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Forced Induction Defined + Basics


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Old 06-15-2008, 05:49 PM   #1
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Forced induction is a term used to describe internal combustion engines that are not naturally aspirated. Instead, a gas compressor is added to the air intake, thereby increasing the quantity of oxygen available for combustion. This compressed air is normally referred to as Boost or charge air.

Introduction
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Forced induction can be used to improve the power, efficiency, emissions, or combinations of same, without much extra weight and minimal modifications to the engine architecture. The two most common forms of forced induction are turbochargers and superchargers, which both compress the air entering the cylinders, but use different methods to obtain the requisite power. Functionally, they are much the same. Since only so much power can be had from a given amount of gasoline, the more gasoline can be burned in the cylinder, the more power can be produced. However, simply adding more gas beyond the optimal air/fuel ratio (commonly called "running rich") does nothing for power. An engine can only take in so much when breathing air at atmospheric pressures, since the capacity and number of cylinders is non-variable. Hence, the only way to get more air into the cylinder, and therefore produce more power, is to increase the pressure at the intake.
All we've considered up to now is increased power, so how does forced induction improve emissions or efficiency? One of the primary concerns in internal combustion emissions is a factor called the NOx fraction, or the amount of nitrogen/oxygen compounds the engine produces. High combustion temperatures lead to a lower NOx fraction, and since gasses heat when compressed, the more gas is compressed in a given volume, the hotter it will get, and the lower the NOx fraction will be. Since forced induction increases the amount of gas being compressed, it increases the heat generated when compression occurs. It should be noted that since colder air is denser, it is most desirable, from a power standpoint, to have cold air coming in, but better from an emissions standpoint if the air is hot. In a perfect world, incoming air would be frigid, and the compression would be high enough to dramatically and rapidly increase cylinder temperatures, reducing emissions significantly.
Two of the commonly used forced induction technologies are turbochargers and superchargers. They differ primarily in the power source for the compressor. It should be noted that there is a difference between forced induction and power adders. A power adder is anything that improves an engine's power output, which does not necessarily mean increasing charge density. Oxidizing technologies such as nitrous oxide injection systems provide improved power, but are not a form of forced induction.

Comparison
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Strengths and weaknesses vary according to the method of forcing induction largely based upon the inherent design functions of both. A turbocharger acts as an obstacle to exhaust gases due to its placement in the exhaust system tract. A supercharger uses torque generated from the rotational mass internal to the engine through the crank pulley. A turbo relies on the volume and velocity of exhaust gases to spool, or spin the turbine wheel. The turbine wheel is connected to the compressor wheel via a common shaft. The compressor wheel compresses the intake charge increasing the charge density by a large factor. The amount of time that it takes a turbocharger to reach the onset of boost is referred to as lag. A supercharger is 'on' all of the time, meaning that it is capable of producing a linear increase of boost up until redline. It is easier to target a desired boost with a turbocharger as there are many forms of boost controllers that allow a user to adjust to desired boost fairly easily. In order to achieve desired boost with a supercharger, a larger or smaller pulley must be installed.

Intercooling
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An unavoidable side-effect of forced induction is that compressing air raises its temperature. As a result, the charge density is reduced and the cylinders receive less fresh air than the system’s boost pressure prescribes. The risk of pre-ignition or "knock" in internal combustion engines greatly increases. These drawbacks are countered by charged-air-cooler, which passes the air leaving the turbocharger or supercharger through a heat exchanger typically called an intercooler. This is done by cooling the charge air with an ambient flow of either air or liquid, the charge air density is increased and the temperature is reduced.

Alcohol/Water Injection
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Additionally, alcohol injection is an effective means of cooling the charge air. Methanol is the preferred alcohol due to its elemental properties, and is normally mixed with water to prevent evaporation. Methanol is typically injected pre-throttle body. Methanol, unlike nitrous oxide or forced induction itself, doesn't add more oxygen to the charge, but by its low evaporation point changes from a liquid to a gas as it is introduced into the air charge The evaporation process uses the heat from the intake charge to complete the phase change. The alcohol is also a fuel in the charge which will cause a rich condition if used in excess. Due to the lower intake temperatures and denser air charge more power is exerted from the engine. Methanol is typically used in conjunction with poor quality fuel(pump gas) in order to run higher than normal boost pressures.

Like was stated above, adding forced induction increases the amount of air an engine can use for combustion, in effect allowing more fuel to be used with the available oxygen. Further, it increases an engine's dynamic compression ratio. As compression ratio increases, so does the threat of knock and therefore the need for higher octane fuel.

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Old 06-15-2008, 05:50 PM   #2
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okay now. that is out of the way lets move onto air cooling.(inter-cooling)




Charged-air-cooler
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A charge air cooler (also known as an intercooler) is used to cool engine air after it has passed through a turbo charger, but before it enters the engine. The idea is to return the air to the optimum temperature for the combustion of the engine.


Air-to-liquid
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Air-to-liquid intercoolers are heat exchangers that reject intake charge heat to an intermediate fluid, usually water/ or coolant, which finally rejects heat to the air. These systems use radiators in other/or stock locations, usually due to space constraints, to reject unwanted heat, similar to an automotive radiator cooling system. Air-to-liquid intercoolers are usually heavier than their air-to-air counterparts due to additional components making up the system (water circulation pump, radiator, fluid, and plumbing).


Air-to-air
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Air-to-air intercoolers are the most common type, both in factory forced induction cars and aftermarket. They are technically simple, rugged and reliable. An air-to-air intercooler consists of a tube and fin radiator. The induction air passes through thin rectangular cross-section tubes that are stacked on top of the other. Often inside the tubes are fins that are designed to create turbulence and so improve heat exchange. Between the tubes are more fins, usually bent in a zig-zag formation. Invariably, air-to-air intercoolers are constructed from aluminum. The induction air flows through the many tubes. The air is then exposed to a very large surface area of conductive aluminum that absorbs and transfers the heat through the thickness of metal. Outside air - driven through the core by the forward motion of the car - takes this heat away, transferring it from the intake air to the atmosphere.

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forced induction, supercharger, turbo, turbocharger


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