internal painting of silencers

COMPUTATIONAL FLUID DYNAMICS

Aniket Gavraskar & Tanmay T. Raha ([email protected]) Government College of Engineering, Aurangabad – 431005.

ABSTRACT

Silencers are provided in an automobile to condition and remove exhaust gases from the vehicle and to reduce noise due to sudden expansion of exhaust gases. They thus bear the brunt of the corrosive action of the exhaust gases which either render them useless or badly impair their functioning, generally within 3 years. To prolong the life of silencers up to 6 years an internal protective coating is needed to be given.

This paper introduces the concept of internal painting of automobile silencers to protect it from corrosion. This report gives a brief description about silencers, their structure, corrosion in silencers, need and difficulties involved in internal painting. Also it tries to encompass all the attributes regarding internal painting, viz. paint formulation, surface pretreatments, suitable paint coating techniques, and paint drying methods.

This concept requires certain modifications to be done in the silencers to pave way for efficient paint application, which has been illustrated in the report. Also an overview of proposed SPM for this purpose has been given. This paper explains how this concept is economically viable with respect to Indian perspective and goes a long way in improving the life of silencers. .

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INTRODUCTION

In today's scenario, the automobile industry is taking giant leaps in development and growth, with ever increasing demands in domestic and international market. This has seen the introduction of newer and better models of automobiles flooding the market. A lot of investment is being done in the field of aesthetics, efficiency improvement, emission control and social needs to avoid public nuisance on road. These efforts have brought out most fuel efficient methods, better control and ergonomics, effective pollution control to be applied in automobiles, which have made them a better value for money and respect for social obligations.

These progress have however overseen an essential component which functions silently, it is the silencer or muffler. Development in the field of silencers has been limited to the field of noise and emission control. Silencer till now has been treated merely as a ‘fabricated item’ in Auto industry. The mentality is that if it gets damaged, it can be replaced easily. Unfortunately not much effort has been taken to improve the life of the silencer, except using more corrosion resistant materials like stainless steels. This however is not enough to make the life of silencer equivalent to that of a vehicle. External plating and painting have been developed mostly to take care of climatic corrosion resistance and improved aesthetics. However internal rust prevention coating was neglected and felt unimportant so far. Corrosion inside the silencer not only reduces its life but also affects engine performance.

A recent survey revealed that the life of the silencers in most cases does not extend beyond 3 years, especially in those vehicles which run for short distances. This made the silencer manufacturers to rethink their strategy and made them to ponder over a cost effective method of protecting the silencer. Thus the concept of internal painting of silencer was developed. (8)

This concept required development of effective paint filling methods keeping in mind the complex internal structure of the silencer as well as development of heat and corrosion resistant paint to suit these requirements. To achieve proper paint coating, certain modifications in the internal structure of the silencer are proposed and some have been implemented. Considering Indian perspective of cost consciousness, limited the scope of materials that can be coated and coating methodology.

The technological improvements, cost effective production methods and increasing market demand for product improvement has encouraged the need for these types of mufflers. Major auto industries, to the likes of Bajaj, TVS, Hero Honda etc. are now adapting to this concept with prompt attention owing to higher and efficient product life, customer satisfaction and the social obligations. (6)

SILENCER BASICS

Silencer or muffler is a device to absorb and damp out high pressure surges of gases introduced into the exhaust of an engine, when exhaust valves open, to a smooth, quiet flow which will emerge into the outer atmosphere without undue noise.

__________________________________________________________________ Introduction to the Concept of Internal Painting of Silencers

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The exhaust system directs gases produced by combustion, out of the engine, ensuring a continuous, smooth flow of exhaust gases which helps in improving the engine performance.A silencer is a combination of tuning chambers, baffle partitions and perforated and

solid tubes designed to create reflected waves that interfere with each other or cancel each other out and thus dissipate noise pulses while moving exhaust gases and vapor from all the cylinders and thereby preventing an undesirable roar.

Parts of a Silencer

CAUSES OF SILENCER FAILURE

During the exhaust cycle the flue gases which normally contains un-burnt hydrocarbons, carbon monoxide- CO (due to incomplete combustion of fuel), carbon dioxide- CO2 (from complete combustion of fuel), oxides of nitrogen- NOx, oxides of Sulphur - SO/SO2 (due to sulphur content present in the fuel), water-H2O , compounds of lead (Pb).

When the engine runs, the transmitted hot air has a temp 100°C to 400°C in various zones. The function of the silencer works better under the hot condition as there is hardly any accumulation and condensation of flue gases. However, after the engine stops and the silencer cools to normal ambient temperature there is condensation of the residual fumes and this initiates the corrosion on the internal surface of the silencer. The water vapor (H2O) and oxides of sulphur SO/SO2 from the flue gasses react forming low concentrated sulphuric acid (H2SO4) which is highly corrosive and attacks


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the welded areas first. This weakens the welding strength and affect the steel material causing the pit marks, which leads to formation of holes or clear apertures.

This in turn reduces the functional life of the silencer due to lot of leakage current of gases and increases noise pollution. At the same time the initial corrosion accelerate further due to more attacks during the heating and cooling cycles of the condensation of the acid fumes. Moisture can also come from the outside in the form of rain.

It has been discovered that the silencer of the vehicle with more working duration period has longer life because the condensation of the acid fumes are less owing to the high temperature and turbulence. However in the vehicles operating for short duration and longer rest periods are more prone to corrosion because the acid fumes obtained from the exhaust gas reactions get time to condense and hence condense for longer duration and amount. On a short trip the water never has a chance to get hot enough to turn back into water vapor and just stays in the system and rusts away the pipes. (9)

NEED OF INTERNAL PAINTING FOR THE SILENCERS

1. As mentioned earlier, when the vehicle is not running the acid droplets formed by the chemical reaction of the exhaust gases, starts condensing or accumulating on the inner side of the silencer. So the metal surface of the silencer starts rusting in the absence of any protective layer. This weakens the welds and causes pits and holes and ultimately the baffle breaks away indicating complete silencer failure. The protective layer of paint will prevent this due to its anti corrosion properties.

2. Noise muffling capacity of the silencer is reduced by corrosion because it causes prominent holes and pits in the welded areas which causes the leakage of sound wave without destructive interference. Sometimes the weld becomes so weak that the baffles and pipes inside the silencers become loose and starts vibrating under the impact of gases. This causes unpleasant noise and failure of silencer.

3. Owing to corrosion in the silencer the perforations of pipes inside it may get chocked and prevent smooth passage of gases which will increase back pressure on the engine reducing its efficiency.

In absence of any surface coating inside, corrosion phenomenon reduces the functional life of the silencer and the engine performance there after. Internal heat resistance paint does the role of protective layer. It can withstand the temperature up to 250°C to 300°C. It can also withstand the acid attacks on it as it is acid and heat resistant. So far it was felt insignificant and hence ignored as far as internal surface coating was concerned. The external painting however continues to attract costumer attention, elegance, aesthetics and where ever the silencer surfaces have exposed for climatic effects.


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CHANLLANGES IN PROPER INTERNAL PAINITING PROCESS

Following difficulties may occur during the painting.1) Improper De-greasing and surface cleaning: Pretreatment processes like de-greasing and surface cleaning may cause problem while cleaning the internal surfaces by virtue of the critical design considerations resulting in such shapes and intricate passages which may entrap cleaning solutions or which are difficult for cleaning medium to reach. Besides it is difficult to ensure that all nook and corner inside the silencer is completely cleaned as swarf and grit particles, traces of grease, welding burrs, and trapped cleaning solution can be left behind. This may also result in setting up local corrosion zones as well as hampering paint adhesion if paint is applied without proper drying. 2)Air entrapment: During paint filling there is a possibility of air entrapment if there is high flow rate of paint causing incomplete paint adherence.3) Protection of Catalytic Converter (CAT): One of the most challenging aspects of internal painting is the protection of CAT. CAT is placed at the end of the inlet pipe. It has a fine honeycomb like structure placed inside a cylindrical enclosure. If paint enters in this mesh, it will get clogged and will hamper its proper functioning. It may even become useless. This limits the opening into the silencer as tail pipe only. Thus there is only one opening for paint filling and removal.

The heat resistant paint to be utilized for painting should be able to satisfy following criteria, 1. Good etching properties. (So that paint adheres to surface quickly and firmly.)2. Resistant to acid attacks. 3. Sufficient resistance to alkali attacks.4. Good heat resistance to avoid being affected by hot flue gases and to prevent the

peel off effect. The classes of paint according to temperature they can withstand are: Class I: Up to 250 °CClass II: 250 °C - 350°CClass III: Up to 450°C and above.

For this requirement Class III paint will be suitable.5. Uniform film coating.6. Better penetration of gaps and crevices.

PRETREATMENT OF INTERNAL SURFACE BEFORE APPLYING PAINT


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Residues of oil, grease, marking inks, cutting oils, welding flux, weld burrs etc. after fabrication operations will seriously affect the adhesion of applied coatings and must be removed. It is erroneous to think that subsequent cleaning operations will remove such contaminants and it is bad practice to permit them to remain on the surface. Surface preparation is typically carried out to clean the substrate before coating, improve adhesion of the coating, and correct surface flaws. Hence following pretreatment operations are performed.

1. Degreasing and Cleaning: It includes removal of oil and other contaminating foreign matter by dipping the component in a emulsion of a mixture of oils in water. 2. Acid Cleaning: It involves use of inorganic acids like phosphoric acid combined with grease solvents plus wetting agents. It removes both rust and grease or any other corrosion products in one operation. This process leaves the surface in condition to provide better initial paint adhesion as there is some roughing action. Washing after Acid Cleaning: It is essential as considerable amount of sludge and acid containing iron salts adhere to the metal when it is lifted from acid solution. If imperfectly washed off, these salts set up corrosion and can contaminate subsequent processing tanks into which they may enter. Residual salts can also accumulate in pores in metal and weld and can subsequently ooze out damaging any finishing coating that may be applied.3. Phosphate Coating: Phosphate coating consists of water solution of Zn or manganese dihydrogen phosphates, free phosphoric acid, oxidizing agent and certain heavy metal ions such as Cu2+. It removes the cleaning agent or excess processing chemical and sludge form any earlier process and improves corrosion resistance.

BEST METHOD OF PAINT APPLICATION

Various paint application methods are discussed below with due consideration to the intricacies involved in the internal painting of silencer. An attempt is made to zero in on the most economical and effective method of paint application.

Spray painting: This method is good for external surfaces. It is not suitable for internal painting of silencer as1. Owing to the arrangement of tubes and baffles we cannot insert the spray gun inside

the silencer. Thus the paint cannot reach inside the chambers making this method unsuitable for this application.

2. For spray painting, a painting containing slower evaporating solvents must be used which may hamper proper drying of paint and thereby a uniform coat may not be achieved in the interior parts of silencer.

3. The paint used for spray painting must have lower viscosity, but heat resistant paint is of high viscosity.


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Electroless Plating (EN): It is a chemical reduction process which depends upon the catalytic reduction process of nickel ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of nickel metal without the use of electrical energy. Uniformity of the deposits, even on complex shapes, irregularly shaped objects, holes, recesses, internal surfaces, valves or threaded parts is the advantage of this process. This process may seem to serve our purpose, its overriding costs is the main stumbling block towards the adoption of this technique.Dipping: It is one of the low cost methods of applying paint coating in which the articles to be coated are completely immersed in tanks containing the paint. This method is not suitable for current application as1. The external surface of the silencer is also unnecessarily coated. 2. Articles should be lowered in the dip tanks fairly slowly to prevent paint splash,

air- bubble formation and carrying entrapped air into the paint, later two may increase oxidation and solvent evaporation.

3. To obtain satisfactory dip finishes it is necessary to have slow withdrawal of the articles from dip tanks. There needs a proper balance of speed of production and the quality of finish desired. (3)

Flow Coating: This method of application involves a solid stream of paint from a nozzle resembling the spray of water from a hose. Because air is not mixed in; there is no atomization as in regular spray painting. The arrangement of nozzles, amount of paint used per article is calculated to have efficient use of paint. The excess paint is collected and again sent to the main reservoir though filter to be used again. Also this method overcomes the losses occurring in case of spray painting. Accommodating some modification this method is found suitable for internal painting of silencers. This is explained in detail later.

INTERNAL PAINTING EQUIPMENT FOR SILENCER

After considering all aspects of internal painting of silencer, we at a stage where the machine for this application can be described. This concept is still under developmental stage and conceptual changes in the machine are also under going the corresponding modifications. While the development of the concept we have considered the silencers used in Bajaj 2- stroke FE (Front Engine) and RE (Rear Engine) type 3-wheelars.

Study of paint filling methodIt has been explained earlier why flow coating is suitable for this application.

To adapt this method for silencers, stream of paint is directly sent either through the tail pipe or open end of silencer after removing inlet pipe and CAT sub-assembly. In this the paint is forced into every part of silencer by gravity or by using a pump. Taking trials of various positions of the silencer for paint filling, it is found that vertical placement of silencer is most effective. This position also is suitable for proper draining of paint through the tail pipe. It may however be needed to shake or tilt the silencer for complete removal of paint.


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The paint is allowed to remain inside the silencer for about 30 seconds depending on the paint characteristics before being drained off. This is necessary for paint to adhere/etch properly on the surface. To ease the paint filling and removal from the silencer certain design modification are suggested later.

Gravity filling method is effective, but it is slow. To improve productivity, pumps can be used for filling of paint. After studying various types of pumps and their characteristics with respect to its effectiveness in pumping viscous fluids like paints, gear and pneumatic pumps are found suitable.

DESIGN MODIFICATION IN SILENCERS TO ACHIEVE SATISFACTORY INTERNAL PAINT COATING

1. In a silencer, after assembly, only tail pipe is available for paint filling and there is no air vent if CAT is masked. To achieve this, a modification is done in silencer in the form of a drilled hole on the cover plate near inlet pipe. It serves the purpose as an air vent as well as helps in proper circulation of air to remove solvent fumes while baking. This hole can also be used for sending in compressed air for fast draining of paint instead of using gravity. After painting has been done this can be closed by nut with asbestos washer or by welding. The diameter of vent hole if required can be reduced.

Modification in Silencer Cover Plate to allow Air Venting

2. During trials it has been found that paint accumulates over the baffles causing non uniformity in paint coating. This problem is solved by making two small diametrically opposite openings near the edge of the baffles as shown in figure. Also a slope may be provided around the openings to ease the passage of


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paint. This may affect the silencer performance but this is yet o be proved experimentally.

3. To ensure proper attachment of paint filling nozzle to the tail pipe, a cone shaped structure may welded to it as shown.

We have considered 2 methods for protection of cat from getting clogged.Removal of CAT In the assembly line, inlet pipe on which CAT is attached, is used for fool proofing and fixing of silencer for carrying out further processes. Instead of removing the CAT we thought of temporarily assemble the inlet pipe and CAT by using 3 bolts instead of 6. This entails following advantages1. This eliminates difficulties in fixing and testing of silencer for leakages. 2. After pre-treatment, inlet pipe and CAT sub-assembly can be removed and proper drying of internal surfaces is achieved.3. Paint filling is to be done through the open end of silencer and removal through tail pipe. This eliminates the need of making extra holes on the cover plates of silencer.4. Paint accumulation on the baffles of silencer can be effectively reduced. 5. Protection of CAT is assured.However, there are certain disadvantages involved in this approach1. Inlet pipe and CAT sub-assembly is to be painted separately. 2. Testing of silencer for leakages is to be done twice once before painting and then again after painting and final assembly.3. Extra man-power is needed for removing of inlet pipe and CAT sub-assembly while painting and again re-assembling them.

Masking of CATThis concept involves the covering of the CAT prior to assembly. The kind of material to be used must withstand a temperature of 1200C but melt at around 1800C and pressure of 2 bar due to forced paint filling. This is because drying after pretreatment is done at 1200C, while baking is done at around 1800C and hence, it is expected that it should retain its characteristics in the temperature range of 120-1800C.The advantages are1. There is no interference in the already established assembly line.2. Testing of silencer for leakages is to be done once only.3. Inlet pipe and CAT sub-assembly is not to be painted separately.Disadvantages1. An extra operation is added for masking of CAT in production line of silencer.2. Modification in the cover plate is needed as explained before.

CONSTRUCTION OF SPM


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1. Upper paint tank: As shown in the fig. upper paint tank is a tank of capacity 50 liters. This is so placed to provide a sufficient head for paint filling. 2. Collection tank: This tank is placed at the bottom to collect the excess paint from the upper paint tank as well as the paint after drain off from the silencers. It is covered from the top to minimize inclusion of dust or any other foreign matter.3. Pump: It is used to pump the paint from collection tank to upper paint tank through the paint up pipe. It has a filter at its inlet to filter out the foreign matter from entering the upper tank. It is made to operate in such order that, after one cycle of filling the silencers is completed and valves are closed, it starts filling the upper tank. 4. Agitator: The liquid paint in contact with the atmosphere tends to oxidize and starts to set at the top. To prevent this agitator is used to create turbulence and avoid paint settlement.5. Over flow Pipe: In order to achieve proper height of paint in the silencer, correct height of paint in the upper paint tank is to be maintained every time paint is to be filled in the silencers. The level is maintained at such a height to perform the aforementioned function. When the pump fills in paint from the collection tank, as soon as the required level is achieved, the over flow pipe comes into operation and any excess paint coming henceforth is sent back to the collection tank.6. Distribution Pipe: It performs the function of distributing the paint coming from the upper tank to the 8 nozzles for filling up the silencers. It has valves provided at its 8 outlets. Also it has a 3-way valve to direct the paint either towards the nozzles or towards the collection tank.7. Painting Nozzle: Painting nozzle is used to produce a stream of paint to fill up the silencers. It connected to the tail pipe of the silencer. Number of nozzles can be provided after making height adjustment in the overflow pipe..8. Silencer holding fixture: This fixture is designed to be foolproof, capable of holding the silencer only in a position that it remains vertical and the tail pipe is open towards the nozzle. It so designed, that it is capable of accommodating other silencers of similar design.9. Drain off channel: The paint is allowed to remain inside the silencer for a time depending upon the paint characteristics and the thickness of the film required. Then the connection of the nozzles is removed and the paint is allowed to drain off through the drain channel into collection tank.There is a large scope of improvement in this design of machine.


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SPM for Internal Painting of Silencer


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PROCESS FLOW DIAGRAM OF SILENCER PAINTING (INTERNAL/EXTERNAL)

DRYING OF PAINT

Surface coatings are applied in liquid or paste form and must become solid to fill their proper functions of protection. The transition of these liquids or pastes to solids or semisolids is called “drying”. Drying operations are classified as air drying force drying and baking according to the temperature at which they are conducted.


FULLY ASSEMBLED SILENCER

AIR DRYING

DEGREASING

SHOT BLASTING OF EXTERNAL SURFACE

SPRAY PAINTING OF EXTERNAL PARTS

CLOSING OF INLET AND TAIL PIPE OF SILENCER

WATER RINSE

COMPLETE DRY OFF

WATER RINSEMASKING OF CATALYTIC

CONVERTER

INTERNAL PAINTING OF SILENCER

BAKING

ACID CLEANING

CHROMIC ACID OR PHOSPHORIC-CHROMIC

ACID RINSE

PHOSPHATING

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The method of drying and time and temperature used for a given operation depend on various factors such as the surface to be finished, coating to be applied and the drying equipment available.

Baking: The method of drying of paint above 80 C is known as baking. In case of internal silencer coating this method is suitable because:1. The substrata (silencer body) and coating being heat resistant are not harmed by

heat.2. To minimize drying time. It takes around 10-15 minutes to completely dry by

baking as compared to 25-30 minutes for forced drying and 30-40 minutes for air drying.

3. To prevent sagging, wrinkles appearing in the applied paint which may be caused by long standing time in case of air- drying and forced drying. Thus it helps to maintain a uniform coating layer.

Hot air ovens: The silencers after flash off are put into the oven and baked at 180- 250 C for about 10-15 minutes. Attempt is being made toward shorter, higher bakes for higher production rates. To achieve this some paints containing heat-stable pigments and vehicles are being formulated to bake at around 300C. There should be proper circulation of dust-free to maintain uniform temperature and to replace solvent vapors by fresh air.

FUTURE SCOPE FOR THIS CONCEPT

The concept of internal painting of silencers has just seen the light development. Earlier this field was not even in the agenda of automobile makers. Development of efficient methods of painting internal parts and reduction in draining period is going on. Simultaneously the development of low cost heat resistant paint is being done for better flow and setting characteristics. Besides this, modification in design of silencers is tried out to suit internal painting without sacrificing its basic function. In the current scenario, the SPM developed for this concept has limited painting

capability as well as specific to a particular silencer design. Efforts are going on to make fully automated SPM with higher painting rate as well as adaptable to various silencer designs. As the benefits of this concept are proven by experimental results, it gains ground in

the fact of providing better silencer functioning and customer satisfaction. The success of initial experiments have developed confidence in minds of Auto industry giants who are enthusiastic in further development in this field to incorporate this concept in their regular production schedule. Bajaj Auto Ltd. Has adopted this concept for it new models like Pulsar K-60 and Caliber 115.


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REFERENCES

1. Jain, K.K., Asthana, R.B., 'Automobile Engineering', Tata-McGrawhill, New Delhi (2001).

2. Shreir, L.L., 'Corrosion', Newnes, London (1963).3. Silman, H., Iseerlis, G., Averill, A.F., 'Protective and Decorative Coatings for

Metals', Finishing Publications Ltd.4. Parker, Dean H., 'Principles of Surface Coating Technology', Interscience

Publishers, New York (1965).5. Fontana, Mars G., Greene, Norbert D., 'Corrosion Engineering', McGraw-Hill,

New York (1967).6. Huminik, John Jr., 'High Temperature Inorganic Coatings', Reinhold Publishing

Corporation.7. www.autoeducation.com8. www.howstuffworks.com9. www.corrosion-doctor.com10. www.rollingmotors.com


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