Forced induction
Encyclopedia
Forced induction is the process of compressing air on the intake of an internal combustion engine
(also known as supercharging). A forced induction engine uses a gas compressor
to increase the pressure, temperature and density of the air. An engine without forced induction is considered a naturally aspirated engine.
Higher compression engines have the benefit of maximizing the amount of useful energy extracted per unit of fuel. Therefore, the thermal efficiency of the engine is increased in accordance with the vapor power cycle analysis of the second law of thermodynamics. The reason all engines are not higher compression is because for any given octane, the fuel will prematurely detonate with a higher than normal compression ratio. This is called preignition, detonation or knock and can cause severe engine damage. High compression on a naturally aspirated engine can reach the detonation threshold fairly easily. However, a forced induction engine can have a higher total compression without detonation because the air charge can be cooled after the first stage of compression, using an intercooler
.
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. This level is government regulated for emissions as commonly seen at inspection stations. High compression causes high combustion temperatures. High combustion temperatures lead to higher NOx emissions, thus forced induction can give higher Nox fractions.
s and supercharger
s. A turbocharger is a centrifugal compressor driven by the flow of exhaust gasses. Superchargers can be found as different types of compressors but are all powered by the rotation of the engine, usually through a belt drive. The compressor can be centrifugal or a roots type for positive displacement compression. An example of an internal compressor is a screw-type supercharger or a piston compressor.
The chief benefit of a turbocharger is that it consumes less power from the engine than a supercharger; the main drawback is that engine response suffers greatly because it takes time for the turbocharger to come up to speed (spool up). This delay in power delivery is referred to as turbo lag.
Any given turbo design is inherently one of compromise; a smaller turbo will spool quickly and deliver full boost pressure at low engine speeds, but boost pressure will suffer at high engine RPM. A larger turbo, on the other hand, will provide improved high-rev performance at the expense of low-end response.
Other common design issues include limited turbine lifespan, due to the high exhaust temperatures it must withstand, and the restrictive effect the turbine has upon exhaust flow.
). This compressor has the advantage of being able to produce the same boost pressure at every engine speed. Ultimately this depends on the flow characteristics of the entire engine but the same volume of air is pumped into the engine for every revolution of the engine. They are not as common as turbochargers because they use the torque produced from the engine to operate. This results in some loss in power and efficiency. A centrifugal compressor is commonly used because it is compact and easier to use an intercooler with. It sits to the side of the engine rather than on top. They do not produce the same pressure at every engine speed. A screw type supercharger uses multiple screws to compress the air between the screws before releasing it to the engine. It generally produces a cooler air output and has a flatter boost curve then a centrifugal supercharger.
. As a result, the charge density is reduced and the cylinders receive less air than the system’s boost pressure prescribes. The risk of detonation, or "knock
", greatly increases. These drawbacks are countered by charge-air cooling, 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 (air-air intercooler) or liquid (liquid-to-air intercooler). The charge air density is increased and the temperature is reduced. In this way an intercooler can greatly increase the ability to run higher absolute compression ratios and take full advantage of using compressors in series. The only drawback of intercooling is its mass and volume of the cooler along with the associated plumbing and piping.
. Methanol is mixed with the water to prevent freezing and to act as a slower-burning fuel. Water injection, unlike nitrous oxide
or forced induction, doesn't add much power to the engine by itself, but allows more power to be safely added. It works by being sprayed into the compressed air charge. The water absorbs heat as it evaporates to cool the charge and lower combustion temperatures. The alcohol is also a fuel in the charge which burns slower and cooler than gasoline. Due to the lower intake temperatures and denser air charge, more boost pressure and timing advance can be safely had without using higher octane fuel. It is most often used in racing applications, however it was also shown to be practical for extended use.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
(also known as supercharging). A forced induction engine uses a gas compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
to increase the pressure, temperature and density of the air. An engine without forced induction is considered a naturally aspirated engine.
Introduction
Forced induction is used in the automotive and aviation industry to increase engine power and efficiency. A forced induction engine is essentially two compressors in series. The compression stroke of the engine is the main compression that every engine has. An additional compressor feeding into the intake of the engine makes it a forced induction. A compressor feeding pressure into another greatly increases the total compression ratio of the entire system. This intake pressure is called boost. This particularly helps aviation engines, as they need to operate at high altitude.Higher compression engines have the benefit of maximizing the amount of useful energy extracted per unit of fuel. Therefore, the thermal efficiency of the engine is increased in accordance with the vapor power cycle analysis of the second law of thermodynamics. The reason all engines are not higher compression is because for any given octane, the fuel will prematurely detonate with a higher than normal compression ratio. This is called preignition, detonation or knock and can cause severe engine damage. High compression on a naturally aspirated engine can reach the detonation threshold fairly easily. However, a forced induction engine can have a higher total compression without detonation because the air charge can be cooled after the first stage of compression, using an intercooler
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...
.
One of the primary concerns in internal combustion emissions is a factor called the NOx
NOx
NOx is a generic term for the mono-nitrogen oxides NO and NO2 . They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures...
fraction, or the amount of nitrogen/oxygen compounds the engine produces. This level is government regulated for emissions as commonly seen at inspection stations. High compression causes high combustion temperatures. High combustion temperatures lead to higher NOx emissions, thus forced induction can give higher Nox fractions.
Types of compressors
Two commonly used forced-induction compressors are turbochargerTurbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
s and supercharger
Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
s. A turbocharger is a centrifugal compressor driven by the flow of exhaust gasses. Superchargers can be found as different types of compressors but are all powered by the rotation of the engine, usually through a belt drive. The compressor can be centrifugal or a roots type for positive displacement compression. An example of an internal compressor is a screw-type supercharger or a piston compressor.
Turbochargers
A turbocharger relies on the volume and velocity of exhaust gases to spin (spool) the turbine wheel, which is connected to the compressor wheel via a common shaft. The boost pressure produced can be regulated by a system of release valves and electronic controllers.The chief benefit of a turbocharger is that it consumes less power from the engine than a supercharger; the main drawback is that engine response suffers greatly because it takes time for the turbocharger to come up to speed (spool up). This delay in power delivery is referred to as turbo lag.
Any given turbo design is inherently one of compromise; a smaller turbo will spool quickly and deliver full boost pressure at low engine speeds, but boost pressure will suffer at high engine RPM. A larger turbo, on the other hand, will provide improved high-rev performance at the expense of low-end response.
Other common design issues include limited turbine lifespan, due to the high exhaust temperatures it must withstand, and the restrictive effect the turbine has upon exhaust flow.
Superchargers
Superchargers have almost no lag time to build pressure because the compressor is always spinning proportionally to the engine speed. A roots type supercharger uses paddles on two rotating drums to push air into the intake. This is considered positive displacement because the pressure is not made inside, as in a piston compressor (also known as reciprocating compressorReciprocating compressor
A reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure....
). This compressor has the advantage of being able to produce the same boost pressure at every engine speed. Ultimately this depends on the flow characteristics of the entire engine but the same volume of air is pumped into the engine for every revolution of the engine. They are not as common as turbochargers because they use the torque produced from the engine to operate. This results in some loss in power and efficiency. A centrifugal compressor is commonly used because it is compact and easier to use an intercooler with. It sits to the side of the engine rather than on top. They do not produce the same pressure at every engine speed. A screw type supercharger uses multiple screws to compress the air between the screws before releasing it to the engine. It generally produces a cooler air output and has a flatter boost curve then a centrifugal supercharger.
Intercooling
An unavoidable side-effect of forced induction is that compressing air, as stated in the first law of thermodynamics, raises its temperatureFirst law of thermodynamics
The first law of thermodynamics is an expression of the principle of conservation of work.The law states that energy can be transformed, i.e. changed from one form to another, but cannot be created nor destroyed...
. As a result, the charge density is reduced and the cylinders receive less air than the system’s boost pressure prescribes. The risk of detonation, or "knock
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...
", greatly increases. These drawbacks are countered by charge-air cooling, which passes the air leaving the turbocharger or supercharger through a heat exchanger typically called an intercooler
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...
. This is done by cooling the charge air with an ambient flow of either air (air-air intercooler) or liquid (liquid-to-air intercooler). The charge air density is increased and the temperature is reduced. In this way an intercooler can greatly increase the ability to run higher absolute compression ratios and take full advantage of using compressors in series. The only drawback of intercooling is its mass and volume of the cooler along with the associated plumbing and piping.
Water injection
Water injection is another effective means of cooling the charge air to prevent detonationEngine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...
. Methanol is mixed with the water to prevent freezing and to act as a slower-burning fuel. Water injection, unlike nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
or forced induction, doesn't add much power to the engine by itself, but allows more power to be safely added. It works by being sprayed into the compressed air charge. The water absorbs heat as it evaporates to cool the charge and lower combustion temperatures. The alcohol is also a fuel in the charge which burns slower and cooler than gasoline. Due to the lower intake temperatures and denser air charge, more boost pressure and timing advance can be safely had without using higher octane fuel. It is most often used in racing applications, however it was also shown to be practical for extended use.