.
As a result of early experience with the Aero type, the A.E.C. concern developed a new type of engine in conjunction with Mr. Ricardo. In these the combustion process is much more efficient and a smokeless exhaust is obtained under all operating conditions. The only time when smoke is apparent is On acceleration, when a slight momentary puff is emitted. The smoothness, general running and economy are also superior. It is significant that not a single complaint regarding smell or smoke has been received about these engines.
The most recent addition to the fleet of experimental vehicles was a Gardner-engined six-wheeled bus. Limitations of space necessitated the use of a five-cylindered engine.
It was found that this engine idled evenly and was quiet, but when running under load was inclined to become progressively rougher and noisier. No fad-consumption figures are yet available.
Fuels and Lubricants.
As already mentioned, commercial gas oil was originally used and then a Diesel oil was substituted, Finding that the performance was not wholly satisfactory, howeyer, a research was made in conjunction with the A.E.C. concern and a large range of fuels tested. It was found that only a small proportion gave satisfactory running. This was also realized by the oil companies, and their researches led to the development of fuels designed for the high-speed engine. To-day it is appreciated that a compression-ignition engine requires as much care in the selection of its fuel as does a petrol engine, and that it will not "run on anything," as used to be popularly imagined.
A fuel is now being used which is proving much more satisfactory,but the stage has not yet been reached where a definite specification can be given, as the only sure test is its performance in an engine.
As a side-line, research tests were made with fuels derived from coal. The results, however, were disappointing, and it would seem that the only means for utilizing homeproduced fuel would be to design a special engine.
The lubrication of the compression-ignition engine is a problem still under investigation, and although a comparatively viscous and high-grade oil was used for the Junkers and Gardner engines, both these types exhibited heavy crankcase oil dilution.
The A.E.C. engine is exceptional in that dilution is low, the crankcase oil containing only about 2 per cent. of fuel. The problem of lubrication of this engine differs from that of a petrol unit in that dilution is rim less and the carbon which accumulates is finely divided and does not form sludge.