27-10-2012, 05:53 PM
Exhaust Flow in an Automobile
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Introduction
The engine is like an air pump; the more air that is allowed to flow through it, the more horsepower that you get out of it. In other words, if you have a free-flowing air intake and exhaust system in your average vehicle, you’ll get more horsepower because of the efficient flow of air into and out of the engine. Fuel requires air to burn and thus to produce energy. The more air that is available for combustion will also improve efficiency otherwise known as gas mileage.
The purpose of this project is to research on the present common air flow systems in today’s automobiles, particularly cars. This will be too general and cover too wide of a topic, as a car also uses air for various other mechanical purposes. So I will focus my project on the exhaust gas flow in a naturally aspirated fuel injected vehicles. Thus, I will not cover turbos, superchargers and nitrous oxide systems (which are forced inductions and are a substantial topic alone). And since some other students are doing projects on the engine itself, I will not spend too much time on how the air undergoes combustion in the engine.
The Exhaust Manifold
The exhaust manifold is a pipe that conducts the exhaust gases from the combustion chambers to the exhaust pipe. Many exhaust manifolds are made from cast iron or nodular iron. Some are made from stainless steel or heavy-gauge steel. The exhaust manifold contains an exhaust port for each exhaust port in the cylinder head, and a flat machined surface on this manifold fits against a matching surface on the exhaust port area in the cylinder head.
Methods on How to Improve Efficiency and Power
After the above discussion of the components in an automotive exhaust system, it is obvious that the principle of the engine as a pump is not being utilized to the fullest. Air is not allowed to flow too freely because of restrictions in the form of the catalytic converter, the resonator, and the muffler. However, these components are necessary by regulations to maintain safe exhaust gas emissions and minimal sound levels (noise suppression). Also, in part, it takes time and money to design an excellent performing and free flowing exhaust system; something that car manufacturers just can’t afford to waste resources on. This is where aftermarket companies come in to create cost effective options for performance minded car owners. Of course, a free flowing exhaust would be expected to make more noise than a normal one. But a good manufactured system has a deep throaty tone, while yielding increases in horse power and also passing emission tests. I will now go through some of the modifications of the exhaust system that would "unleash" some horsepower and efficiency, while still being street-legal.
Dual Exhaust Systems
For engines with the "V" type configurations, it would be more efficient to use a dual exhaust system than the "Y" pipe. In other words, two pipes (instead of one) connect the exhaust manifold/header to two catalytic converters, two resonators, and two mufflers. Thereby each manifold will have their own resonators, catalytic converters, exhaust pipes, mufflers, and tailpipes. The advantage of a dual exhaust system is that the engine exhausts air and gases more freely, thereby lowering the back pressure, which is inherent in an exhaust system. With a dual exhaust system, a sizable increase in engine horsepower can be obtained because the "breathing" capacity of the engine is improved, leaving less exhaust gases in the engine at the end of each exhaust stroke. This, in turn, leaves more room for an extra intake of the air-fuel mixture. The disadvantage of a dual exhaust system is that it would be costly due to the additional components. No doubt the addition of another exhaust system adds more weight to the car, but the increase in horsepower is substantial enough to outweigh the horsepower losses through additional weight.
Removing the Resonator
The resonator does not function also as emissions control device, so removing it and putting a straight pipe connecting the catalytic converter and the exhaust pipe will not cause the car to fail emissions test. Instead, some horsepower can be realized and not to mention the loudness of the exhaust. However, with a tuned muffler, the sound can be toned down to a deep throaty sound that is not irritable.
Upgrading to Larger Pipe Diameter
The factory exhaust pipes have diameters around 1.5" to 2" (some 2.25" for newer larger engine cars). Increasing the diameter of the piping will also increase the average path/cross-sectional area that the air can pass with a minute decrease in velocity. As mentioned before, if the diameter (and hence cross-sectional area) of the pipe is increased too much, the velocity of the air flow will decrease more than the area increases, so flow would be adversely affected and power would be lost.
So, depending on the size of the engine, the optimal size pipe to upgrade to varies from 2" to 2.5". On average, a naturally aspirated 2.5 liter engine would suffice with 2.25" exhaust piping from the catalytic converter back to the muffler inlet.
Mandrel Bent Versus Crush Bent Piping
Another way to upgrade the exhaust piping from the catalytic converter back to the muffler is to have the exhaust piping mandrel (heat) bent instead of the conventional crush bending. As the name suggests, mandrel bends are achieved through the heating of the piping before bending whereas crush bent just literally mean that the piping is bent entirely by force. However, the main difference between mandrel bent and crush bent piping is the ease of flow. Mandrel piping keeps the pipe at a constant cross-sectional area throughout a bend which makes exhaust flow easier. On the other hand, crush bending deforms the pipe at the bend(s), which can restrict the exhaust flow. The disadvantage of mandrel bending is that it is relatively expensive, because of the costs involved in operating a mandrel bending heat machine. A popular alternative is to get piping with larger diameter and then have it crush bent. This way, it kind of evens out the differences in air flow ease, especially if that particular exhaust pipe configuration has a lot of bends and 90 degree bends.