The Oil Engine Discussed
Page 36
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AT a joint meeting of the Institution ' of Mechanical Engineers and the institution of Automobile Engineers at Leeds on March 23, Mr. W. H. Worrall spoke on "The Armstrong-Saurer Commercial Vehicle." Time allowed of reference only to the engine.
There is no need to recapitulate the paper, as it dealt with features well known to our readers ;•there were, however, many details which contain an element of novelty. In the latest Britishbuilt chassis the air cell differs materially from its prototypes in that the angle of injection is more nearly vertical and the spray is directed 'farther into the cell. A compression ratio of 16.5 to 1 is adopted ; the mean thermal efficiency is nearly 30 per cent., whilst the fuel consumption is of the order of .437 lb. per b.h.p.-hour.
A point in connection with the rollerbearing crankshaft is that the rollers are so placed as to obviate a sequence of peak loads in any one restricted area. Lubrication was dealt with at length ; the author stated that oil engines use a greater quantity of oil than petrol types; he regards 800 m.p.g. as a good average figure, but remarked that oil dilution by fuel is now down to 3 per. cent. One of the most vital factors in lubricant specification is the pour point, which—if too high—impedes starting. . In his opinion almost every case. of failure to derive satisfaction from compression-ignition engines may be attributed to the use of poor oil.
An interesting itenem-as that failure to control lubricant passing the piston rings may result in an engine continuing to develop elight, power after stopping fuel injection, due to surface combuotion of the oil. which has reached the cylinder. An indicator diagram was shown to demonstrate this, the conditions being_ that the fuel pipes from the pump were actualig disconnected. The fact that 110 noticeable 'ovality occurs in cylinder wear, according to Mr. Worrell, points to corroboration of Mr. Ricardo's theory of liner wear being due to rapid oxidation caused by the products of incomplete combustion.
A series of comparisons.between petrol and oil engines, both of Armstrong-Saurer make, produced some interesting figures. The former costs £4 per b.h.p., and the oil engine 15.08 per b.b.p. Weights are 14.38 lb. per b.h.p. (petrol) as against 16.97 lb.; owing to the higher stresses in the oiler the reciprocating masses weigh about 12 per cent. more.
Special audiometer tests for noise showed that; over a range of speeds, the compression-ignition type is some 20 per cent. more noisy.
Approximately 3,000 Saurer oil engines are in service, of which 300 have been built at the Scotswood Works., It is probable that, at an early date, it will be poseible to arrange a scheme for Armstrong-Saurer owners, whereby maintenance at a fixed price per mile will be available. In this way uncertainty can be banished and operating costs established on a definite basis. It is hoped also to reduce unladen weights of the complete vehicle.