Forced induction is a term used to describe
internal combustion engineThe 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 pressure gases, which are produced by the combustion, directly applies force to a movable...
s that are not
naturally aspiratedA naturally-aspirated engine is a reciprocating internal combustion engine that depends solely on atmospheric pressure to draw in combustion air...
. A
gas compressorA 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...
is added to the air intake instead, thereby increasing the quantity of oxygen available for combustion. This compressed air is normally referred to as Boost or charge air.
Forced induction is used to improve the power, efficiency, and emissions without much extra weight and minimal modifications to the
engineAn engine is a machine that produces mechanical force and motion from another form of energy . It is also referred to as a prime mover. An automobile makes use of several motors to start the car and drive the car's various pumps – but the power plant that propels the car is called an engine...
architecture.
Forced induction is a term used to describe
internal combustion engineThe 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 pressure gases, which are produced by the combustion, directly applies force to a movable...
s that are not
naturally aspiratedA naturally-aspirated engine is a reciprocating internal combustion engine that depends solely on atmospheric pressure to draw in combustion air...
. A
gas compressorA 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...
is added to the air intake instead, thereby increasing the quantity of oxygen available for combustion. This compressed air is normally referred to as Boost or charge air.
Introduction
Forced induction is used to improve the power, efficiency, and emissions without much extra weight and minimal modifications to the
engineAn engine is a machine that produces mechanical force and motion from another form of energy . It is also referred to as a prime mover. An automobile makes use of several motors to start the car and drive the car's various pumps – but the power plant that propels the car is called an engine...
architecture. Two of the commonly used forced induction technologies are
turbochargerA turbocharger, or turbo, is a gas compressor that is used for forced-induction of an internal combustion engine. A form of supercharger, the purpose of a turbocharger is to increase the density of air entering the engine to create more power...
s and
superchargerA 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 provided and more work to be done per cycle,...
s. Since only so much power can be had from a given amount of gasoline, the more gasoline that can be burned in the cylinder, the more power can be produced. An engine can only take in so much when breathing air at atmospheric pressures, since the capacity and number of cylinders is non-variable. The only way to get more air into the cylinder, and therefore produce more power, is to increase the pressure at the intake.
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. Forced induction increases the amount of gas being compressed, and it increases the heat generated when compression occurs. 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.
Comparison
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
redlineRedline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine...
. 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.
Because a supercharger pump is driven by a pulley on the engine, the supercharger charge pressure is linearly proportionate to engine RPM. In other words, at 1/2 of peak RPM, a traditional supercharger only produces half of its peak boost. On the contrary, a turbocharger can be set up to produce near-peak boost levels at lower RPMs, which produces considerably more torque in an application. Emerging supercharger technologies are using techniques such as allowing internal viscosity to control a maximum pump output. This is advantageous because the supercharger can be set to reach peak boost at the onset of slip. Therefore, the linear power increase happens sooner, and the supercharger can be set to boost at higher pressure levels throughout the entire engine speed range. At present, however, for a given size, a turbocharger can usually be set to produce more power under the curve than an equivalent supercharger, but superchargers do not suffer from boost lag.
Intercooling
An unavoidable side-effect of forced induction is that compressing air
raises its temperatureThe first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed , but cannot be created or destroyed. Alternatively:-Description:...
(see also
Combined gas lawThe combined gas law is a gas law which combines Charles's law, Boyle's law, and Gay-Lussac's law. These laws each relate one thermodynamic variable to another mathematically while holding everything else constant. Charles' law states that volume and temperature are directly proportional to each...
). 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 detonation, or "
knockKnocking 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...
" in internal combustion engines 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
intercoolerAn 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...
. This is done by cooling the charge air with an ambient flow of either air (air-air intercoolers) or liquid (liquid to air intercoolers). The charge air density is increased and the temperature is reduced.
Water Injection
Water injection is another effective means of cooling the charge air to prevent
detonationKnocking 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...
. Methanol is mixed with the water to prevent freezing and to act as a slower-burning fuel. Water injection, unlike
nitrous oxideNitrous oxide, commonly known as happy gas or laughing gas, is a chemical compound with the chemical formula N
2O. At room temperature, it is a colorless non-flammable gas, with a pleasant, slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
or forced induction itself, doesn't add more oxygen to the charge, but 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 power is exerted from the engine. Water injection is typically used in conjunction with
low-octaneThe octane rating is a measure of the resistance of gasoline and other fuels to detonation in spark-ignition internal combustion engines. High-performance engines typically have higher compression ratios and are therefore more prone to detonation, so they require higher octane fuel...
fuel in order to run higher than normal manifold pressures.
Diesel engines
Diesel engines do not have preignition problems because fuel is injected at the end of the compression stroke. Therefore higher boost pressures can be used.