Quiet progress
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Low-noise 80 dB(A) trucks are already here — but will they really make our roads that much quieter?
People have talked about the possibility of an 80dB(A) truck since the early 1970s. It became a realty in 1991 when Austria introduced its night-time driving ban on vehicles that couldn't meet the 80dB(A) limit. But what's the cost to the operator and the payoff to the driver and to the citizen for whose benefit it was designed?
Competitive and legislative pressures have continually obliged engineers to reduce the noise levels of their truck products and the results would have been impressive even if the trucks themselves hadn't changed. But when other factors are taken into account, such as increased weights, more powerful engines and a tougher test procedure, the results are amazing.
The quest for noise reduction took a jump forward in the late 1970s as the lower speed in-line engines came back into favour at the expense of the generally higher-revving vee eights. As turbochargers were developed further, in-line engine speeds came down again, to as low 1,800 rpm without harming power and torque. Cooling fans became thermostatically controlled so they worked only when required, reduced noise and saved valuable fuel.
Engine blocks were redesigned when the occasion and resources allowed and with the help of computer-aided design techniques engineers were able to move any surplus metal to where it did most good and thus stiffen up the block or substantially reducing the vibrations which give rise to much of the noise. Designers also moved away from chain-driven auxilliaries on the front of the block, using gears instead to reduce noise and improve their working life.
But all of this work barely offset the increase in noise which resulted from the steady demand for more and more powerful engines. At the beginning of the 1970s, a 32tonner typically had an engine of 135kW (180hp), about half that of today's 38-tonner.
The accelerating drive-by noise level for vehicles with engines of more than 150kW (201hp) which was 91dB(A) through the late 1970s dropped to 88dB(A) during the 1980s and dropped again to 84dB(A) in October 1990. However, these figures don't tell the whole story. In 1985 the test procedure was revised so that any driving gear could be tested making it that much harder to pass. Manufacturers generally agree that changing the test method in this way had the same effect as lowering the limit by up to 5dB(A) depending on the model.
REFINEMENTS
Despite this tightening of the test procedure — and the repeated lowering of the limits — the manufacturers achieved the results by further refinements to the engines rather than having to add heavy, expensive shielding. But this is certainly not the case with the 80dB(A) level which is proposed for adoption by the EC in 1996. Manufacturers already have experience of 80cIB(A) in Austria where trucks using the Brenner pass during the hours of darkness have had to comply with tough new limits since 1991. Vehicles of below 150kW (201hp) have had to meet 78dB(A) and those over to 80dB(A). Vehicles which comply display a white "1.7 on a green oval.
'lb achieve these very low levels, the engine and gearbox are almost fully enclosed by noise insulating panels. They are positioned close to the noise generating surfaces for best effect but not so close as to stop cooling air from passing through. The panels extend from the bottom of the chassis and pass under the engine and gearbox. They are made of steel, lined with a non-asbestos noise absorbing matting and held in place by bolts which screw into supporting brackets.
In the case of MAN, the complete kit, which includes the encapsulating panels, a vertical exhaust stack with a modified silencer and specially developed low noise ryi-es, costs about £1,000, depending on the model type. Some British operators are already running 80dB(A) vehicles and finding out how the kits affect servicing. Les Evans, assistant workshop manager for Sheddick Transport which runs nine MAN 17.422 "Silent" tractors says: "The shielding works well and isn't that unfriendly. You have to be careful when draining oil or changing oil filters not to drop oil inside the shield and contaminate it, but otherwise it's not a problem."
Now that these very low noise vehicles have been developed and are available today—at a cost—they will almost inevitably become the norm by the end of the century But will they achieve their purpose and provide a better quality of life for the average citizen?
The standard noise level test, which was conceived at a time when the most annoying noise was produced by the truck's engine when it was accelerating in city traffic, now looks very unrepresentative of today's traffic which is more and more concentrated on urban main roads and motorways. Today the traditional culprit, the engine, is rapidly being overtaken by tyre noise as the main noise source.
According to a study carried out by MAN, a 38-tonne 13-active unit equipped with a lownoise pack that would meet the 80dB(A) requirement of the accelerating drive-by test will produce 78c1B(A) at a steady 60krn/h (37.3mph) and 82.5dB(A) at a steady 801unth (49.7mph). A standard unit that would record 84d}3(A) in the accelerating drive-by test is
only 35dB(A) worse at 60km/h and just 1.5 dB(A) worse fif.
These tests show that the differen between the low noise and standard reduces as the speed increases and noise builds up. When the steady sj were repeated with the vehicles sim rolling through the test area, withou power, the differences were even sm 80krri/h, for example, the difference the 80dB(A) truck and the standard was just 0.5dB(A).
ROAD SURFACE
When repeated with the vehicles ful the results were even more telling. B trucks recorded well over 80dB(A) a and 80kni/h even when rolling withl power. So, despite all the work put i design and development engineers years, the industry has reached an ii over the noise created by the tyres n over the road surface.
Work done by a number of manui Sunia and Iveco included, has short the actual noise level recorded in ttx accelerating drive-by test can vary 5dB(A) depending on the type of tyt tread pattern used. lithe vehicle am tyres are kept the same, results vary 5dB(A) on different test surfaces. A: Stmnia shows that tyre noise increa: about 7dB(A) when the truck is rum wet road surface.
All of this detailed development v shows that, to the man in the street, little or not if it's an 80clB(A) truck. 601cm/h the tyre noise is the domina factor and certainly tread pattern ar surface will make a greater different the engine is working hard or simpl coasting. In the future the greatest r in noise will almost certainly come f improvements in the road surfau than the trucks travelling a by Gibb Grace