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C. H 13RADBURY

26th July 1968, Page 56
26th July 1968
Page 56
Page 57
Page 56, 26th July 1968 — C. H 13RADBURY
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MR. BRA DBURY says he was peering into the more distant future when he made various forecasts regarding engine design in his chairman's address to the Automobile division of The Institution of Mechanical Engineers in the paper entitled "A saga of sound and vibration".

After dealing in this paper with the potential of the variable-compression-ratio piston and ultrasonic fumigation as aids in the development of the low-compression diesel Mr. Bradbury concluded: "We see the beginning of a low compression era.

"We can see—probably a little dimly as yet—a low compression, all fuel, stratified-charge, automotive engine with electric ignition and with fuel atomized ultrasonically or by a tow-pressure jet into the open air intake. -If such an engine can be made to work satisfactorily from the standpoint of the operating cycle then an engine as quiet as the present-day petrol engine, but with diesel economy, is in sight."

Attempting to merge recent developments with present-day trends and coupling these with thoughts for the foreseeable future, Mr. Bradbury looks first at the question of stroke/bore ratio. He feels that the oversquare engine has now reached its zenith and that the square or even under-square engine will resume its place in the pattern of development.

He points out that the over-square engine limits the designer in his choice of combustion-chamber configuration. The most desirable bowl shape approximates to that of a hemisphere; the size of the bowl resulting from the use of a short stroke is such that the spray paths are short and the air vortex motion induced by intake swirl cannot be used effectively. Over-penetration is a problem particularly at higher outputs, and the deposition of fuel on the piston surface is a bad feature leading to exhaust smoke.

Commenting on the MAN system in which fuel is deliberately deposited on the surface of the piston bowl, Mr. Bradbury observes that he is not convinced that this method of reducing engine noise is conducive to good fuel economy. He points Out that the system is, in fact, a variant of the older type of hot-bulb engine.

The ability of the short-stroke engine to run at high rotational speeds with moderate piston speed and the possibility it offers of obtaining better breathing by the use of larger valves are advantages that should not be overlooked, says Mr. Bradbury. But he emphasizes that these advantages must be considered in relation to the combustion problems mentioned.

A problem that arises in connection with the development of high specific-output engines, cited by Mr. Bradbury, is that of obtaining satisfactory fuel injection over the whole load /speed range. All automotive engines must spend some time idling, and atomization must be satisfactory at idling speeds. The greater the "span of output" the more difficult it is to design an injector that will give a satisfactory performance over the whole load range.

The problem has been tackled in mediurn-speed industrial and .similar engines by using a nozzle which is satisfactory for the lighter-load end of the range and by increas ing the injection pressures considerably at high outputs. These high pressures pose problems with regard to the elasticity of the injection equipment components; long fuel pipes are a definite disadvantage.

It can be foreseen that inside the next 10 years with engines operating at a b.m.e.p. of 300 p.s.i. and over, this problem will have to be dealt with by completely eliminating high-pressure fuel pipes by using either unit injectors or some elaborate form of common-rail injection.

To maintain maximum cylinder pressures at reasonable levels the engine of the future must work at low compression at the higher end of the load range; and it is assumed that the growing popularity of the turbocharger will be further enhanced and that it will eventually become "almost a standard fitting". The variable-compression-ratio piston, developed more than 10 years ago, is being only tardily accepted and it is as yet difficult to foresee any material extension of its use in engines of the automotive type.

For smaller industrial engines a two-ratio system has been employed for a number of years, usually with manual control, and there is no doubt that automatic control could be designed and developed. Having a changeover point would, however, be a material disadvantage for a variable-speed, variable-load engine. The simplest solution must lie in a form of dual-compression-ratio engine where part of the combustion chamber is sealed off before the end of the compression stroke, the part which is sealed off having a high compression ratio and the remainder having a low-compression ratio. By this means combustion can be initiated in the first chamber and can be continued in the second.

This system, which is already under investigation in this country, has the advantage that it can be made to burn any type fuel; it can use aspirated or injected fuel and the load on the piston need be no greater than that of an equivalent petrol engine. It would, of course, enter the realm of that other type of engine which has proved the bane of so many inventors for so many years, namely the stratified-charge petrol engine. It would in fact form the bridge between the conventional petrol engine, the stratified-charge engine and the diesel.

In his address to the Institution, Mr. Bradbury mentioned the possibility of developing ultrasonic fuel atomization for general use. Fuel atomized ultrasonically has little or no penetration but has remarkably uniform droplet size. Although its application to reciprocating engines is at the moment somewhat unpredictable, Mr. Bradbury foresees the use of ultrasonically atomized fuel in petrol engines where the control of droplet size can facilitate complete vaporization and hence the elimination of many of the harmful constituents of presentday petrol-engine exhausts.

Commenting on the prospects of the constant-horsepower engine, Mr. Bradbury says its future may well depend on operator demand. There is a tendency with presentday high-output engines and with multispeed gearboxes for drivers to attempt to run at constant engine rpm and to adjust the engine speed to vehicle speed by frequent gear changes.

The use of a constant-horsepower unit with, say, only three speeds would initiate a Phange in drivers' habits but would make the overall task of driving a heavy vehicle much less onerous. Mr. Bradbury feels that the constant-horsepower engine must indeed take its place in the transport industry in the not too distant future but that such an angine must be basically simple; it must achieve its performance with simple or some 7orm of compounded turbocharging. The ise of intercoolers will be vital but will not present any major problems.

One of the principal weaknesses of an engine designed for high output is the cyinder-head gasket and difficulties arise bemuse of the break in the engine structure at 3asket level. Cylinder-head studs cause distortion and obstruct the use of effective porting. Effective cylinder cooling is almost absent at the point where it is required most.

Reverting to a practice that was adopted m certain earlier cars of casting the cylinder lead and cylinder block in one piece would low appear to be logical. Design progress las enabled extremely reliable valves and ;eats to be produced. The necessity for vithdrawing the whole of the cylinder block )f a one-piece engine for a top overhaul vould become less and less frequent, in part )ecause of the adequate cooling that could )e employed.

Mr. Bradbury refers to the noise problem as one that "appears to be outrunning all iforts to keep it under control". Cornlustion at low engine speeds can be a ;erious problem but as engine speeds rise it pecomes less and less of a problem; and it an be shown, by analysis, that at very high ;peeds the diesel is as noisy when motored as when running under load.

Noise must therefore be tackled from the )oint of view of radiation and this will nvolve attention to the materials used and o the scantlings. Cast iron, steel and alumiaium have been used in the construction of liesel engines for many years; Mr. Bradairy feels that alternative forms of material at' high damping capacity will find a place in he pattern of diesel development in the next .0 years.

Scant consideration is given, Mr. Bradnary emphasizes, to a particular source of [Oise that could be a major contributory actor to engine sound emission in the highT speed range, namely that of air turaulence in the crankcase. In a conventional our-cylinder four-stroke engine the full pison swept volume is displaced from No. Ito sTo. 2 cylinder and vice versa, and from No. I to No. 4 and vice versa every revolution at' the engine. And during this displacement he air has to pass at a high speed across elatively sharp-edged protuberances formed ay the crankshaft, connecting rod and variaus projecting static obstructions in the rankcase. The wave motions set up in this aighly turbulent air find their response in 'axiom component parts of the engine and toise emission is the result. It can be oreseen that in the not too distant future he same care will be applied to streamlining he inside of the crankcase and its contents s is applied to such components as the adiator fan and so on.

With reference to V engines Mr. Bradbury believes that the V8 engine, already well established, will continue to gain ground in spite of the resultant balancing difficulties. Ultimately, however, an attempt will be made to reduce the V-angle and make the engine more acceptable from the installation point of view. The future for the six-cylinder V is not so certain. The saving in length over the in-line six is more marginal than that provided by the eight-cylinder V engine over the straight-eight, and the in-line six has many inherent advantages which units of V formation cannot match.

In a last word on probable long-term engine developments, Mr. Bradbury supports the common view that the gas turbine has a future for vehicles requiring more than 350 bhp and operating at a high average load factor. He also feels that the Wankel type rotary engine will find acceptance in certain spheres. This engine has progressed to a point where the considerable thought and expenditure devoted to it must produce a viable power unit, despite the fact that many development problems have not as yet been solved.

Mr. Bradbury maintains that the Stirling all-fuel engine cannot be completely ignored. Its attributes as far as silence and clean exhaust are concerned make it an attractive proposition and there is no doubt that certain American developments are leading towards an engine that might be commercially acceptable on the roads.

While Mr. Bradbury is primarily a specialist in engines and engine-mounted components, he has an interest in other aspects of vehicle developments, namely in braking and retarder systems. Anti-locking devices will, he believes become mandatory in less than 10 years' time and he considers that intense research must be put into the design and development of suitable systems at the earliest possible date.

For the heavy commercial vehicle, and indeed for the passenger coach, a braking system not associated with brakes on the vehicle wheels will find increasing application. Mr. Bradbury is convinced that the retarder that will ultimately be adopted will be one which fulfils certain fundamental conditions. It must be capable of actuation through the engine gearbox, that is it must be on the engine side of the gearbox; it must be capable of being applied automatically and of dissipating its heat through heat exchangers that are already associated with the engine. These conditions boil down to some means of making the engine act as its own retarder. The Jacobs system, which is applicable to certain American-designed engines, fulfils these functions.

The Jacobs has limited application; the design and development of a system providing the necessary functions and applicable to almost any type of engine should be possible. Mr. Bradbury is convinced that once a suitable system is devised (some thought has already been givgn to this problem) wide adoption will autvirmatically follow.


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