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Projected short-term improvements
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Further short-term improvements can be expected from the engine itself on which there has been separate development activity, in two basic forms.
First, errors in the advance settings of the fuel injection equipment produce large increases in the noise emitted as well as actual increases in consumption and smoke e sion. Improvements will achieved not only by tighte production tolerances in area but also by changes ix design of the fuel injec equipment, which in turn enable these tighter tolera 3e achieved more easily. rndly, the more general tice of turbocharging Ares energy from the Lust and tends to produce a lther combustion process, :e a quietening effect.
iese changes result in a .ction of the noise emitted L the surfaces of the engine k and sump and make a big rence to the overall noise . when the engine is inad in the vehicle.
3spite the previously men)41 limitations on cooling 3m development, there is in room for improved effici • in this area. A small imement in the radiator matrix iency, and a development Le aerodynamic fan with a 2a1 exit pattern will enable horsepowers to be reduced e maintaining the same heat :tion rate. Also the choice 3.n in a heavy commercial cle is a function of its ability aject the waste heat from mgine at full power while vehicle is climbing a steep n a following wind. This is rent from the passenger dtuation for which it is only ired to reject the waste generated when the car is in traffic. It leads to the tion where the fan on a is only required during a 1 percentage of its operating Ind makes the thermostaticontrolled fan an attractive osition. Not only is there a absence of fan noise but a fuel saving arises through saving to drive the fan con)usly, involving a power ig between 7 ami,17 bhp.
L alternative to the thermo:ally controlled fan drive te radiator shutter, which 'ugh not completely elimiLg the power required, puts an into a stalled condition a the shutter is closed. This save up to 40 per cent of an horsepower as well as iding a barrier to the sound ; radiated from the engine 3artment. Again, due to the city of the cooling system ired for a heavy truck, the ers are not usually open 'ore than about 10 seconds &every 60 during an urban ley. Although shutters save about 40 per cent of the iorsepower they have two benefits, that of controlling engine temperature more ly and secondly of iming the vehicle aeromics when closed. This is that the fuel saving !ved is about the same as that of the thermostatically controlled fan.
Short-term reductions in truck noise levels are likely to be achieved by employing all the palliatives mentioned above. They could lead to certain vehicles being produced in which the specification of derated engines, and limited encapsulation, could provide sufficient performance for local delivery duty in urban areas.
Sufficient flexibility exists in bus design to permit the separation of the engine and cooling system, thus avoiding large openings in the engine compartment through which noise is transmitted. Also city buses with derated engines and limited performance allow a degree of engine encapsulation for reducing noise in highly populated areas.
Long-term improvements
The limitation to these reductions lies in a number of areas. First, the changes that have been brought about in recent years have been successful largely because there were some areas of a vehicle which radiated noise with an exceptionally high pressure level. By identifying these areas and by carefully working on them to reduce their noise level to a point at which there is no contribution to the overall vehicle noise level, a significant reduction to that overall noise level is achieved. However, it can be readily understood that this very reduction brings many more parts of the vehicle within the 10dB(A) range and means that the next step becomes much more complicated and significantly much more expensive.
It should be re-emphasised that existing standardised test procedures and legislation were originally designed to deal with the maximum noise level that it was possible for a vehicle to produce. This level has in fact been successfully reduced to nearly a quarter of its original value. Although this reduction: has had a significant effect on noise levels in urban environments, vehicles do not spend much of their operating life emitting their maximum noise level under 'these conditions and it will be necessary for new test procedures and legislation to be developed which will reduce urban noise levels as successfully as maximum noise levels. In the longer term, basic changes to the design of the cab, the cooling system and possibly to the legislation on dimensions will assist in the reduction of noise in the urban environment.
Truck design for noise reduction
Once the noise levels required by the next phase of legislation, projected at the moment for the end of 1977 in the Common Market directive 70/157 EEC, are attained, the limits of development will have been reached within current design parameters. More basic design changes will be required for the next phase of the work.
New designs of truck cab will require noise shielding built in as part of the wheel arch. They will require engine compartments with adequate space for noise and insulation materials with sufficient clearance around the cooling system to enable air entry and exit conditions to be optimised. If possible the engine compartment and cooling system will be physically separated, as in buses. All this would be more easily attained if some of the dimensional restrictions were relaxed.
Future developments in combustion and a better understanding of the variation in noise levels experienced within a batch of production diesel engines will enable further reductions in the basic source of the noise, also developments in the field of ancillaries such as Lyre equipment will enable full advantage to be taken of the reduction in engine noise levels.
The growing popularity of automatic gearboxes enables vehicle noise to be minimised in a particular situation by ensuring that the vehicle is using the highest gear ratio commensurate with the demand from the driver. That is to say that although the driver can cause the vehicle to emit its maximum noise level, the automatic gearbox will ensure that the optimum gear ratio is selected so that, if he is prepared to moderate his demand, a lower noise level is automatically obtained without the requirement for a conscious effort from the driver to select that ratio.
National projects are currently under way to research into the best possible ways of achieving a quiet heavy vehicle. The problem is being tackled from two points of view: that of the engine designer and that of the vehicle designer.
Vehicles are being prepared by the Motor Industry Research Association under a research contract to the Government which, within the confines of traditional commercial vehicle practice, will have as low a noise level as is possible to obtain. This is being achieved by optimising the design of the cooling system, exhaust system and engine enclosure to obtain minimum external noise levels (fig 1) while at the same time Southampton Institute of Sound Vibration Research are undertaking a more fundamental approach to engine noise emission.
Quiet buses—present and future
Developments in bus design will allow partial or complete separation of engine and cooling system with basic changes to some dimensions which will ease the problems associated with engine encapsulation without the fire hazards which tend to be associated with trying to reduce noise in current designs. Current examples are shown in fig 2, 3 and 4.
Furthermore, the develop ment of integral construction for bus structures enables better control of the final product so that problems of maintaining an adequate standard of sealing are reduced and boundary disputes between chassis and bodybuilders are avoided.
Buses of advanced design, with complete cooling system separation, will bd ready to go into service at the end of the decade and will largely be aimed at city and urban operation where they will operate with an external noise level well under 80dB(A) and should subjectively make a significant difference to the overall noise level in these areas.
Further possibilities for vehicle external noise reduc tions will require basic develop ments in engine design, possibly in line with the proposals of Southampton ISVR whose research engine, instead of being a "cast iron box" with the other components assem bled into it, is a cast iron backbone locating the components with a "damped steel box" wrapped around it and containing the coolant, lubricant and radiator noise.
Other components which have up to now not significantly contributed to the overall noise level of a vehicle will require attention. For instance, tyres, which have thus far received attention on motor cars largely to reduce the transmission of structure-borne noise to the interior of the car, will require that attention extending to truck tyres with the object of reducing the external drive-past noise. This reduction may have to be achieved at the expense of tyre life and, for this reason, legislation will be required before operators, and therefore manufacturers, will be prepared to accept the reduction.
Unfortunately at this time there is no clear agreement either internationally or nationally on the form that future legislation should take. Society must decide fairly quickly whether it continues to be worried only by the single exceptional vehicle. This is typified by the noisy motorcycle accelerating away from the traffic lights in the middle of the night. Alternatively, Society might decide that it would now be more beneficial to concentrate on reducing the ambient noise levels in the more densely populated urban environments. This will inevitably involve the improvement of analysis techniques and the development of procedures which more nearly represent the actual mode of operation of the vehicle.
Internal noise reduction Considering firstly the noise levels at which hearing and general health become impaired, this depends on the length of the exposure as well as the pressure level of the sound to which the subject is exposed. British Standard Code of Practice 1972 states that the tolerance level of the driver is approximately eight hours/day at 90dB(A) or four hours/day at 93dB(A) or two hours/day at 96dB(A). After these durations, hearing tends to be permanently impaired.
The reasons for reducing internal noise levels of motor vehicles are twofold. It is of the utmost importance that the hearing and general health of the driver is not impaired as he goes about his daily task. Further reductions are then necessary to improve the environment of the driver and his efficiency at work and reduce the prob lems of recruitment and industrial relations.
How far the second goal can be pursued is largely a matter of economics; if taken too far it can have a diminishing return and become very expensive.
Remembering that maximum noise level in a vehicle is only experienced while the engine is delivering full power, which in the case of a diesel engined vehicle is not necessarily at full speed, a maximum internal noise of 90dB(A) even for alongdistance lorry driver would theoretically be low enough to prevent any permanent injury.
Fig 5 shows the noise level recorded in a vehicle before and after noise suppression treatment. On the right can be seen the overall maximum noise levels measured on the sound pressure level meter in dB(A), while the long lines in the centre represent the noise, recorded at a moment in time and analysed into its constituant parts measured in dB. The information that can be gained from this analysis is useful determining which part of vehicle should be attacked fi The lefthand of the line are low frequency impulses duced by the firing freque of the engine and exha whereas the righthand enc the line shows the high-pitc whine of the turbochar turbine.
The reduction in noise h gained during this exercise produced by careful sealin, the cab, followed by the appl tion of accoustic trim motel to the roof, floor and em cover.
The importance of car sealing is underlined by E in which the noise-suppreE vehicle had its gear lever gE displaced to expose a si opening to the engine comp ment, thereby losing nearts the previously gained adi tage. This demonstrates important facts, first that r of the noise can get thrc even the smallest hole secondly to recognise fully problem of maintenance as • as the initial engineering. / of expensive suppression v can be spoilt unless the f replaces any seals that disturbs.
Fig 7 shows the effect o. different parts of the supy sion kit that were applied tc vehicle. Note that the effec the bonnet insulation is gre than the effect of the floor which in turn is greater I the effect of the roof hr They all contribute to the o all result, however, and con a vehicle which 4.without treatment at all would be toi unacceptable into one whic subjectively quiet inside.
It should be rememberE this context that even the a age family car can proi sound pressure levels of 8 86dB(A) at 70 mph. The p lem with the truck is aggrav by the fact that it spent much greater part of its li or near maximum power therefore requires a hi degree of suppression thar average family car.
Internal noise levels on vehicles being produced a moment are between 84 86dB(A), generally; Br Leyland Truck and Bus Div are aiming to ensure that new model will have an intE noise level between 81 83dB( A).
These short-term impr ments will be gained main] exclusion and absori whereas the longer-term tions will involve produ engine and transmission with a much lower noise oul as described in the earlier of this article.