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The 80-decibel truck is not yet a practical proposition

30th June 1972, Page 28
30th June 1972
Page 28
Page 29
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Page 28, 30th June 1972 — The 80-decibel truck is not yet a practical proposition
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Which of the following most accurately describes the problem?

The world's automotive engineers meet in London to discuss quieter, safer, cleaner and better-performing vehicles for tomorrow

FISITA — the International Federation of Automotive Engineers — held its 14th international congress at the Royal Festival Hall, London, this week with the British member society, the Institution of Mechanical Engineers, acting as host. As might have been expected, the environmental problem was studied by separate sections on vehicle noise and emissions; other sections covered safety, costing, power plants and the future role of the petroleum industry.

P. E. Waters, research fellow, Institution of Sound and Vibration Research, Southampton, in his paper "Commercial vehicle exterior noise" described how the noise emitted by a truck is determined principally by its speed and its gross weight. Noise due to speed may be as a result of road speed or engine speed and although the two are related by gear ratio the relationship between noise and speed will be different for different gears.

Work which has been carried out at Institute of Sound and Vibration Research over the past five years has determined the importance of each contributed noise source to the overall vehicle noise, and given rise to some definite conclusions.

Transmission noise increases four times if the road speed doubles. Examples of engine-speed dependent noises are given as engine mechanical noise which increases as the cube of the engine speed; air intake and exhaust noise which both increase to the power 4.5; and cooling fan noise which increases to the power 5.5.

Exhaust and intake noise show some dependence on load, due mainly to the particular silencing system and engine design. Engine combustion noise also shows a slight variation with load; 3 dBA change or less has been measured between no load and full load for a direct-injection diesel engine.

It is possible mathematically, once all such relationships are known, to derive the overall noise of any required vehicle and studies undertaken so far have proved to be correct to within plus or minus 2 dBA. Power unit or tyre rolling noise are the main noise sources in a commercial vehicle, but the mathematical model used suggests that in current vehicles the power unit noise is the greatest problem. Is the 80 dBA truck a possibility? This question was asked of Mr Waters and he thought that at the moment it was not. At the 80 dBA level many noise sources became a problem, and as yet, there is no known way of reducing the noise of these components further.

The cooling fan was given as an example; Mr Waters did not know how it could be quietened below 85 dBA. Mr Roche from the USA gave a very good insight into the problems of fan design and thought that with good aerodynamic design it might be possible. Much fan noise was caused by vortex shed at the blade tip and root; this could be prevented by using a close-fitting shroud and a built-up hub. Also, the more efficient a fan became the smaller it could be made and therefore the lower the tip speed became; this too would help quieten the fan.

Truck tyre noise A paper presented by a Japanese delegate, J. Sakagami, was entitled "The experimental study of truck tyre noise". This paper described work done by the Japanese Automobile Research Institute into the question of truck tyre noise. It has been shown that at speeds up to 48 mph tyre noise is not a major factor but at speeds above this some tyres, particularly those with lug or snow pattern treads, determine the overall level of vehicle noise. These same tread patterns also create more noise than other types when the vehicle is laden. A full payload can increase noise level by 5 to 8 dBA whereas when a vehicle is running empty, all tread patterns give about the same noise level. Tests carried out to determine the effect of tyre pressure showed that for a ribbed-type tread the noise level rose with an increase in contact patch area, ie a lowering of the inflation pressure.

The effect of the road surface on the noise level is more complicated and no simple correlation was found. In the case of a dry surface, noise does increase slightly with increasing road roughness. For a wet road, noise level was found to remain constant regardless of road surface as the splash noise masks the tyre noise.

M. Rapin of Peugeot asked whether the findings related to cross-ply or radial tyres and how the types compared as regards noise. Mr Sakagami replied that all the work presented in his paper had been derived from cross-ply tyres and that he had no information on radial types.

Mr Harland of the Transport and Road Research Laboratory said that they, too, were involved in tyre noise work, but not so far advanced as Mr Sakagami. Mr Harland's findings showed that lug and ribbed tyres were equally noisy in the dry on a typical motorway surface, and that the transverse groove (lug) pattern was noisier than a smooth tread in the wet.

A delegate from Dunlop, Mr Turkey, said that work which his company had done had shown up a very significant difference between cross-ply and radial tyres at up to 50 mph in dry conditions.

Dr. Austin of CAV queried the various levels of tyre noise given in a variety of papers, and asked the authors to comment. It was generally agreed among the authors that different test conditions could produce results varying from noise proportional to V2 to V4, ie noise proportional to V3 representing a realistic compromise.

Individual noise sources A third paper on vehicle noise by co-authors D. Anderton, R. J. Price and R. J. Varley was entitled "Vehicle design and development trends for low noise emission". This paper also considered each noise source in turn and determined how big is its contribution to the overall noise level. The work was presented in the form of a graph termed a "noise balance", from which the relative importance of each noise source is easily seen. A noise balance for a 170 bhp prototype truck is shown in fig 1.

Having identified the sources, those that contribute most to the overall vehicle noise can be studied with a view to reducing their noise output. The authors describe how the individual noise sources are identified and measured, and conclude that in many applications reduction of noise by treatment of inlet, exhaust and fan has reached its limit, and go on to say that in the short-term engine noise can be further controlled only by shielding techniques, and in the longterm by major redesign of engine structure. Also tailoring of the combustion process itself becomes of the utmost importance.

Mr Russell of CAV wanted to know why the noise of an engine decreased as its compression ratio increased. Mr Price replied, for the authors, that the results given in the lecture had only been measured on one test engine and were not necessarily general.

Mr Bisiker of Shell-Mex and BP Ltd said that as an operator of a large vehicle fleet he was anxious that any under-cab noise insulating material used should be non-fuel absorbent and if possible non-flammable as one vehicle in his fleet had experienced a cab fire through oil and grease-soaked insulation igniting. Mr Price agreed that this was an important practical point and said that the USA may soon introduce a flammability standard for automotive materials.

Mr Smith of London Transport complained that although his buses could get through the British Standard noise test their engines none the less caused lowvibration rattles in house windows, and he wondered if the dBA scale was the right one to use. Replying, Mr Anderton said that low frequencies could be absorbed in a properly designed exhaust system, but these were necessarily large and expensive. Mr Waters took the opposite view and cast further doubt on the suitability of the dBA scale; he suggested that dBB or even dB lin scales might give a better result.

M. Thiry, a French delegate, said that a dBB scale might satisfy the scientists but not the politicians, as it was a numerically bigger figure than the equivalent dBA reading. Mr H. Perrin of the DOE said that he was aware of the shortcomings in the dBA scale and the problem was under consideration by his department at the moment.

One whole section of the congress was devoted to the discussion of commercial vehicle power plants and among the nine

Fig 1 Noise balance for 170 bhp vehicle.

papers presented were two describing unusual engine designs.

Diesel Wankel engine

The amount of design work necessary to produce a new type of engine is tremendous and a paper by D. P. Hutchinson, D. Millar, B. Lawton and J. E. C. Chandler entitled "Preliminary investigations on a rotary compression ignition engine" give a good insight into some of the exploratory design studies which have already taken place in the case of the diesel Wankel engine.

The engine used in the studies was the Rolls-Royce R2 experimental engine. The layout of this engine is shown in fig 2.

The engine has three major units, the compressor. the high-pressure unit and the expander. Air is drawn into the pre-compressor unit, compressed and transferred to the high-pressure unit where it is further compressed before combustion and initial expansion takes place. The gases then pass to the expander where final expansion occurs before being released to exhaust. Gas flow through the engine is controlled by inlet, compressor transfer, expander transfer and exhaust parts; these serve the same purpose as the timing and valve gear in a conventional four-stroke diesel engine and have to be adjusted correctly to dye the best power output.

The R2 engine, a three-rotor design, lends itself to this experimental work, and allows the different air flow factors to be studied independently. For example, the angular relationship between the three rotors can be altered easily by adjusting the drive gears on their shafts. Rolls-Royce is currently developing a more compact two-rotor design which owes a lot to the R2 investigations and should be considerably more efficient. Full-speed, full-load performance of the R2 engine is, at present limited by two factors. The present design of combustion chamber produces an inherently high air /fuel ratio and internal friction is too high. The mechanical efficiency of R2 will be about 66 per cent at full load, but improved designs incorporated in the two-rotor engine will improve its efficiency considerably.

A challenge to the gas turbine

Possibly the most interesting engine paper was that presented by M. Calovolo entitled. "The hybrid power plant: a comparison with the regenerative gas turbine".

It is not inconceivable that within a few years engines of 400 bhp output will be needed and Mr Calovolo argued that diesels using turbo-charging and an increasing number of cylinders will not provide an economically acceptable answer to the problem. Nor is the widely accepted alternative, the regenerative gas turbine, the perfect answer. For it to be efficient, it requires large inter-coolers which determine the size of the engine and make it too big to suit the conventional in-chassis positioning.

Mr Calovolo suggested as a possible compromise a hybrid engine, ie a high-pressure engine in which only the low-pressure part of the cycle is handled by a turbine, leaving the high pressures and temperatures to be dealt with by a conventional engine. Such a hybrid engine, it is argued, could have an improved specific fuel consumption and reduced size for a given output.

The author proposed that the high-pressure side of the turbine be dispensed with and the required mass flow of air be provided by a one-stage centrifugal compressor driven by a high-speed four-stroke diesel. A detailed comparison of equivalent 400 bhp designs of hybrid and regenerative gas turbine designs follows in the paper and it is concluded that although the hybrid engine is more complex, it does combine the characteristic torque/rpm behaviour of a free turbine engine, with the specific fuel consumption and compactness typical of a high-speed turbocharged and after-cooled diesel. Furthermore, the hybrid would fit easily into a conventional chassis and could use low-cost materials in the power turbine, operating at low temperatures. The design, it is foreseen, could be developed to give up to 700 bhp before meeting serious installation problems.

Vehicle emissions Two papers suggested that probably the best way of controlling exhaust gas emissions is the dual-fuel principle of engine operation in which LPG is used and comprises up to 30 per cent of the fuel throughput under heavy load. These papers were, "Techniques for reducing exhaust emissions from diesel engines" by D. Lyon, J. M. Tims and K. Muller, of the UK; and "Diesel oil and LPG fuel feed of in engines to reduce urban bus exhaust smokiness" by Italy's M. Oggero (the late Dr lng C. Martini was co-author). The latter paper mentioned the experimental operation of a bus with a diesel /LPG engine in regular service.

The authors of the UK paper claimed that employing LPG in conjunction with retarded injection timing can considerably reduce the smoke and nitrogen-oxide levels of the exhaust. The method is compared favourable with mechanical modification of the engine which may reduce particular pollutants of the exhaust gas but increase the emission of other pollutants.

While the quality of fuel could be favourable to a reduction of certain pollutants, wide variations in the response of different types of engine to a change of quality limit the potential of varying quality to control pollution. Anti-smoke additives

Fig 3 Comparative outlines and dimensions of 400 bhp hydrid (top) and a regenerative gas turbine (bottom) engines. Radiators, oil coolers, silencers, etc, are not shown.

are effective in their particular role but. commercial acceptance has been limited. Exhaust recycling is an effective means of reducing nitrous oxides but it increases smoke at maximum load and could increase deposits in a supercharger or in the engine. Employing indirect injection reduces the emission of hydrocarbons but raises fuel consumption by 10/20 per cent. Retarding the injection reduces toxic emissions at the cost of increased smoke and a loss of efficiency. While turbocharging reduces the smoke level it may foster the production of nitrous oxides.

Employing LPG in a diesel engine is a more practical alternative to fumigation (or atomization) of diesel fuel, which creates difficulties in commercial applications. In a dual-fuel diesel, injection pump output is limited to about 75 per cent of its normal delivery at higher loads and LPG is introduced into the intake in a quantity sufficient to restore the original power. Its use, typically, reduces smoke emission by up to 60 per cent at high loads. The cost of conversion is £100 /£160.

M. Oggero claimed that extensive tests, including overload testing, had established the reliability of the diesel /LPG engine. Operated by Turin public transport department, the diesel engine under consideration is a six-cylindered unit of relatively old design with a bore of 130mm and a stroke of I45mm that develops 173 bhp at 1900 rpm. The use of LPG is effective above 60 per cent full load, and in addition to giving a lower smoke emission it improves fuel consumption and reduces the exhaust temperature. With an LPG content of more than 25 per cent, however, the peak cycle pressure and operational roughness are increased. Intake of LPG is, therefore, limited to this percentage (by weight) and

the supply of LPG is started at 70 per cent full load. The original injection timing is retained as it had been proved in tests to be the most suitable.

The claims made for the diesel /LPG engine in the UK and Italian papers were challenged by technicans of the Daimler-Benz and Volvo companies during the discussion, mainly on the score that the use of LPG involved complications, increased wear and tear or was dangerous.

But the claims for the diesel /LPG unit were supported by Dr. P. J. Agius of the Esso Petroleum Co Ltd. Field tests had shown, said Dr Agius, that the use of LPG significantly reduced deposits in the engine and extended the life of the lubricating oil.

LPG complicated the mechanics of a vehicle, it increased wear and tear and its use could be dangerous, said Mr P. S. Berg of the Volvo truck division, and a low smoke level could readily be obtained by turbocharging or by refinement of the engine. Mr D. Koppenhofer of Daimler-Benz gave an example of converting to dual-fuel operation for a municipality, a move that had resulted in very rapid wear. It had also increased CO and NO levels, and its use was noi considered advisable by his company.

In reply, Mr Lyon said that dual-fuelling was advantgeous if it were used to reduct the smoke level at the same power output. 11 should not be employed to increase powei at the same smoke level.

Mr G. Antonucci, of Fiat, mention& service tests that had proved the worth a LPG. They had enabled torque to Ix increased by 25 per cent and power to bo improved by 20 per cent at maximum rpn — which could also be increased by 2( per cent.

Better performance, reducec pollution The paper by G. Torazza of Italy "A variable lift and event control device fo piston engine valve operation" dealt with th

work of the Fiat research department on variable valve-lift mechanisms and was given during the session on vehicle emissions. The potential advantages of the system for diesel engines as well as the spark-ignition type mentioned by the author promise to provide a substantial improvement in performance in addition to a reduction in pollution.

The author said that if an engine were designed to operate with variable valve timing its higher cost may well be offset by improved performance. It may be possible to reduce the piston displacement for a given output, to increase the powerto-weight ratio, to reduce the number of gear ratios required, to improve cold starting, give more rapid warming up and to raise the efficiency of high-altitude operation. In the case of a petrol engine, better mixture homogeneity may be obtainable together with internal exhaustgas recirculation by using negative valve overlap. The breathing capacity and turbulence of a diesel engine may be improved and its cranking speed for starting reduced.

Automatically controlled by variations in engine load and rpm, the Fiat system described by the author incorporates an hydraulic servo mechanism and has been applied experimentally to twinoverhead-camshaft engines. The camshaft in each case operates a rocking cam bearing on a follower, the pivot point of which can be moved through an arc to vary the lift of the valve.

After congratulating Mr Torazza on his paper, Mr D. Downs of Ricardo said that the research work involved had obviously been justified, but he questioned the cost-effectiveness of the system.

In reply, Mr Torazza said that the increase in cost should not be more than £10 /£15 and he pointed out that its greater torque potential might enable a smaller engine to do the job of a bigger, more costly unit. Mr Torazza particularly mentioned the benefit of the system in the case of the diesel engine.

Meeting 1976 legislation In a paper on "Comparative study of methods of reducing exhaust emissions to very low levels", C. D. Haynes and R. Lindsay of the UK referred to the work on emission control mide in a joint programme of research by BLMC and Shell. The authors claimed that the most likely system to be adopted for petrol engines is control by the all-catalyst method. Catering for the "extreme" reduction in the emission of nitrous oxides, that would be necessary to meet 1976 legislation in America, is the over-riding factor in evaluating a system.

For small reductions in NO levels, exhaust gas recirculation is very effective. Reliance on this method as the main means of reducing NO emissions would appear to be unacceptable in the long term unless applied to a vehicle operating at a low NO level. It could also increase fuel consumption.

Employing a different form of power unit, such as a Wankel or stratified-charge type, or an alternative fuel such as LPG or LNG may reduce emission control problems, but it will not alter the principles of the main methods of control. An all-catalyst system applied to research vehicles for limited mileages has been shown to give results that compare favourably with the values required. There is a durability problem to overcome and the cost could be high, but drivability should be unimpaired.

Major improvements in the design of catalytic converters will be necessary to meet the stringent 1976 emission standards, according to J.C.W. Kuo, C.D. Prater, D.P. Osterhout, P.W. Snyder and J. Wei, of America ("Predicting performance of future catalytic exhaust emission control systems"). Even a catalyst that is 1000 times more active than those used in a test will not be adequate.

In the opinion of M. Camarsa, B. J. Kraus and G. Marcenaro of Italy ("Mechanical modifications for emission control under European conditions"), exhaust recirculation in a petrol engine could reduce NO by 50 per cent (with 10/15 per cent recirculation) but it does not reduce carbon monoxide. While modifications that enabled a lean mixture to be used will not meet the 1976 American standards, they could provide a very substantial improvement over existing ECE requirements and should be taken into consideration in deciding future control limits in Europe.

Burden of US regulations An improvement in air-pollution control in Europe was called for by M. A. I. Jacobson, of the UK, in a paper "The air pollution problem in Europe: a plea for practical solutions". It would be against the interests of' the UK and most European countries to take on the burden of the prospective American regulations, he said.

A plea that anti-pollution regulations should be based on local conditions was made by Dr. J. H. Weaving, BLMC atmosphere pollution control research laboratory, during an early discussion on vehicle emissions. It was desirable, he said, that requirements in California should not "overinfluence" the anti-pollution regulations of other countries. The low level of nitrous oxide required in California in prospect would be very difficult to apply on a worldwide scale.

In reply, Mr Lindsay pointed out that "differential strategies" were acceptable if the areas being considered were far enough apart. Differentiation of pollution requirements between countries in Europe was very undesirable. It would create manufacturing and cost chaos.

Electronic anti-skid devices A warning that applying a low-cost design of anti-wheel-locking system to a vehicle could create worse hazards than conventional systems was given by P. Muller and A. Czinczel of Germany, in their paper, -Electronic anti-skid system — performance and application" in which particular mention was made of the Teldix system.

The authors claimed that systems can be produced for application to all the wheels of a vehicle (they dealt specifically with two-axle vehicles) to reduce stopping distances on wet roads and give a retardation on a dry road equal to the best obtainable with a standard system in addition to "assuring full vehicle steerability and stability when braking in a turn or during an evasive manoeuvre".

Employing rear-axle control prevents swerving only under certain conditions, but its use eliminates the possibility of applying the brakes fully on a corner or during a compulsory swerve. Incorporating pulsetype wheel-speed sensors, two front-wheel accelerometers and an electronic logicprocessing control unit, the Teldix system is said to provide control of wheel locking in a fit range of 0.1/0.9.

The authors stated that controllability in the range between 0.1 du and 0.2 is of particular importance. Failure to control skidding at a coefficient of less than 0.2/4 could produce greater hazards than braking with a conventional system.

A further FISITA report will appear in CM next week.

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