Everything you need to learn about reducing and controlling automobile noise.
Introduction:
The problem of automobile noise is growing from year to year. Growing urbanisation, the rising number of automobiles and trucks and the quantity of traffic have had the effect of increasing noise levels generally.
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This points to the fact that noise-level estimates must actually be qualified, since sound levels have increased substantially in certain urban areas (particularly in suburbs) over the last ten years, but they have risen only slightly in the central core of cities, where any increase is limited by congestion. The phenomenon with regard to road traffic is limited by congestion. The phenomenon with regard to road traffic is therefore more one of noise extension in time and space, rather than one of noise intensification.
Quiet areas have become noisy and the traffic-free period during the night has shortened. London enquiry showed that the duration of an average ‘noise night’, that is, the time during which sound level is lower, was surprisingly short: only 5½ hours (from midnight to 5:00 or 6:00 a.m.). This period of quiet began before midnight only in about 25 per cent of places where measurements were made and extended beyond 6:30 a.m. only in 11 per cent of cases.
Even more disturbing, however, is the fact that measurements made in London since 1991 show that this quiet period is becoming ever shorter since night traffic is steadily rising, the period of quiet is steadily diminishing. Such a trend means that ultimately the night noise levels may approach those currently recorded during the day, at least in big cities.
Thus, while day traffic is usually already so congested that there is little likelihood of any further rise in noise levels recorded during daytime traffic peaks, night-time levels may still rise substantially, since night traffic is still far from being saturated.
Foreseeable Trends:
To predict what the levels of noise from motor vehicles will be over the next few years is, like all forecasts, a hazardous enterprise, which requires the consideration of a number of factors:
1. Technical evolution of motor vehicles and progress in reducing their noise.
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2. Population and urbanisation trends.
3. Economic trends and trends in motor vehicle ownership.
4. Traffic trends, particularly in urban areas.
5. Changes in legislation.
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6. Changes in public attitudes toward the noise problem.
Several of these factors are very difficult to predict. Others, such as the progress in noise reduction made possible through technical evolution in motor vehicle design are amenable to reasonable forecasts. Here we shall make the assumption that in the near future noise emissions from vehicles will be similar to those of today and we shall proceed to consider future trends in the development of traffic and of the annoyance caused by noise.
From these facts and forecasts it is thus evident that all conditions are fulfilled for a substantial rise in urban motor traffic during the coming years, an increase which should be especially noticeable in metropolitan areas of small density—such as the near and distant suburbs—which are poorly served by public transport. This trend may be checked as a result of the current energy crisis, if motor vehicle traffic is strongly controlled and public transport vigorously supported by governments.
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The rise in noise levels over those of the present levels can be very roughly predicated from estimates of the growth of the motor vehicle numbers, the annual mileage covered, and the average vehicle speeds in urban areas. In France, for example, such computations indicate an increase of 2-3 dBA between 1985 and 1995.
However, in view of the fact that local conditions play a decisive role where transport and its disagreeable aspects are concerned, it is clear that estimates of this kind are of but limited value. In the saturated centers of big cities, while daytime noise is unlikely to increases much further, it will rise appreciably by night. On the other hand, in suburban areas noise will increases significantly, especially in the neighborhood of fast highways, which are increasing in number.
The more scattered the dwellings in suburban areas, the more marked will be the increase in noise. In fact, contrary to what happens in city centers, where the buildings are contiguous and rooms not opening directly onto the streets are thus protected from noise, in residential suburbs the buildings, are separated from one another, thus promoting the transmission of noise to all sides of a building.
Sources of Noise in Motor Vehicles:
The noise depends primarily upon two groups of factors:
1. Engine speed.
2. Vehicle speed and how the vehicle is used.
Noise related to engine speed has a number of components: intake and exhaust noise, cooling-fan noise, noise emitted by the engine proper and noise from that part of the transmission (gearbox) which rotates at engine speed. The predominant engine noise is, in general, in the frequency range of 300-4000 Hz.
The sources of noise related to road speed are that part of the transmission affected by engagement of the different gears and the rolling of the tyres at higher speeds, aerodynamic noise may be a factor. Certain operational factors, such as the load, age and general condition of the vehicle and the fuel used, also have an influence on the noise emitted.
i. Intake and Exhaust Noise:
If they are not silenced, exhaust and intake noises are predominant in an internal combustion engine. Intake noise is produced by the opening and the closing of the intake valves—by the high-speed airflow through the valve seat. The process sets in vibration gas columns at high pressure which communicate directly with the atmosphere.
Internal combustion engines are usually cooled by either centrifugal or axial fans. Axial fans, which are the most common, are used exclusively in water-cooled engines to draw air through the radiator, while centrifugal fans are sometimes used for air-cooled engines.
All fans produce broad-band aerodynamic noise, originating primarily from lift fluctuations on the blades due to vortex shedding at the trailing edges. Any large-scale turbulence in the flow ahead of the blades will set up additional lift fluctuations which can significantly increase the broad band noise radiated. As a broad generalisation, the intensity of fan noise increases very rapidly with the speed of the blades—for example, by 55-60 dBA when the blade tips increase in speed tenfold.
Combustion is the primary noise source in the internal combustion engine. In diesel engines, the gas temperature in the combustion chamber after compression is sufficiently high to cause self-ignition of the injected fuel, while in gasoline engines; the gas mixture entering in a combustion chamber is ignited by an electric spark.
Engine noise is primarily determined by three parameters:
a. Engine Size:
Engine noise increases faster than engine capacity for example – by about 17.5 dBA for a tenfold increase in total cylinder capacity.
b. Engine Speed:
Noise from diesel engines increases by approximately 30 dBA for a tenfold increase in engine speed, while that from gasoline engines rises by approximately 50 dBA. The tendency of modern engines to be over square (very short strokes) to save space increases both their bore and speed and hence their noise.
c. Engine Load:
For diesel engines, the effect of load on the gas forces resulting from combustion is, in broad terms, less important than the other two effects—but this effect is substantial for gasoline engines.
The variations of sound intensity with speed and size lead to the general observation that for the same horsepower, the larger but slower engines are considerably quieter. For instance, it has been shown that a 30 litre/cylinder engine running at 500 r.p.m. and developing 600 h.p. has the same level of noise as an engine of 0.4 litre/cylinder developing only 40 h.p., at 4000 r.p.m.
Yet the ratio of power is 150: 1. Thus, noise is generally independent of horsepower—the amount of work done per unit of time; the main parameter that governs engine noise is rather the operating speed or the shortness of the time interval within which the machine perform one cycle.
Investigations into the effect of the number of cylinders show that there are no appreciable differences in noise between four cylinders in line, six cylinders in line and a V8 arrangement for engines of the same bore running at the same speed. However, the choice of the stroke/bore ratio has an important effect: engines with a larger bore have a higher overall noise level.
i. Transmission Noise:
The source of transmission noise is not altogether clear. In most cases, the gearbox is closely coupled to the engine; therefore, it is likely to be excited by vibration forces transmitted from the engine or by vibratory forces caused by gear meshing. Levels of gearbox and rear-axle noise in trucks under ISO test conditions are of the order of 75-85 dBA in the worst cases.
Much of the rolling noise from tyre is of an aerodynamic nature due to the compression and decompression of the air trapped between the treads as the tyre rolls over the road surface. Thus, it depends both on the design of the tyre the width of the tyre, the dimensions and shape of the treads, the tyre pressure—and on the operational factors—the speed of the vehicle, but also the weight and load of the vehicle and the nature of the road surface (whether smooth or rough, dry or wet).
In general, tyre noise is greater for trucks than for automobiles, but for a given type of vehicle increases only moderately with load. In particular, trucks, the noise:
1. Increases with tyre width (almost in direct proportion, other factors being equal).
2. Increases by about 30 dBA for a tenfold increase in vehicle speed.
3. Increases as the depth of grooves in the treads decreases, that is, as the tyre becomes worn out.
4. Varies with the design of the tread. Treads with unvented or poorly vented cavities are particularly noisy. Longitudinal treads are quieter than transversal treads, which in turn are quieter than recaps. A recap may be noisier by as much as 20 dBA than a longitudinal tread tyre and a tyre with a partially worn out transversal tread by as much as 10 dBA with respect to the corresponding tyre with a longitudinal tread. Randomisation of tread patterns can spread pure tones in the frequency spectrum, with a concomitant reduction in community annoyance.
1. Payload:
Both in commercial vehicles and cars a reduction of payload does not necessarily result in a reduction of noise. In most cases noise levels obtained in constant-speed drive-past tests are lower for fully loaded vehicles than with the driver alone. Traffic situations, rather than any inherent characteristic of the vehicle, are usually responsible for increases in noise when the vehicle is fully loaded.
On gradients, heavily loaded vehicles tend to be in a lower gear at high engine speed than their unloaded counterparts and spend a longer time at high engine speeds when accelerating. Vehicles deteriorate with age. Tests in Germany and Austria have shown that 3-4 year-old vehicles emit 2-3 dBA more than new ones. The maximum increase in noise due to age is 3-4 dBA both for diesel and gasoline vehicles.
2. Fuel Used:
The kind of fuel used does not seem to affect the noise from gasoline engines unless the octane rating is insufficient, in which case backfiring and ‘pinking’ can occur. For diesel engines, on the other hand, the cetane rating (which varies from one country to another) has a significant effect: noise increases of 2-3 dBA can be observed if the cetane rating is low. These increases are not uniform, as some engines are more sensitive than others to the cetane rating.
Engine Speed versus Vehicle Speed:
For commercial vehicles, the influence of vehicle and engine speed can be described from measurements of the noise produced at various constant vehicle speeds.
For Diesel Trucks:
Noise level in different gears does not change much, in spite of considerable change in vehicle-road speed.
1. If the noise levels are plotted against engine speed, they reduce almost to a single line.
2. The same rate of increase of noise (11 dBA for a doubling of speed) is observed with vehicle speed at any chosen gear.
3. Rolling noise, whose principal component is tyre noise, is appreciably below engine noise.
For Automobiles with a Gasoline Engine:
1. The trends are similar to those for trucks – the noise is again to a large extent controlled by the engine (although other sources such as road and transmission contribute to a much greater extent than in the case of diesel trucks).
2. Noise levels plotted against engine speed do not reduce to a single line as distinctly as those of diesel commercial vehicles.
3. The rates of increase of noise with engine and road speed is substantially higher than for trucks (an increase of 15 dBA when the speed doubles and of 50 dBA for a tenfold increase in speed— versus 37 dBA for trucks).
Noise Reduction without Radical Changes in Design:
A motor vehicle is a system; the suppression of a single source of noise, such as exhaust noise, does not necessarily suffice to solve the noise problem. If a particular type of noise predominates, its suppression or reduction could well unmask other significant sources of noise. Thus, all dominant sources of noise in the vehicle must be dealt with if the total noise emitted by the vehicle is to be reduced.
The development of a new vehicle and engine costs both time and money. Some 5-7 years are needed between the design of a new model and the beginning of quantity production. The manufacturer therefore hopes that a vehicle will continue in production over a number of years without changes to the vehicle or engine that would necessitate major changes in the production plant.
Yet, as legislation on pollution and noise becomes more severe, changes in vehicle design will be impossible to avoid. Certain changes are not of a radical kind and are feasible when standards are not yet too severe, or can be adopted as interim measures before basically new models are developed. These changes include more effective control of engine noise by appropriate engine design, total engine enclosures, appropriate design of the entire vehicle, appropriate design of the intake and exhaust system and of tyres.
Control of Engine Noise—the Case of the Diesel Engine:
At moderate speeds that is, in urban areas—the noise of both gasoline and diesel vehicles may reasonably be assumed to depend primarily on the engine speed. Since diesel engines are considerably noisier than gasoline ones, this section will concentrate on them as an example of measures that can be taken to reduce engine noise.
Most considerations are also quantitatively valid for gasoline engines. To place the control of engine noise in perspective, it is useful to note at this point that the engine cost represents only 10-20 per cent of the total vehicle cost. Operational factors are also important. For instance, intake noise decreases markedly with engine load—by 10-15 dBA in diesel engines and by as much as 20-25 dBA in gasoline engines.
1. Choice of Engine Design Parameters:
To reduce noise, a correct initial choice of design parameters becomes very important. For a given horsepower, a quieter engine is obtained by the choice of smaller bore, larger number of cylinders and lower rated speed (for the same horsepower, the greater the cylinder capacity the less the noise). A design taking into account these parameters can, however, result in an increase of total engine weight for the same horsepower, but this could be offset by turbocharging.
2. Control of Noise due to Combustion:
Considerable changes can be made in a combustion system by modifying the injection arrangements— the number of sprays, the rate of injection and the injection timing.
A critical balance is needed to reduce combustion noise, since any change in injection characteristics will also result in a change in exhaust emissions and fuel consumption.
Usually to retard injection in a direct-injection diesel engine effectively reduces noise but adversely affects fuel consumption and smoke emission. Similarly, in a gasoline engine the use of lean mixtures to reduce emission results in a more rapid cylinder pressure rise and more noise.
3. Noise Reduction by Turbocharging:
The initial pressure rise in the cylinder is the determinant factor in engine noise. Thus, engine load should have no effect on noise and engine power can be increased without corresponding increases in noise by pressure-charging, rather than by increasing cylinder bore and rated speed.
Alternatively, to reduce the noise of an existing engine, the speed can be decreased, while the load is increased by turbocharging, to maintain the same power. With its higher combustion chamber temperatures resulting in a shorter ignition delay period, it is even possible that, other factors being equal, a pressure-charged engine will be nominally quieter than a normally aspirated engine.
A large area of the external surface of an engine, often as much as 60 per cent, consists of casings or covers, such as the oil pump, front timing and valve gear cover. If cover noise could be completely eliminated (it can be done experimentally with lead sheet shielding) the overall engine noise can be reduced by 2-6 dBA, depending on the type of engine.
Thus, there is considerable scope for appreciable noise reduction through improved cover design. Another satisfactory method of reducing cover noise is vibration isolation, in which the main area of the cover is isolated from its fixing flange by a bonded rubber strip. This method offers considerable production difficulties and is not reliable enough at present for oil sump applications.
In general, an improved cover should have a natural frequency falling outside the 300 4000 Hz range of predominant engine noise. The cost of covers represents only a small fraction of the total engine cost (on the order of 1 per cent). Therefore, even if the cost of an improved cover should increase by 100 per cent or more, the total engine cost would not be affected significantly.
5. Engine Shielding and Enclosure:
Close-fitting shields can be used to reduce noise emitted by the basic engine casting—the cylinder block and the crankcase structure; the resulting overall engine noise reduction can reach 2-3 dBA.
Engine noise can also be reduced by enclosing the engine—by fitting an acoustic box around it. The main difference from the close-fitting engine shields is that the enclosure is some distance from the noise-radiating surface of the engine (usually near the walls of the vehicle forming the engine compartment).
Thus the enclosure has to deal with both normal and random incidence noise and is at a disadvantage with respect to the shields, which have to deal only with normal incidence noise, because of their close proximity to the noise-radiating surface of the engine. The attenuation of a panel at random incidence is generally about 10 dB less than at normal incidence.
Moreover, because of the large reverberant volume, the enclosure must always be lined with absorbent material. The use of a large quantity of porous material around the engine constitutes a serious fire risk which may be reduced to some extent by the use of fire-retarding forms and perforated metal guards.
A reduction of 6-10 dBA is possible by complete enclosure with absorbent lining which underscores the importance of the sound-absorbing material. The achievement of a wholly satisfactory design is impeded, however, by a number of factors.
In addition to the fire hazard, the enclosure causes poor cooling of the engine surfaces (particularly the oil sump), difficulties in passing through controls and pipes, increases in weight (about 50 kg), poor accessibility for engine servicing and finally, is costly in both material and installation. In reality, an effective enclosure that eliminates overheating and many of the other problems can probably be achieved only through drastic changes in the configuration of the vehicle—for instance, by locating radiator and fan ways from the engine.
1. Intake Noise:
Intake noise can be lowered by appropriate silencers that reduce the vibrations transmitted to the air by the aspiration of the engine. The whole intake system needs to be considered—the inlet manifold, air filter and intake silencer. The latter require space, making the problem more severe in automobiles than in trucks, because the limited space in the engine compartment demands very compact air-intake silencers.
2. Exhaust Noise:
Making better exhaust system would be neither difficult nor expensive. However, disagreement persists as to the method for producing quieter exhaust system. The methods generally used depend more on an empirical approach than on theory: every model poses a specific problem calling for a specific solution.
Exhaust noise is affected by a variety of factors and mechanisms, which demand in turn different approaches for reducing it.
These approaches fall into the following basic categories:
i. Approaches concerning the engine itself and
ii. Those covering the design and positioning of the muffler.
In the first type of approach, exhaust noise can be reduced by as much as 10-15 dBA by modifying the exhaust valves (e.g., a fluted exhaust valve to divide the flow of the exhaust gas at the valve seat between a number of separate channels), or by an appropriate design of the exhaust cam (giving a very gradual initial opening so as to release the high-pressure gases through a narrow opening).
In the approaches aimed at modifying the manifolds and the exhaust muffler, various investigations have shown that the noise emitted can be reduced by adopting additional expansion chambers or a second exhaust system and by correct positioning of the exhaust pipe. For instance, on diesel-engine commercial vehicles, when sufficient space is available the addition of a second exhaust chamber appears to reduce noise by 3-4 dBA.
A quieter exhaust system should not lower performance (it is generally assumed that the exhaust system should not absorb more than 5 per cent of the maximum effective engine power). These systems should not be allowed to deteriorate with age and use and need to be compatible with air pollution standards and measures, such as special mufflers for reducing the emission of pollutants.
In cases where exhaust noise predominates over other sources of noise, as in sports cars and motorcycles, the design of quieter exhaust systems has the advantage of allowing fairly rapid correction, since ‘retrofit’ replacements of the exhaust system are possible at acceptable cost for vehicles in use, whereas to replace the engine is a much more difficult task.
There are substantial differences in the characteristics and sound levels of different categories of vehicles (motorcycles, automobiles, trucks, tractors, buses, etc.). However, the overall design of a vehicle is based upon functional consideration and therefore does not lend itself easily to adequate acoustic treatment.
In automobiles, there is a large variety of possible engine installations, but by far the most common is the front axial position. In this position, the engine is well shielded by the engine compartment, except at the front where the radiator is a weak point. Ground clearance is low.
In buses, the body is low and gives a small ground clearance, thus making a fairly effective screen for the engine. The engine can be installed in a variety of positions, the most common being a horizontal position between the longitudinal chassis members or a transverse position at the rear. Both these positions permit adequate shielding and enclosure. They also allow the radiator and fan to be placed at a certain distance from the engine itself.
In many heavy trucks, the engine is behind the cab or under it. In cases where the vehicle is of the tractor type, the rear of the engine and the gearbox protrude behind the cab and are fully exposed. In general, because of operational requirements, trucks have a high ground clearance. The engine surfaces are usually exposed between the chassis and the front wings. The radiator, which is large, is at the front of the vehicle, very close to the front engine surfaces.
In light trucks, the engine is often positioned on the centerline of the vehicle between the driver’s and passenger’s seats. As most light trucks are designed to operate on good roads, the body of the vehicle extends down to the normal road clearance, about 15-30 cm from the ground. Thus, the body makes a fairly effective screen for the engine. In certain cases, the front of the vehicle is well forward of the radiator and also provides a kind of screen.
In brief, therefore, in common types of heavy trucks, the vehicle walls do not shield the engine. On the other hand, in light trucks, in cars and buses, the engine noise is substantially masked. For instance, if a bus and a heavy truck of the same power are compared, the bus is found to emit 4—5 dBA less than the truck.
Just as for exhaust silencers, the tyres of vehicles in operation could be replaced by quieter ones, especially since a set of tyres has to be changed several times during a vehicle’s lifetime.
Since tyre noise becomes predominant only at high speed, the effect of its reduction is bound to be felt more on highways than in the cities or other densely built-up areas. In urban centers, the beneficial effects of reducing tyres noise will be felt only in the long run, after other sources of vehicle noise have been significantly reduced.
Vehicle speed, type of road surface, wheel loading and tread depth, to a varying extent, all influence tyre noise. For instance, noise increases as tyres wear out, that is as the tread depth diminishes. Thus, frequent replacement of tyres, although costly, can reduce somewhat tyre noise. Also, changes in tread design can eliminate certain frequencies and harmonics.
Current efforts in tyre design are concentrated primarily on designing new tyres to improve safety and roadholding quality rather than to reduce noise. Tyres should, of course, not be made quieter to the detriment of safety, but research is needed to find tyre designs that can be both safe and less noisy.
Noise from motorcycles is a particular problem, since the inherent features of these vehicles can vary substantially according to cylinder capacity, whether they employ a two-stroke or four-stroke cycle, etc.—and since they can be variously used and converted (e.g., changes to the exhaust system) by the driver.
The noise emitted by motorcycles comes primarily from the exhaust and to a lesser extent, from the air intake and from the engine itself, which diffuses noise in all directions. Thus, the simplest and more effective step in quieting motorcycles is to provide them with more effective silencers. Problems of space and weight arise; however and also have an adverse effect on performance, especially in two- stroke engines.
Investigations have shown that noise from a motorcycle could be reduced from an original value of 94-80 dBA by damping the crankcase and cylinder cooling fans and modifying the exhaust and air-intake systems. Acceleration was, however, then reduced, especially at low speeds and too much space was taken up by the various altered components.
Radical Changes in Design:
If significant noise reductions of the order of 10 dBA are desired (primarily in diesel engine), they can be obtained only by radically changing the basic design of the engine. The design objective is to achieve an optimum combination of all possible parameters influencing noise—engine configuration, number of cylinders, bore, bore/stroke ratio, operating cycle, speed, degree of pressure charging, etc. Should compromises become necessary with other requirements, such as emission control or fuel economy, an engine that is designed especially for low noise characteristics has the advantage of offering, from the beginning scope for overall improvement.
New Designs for Conventional Engines:
When intake and exhaust noise are properly suppressed, the noise from an internal combustion engine is transmitted by its basic components, such as the engine crankcase and cylinder block. Experiments carried out in the United Kingdom show that noise reductions of as much as 10 dBA can be obtained by careful design of the crankcase and the structure of the cylinder block. Two designs are possible: either a stiff load-carrying framework with separate, highly damped outer walls, or engine walls of enormously increased stiffness, using magnesium or other light metals, so as not to increase engine weight.
Experimental engines have been designed following these criteria, to show their possibilities. They have not, however, reached the production stage because they require costly materials and are quite possibly considered by engine manufacturers to represent too drastic a departure from present designs (a consideration that may lose some of its significance with the advent of new types of engines, which require major plant changes).
More conventional materials and construction methods have been employed by the UK Institute of Sound and Vibration Research in a ‘crankframe’ engine—an experimental structure of entirely new design, whose basic principles can be adapted to most engines.
The crankcase walls were completely eliminated and the crankshaft was supported by a crankframe enclosed by a non-rigid, highly damped sheet metal cover. The cover also formed the oil sump, which was bolted to the lower deck of the cylinder block where the vibration levels are low.
The result was a considerable reduction in noise and vibration with respect to that of a normal engine using the same basic running parts. A British firm has shown that by incorporating the concepts of Priede and his colleagues at the Institute of Sound and Vibration Research, a production type V8 diesel engine can be designed which would emit 4-9 dBA less than existing diesel engines of the same power.
A study by Cummins Engine Company has shown that a 4-5 reduction in present diesel-engine noise levels can be obtained with a reasonable increase in cost. In general, it seems that for standard diesel engines of 300 h.p., a reduction of 5-6 dBA can be achieved at a cost of approximately 5 per cent of the engine cost—or of about 1 per cent of the total cost of the vehicle. For reductions greater than 5 dBA, costs begin to increase significantly.
Because of its importance, the problem has attracted considerable government attention. For example – the German Ministry of Transport is sponsoring a broad, government supported research programme, carried out by the Association of German Engineers (VDI), for the purpose of reducing air pollution and noise from motor vehicles. This programme included cost-benefit analyses and extends to psychophysiological considerations.
Similarly, in the United Kingdom, the government, including the Department of the environment, is supporting an ambitious research programme with the goal of developing a less noisy commercial vehicle—a diesel-engine truck whose total noise, under all traffic conditions, will not exceed 80 dBA.
Thus, the study covers not only the engine but also the exhaust system, cooling fan, the tyres and the body, as well as problems of pollution and safety. Several years are likely to pass before this programme can yield any concrete results as to feasibility and cost of quantity production.
The breadth of the British programme serves to stress once again that vehicle noise reductions must be considered in a systems context. To achieve an overall reduction of motor vehicle noise, all the major sources of noise (intake, exhaust, engine structure, fan, etc.) must be reduced together. This is the reason why the total costs of reducing noise at source may be more substantial than they would appear at first glance.
New Designs for Unconventional Engines:
It is very unlikely that the internal combustion engine will be replaced by another mode of propulsion. On the other hand, it is certainly possible that before the end of the century other kinds of engines will be developed commercially, if they prove capable of replacing the piston or the internal combustion engine to advantage.
Electric motors and steam engines (or other engines based on the Stirling or Rankine principle) would be appreciably quieter than conventional internal combustion engines; on the other hand, gas turbines and rotary piston engines, such as the Wankel engine, do not seem to offer a better noise emission solution than conventional ones.
Noise is particularly minimal in an electric motor, so that the noise from an electric vehicle is virtually equivalent to its rolling noise—the noise from the tyres. The use of electric motors in vehicles would thus substantially reduce noise in urban areas. However, with today’s battery systems, engine performance (acceleration, speed, payload, distance traveled without refueling, and maintenance record) cannot match that of internal combustion engines. Today, electric vehicles could best be used and are used in certain countries as delivery vehicles or as garbage trucks, where distances are short and a return to base is possible for battery recharging.
The Stirling and Rankine engines also produce much less noise than a conventional engine (about 20 dBA less for a Stirling engine) and therefore can offer definite noise-emission advantages.
Neither the gas turbine nor the Wankel engine seems to offer at this moment advantages over conventional engines. This is particularly distressing in the case of the gas turbine; because they are already being used experimentally on some heavy trucks the vehicles for which engine noise is the most important component of noise.
Noise from a gas turbine engine is less uneven and comprises less pulse than a piston internal combustion engine. Since the gas turbine is designed to operate at high speed, its idling speed is high, thus at traffic lights, for instance, the noise from a stationary vehicle with a gas turbine engine is louder than that with a conventional one. In general, noise from a turbine-driven truck corresponds roughly to the limits currently imposed by some countries; considerable efforts need to be made to reduce inlet and exhaust noise if the gas turbine is to become less noisy.
Noise, Safety and Air Pollution:
In certain cases noise abatement requirements will conflict with pollution control or safety requirements. Thus, a less noisy vehicle might turn out to be more air polluting or less safe or vice versa. For example – in direct-injection diesel engines, where the combustion chamber is in the piston crown, if the engine timing is advanced to reduce smoke emission, noise increases, typically by some 3 dBA.
On the other hand, with an indirect-injection diesel, where the fuel is injected in a pre-combustion chamber, noise can be reduced by retarding the injection timing, with no increase in smoke emission. Similarly, radial ply tyres, which are increasingly in use because of their roadholding qualities, are somewhat noisier than others on stone-paved roads and on roads made of concrete slabs.
However, there is no necessary conflict between the safety and tyre noise, because tyre noise does not seem to increases as adherence increases (even if it varies with tread design); on the contrary, as treads wear out, noise increases and adherence decreases.
In any case, research is necessary to produce engines that are both quieter and less air polluting and to produce tyres that are both quieter and safe. At this moment tyre manufactures are pre-occupied primarily with increasing safety and car manufactures do not have enough information concerning the effect of several anti-air pollution measures on noise (even if it is unlikely that such measures will prove incompatible with noise reduction).