Efficient Noise Reduction Techniques For Muffler Design In Student Formula Competition Vehicle Essay.

Noise is one of the biggest challenges that were proposed in the student formula competition. In fact, most people in cities and major towns are seen to be more visibly anxious and many are facing hypertension due to this noise pollution. Many of people who are exposed to these loud noises from a younger age have to face many problems by the time they reach old age and always suffer from noise-induced health conditions which make them, first uncomfortable and secondly, vulnerable to permanent hearing damages. Many innovative steps have been put forward to reduce these impacts through an integrated control and management systems which mainly focuses on noise reduction techniques which gets produced from the sources, during propagation and also at the receiver end. This technique seems to be a drop in the ocean when increased industrial and transport activities in major towns and cities are taken into account. The current maximum noise level recommended by OSHA (occupational Safety and Health Administration) is 90dB for an eight-hour exposure. Hardly have motorists especially “matatu” drivers achieved this. Many PSVs have malfunctioning exhaust systems but many of them chose to operate their exhaust systems without proper design to go through. However, the exhaust machines that are currently in use have better design so that the sound level can be measured but this is not enough and more had to be done. The most remarkable thing that was designed to reduce these sound were the Sound cancellation techniques used by the silencer designer. Therefore, this project seeks to combine the three major techniques used to come up with a more efficient exhaust system for the student formula competition vehicle. These techniques are sound absorption, sound reflection and restriction. It is also expected to give minimal exciting sound if any at all is produced. In addition, engine power and noise reduction will not be traded off because in fact the engine efficiency will be improved because back pressure will be reduced.  Now, it should be noted that the muffler is the main component of the system hence the special focus.

The main objectives for this project include:

(i)  To effectively reduce the noise levels by a double digit

(ii) To minimize back pressure while ensuring maximum scavenging of the exhaust gas/air

(iii) Tuning the remaining audible sound to an exciting sound

The new design that is proposed is solely aimed to combine all the three sound attenuation methods so that they can work up on personal merits and suppressing the demerits so that they can achieve a set of objectives. The three attenuation methods are absorptive, reflective and restrictive.

The Scope of the Design

From the beginning of the time, mufflers came pre-installed within the exhaust system for most of the internal combustion engines, though these mufflers were not designed to serve any kind of primary exhaust functions. The main purpose of designing these mufflers were that they tends to provide an acoustic soundproofing device whose main purpose was to reduce the loudness of the sound pressure  that got created by the engine by way of Acoustic quieting. The majority of the sound pressure that got produced by the engine is emitted out of the vehicle using the same piping that is used by the silent exhaust gases which were responsible for absorbing a series of passages and chambers that gets lined up with roving fiberglass insulation and/or resonating chambers harmonically tuned to cause destructive interference wherein opposite sound waves cancel each other out. An undesirable side effect of using this muffler is that there is higher chance of that engine efficiency will get decreased as it increases the back pressure. This is because the engine exhaust must share the same complex exit pathway built inside the muffler as the sound pressure that the muffler is designed to mitigate. Performance mufflers have three simple goals for perfect operation. First, to absorb and dissipate noise pulses. Second to move exhaust gases, and finally to maintain power and performance of the engine while achieving the first two goals.

Designing and engineering mufflers that efficaciously contain, absorb and dissipate noise pulses and maintain power with cost efficiency at the same time is no easy task. Addition of these mufflers can significantly increase the performance of the car but on the cost of decreasing the system efficiency and this is happened by reducing engine heat and boosting exhaust flow. Performance mufflers can also be considered in putting in the exhaust pipe that is the tail pipe which will improve the exhaust and would also produce some exciting sound for the car lovers.

Nevertheless, all the existing exhaust systems fall into three basic categories namely:

Absorptive

Reflective

In this review, we shall discuss the merits and demerits of each of these categories of exhaust system. The emphasis will be on the muffler design as it is the most important element in the exhaust system.

N/B: It should be noted that due to space limitation, discussion of these muffler types may not be very exhaustive owing to several mufflers in existence. However, basically, as earlier noted, they fall into 3 main categories, namely: 

Literature Review

As previously discussed, these can attenuate sound by dissipating the acoustic energy that is being produced through the porous materials such as mineral fibers and fiber glass. It can also achieve sound levels from middle to the high frequency ranges. The material that is used in this process is flow resistant and also helps in allowing the heat transfer that occurs between the flowing gas and the material itself.

Courtesy of different techniques, we have different types of mufflers classified over

an elaborate range of parameters. These include: 

Design and construction methods

Vector muffler - for larger diesel trucks, uses many concentric cones

Spiral baffle muffler - for regular cars, uses a spiral-shaped baffle system 

Aero turbine muffler - creates partial vacuums at carefully spaced out time intervals

to create negative back pressure, effectively ‘sucking’ the exhaust out of the combustion cylinders

Muffler is generally a straight steel pipe with perforations, which helps in some sound canceling by creating very little back pressure. The tube is fully wrapped in glass insulation which is further protected by a steel shell and is also referred to as a glass pack. This muffler became very popular in the 1960s and 1970s when it was used up in muscle cars like the GTO or classic older Chevy. The muffler has a characteristic that have an explosive sound that earned it the street name cherry bomb. These mufflers are still popular for hot rods and classics and many street racers use these mufflers to boost their cars.

In this case bulk of gas is used to damp the vibrations caused by the reciprocating combustion

engine. Damping of low frequency waves is achieved by use of baffles to cause turbulence hence

breaks up the sound waves. It requires a large drum hence occupies a large space. 

Sound absorption materials like fiber glass are used to absorb sound waves . The outer material is

made of corrosion resistant material because of the high temperatures ,steam and the acidic

exhaust material from the combustion engine. 

It consists of a through straight perforated tube. Its surrounded by a sound absorbing material but

heat resistant material like fiber glass. This damps the high frequency waves. When the sound

comes from the outer wall of the chamber it is out of phase with the middle sound stream hence a

destructive interference hence the peak is reduced. 

Other types of mufflers are :

Method of Operation

Composite type

Spring/Spiral type

Reflection type

Discussion in this section will be on the theory behind the sound propagation and noise attenuation. It explores the various physical relationship that exist in sound propagation. 

By altering the high and low air pressure, pulses are formed which results in the making of Sound. These pulses of sound make their way at the speed of sound through the air. 

The same thing happens inside the exhaust valve too as it opens and burst a high pressure gas which suddenly then enters the exhaust system. The molecules that are present in gas collide with each other at a very low pressure and repeatedly which helps them to stack up on to each other. In turn they can stack up on the molecules a little further down the pipe which leaves the low pressure area behind. When these pressure pulses reach our ear, the eardrum vibrates back and forth. The brain interprets this motion as sound. 

Two main characteristics of the wave determine how we perceive the sound:

Sound wave frequency - A higher wave frequency simply means that the air pressure fluctuates faster. The faster an engine runs, the higher the pitch we hear. Slower fluctuations sound like a lower pitch.

Air pressure level - The wave amplitude determines how loud the sound is. Sound waves with greater amplitudes move the eardrums more, and we register this sensation as a higher volume. 

During the pressure, if the crest of one pulse meets the crest of another wave pulse of the same frequency at the same point, the resultant wave that is formed is the displacement of the sum of the two waves. These two waves in return create a single phase which is twice the amplitude that was present initially. This can be explained using the example of the eight cylinder engine where eight pulse waves of the same frequency are present at some point which in turn results in creating a lot of sound.  

Interference mainly occurs when there is a displacement present between the two waves of the same frequency. The muffler design  is created by using a simple physics law that is when the first wave is at its maximum and the second one is at minimum and if these two combines together and reaches our ear, the person will not be able to hear anything because it always sums to zero. By using this simple technique, noise cancellation techniques are created. 

Formulas that are proposed here are mainly used for targeting specific sound frequencies which are generally based on the number of cylinders of the engine and firing rate that is present in the cylinder of the engine that is being designed by the engineer. It mainly depends on space that is both the amount of space which is available in the platform and the volume of the muffler required for the engine size and rpm. 

The engine performance specification under design consideration include:

Speed= 5500 rpm

Power=185HPX0.746kW=138.01kW 

The engine firing frequency is obtained by:

Firing frequency(F)= RPM X  n ….....(1)       where n is the number of cylinders(8 cylinders)

120

and RPM is the engine speed(Revolutions per minute)

In our chosen engine, the rpm=5500 and power=185hp(operating from a maximum value)

substituting these values gives F=366.67........(2) 

From the table below(P re-calculated under similar design conditions), we can select the size of the muffler pipe(Diameter)

The focus, as shown earlier, is on the muffler as it is the main component of the system. The proposed ideas include parallel, series and concentric design of the muffler. The connecting pipes are also considered where H-, Y- or X-configurations were reviewed.

The selected design finally was the Y-type connection and the muffler selected was the hybrid ( which is a mix of parallel and series muffler configuration.

The final design is to extend the primary pipe by 30 cm, and the pipe will be routed down the engine, for the secondary pipe the diameter will be 42mm and the length 30cm then bend up(angle 75 degrees) by 10 cm ,then another pipe with length 30 cm will be welded as it is shown below, then  routed up to the right side of the vehicle, connected to the muffler.