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FISITA CONGRESS
Page 86
Page 87
Page 91
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Diesel Combustion, Smoke Metei rid Braking Problems Reviewed
DIESEL engine combustion, methods of measuring smoke density and problems associated with vehicle braking and control were the main subjects of interest to commercial-vehicle manufacturers and operators reviewed in 13 papers at the Ninth International Technical Congress of the Federation Internationale des Socidtes D'Ingenieurs des Techniques de l'Autornobile (FISITA) in London this week. The congress was organized by the Automobile Division of the Institution of Mechanical Engineers, 1 Birdcage Walk, London, S.W.I.
A number of papers related in the main to problems normally associated with the performance of private cars travelling at high speed, but it is possible that the discussions will have provided material pertinent to the design and operation of heavier vehicles. A full account of the discussions will be published in next week's issue of The Commercial Motor.
Variations in Swirl Rate
After making the statement that very little is known about the mechanism of combustion and the ways in which it can be influenced, Dr. F. Pischinger, of the Prof. H. List company, Graz, Austria, described tests with a singlecylinder research engine of the conventional direct-injection, piston-cavity double-vortex type with a four-hole injector, and emphasized the importance of impingement of the fuel spray on the combustion chamber wall, combined witl rotary movement of the air. Dr. Pischinger read a paper on "Development Work on a Combustion System for Vehicle Diesel Engines."
Only the outer droplets of the fuel spray, stated the author, were diverted by the turbulent air moving transversely to the spray, the bulk of the spray forming the inner core being only slightly deflected. Ignition normally started with the "spray mantle" which quickly evaporated and mixed with the air, the core of fuel being distributed on the wall and incompletely mixed with the combustion air.
After the start of combustion, however, thermal mixing of the wall-distributed fuel occurred by virtue of the tendency of the burnt and burning gases to flow towards the centre of the centrifugal force, created by the swirl, on account of their relatively low specific gravity compared with the unburnt air, which tended to move outwards to the periphery and thus supply the unburnt fuel with oxygen for combustion. The low temperature of the walldistributed fuel prevented premature decomposition and the formation of soot in the absence of sufficient air. Whilst a higher swirl rate reduced the portion of the fuel reaching the wall zone, excessive swirl produced undesirable disturbances and combustion deteriorated.
Tests had shown that the duration of injection decreased sharply at low r.p.m. and also at speeds above 1,600 r.p.m. From 1,000 r.p.m. to 1,600 r.p.m. the swirl rate should increase, but above this speed a reduced swirl rate was required. It was impossible to obtain the most favourable swirl rate over the entire speed range with a constant-swirl port, and the choice of swirl rate was necessarily a compromise. An improvement might be obtained by designing the intake port so that there was a non-linear relationship between swirl rate and r.p.m.
Measuring Smoke
In his paper "The Measurement of Smoke Density of igX1.1811St Gases from Vehicle Diesel Engines," Dr. F. P. Malschaert, director of the Technical Automobile Laboratory, Ghent University, Belgium, outlined tests carried out in the laboratory at the instigation of the Belgian Research Foundation for Road Safety to compare methods of smoke measurement which could be employed at the 47 vehicle inspection stations in the country, established by local authorities.
Having made a comparison betWeen the Bosch filtertype smoke-meter and the Hartridge light-ray meter, Dr. Malschaert described a method of testing vehicles for smoke emission employed at Ghent University in which the front wheels of the vehicle are raised on a sling and the rear wheels are rotated under power when standing on concrete rollers equipped with brakes. The system would have been too costly for use in the 47 inspection stations in Belgium, whilst mounting the wheels on free rollers and using the vehicle brakes to absorb power would not have been acceptable to the majority of users. The simple method of accelerating the engine in neutral had therefore been adopted, the inertia of the moving parts providing the reactive force. This method had the advantage of convenience and it was easily repeatable. Tests had been applied to direct-injection and precombustion-chamber engines up to 12 litres capacity and many road tests had been made of manufacturers' vehicles.
A number of precautions were necessary when making a smoke test, said the author, which included establishing an engine working temperature of 70-85 degrees C. and ,urging the exhaust system before the test by a series of icceleration " bursts" up to .a maximumgoverned speed. When testing the engine of a service vehicle, allowance lad to be made for various " tricks " which had been levised by operators to reduce smoke density. These ncluded limitation of accelerator-pedal travel, diversion DI some of the gas through a secondary system (such as a 'eater or another exhaust pipe), the introduction of extra dr to the pipe by means of a venturi and reducing the peed of the gases below the probe requirement by ncreasing the size of the pipe. It should always be remem)ered, said the author, that diesel engines were irregular ;moke producers and a smoke meter should not be :ondemned because it did not always show the same reading Df an engine operating under the same conditions.
Conclusions that could be drawn from the tests showed hat injection timing was of Special importance with regard :o smoke emission, that automatic advance could eon;iderably reduce smoke density (without affecting power or :uel consumption), that smoke increased with altitude and .hat the use of an additive could reduce the density whilst ;iving a slight improvement in power and consumption.
In the case of the vehicles tested at the inspection aations, drivers were advised that their engines were in ;ood order if the Hartridge reading was lower than 45 mits, whereas they were warned of the annoyance caused o other road users and the risks involved if the reading .as between 45 and 75 units. If the reading was above 75, the drivers were instructed to return their vehicles at stipulated time, after repair or adjustment, for another :heck.
Stressing the importance of the smoke meter to mainenance engineers, the author suggested that it could be idvantageously employed for programming vehicle inspecions and in arranging maintenance schedules.
Paths to Powerful Braking
Three papers were read on braking systems and probems, "The Disc Brake," by Dr. R. C. Parker, of Ferodo, Ltd., Chapel-en-le-Frith, "Contribution to the Study of the Dynamics of Automobiles under Braking Conditions," Dy Mr. M. I. Odier, of S.A. Franeaise du Ferodo, and Vehicle Vibrations During Braking," by Dr_ D. Giacosa md Dr. E. Cordiano of the Fiat company, Turin. Prof. N. Kamm, scientific adviser of the Battelle Institute, Frankfurt, Germany, also reviewed methods of increasing ,raking efficiency (with particular reference to slippery -oad surfaces) in his paper "Future Development Towards kchieving Maximum Safety in Motor Vehicles."
Particular advantages of the disc brake, mentioned by Dr. Parker, included its stability compared with the drum Drake, derived from the thermal expansion of the disc in he direction of the friction material. Given caliper -igidity, this enabled a high displacement ratio to be .=!mployed without a self-energizing device and the output the brake varied proportionately to the adventitious :hange in friction. The output of a drum brake varied iccording to a power of the friction change, whereas ; disc Drake's output varies proportionally to changes in friction.
Whilst the disc could distort and thus affect brake outnut, this fault was more easily avoidable than distortion
of a drum. A greater resistance to fade was offered by the disc brake, despite the fact that, for a given effort, the temperature of the disc was higher than that of a drum. This was not a disadvantage, however, provided that the cooling rate of the disc was relatively more favourable. Disc cooling was mainly by ,convection if a temperature of about 500° C. (932" F.) was not exceeded. The relative efficiency of disc brakes was greater at high speeds; and fade resistance was also favoured .by the flat surface of the friction material. The designer selected a pad with H. high coefficient of friction and provided a system giving the required thrust. An external servo might be needed to augment the overall ratio without attendant lost motions.
Dr. Parker described Dunlop, Girling, Lockheed and Citroen disc brakes in detail and summarized the factors conducive to the general stability of the disc brake, saying that these included the absence of self-energization, the favourable configuration of the disc and the development of improved friction materials. Other advantages were a less variable pedal position, ease of pad replacement and of maintenance generally and a reduction in unsprung weight. The provision of a handbrake still presented problems, and more careful adjustment of the wheel bearing was required. Although self-energizing devices might be later applied to the caliper mechanism, they should be used with discretion as too much self-energization would affect stability. As braking duties increased, designers would need either to introduce forced cooling or insulate the hydraulic cylinders from heat.
Dealing mainly with front suspensiOn systems of the wishbone type, Dr. Giacosa and Dr. Cordiano stated that to obviate vibrations produced by braking it would be necessary to provide a system that was non-sensitive to shape irregularities of its component parts, which resulted from machining tolerances and thermal deformations. The disc brake had the best characteristics since shape defects had no influence on braking stability, but marked improvements could be obtained in the design of drum brakes. A floating shoe Was self-adapting to changes in drum profile and modifications to shoe geometry could advantageously be employed. These related to facing extensions with respect to shoe abutments and location of the abutments, and enabled a reduction to be made in the displacement of the contact point relative to the facing material. A twoleading-shoe brake based on these principles had been evolved and offered uniform distribution of -pressures over the facing arc, the penalty being a loss of efficiency of about 20-30 per cent.
Referring to the distribution of braking power, Mr. Odier emphasized the importance of varying the braking force of the front brakes in accordance with the deceleration rate and weight distribution 041 the axles and also with the speed of the vehicle. A pressure limiter or a pressure distributor might be employed to improve brake balance, or braking of the front or rear wheels could be delayed in some way. Anti-locking devices on the rear wheels might also be used.
Inserting wedge-type shoes by mechanical means between the rear tyres and road surface was recommended for consideration by Prof. Kamm for operations on slippery ads. Whilst this development would have to be perfected, id the author, it would enhance braking effort and steering ,ntrol in extreme conditions. Although the shoes would oduce " scratches " on the road surface, this disadvantage Duld be more than offset by the reduction in accidents e system would affOrd.
Dealing with other design features that promoted tbility, the author claimed that front-wheel-drive haneed directional stability on curves. The centre of avity of the laden vehicle should be well forward of transverse centre line and the preponderance of braking ould be on the front wheels.
inlet High-performance Pump
Features of an hydraulic pump that has so far been nited to accessory applications, but which should have a tiversal scope, were described by Mr. Hubert M. Clark, sistant chief engineer, of the Thompson Products CornLay, Michigan, U.S.A., in a paper entitled "The Slipper ■ •pe Pump for Automotive Hydraulics." The pump is
basically of the vane type in which the rotor revolves in an eccentric housing, the novel feature of the unit being the use of slipper pumping elements in place of vanes. Each slipper is spring-loaded and the contact surface is radiused so that the resultant wedge action promotes the formation of a fluid film.
Advantages claimed for the pump include improved performance and reliability, quietness of operation and low manufacturing cost. A dual-outlet version of the unit can be employed in place of two separate pumps, with a consequent saving in cost and space.
The author paid tribute to the outstanding benefit of electronic instrumentation in analysing variables and in the detailed examination of dynamic characteristics. Research data indicated that slipper-type industrial pumps could be continuously operated at pressures of 3,000 lb. per sq. in. and at speeds up to 20,000 r.p.m., volumetric efficiencies being in excess of 95 per cent. Units had been produced for power-steering systems and for supplying fluid to the control systems of automatic transmissions.