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The Bentley High-Temperature Compound

1st July 1909, Page 4
1st July 1909
Page 4
Page 5
Page 6
Page 4, 1st July 1909 — The Bentley High-Temperature Compound
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Steam Engine.

Our readers will remember that the New Lowca Engineering Co., Ltd., of Whitehaven, entered a steam tractor for the recent competition of the War Department, hut that, unfortunately, the engine was not completed in time to take part in the competition. The tractor has since been completed, and has given extraordinary test results on the road as regards tractive power and high economy in fuel and water. As this appeared to be due very largely to its new form of engine, the invention of Mr. II. Bentley, the managing director of the company, the engine, at the suggestion of Colonel Crompton, was removed from the tractor, mounted on a special bedplate and coupled direct to a dynamo, in order that a bench test might be carried out underconditions of unimpeachable accuracy. The preliminary tests, undertaken by the Company's staff, turned out to be so satisfactory that an invitation was issued to engineering friends and representatives of the technical Press, to witness a set Of bench trials which took place at Lowea on Friday, the 18th ultimo.

The engine was designed to develop 15h.p. under ordinary working road conditions on the tractor. It is of the inverted-inclined type, having four high-pressure and four low-pressure cylinders. hut only two cranks, which are set at 180 degrees apart. It will be seen that the arrangement greatly facilitates the accurate balancing of all the moving parts. The highpressure pistons, two inches in diameter, are attached by trunks, tandem fashion, to the low-pressure pistons, and these are five inches in diameter. The ratio of effective area of each high-pressure piston to each low-pressure piston is as 1 is to 5-1-, so that, at the most-effective point of cut-off of the high-pressure pistons, a ratio of expansion as high as 20 to 1 can be obtained ; steam of the high pressure used, 7001b., can thus be expanded so as to give a mean effective pressure of 40Ib. on the low-pressure cylinders. The steam distribution is simple ; it consists of lift valves for controlling the high pressure, but the exhaust of the low pressure is effected by the uncovering of the port A, in a manner somewhat similar to the arrangement in the well-known Knight internal-combustion engine. The steam, after expansion in any high

pressure cylinder, is led through a pipe to the top of the low-pressure piston in the adjacent pair of cylinders on the same side of the crankshaft. Ball bearings are used throughout, and in practice it is found that the ratio of b.h,p. to i.h.p. is exceedingly high. A refinement has been introduced, in order to reduce the friction and increase the durability of the lift valves; they are slowly revolved by the gear wheels shown at B. Each pair of cylinders is provided with a third camoperated valve, by means of which the back pressure in the low-pressure cylinders may be relieved when the

engine is running with little or no load; all knocking, as the result of the compressing of the steam above tli.! low-pressure pistons is thereby entirely avoided.

In engines which use steam of exceptionally-high pressure and temperature, the usual difficulties met by Serpollet and others have been the main-. tenance of the pistons in a steam-tight condition, and the prevention of the passage of any steam into the crankchamber. It will be seen that this is effected by three sets of rings on the high-pressure piston, and six sets of rings on the low-pressure pistons.

For the purposes of the test, it will be seen that the engine was mounted on a substantial hedplate and coupled direct -to a dynamo capable at its full power of giving off 60 kilowatts. An efficiency curve was prepared for the purposes of this test by Messrs. Crompton, the manufacturers of the dynamo, and the tests were carried out at a point very close to one-third the output of the dynamo, at which figure the efficiency is exactly 80 per centum: In order, therefore, to convert the watts measured at the terminals of the dynamo into b.h.p., it is necessary to use a multiplier of .001674. The fuel and water were measured by the engine's being started and brought up to its load, and the accurate filling of the water and fuel tanks to an agreed mark. The test was then started, and it continued for. 30 minutes. During that period, readings of revolutions, amperes, volts. watts, steam pressure at throttle and temperature at throttle were taken, and these averaged for the trial. At. the end of the 30 minutes, water and' fuel were shut off, and sufficient water and fuel were then weighed in to refill.

the tanks to the agreed mark. Experience has shown that, by these arrangements, practically as great accuracy can be obtained for the half-hour test as with a test of much longer duration. The temperature was measured by a constantan-copper couple

electric pyrometer. Messrs. Crompton are responsible for the accurate calibration of the ampererneter, voltmeter and pyrometer that were used, and Messrs. Avery for the two weigh

bridges. The readings in the third test, on the 18th ulto., were taken by Colonel Crompton himself and checked by representatives of the technical Press who were then present.

The accompanying table gives a summary of the principal particulars, and it will be seen that the three sets of tests show remarkably consistent results in the quantity of water used per b.h.p. per hour by the engine. The fuel consumption is not so consistent, as the boiler, being worked so very closely to its maximum h.p., did not give quite such good results on the third test as on the others, probably due to the fact that at one period during the third test it was worked considerably beyond its capacity, so that neither the temperature nor pressure were kept so steady as in the other two tests. The actual result of the test-13.31b. of water per b.h.p. per hour—is, we believe, practically a record, but our representative was assured by Mr. Bentley—and by Mr. G. K. Field, who has driven the tractor on the numerous road tests which have taken place since it was completed— that the actual figures obtained on the road when the engine was worked at a slightly-lower rate of output, and with the vacuum that was then obtainable when the condenser was used, were probably somewhat better : they are satisfied that the water per b.h.p. used on the road could not have exceeded 12th., and that the boiler, being much more easily worked, gave a rate of evaporation of about 10i-lb. Since the average consumption measured in lb. of fuel per train-mile was under 51b., it is evident that the road figures must have been somewhat better than that obtained on the bench test. We, however, think that the engine is still rather too large for the boiler which was used, and that still-higher boiler economy would he obtained by an increase of the length of the tube and, hence, of the evaporative surface exposed to the action of the hot gases.

The burner used was extremely efficient. Everyone present was much impressed by the good colour and marked steadiness of the flame, and the comparative absence of the roaring noise which is usually associated with paraffin burners using oil at the rate of 481b. an hour. The flame was white, inclining to red at the tips, and appeared to be very easily controlled.

Mr. Bentley's engine shows that he has attained his object, which was to prove that the results of Serpollet, 1Vhite and others could he surpassed if the high expansion of the steam used by White were coupled with tie lift valves and higher temperatures used by Serpollet. The tandem arrangement admits of an abnormally-large range of expansion: the cylinder ratio, in this ease, is as high as 5,'t to 1, and we are unaware that so high a ratio has ever been used on a tandem-compound engine. The whole of the bearings, being continuously in thrust, all difficulties connected with gland packing, or the taking-up of big-end brasses, which are inseparable from the double-acting arrangement, are avoided.

Another admirable feature in the engine was pointed out to us. Users of compound engines for road-transport purposes have found it desirable to fit a " simpling " arrangement, by which, at starting under very heavy load, the boiler pressure can be admitted direct to the low-pressure pistons. The arrangements used by White and by traction-engine builders are less simple than they might be ; they necessitate separate valves and steam passages, with the attendant disadvantages of an increased number of joints and exposed radiating surfaces. All these losses are avoided in the Bentley arrangement, which consists of a prolongation of the camshaft, so arranged that, when the reversing lever is placed in its extreme position at either end of the stroke, the high-pressure valves are continuously held up, and the high-pressure steam is admitted through the high-pressure cylinders, which thus form part of the steam pipe, to the low-pressure pistons.

The engine and boiler have now been replaced on the tractor, and road tests are being carried out at Lowca, on the steep gradients which exist immediately outside the works, and on long lengths of the undulating road which are found near the coast line of West Cumberland. With the data that have been obtained from the bench tests, the Lowca directors believe they will now be able to show exceedingly good road results, on account of the high transmission efficiency obtained in the tractor, the reduction of road rolling resistance by the use of 7ft.-diameter wheels, and the exceptional springing arrangements which have been employed in the tractor itself, a description of which we reserve until after a representative of this journal witnesses such tests of the tractor on the road.


Organisations: War Department

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