s the Single Sleek
Page 24
Page 25
If you've noticed an error in this article please click here to report it so we can fix it.
lye Engine the Unit of the Future?
Reliability, Silent Running and High Power Output with Economy in Fuel are Characteristics which Throw Out a Serious Challenge to the Poppet Valve
IT is by no mere accident or whim of the designer that the single sleevevalve"-type of engine has reached such a position as to be one of the outstanding achievements of the British aircraft industry. The report of the Aeronautical Research Committee (1925-6) laid stress on the advantages of the single sleeve-valve, and, in the light of this report, the Bristol Aeroplane Co., Ltd., in collaboration with the Air Ministry, decided to investigate its po-ssibilities, To them is due the credit of developing the single ..leeve-valve aero-engine to its presenthigh state of perfection, The success.of the Bristol engines has encouraged other aera-engine builders to adopt the single-sleeve principle, as witness the new Napier 24-eylindered H-type unit installed in the Typhoon fighter-bomber. Moreover, it is no secret that further developments may be expected from other prominent armengine. factories, where progress has passed the drawing-board stage.
The versatility of the single sleevevalve engine is evidenced by the wide range of aircraft into which it is installed. This extends from the longdistance heavy bomber, requiring lengthy periods of reliable running, coupled with maximum economy of fuel, to the high-speed fighter, in which ability to manceuvre is a first essential, its engine often operating under full boost and maximum-output conditions. Then there are many intermediate types, such as flying-boats, light bombers, torpedo aircraft, reconnaissance machines, trainers, and so on.
.Stich signal success of the single sleeve-valve engine when applied to airgait will doubtless .raise in the minds of. road-transport operators the ques.
, A22 lion as to why it ha e been neglected by the makers of commercial-vehicle engines. It is the purpose of this article to answer this, together with others that may probably arise, and to indicate the developments which have been taking place in recent years to make this type of engine a sound proposition from the operator's point of view. Unfortunately, the intervention of the war has somewhat retarded manufacturing progress, but it can be safely claimed that a most promising position had been reached just before the outbreak of hostilities.
Some of the Chief Features of the Type The older members of the industry will recollect the impression created by the Scottish-built Argyll .car, particularly by virtue of its silent, smoothrunning engine. Others may have come into contact with prime-movers of similar type employed in widely differing duties,' such as for marine propulsion, for rail traction, in motorcycles, and for industrial and electrical purposes.
The secret of these engines was the `single-sreevc valve, invented by Mr. Peter Burt, a well-known Scottish engineer. Mr. Burt was no newcomer to the industry, having been engaged in the development of internal-combustion engines so far back as 1887, and the single-sleeve valve, patented in 1909, was based upon his accumulated experience.
It must be borne in mind that the aero-engine designer has 'a certain latitudewhich is denied to the vehicleengine .designer, and that many methods employed in the former would not be desirable in commercial types For example, some of the constructional characteristics of the early single .sleeve-valve engines are well suited to aero-enigne design, but it was these .same characteristics which were a contributory cause of the decline of this type of engine for road service. The fact remains, however, that the basic principles laid down in the original single-sleeve patents have not, so far, been seriously challenged by any other form of valve gear.
The advent of the single sleeve-valve unit marked .a definite advance in engine design and remains, even to-day, the outstanding solution of the positively operated valve, performing the dual duty of admitting the charge to the cylinder and of providing an exit for the exhaust gases, with a coinplete absence of tappets, valve springs and hammering valves, such as is found in its poppet-valve contemporary.
Following the publication of the patent specification, a number of successful designs was introduced, amongst the most prominent, for road, vehicles, being the Argyll, Vauxhall, PiccardPictet, Barr and Stroud, Caledon and Arrol-Jahnston, 1t has to be admitted, however, that the early single sleeve-valve engines suffered from certain inherent defects for which the constructional design was mainly responsible, and which were as fo1lowe—(1) InacceSsibIe valve ports increased the cost of manufacture; (2) port positioning resulted in unequal heating and consequent unequal expansion of the cylinder; (3) sleeve drive position tended to encourage sleeve breakage; (4) inaccessibilita of the sleeve drive. Added to these design features was the difficulty of restraining the oil flow along the sleeve and into the combustion chamber, with consequent high consuinption of lubricating oil.
The single-sleeVe valve suffered 'a further setback in the years following the 1914-1918 war, when the demand for popular-priced cars arose, and it clras unable to compete with the poppet-valve engine, owing to the fact that it was not then so adaptable to quantity-production methods.
As previously stated, the basic principle was so well established that the
problems resolved themselves into those of a constructional nature, and of the means for adapting the design to quantityaproduction methods, for the solution of which Mr. Burt has obtained further patents. In order to confirm that the new patents satisfactorily overcame the early defects, and that the new principles could be incorporated in a practical design, a Wellknown concern of engineers was commissioned to design and build a singlecylindered experimental unit. This test engine was built with essential dimensions and speed characteristics similar to the company's standard poppet-valve experimental unit, which was used for development work in connection with the multi-cylindered engineS then in production.
The. first point established by the• production engineers of the company, and confirmed by independent authorities, was that it would now be possibli, to design commercial, single sleevevalve engines, which could be produced by quantity-production methods at less cost than poppet-valve engines.
Operating on the now well-known compression-ignition principle, the experimental engine here illustrated shows the original injection equipment which consisted of a Beardmore fuel pump and injector, but this was later replaced by equipment of C.A.V. make.
Engine Built from Existing Parts In order to reduce the cost of construction, the crankcase was casf from obsolete patterns and core-boxes adapted to suit, whilst the crankshaft was machined from the end of a broken six-throw shaft. The connecting rod and all bearings, etc., were drawn from the poppet-valve engine stock, whilst the circulating-water pump was the marine version of the standard engine pump.
Referring to the list of defects already mentioned, the new design overcomes these in the following manner.—(1) All ports are on a joint face and are, therefore, completely accessible for machining: Indeed, with a modified port shape recently developed, all ports may be accurately cut on a simple machine in a few minutes. (2) Ports are now staggered, alternately inlet and exhaust, resulting in equal heating and expansion of the metal. (3) The sleeve drive is located approximately at the centre of the sleeve, which is now guided -above and below the drive. it is fully accessible, and the sleeve can be withdrawn when the top bleck has been lifted after removing one nut. (4) The Construction permits of complete accessibility. All ports are exposed for .cleaning by the simple removal of the cylinder head. Decarbonizirii consists of. scraping around the ports with an old knife and is completed in a few minutes.
Further, the oil control effected by the two scraping edges around the sleeve prevents an excess of oil being
passed into the combustion chamber, whilst allowing just sufficient for proper lubrication . of the sleeve. Although the experimental engine operated on the compression-ignition cycle, the principles pf the design are equally applicable to petrol engines. With the C I. single sleeve-valve engine, a simple form of combustion chamber is possible, swirl being obtained naturally without the use of masked valves or other expedients.
When designed as a petrol engine, a higher Compression ratio than is normal for such a unit maybe used which, with a simple combustion-chamber form,. will proxr.ide a higher output with greater economy than can be obtained with the use of poppet valves.
Tests were continued for a period of over 18 month,s, and were made on a Froude hydraulic dynamometer, but had to be discontinued when the test house was dismantled to make room for more essential work. During the whole period of trial only one mechanical fault developed, and this was due to the designer being too optimistic with regard to the lubrication of the ball joint of the sleeve driving gear. On the first day of running the ball drive seiied and caused the failure of the driving extension pin. b Rectification-of this error was quickly and easily made and no further mechanical defects developed. It is -doubtful if any experimental poppetLvalve engine has been so free from trouble.
The results obtained on the test,,ped were in line With those claimed by the Bristol Aeroplane Co. Ltd., i.e., increased power with lower fuel consumption.
The following table, which is also given in graph form, shows the improvement .in performance over the equivalent poppet-valve engine:— Clear Exhaust of the Sleeve-valve Engine
The exhaust of the poppet-valve engine became distinctly shady at about .1,300 r.p.m,, and, progressively darkened as the speed increased, whereas with the single-sleeve engine it was clear throughout. It will he noticed from the citrve that the output from the poppet-valve unit decreased above 1,600 r.p.m , whilst the sleeve: valve engine curve continued to rise.
Special note should be taken of the character of the fuel-consumption curve of the sleeve-valve engine, particularly at the lower and upper limits of the speed range, which would have a marked. effeet on the m.p.g. of a variable-speed engine such as is used in .road vehicles.
Extensive tests proved that the lubricating-oil consumption is ngw on a par with other types of commercial engine.
Tests carried out just before the cessation of the trials resulted in• a further increase in outpnt, .without the engine showing any sign Of distress, and still' with a clear exhaust. These later results are indicated by the dotted curve on-the graph, but, unfortunately.. it was not possible, at the time, to take . fuel-consumption measurements; it may, however, be accepted that a further improvement would result.
The above is a brief account of the work which has been done and of the investigations made. They would appear to indicate that the revival of the single sleeye-valve engine for commercial vehicles is a definite possibility. Further, an analysis of the conditions which, apply lead to the conclusion that the solution of the problem of the small oil engine for light lorries lies in this direction.