 Techline-Sept 09-PERFORMANCE OBSTRUCTIONS - Part III: Exhaust systems
This article is continued in the next thread.
This month we finish this excerpt from Chapter VIII: Eliminating The Performance Obstructions from Donny’s Unauthorized Technical Guide to Harley-Davidson 1936-Present, Volume II. (Some content has been altered to fit AIM’s style and format.)
Exhaust systems perform several functions, such as the removal of spent gases, quietening noise, and reducing emissions. The header pipes also cool down the gases that would otherwise burn the exhaust valves. These hot exhaust gases are comprised of unburned hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), sulfur dioxide (SO2), phosphorous (P), and sometimes very small parts of the heavy metals molybdenum (Mo), and lead (Pb). Catalytic converters have been in use on California Harleys for some time now and may spread to the other 49 states before 2010 when these extra-restrictive EPA emissions standards become law nationwide. Catalytic converters use metals like platinum, palladium, and sometimes nickel to create a reaction with the toxic exhaust gases to convert them to, if not harmless, at least less dangerous substances. Global Emissions Systems has reinvented the catalytic converter with newer technology based on current methodology to reduce up to 100 percent of HC, up to 99 percent CO, and reduce NOx and CO2 in gasoline combustion, as well as other fuels. GES uses slightly different metallurgy in different quantities to achieve its emissions goals. The cost is around $1,000 to $2,000 per vehicle. The GES converters measure about 6" x 8" for a car, so they are very adaptable.
Exhaust Pulses
To gain a better understanding of how mufflers and headers do their job, let’s talk about the dynamics of the exhaust pulse. The exhaust gases do not come out of the engine in one continuous stream. As the exhaust valves open and close, the exhaust gases flow with a burst of pressure, then stop, then come with another pressure burst as the exhaust valve opens.
Of course, this process occurs thousands of times per minute. For an exhaust pulse to travel, its leading edge must be at a higher pressure than the surrounding atmosphere. The body of a pulse is close to ambient pressure while the tail end is lower than ambient, which produces a negative pressure or trailing vacuum. This pressure differential is what keeps a pulse moving. Therefore, the exhaust coming out of the pipe is actually a series of pulses. An H-D exhaust can make up to 50 pulses per second. The low-pressure tail of an exhaust pulse will most definitely attract the high-pressure leading edge of the following pulse, effectively scavenging it. The header tuning in a performance exhaust specifically allows the exhaust pulses to line up and scavenge each other.
Exhaust scavenging also sucks in the fresh intake air and fuel by the fast exhaust exit of the combustion chamber. The trailing negative pressure of the exiting spent gases assist greatly in drawing in the new charge at higher rpm levels. However, as engine rpm changes, the lining up of the exhaust pulses also changes. Therefore, the header pipe design will be for certain rpm ranges. Stock mufflers are so restrictive that, as a side benefit, they ram the exhaust pulses together, which results in more low-end torque. The downside of this extra torque production is that the engine must work harder to finally expel these exhaust pulses. The stacking of exhaust pulses assists the rising piston on the exhaust stroke, pushing out the spent gases by scavenging exhaust from the combustion chamber, which also helps pull in the new intake charge. Therefore, while extra low-end torque is always welcome, its benefits can be increased by minimizing the power losses due to expelling the spent gases.
Mufflers & Noise
Sound is a pressure wave that forms from pulses of alternating high and low air pressure, which travels through the air at the speed of sound. The speed of sound is how much distance such a wave travels in a certain amount of time depending on how it expresses. In dry air with a temperature of 70 degrees Fahrenheit (21 degrees Celsius), the speed of sound is 770 mph (1,232 km/h) or 1,130 ft/s (344 m/s). The speed of sound changes in different mediums such as water.
In a Harley engine, sound pulses are created when an exhaust valve opens and a burst of high-pressure gas suddenly enters the exhaust system. The higher-pressure exhaust gas molecules collide with the lower-pressure molecules already in the header pipe, causing them to stack up on each other. They, in turn, stack up on the molecules progressively farther down the pipe, leaving an area of low pressure behind. Therefore, the sound waves make their way down the pipe much faster than the actual gases do.
When these pressure pulses reach the rider’s ear, the vibration in the eardrum interprets them as sound by the brain. Sound wave frequency determines the pitch of the sound. Higher wave frequencies mean that the air pressures are oscillating faster, much like the high-pitch whine of a Japanese bike winding out. Therefore, when hearing a lower pitch, it indicates slower fluctuations or slower wave frequencies. However, this obviously affects Harleys because as rpm increase, the pitch of the exhaust pulse fluctuations increases. A pushrod-operated Harley redlines at around 5500 rpm, while an overhead cam Japanese bike is at least double that, which explains the difference in sound the bikes make near their redlines. Lower rpm result in slower wave pulse fluctuations, which give that Harley rumble we all love.
The loudness of the noise is determined by the sound wave’s amplitude, which understandably vibrates more in the eardrum. Picture a rolling wave with its peaks and valleys. We can draw an imaginary line equidistant from the crests and valleys dividing them. Amplitude is the distance from this centerline to the crests and valleys. The brain registers an increase in amplitude as a higher volume noise. Therefore, the wave’s amplitude determines how loud the sound is. Sound waves with greater amplitudes move our eardrums more, and we register this sensation as higher volume. However, these differing pitch waves need silencing and there are three main silencing techniques: absorption, restriction, and reflection. A muffler may use one, two, or all three systems to eliminate, reduce, or change the tone.
It’s possible to add two or more sound waves together to cancel the sound from both, or at least end up with less sound. The eardrum vibrates with the total sound, which is the sum of the differing sounds. It is like listening to a band or orchestra. The eardrum hears the totality. Now, some sections of a piece of music may be dominated by one instrument such as a guitar or drum riff, which is the eardrum vibration that the brain interprets and thus hears. Other sections will merge all the different vibrations or sounds, and the brain will interpret the sum total because there is only one vibration in the ear. If two sound waves are exactly opposite or not synchronous, or, in the vernacular, out of sync or out of phase with one another, so that the high part of one intersects with the valley of the other, they will add up to zero.
Zero vibration cannot vibrate the eardrum. Therefore, there is no vibration for the brain to interpret as sound. This is destructive interference, since the crests of one marry the valleys of the other. It is not destructive in terms of controlling exhaust noise. However, if the two identical waves are synchronous with one another, so that the crest of one intersects with the crest of the other and the valley of one overlaps the other, they will double in amplitude. When two identical waves come into sync or phase with each other, where the crests and valleys overlap, they add up into another wave with the same frequency but doubling the size of the valleys and crests.
This is constructive interference, since the two waves complement each other and join. This is not constructive from the point of view of controlling exhaust noise. The point is that sound waves add and subtract, which is the principle that mufflers work on.
The designer wants to put destructive wave interference to work as a noise controller. All three silencing methods: absorption, restriction, and reflection use a variation of a perforated tube that fits in a chamber that may have a packing material like steel wool or fiberglass. Hence, we have the term glass packs.
Muffler Types
The absorption method is the simplest of the three, but probably the least effective at quelling engine noise. An absorption muffler directs the exhaust gas pulses straight through a perforated pipe. Surrounding this pipe is a layer of glass insulation that absorbs some of the pressure pulses. A steel housing surrounds the insulation. This housing may absorb some pressure pulses if it has multi-layer construction. The benefit of absorption is that this method easily lets the exhaust through. The absorption method reduces backpressure. Absorption mufflers produce much less restriction, but do not reduce the sound level as much as conventional mufflers. Some other mufflers have a straight-through design that opens into a chamber inside the muffler that has a much larger diameter than the header pipe.
Remember Bernoulli’s Principle: when the fast-traveling exhaust pulses enter the larger chamber they slow down because the same volume must move through in the same amount of time. There is more room so the gases slow down. During this slowdown period, the exhaust gases have extra time to dwell in the sound insulation thus absorbing more noise. Then the gases exit the muffler from a smaller pipe, speeding up again. The same volume must then move faster to travel through in the same time because there is less room to move in.
Better quality mufflers are constructed in three layers. There are usually two thin layers of metal with a thicker and slightly insulated layer between them. Thus, the muffler body can get rid of some noise by absorbing some pressure pulses.
A restrictive muffler doesn’t require as much engineering expertise as one that muffles through reflection, and is usually the least expensive to manufacture. Thus, we find restrictive mufflers on almost all OEM exhaust systems.
The most sophisticated type of muffler of the three is the reflector. This muffler often utilizes absorption principles in conjunction with reflection to make an effective, high-performance silencer. Sound is really a traveling wave. As we know, when two identical frequency waves interact out of phase, they cancel each other out. All that is left is resultant low-grade heat without noise.
Muffler construction involves two perforated tubes within a separate expansion chamber that is part of the muffler. The exhaust gases leave the header pipe and feed into one perforated tube known as the inlet tube. The pulses bounce off the dead end of this tube, and the gases reflect out through the perforations into the rest of the chamber containing the other perforated pipe, known as the outlet tube, which leads into the next chamber. Thousands of small pressure pulses bounce around in the main chamber, canceling each other out to some extent, in addition to the muffler’s housing absorbing them. The gases reflect through the perforations of the second tube and leave that chamber to enter the next one, known as the resonator chamber. They then exit the pipe out of a narrower tube, which is usually the same (or close to) the diameter of the header pipe, known as the restrictor pipe. Thus, we see these exhaust pulse gases speeding up, slowing down, speeding up, slowing down, and finally speeding out of the narrower restrictor pipe. The reflections destructively interfere with each other, canceling themselves out if they are identical and in phase, or, at the least, subtracting one from another if they’re not identical. Reflection design attempts to preclude in-phase waves that constructively interfere, thus magnifying noise.
Resonators are a kind of mini-mufflers and are similar to glass packs. The main difference is that there is no sound-absorbing fiberglass or steel wool in a resonator. Their main method of silencing is not absorption, but the reflective, out-of-phase wave destructive principle, whereby identical waves cancel each other out while waves that are not the same subtract from each other. The resonator chamber plan has a specific length and volume capacity. Mathematical design calculations determine length and volume to allow specific waves that will cancel out specific targeted frequencies of sound. There are many sound wave frequencies produced from differing rpm levels that propel down the header pipes into the muffler(s). These mixtures of sound frequencies cannot all cancel each other, even with the assistance of perforated tubes, expansion chambers, and resonator chambers. Therefore, the muffler design’s purpose is to destructively interfere with or cancel the noisiest waves.
Backpressure
An important characteristic of mufflers is the amount of backpressure they produce. Mufflers with separate expansion chambers and perforated tubing create many turns and holes through which the exhaust must travel. This, in turn, creates muffler restriction and produces greater backpressure. Too much restriction or backpressure takes away somewhat from engine power. EPA dictates are taking exhaust design away from simple, low-backpressure, absorption mufflers that do not repress horsepower to more complex, reflective, and especially restrictive ones that create too much backpressure in terms of horsepower suppression.
Turbochargers introduce some backpressure to an exhaust system, thus making it a little quieter. All of the typical scavenging rules still apply.
Common mythology is that a turbocharger has a much hotter exhaust system than a naturally aspirated Harley. Controlling the temperature of exhaust gas occurs through the air/fuel mixture, and ignition and cam timing, and not the turbocharger that mounts off the exhaust manifold. When designing an exhaust system, turbocharged engines follow the same rules as naturally aspirated engines, with the only difference being that it will require less silencing. However, one of the silencing methodologies is restriction, and a turbocharger turbine that is part of the exhaust is just that, a restriction.
This article continues in the next thread. Check the back issue for extra information and photos. |
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