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Quiet

28th January 1977
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Graham Mon tgomerie finds that silencers are an exhausting subject...

TO MOST OPERATORS the silencer is the part that is tacked on to the exhaust system and lives out of sight and out of mind somewhere underneath the vehicle.

As it doesn't require any routine maintenance it is easy to forget about the silencer and its role in helping to make the truck environmentally acceptable. In the eyes of the general public. trucks are too big, too heavy and too noisy — and the last point is where the silencer comes in But how does a silencer work? What goes into that iarge cylinder to cut down the initial shattering noise of a big diesel to a level acceptable to the man in the street?

To find out the answers to these and many more questions I spoke to the engineers at Cheswick and Wright, part of the Tube Investments group, who last year celebrated 50 years in the production of silencers_ Trying to silence the exhaust noise from an internal combustion engine comes down to one basic aim — to extract energy from the exhaust gas. and one way to do this is to allow the gas to expand.

With most engines the velocity of the gas in the pipe reaches around 100m/sec (330 ft/sec) and this generates a shock wave velocity of up to 400m/sec (1,970ft/sec).

To reduce the inherent noise due to the frequencies of the gas, the wave must be allowed to expand into a volume of large cross-sectional area — and this is where the silencer comes in.

The expansion chambers in a silencer are arranged to cause a reversal of the gas flow. This creates frequencies which are identical to but in the opposite direction to the original which partially nullifies the exhaust noise.

When dealing with gas flow in a pipe it is obvious that the smaller diameter of the pipe. the faster the gas will travel and this raises the frequency of the emitted noise. To the human ear, however, it is usually the lower frequencies which are the most objectionable.

Owing to the restrictions placed on the physical size of a silencer by the truck manufacturers, it is not possible to cut down the lower frequencies merely by the use of expansion chambers. Thus a tuning element known as a Helmholtz Resonator is often used to get rid of this low frequency in a particular engine.

The Helmholtz Resonator has a calculated volume which allows the exhaust gas to pass across its open neck. The dimensions can be calibrated for a particular engine so that the resonant frequencies created in the chamber are out of phase with those in the main body of the silencer. This has the effect of cancelling out the latter before they are transmitted. I was given a simple analogy to illustrate the principle in the form of a milk bottle. The Resonator works in a similar way to blowing across the top of the bottle as the exhaust gas passes across the neck of the chamber except that it reduces or eliminates the unpleasant frequencies instead of producing a note.

It is possible to get combinations of both absorption and reflective type silencers. Absorption silencers are most effective in silencing high frequency noises and work by absorbing the sound waves in mineral wool packing and converting them into heat energy.

The main problem with absorption silencers concerns this packing or rather how long the packing will last. Because of type approval regulations, the vehicle manufacturer has a responsibility to test the materials to see that they fulfil a durability cycle on the test bench or on the road for 10,000km (6,200 miles).

With the small petrol engine it is more straightforward to leave the absorption material out and prove that the silencer will still meet the regulations without it. This, however, is not possible with trucks, When the material is used in trucks it is put there to reduce the inherent engine noise.

Two types of absorption material fulfil all the basic requirements for a silencer. Long-strand glass silk is the most popular; basalt fibre, the alternative, is used only if there is likely to be a temperature problem. This happens mainly with petrol engines.

For the -outside"' material, aluminised steel is the most popular both from the aesthetic point of view and that of combating corrosion. Diesel engine silencers do not suffer from internal corrosion to the same extent as those for petrol engines owing to the differences in operating characteristics.

With the diesel, the journey is more likely to be a long run rather than a stop/start operation. A protective coating of soot quickly forms inside a diesel silencer with the result that the internal components do not corrode as much, allowing ordinary mild steel to be used.

In the opinion of Cheswick and Wright, the operator will not gain much from the use of stainless steel as a silencer material This is because fatigue kills more silencers than corrosion ever does and if a stainless steel silencer failure is caused by fatigue. the extra resistance to corrosion is wasted.

Cheswick and Wright manufacture silencers, the ends and the case of which are rolled, with the only welding being where the stubs go. If the end plates are welded it destroys the aluminium coating.

Truck manufacturers in the main want the silencer to fit down the frame for less cab noise and less vulnerability. This arrangement also helps the truck to meet external noise requirements as the pipe exits downwards and not straight at the microphone.

Operators stuck with complying with petroleum regs have to use silencers which are mounted transversely at the front because all the -hot partshave to be kept in front of the fire screen. The only real attraction to putting a silencer up at the front is that it avoids the space problems associated with air tanks, batteries, etc.

Because of the different operating characteristics of the various engines on the market (maximum revs, power output, number of cylinders and many others) it is necessary to use a silencer which has been developed to suit that particular engine.

The performance of each engine in the UK is governed by BS Au 141a, which among other things quotes the maximum back pressure for an exhaust system. This ensures that when the engine is installed in the truck it can meet the 6bhp/ton minimum power requirment without excessive smoke.

Having stated the maximum back pressure for certification purposes, the manufacturer must not exceed it. As the back pressure applies to the total exhaust system it can be affected by a number of variables including, for example, a long tail-pipe.

When Cheswick and Wright design a silencer the engineers go for the worst combination of conditions possible in a truck which comprises a long tail-pipe, a multitude of bends and a butterfly valve type of exhaust brake.

Cheswick and Wright truck silencers are supplied to Bedford, ERF, Fodens, Ford, Leyland, Scammell and Seddon Atkinson and are also used on engines from Cummins, Rolls-Royce, Gardner and Perkins.

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