FORCED INDUCTION

What is forced induction?

Forced induction is a process of delivering the compressed air intake of an internal combustion engine. A forced induction uses a gas compressor to increase the temperature, pressure and density of the air. An engine with forced induction is called natural aspirated engine.

Why forced induction?

A forced induction is used in automotive and aviation industry to increase engine power efficiency. A forced induction engine is usually comprised of 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.

Types of forced induction systems:-

Normally forced induction in an internal combustion engine can be done in three ways. They are

1. With the help of Turbocharger.2. With the help of Supercharger.3. With the help of Nitrous oxide (NO2).

Fig 1. Engine with forced induction

Turbocharger:-

Fig 2. Turbocharger

What is a turbocharger?

Turbocharger is a turbine driven forced induction device that makes an engine more efficient and produce more power for its size by forcing extra air into the combustion chamber. A turbo charged engine is more powerful and efficient than natural aspirated engine.

Why turbocharging?

The turbine forces more air, and proportionately more fuel, into the combustion chamber than atmospheric pressure alone so there is an increased performance in the engine. Turbochargers were originally known as the Turbosuperchargers when all the forced induction devices are classified as the superchargers. As already told that it’s a device that make an engine more efficient and produce more power for its size by forcing extra air into the combustion chamber. It is used to increase the fuel efficiency without increasing the power. This is achieved by recovering the waste energy and feeding back to the engines intake.

Turbochargers are commonly used on truck, car, train, aircraft, and construction equipment engines. They are popularly used with Otto and Diesel cycle internal combustion engines. Modern turbochargers can use waste gates, blow off values and variable geometry.

They have also been found useful in automotive fuel cells.

How it operates?

The amount of air actually inspirited into the engine than the theoretical amount that if engine could maintain the atmospheric pressure is called volumetric efficiency. The objective of the turbocharger is to increase the volumetric efficiency by increasing the density of intake air.

Fig 3. Operating of turbocharger

The turbo charger compressor draws in the ambient air and compresses it before it enters the intake manifold at increased pressure. The power needed to work i.e., spin centrifugal compressor will be attained from the engines exhaust gases. Due this spinning action more air will be inhaled by the engine into its cylinders on each intake stroke.

The increased temperature from the higher pressure gives a higher Carnot efficiency.

Types:

A turbocharger’s performance is closely tied to its size. Large ones take more heat and pressure to spin the turbine creating lag at low speed. Unlike big ones, small ones spin quickly but they don’t develop the more power. To overcome this difficulty different schemes had come such as

1. Twin turbochargers.2. Twin scroll turbochargers.3. Variable geometry turbochargers.

1. Twin turbo chargers:

Twin-turbo or bi-turbo designs have two separate turbochargers operating in either a sequence or in parallel. In a parallel configuration, both turbochargers are fed one-half of the engine’s exhaust. In a sequential setup one turbocharger runs at low speeds and the second turns on at a predetermined engine speed or load. Sequential turbochargers further reduce turbo lag, but require an intricate set of pipes to properly feed both turbochargers.

Fig 4. Twin turbocharger

Two-stage variable twin-turbos employ a small turbocharger at low speeds and a large one at higher speeds. They are connected in a series so that boost pressure from one turbocharger is multiplied by another, hence the name "2-stage." The distribution of exhaust gas is continuously variable, so the transition from using the small turbocharger to the large

one can be done incrementally. Smaller turbochargers have less turbo lag than larger ones, so often two small turbochargers are used instead of one large one. This configuration is popular in engines over 2,500 CCs and in V-shape or boxer engines.

2. Twin scroll turbochargers:

Twin-scroll or divided turbochargers have two exhaust gas inlets and two nozzles, a smaller sharper angled one for quick response and a larger less angled one for peak performance.

Fig 5. Twin scroll turbocharger

With high performance cam shaft timing, exhaust values in different cylinders can operate at the same time, overlapping at the end of the power stroke in one cylinder and the end of the of exhaust stroke in another. In twin-scroll design, the exhaust manifold physically separates the channels for the cylinders that can interfere with each other, so that pulsating exhaust gases flow through separate spirals.

3. Variable geometry:

Variable-geometry or variable-nozzle turbochargers use moveable vanes to adjust the air-flow to the turbine, imitating a turbocharger of the optimal size throughout the power curve. The vanes are placed just in front of the turbine like a set of slightly overlapping walls. Their angle is adjusted by an actuator to block or increase air flow to the turbine. This variability maintains a comparable exhaust velocity and back pressure throughout the engine’s rev range. The result is that the turbocharger improves fuel efficiency without a noticeable level of turbocharger lag.

Fig 6. Variable geometry turbocharger

What is a lag in the turbocharger?

Turbocharger lag ("turbo lag") is the time required to change power output in response to a throttle change, noticed as a hesitation or slowed throttle response when accelerating as compared to a naturally aspirated engine. This is due to the time needed for the exhaust system and turbocharger to generate the required boost. Inertia, friction, and compressor load are the primary contributors to turbocharger lag. Superchargers do not suffer this problem, because the turbine is eliminated due to the compressor being directly powered by the engine. Turbocharger lag is most problematic in applications that require rapid changes in power output.

Boost thresh hold:

The boost threshold of a turbocharger system is the lower bound of the region within which the compressor operates. Below a certain rate of flow, a compressor produces insignificant boost. This limits boost at a particular RPM, regardless of exhaust gas pressure. Newer turbocharger and engine developments have steadily reduced boost thresholds.

Turbochargers start producing boost only when a certain amount of kinetic energy is present in the exhaust gasses. Without adequate exhaust gas flow to spin the turbine blades, the turbocharger cannot produce the necessary force needed to compress the air going into the engine. The boost threshold is determined by the engine displacement, engine rpm, throttle opening, and the size of the turbocharger. The operating speed (rpm) at which there is enough exhaust gas momentum to compress the air going into the engine is called the "boost threshold rpm". Reducing the "boost threshold rpm" can improve throttle response.

Is any cooling required for turbocharger?

When the pressure of the engine's intake air is increased, its temperature also increases. In addition, heat soak from the hot exhaust gases spinning the turbine may also heat the intake air. The warmer the intake air the less dense, and the less oxygen available for the combustion event, which reduces volumetric efficiency. Not only does excessive intake-air temperature reduce efficiency, it also leads to engine knock, or detonation, which is destructive to engines.

Turbocharger often makes use of an intercooler (also known as charge air cooler), to cool down the intake air. Intercoolers are often tested for leaks during routine servicing, particularly in trucks where a leaking intercooler can result in a 20% reduction in fuel economy.

Advantages and Disadvantages of turbochargers:

Advantages of turbocharging:

1. More power over natural aspirated engines.2. Reuse excess exhaust heat.3. A turbocharger is smaller, lighter and easier to fit than a supercharger, and it is more

consistent than, for example, a nitrous oxide kit.4. Fuel economy is often better and reduced emission from the engine.5. Torque characteristic can be improved.6. The high altitude performance of a turbocharged engine is significantly better.

Disadvantages of turbocharging:

1. Complicated to install in the engines.2. Turbocharging system can’t be handled by all vehicles and may also cause a break

down.3. These are quite expensive to add to naturally aspirated engines.4. Increased fuel consumption.5. Lag is a drawback in this system.6. Shorter engine life and lower engine performance are the drawbacks of a

turbocharged engine.

7. Drivability may be compromised, particularly when the boost threshold is approached and suddenly a surge of power is too much for the tyres to cope with, causing understeer/oversteer.

Superchargers:

Fig 7. Supercharger

What is a supercharger?

A supercharger is an air compressor that increases the pressure or density of air supplied to an internal combustion engine. This gives each intake cycle of an engine more oxygen, letting it burn more fuel and do more work, thus increasing the power. Power for the supercharger can be provided by means of a belt, gear, shaft or chain connected to the engines crankshaft.

Why supercharging an engine?

A supercharger have almost no lag time because the compressor is always spinning proportionally to the engine speed. Unlike the turbochargers they use the torque from the engine to operate. Most of the superchargers are having the positive displacement devices, the compressor has the same advantage of producing the same pressure ratio at any engine speed, and they are more efficient and produce cool air as output. Dynamic compressor supercharger is having more thermal efficiency than positive displacement type.

How it operates?

There are two main types of superchargers defined according to the method of gas transfer. Positive displacement type and dynamic compressors type. Positive displacement

type blowers and compressors deliver almost constant level of pressure increase at all engine speeds (RPM). Dynamic compressor do not deliver any pressure at all speeds, it will increase its pressure with increased speed.

Positive displacement type superchargers:

Positive displacement type deliver a nearly fixed volume of air per revolution at all speeds.

Major types include:

1. Roots type superchargers2. Twin screw type superchargers.

1. Roots type superchargers:

Roots type supercharger use meshing lobes. The roots type supercharger is the oldest in design. As the meshing lobes spin, air trapped in the pockets between the lobes is carried between the fill side and the discharge side. Large quantities of air move into the intake manifold and “stack up” to create the positive pressure. For this reason, roots superchargers are really nothing more than air blowers and the term “blowers” is still often used to describe all superchargers.

Fig 8. Roots type supercharger

Roots superchargers are usually large and sit on top of the engine. They are popular in muscle cars and hot rods because they stick out the hood of the car. However, they are least efficient supercharger for two reasons: they add more weight to the vehicle and they move air in discrete bursts instead of in a smooth and continuous flow.

Roots type supercharger is basically an external compression type. External compression refers to pumps that transfer air at ambient pressure into the engine. If the engine is running under boost conditions, the pressure in the intake manifold is higher than that coming from the supercharger. That causes a backflow from the engine into the supercharger until the two reach equilibrium. It is the backflow that actually compresses the incoming gas. This is an inefficient process and the main factor in the lack of efficiency of Roots superchargers when used at high boost levels. The lower the boost level the smaller is this loss, and Roots blowers are very efficient at moving air at low pressure differentials, which is what they were invented for (hence the original term "blower").

2. Twin screw superchargers:

A twin-screw supercharger operates by pulling air through a pair of meshing lobes that resemble a set of worm gears. Like the roots type superchargers, the air inside a twin supercharger is trapped in pockets created by the rotor lobes. But a twin screw supercharger compressed the air inside the rotor housing. That’s because the rotors have a conical taper, which means the air pockets decrease in size as air moves from the fill side to the discharge side. As the air pockets shrink, the air is squeezed into a smaller space.

Fig 9. Twin screw supercharger

This makes twin-screw superchargers more efficient, but they cost more because the screw-type rotors require more precision in the manufacturing process. Some types of twin-screw superchargers sit above the engine like the Roots supercharger. They also make a lot of noise. The compressed air exiting the discharge outlet creates a whine or whistle that must be subdued with noise suppression techniques.

Unlike the roots type superchargers it is an internal compression type. Internal compression type refers to the compression of air within the supercharger itself, which, already at or close to boost level, can be delivered smoothly to the engine with little or no back flow. This is more effective than back flow compression and allows higher efficiency to be achieved. Internal compression devices usually use a fixed internal compression ratio. When the boost pressure is equal to the compression pressure of the supercharger, the back flow is zero. If the boost pressure exceeds that compression pressure, back flow can still occur as in a roots blower. Internal compression blowers must be matched to the expected boost pressure in order to achieve the higher efficiency they are capable of, otherwise they will suffer the same problems and low efficiency of the roots blowers.

Dynamic compressor types:

We usually use centrifugal type dynamic compressor type supercharger.

Centrifugal type supercharger:

A centrifugal supercharger powers an impeller -- a device similar to a rotor -- at very high speeds to quickly draw air into a small compressor housing. Impeller speeds can reach 50,000 to 60,000 RPM. As the air is drawn in at the hub of the impeller, centrifugal force causes it to radiate outward. The air leaves the impeller at high speed, but low pressure. A diffuser -- a set of stationary vanes that surround the impeller -- converts the high-speed, low-pressure air to low-speed, high-pressure air. Air molecules slow down when they hit the vanes, which reduces the velocity of the airflow and increases pressure.

Fig 10. Centrifugal Supercharger

Centrifugal superchargers are the most efficient and the most common of all forced induction systems. They are small, lightweight and attach to the front of the engine instead of the top. They also make a distinctive whine as the engine revs up -- a quality that may turn heads out on the street.

Any of these superchargers can be added to a vehicle as an after-market enhancement. Several companies offer kits that come with all of the parts necessary to install a supercharger

as a do-it-yourself project. In the world of funny cars and fuel racers, such customization is an integral part of the sport. Several auto manufacturers also include superchargers in their production models.

Roots blowers tend to be only 40–50% efficient at high boost levels; by contrast centrifugal (dynamic) superchargers are 70–85% efficient at high boost. Lysholm-style blowers can be nearly as efficient as their centrifugal counterparts over a narrow range of load/speed/boost, for which the system must be specifically designed.

Effects of temperature on supercharged engine:

Supercharging an engine can cause a spike in temperature, and extreme temperatures can cause detonation of fuel air mixture and damage to the engine. In this case of air craft, this causes a problem at low altitudes, where the air is both denser and wormer than at high altitudes. With high ambient air temperatures, the detonation could start to occur with the manifold pressure gauge reading far below the red line.

A supercharger optimized for high altitudes causes the opposite problem on the intake side of the system. With the throttle retarded to avoid overboosting, air temperature in the carburetor can drop low enough to cause ice to form at the throttle plate. In this manner, enough ice could accumulate to cause engine failure, even with the engine operating at full rated power. For this reason, many supercharged aircraft featured a carburetor air temperature gauge or warning light to alert the pilot of possible icing conditions.

Advantages and disadvantages of superchargers:

Advantages of superchargers:

1. The biggest advantage of having a supercharger is the increased horsepower.2. Superchargers do not suffer lag.3. Installing a turbocharger requires extensive modification of the exhaust system, but

superchargers can be bolted to the top or side of the engine.4. Finally, no special shutdown procedure is required with superchargers.5. With the introduction of superchargers, airplanes were able to fly higher without

losing engine performance.6. They draw their power directly from the engine and use a compressor to blow

pressurized air into the combustion chamber.

Disadvantages of superchargers:

1. At low engine RPM, the supercharger blasts air into the cylinders to enhance low-end torque.

2. Because the crankshaft drives them, they must steal some of the engine's horsepower.3. Supercharging puts an added strain on the engine, which needs to be strong to handle

the extra boost and bigger explosions.4. Superchargers also cost more to maintain.

Turbochargers vs. Superchargers:

Fig 11. Supercharger and Turbocharger

In contrast to turbochargers, superchargers are mechanically driven by the engine. Belts, chains, shafts, and gears are common methods of powering a supercharger, placing a mechanical load on the engine. For example, on the single-stage single speed supercharged engine, the superchargers uses about 150 horsepower (110KW). But the benefits are more than the cost for the 150 hp to drive supercharger the engine generates an additional 400 horsepower, a net gain of 250 hp (190KW). This is where the principle disadvantage of a supercharger becomes apparent; the engine must withstand the net power output of the engine plus the power to drive the supercharger.

Another disadvantage of some superchargers is lower adiabatic efficiency as compared to turbochargers. Adiabatic efficiency is a measure of a compressor's ability to compress air without adding excess heat to that air. The compression process always produces heat as a byproduct of that process. Roots superchargers impart significantly more heat to the air than turbochargers.

By comparison, a turbocharger does not place a direct mechanical load on the engine, although turbochargers place exhaust back pressure on engines, increasing pumping losses. This is more efficient, because it uses the otherwise wasted energy of the exhaust gas to drive the compressor. In contrast to supercharging, the primary disadvantage of turbocharging is what is referred to as "lag" or "spool time". This is the time between the demand for an increase in power (the throttle being opened) and the turbocharger(s) providing increased intake pressure, and hence increased power.

Roots blowers tend to be only 40–50% efficient at high boost levels; by contrast centrifugal (dynamic) superchargers are 70–85% efficient at high boost. Lysholm-style blowers can be nearly as efficient as their centrifugal counterparts over a narrow range of

load/speed/boost, for which the system must be specifically designed. Mechanically driven superchargers may absorb as much as a third of the total crankshaft power of the engine and are less efficient than turbochargers.

The thermal efficiency, or fraction of the fuel/air energy that is converted to output power, is less with a mechanically driven supercharger than with a turbocharger, because turbochargers use energy from the exhaust gas that would normally be wasted. For this reason, both economy and the power of a turbocharged engine are usually better than with superchargers.

The main advantage of an engine with a mechanically driven supercharger is better throttle response, as well as the ability to reach full-boost pressure instantaneously. With the latest turbocharging technology and direct gasoline injection, throttle response on turbocharged cars is nearly as good as with mechanically powered superchargers, but the existing lag time is still considered a major drawback, especially considering that the vast majority of mechanically driven superchargers are now driven off clutched pulleys, much like an air compressor.

When you look at the boost pressure curve of Turbochargers and Superchargers, the Superchargers don't look too impressive. This is because they produce a more ‘constant’ pressure curve. This results in very good drivability. Turbochargers run very hot, just short of melting down. Therefore, the Turbocharger surrounding must be well insulated from the radiating heat. It is advisable to idle the engine for a minute or two after heavy use before switching off. Turbochargers are water-cooled.

Head to head comparison:

Cost:The cost of supercharger and a turbocharger systems for the same engine are

approximately the same, so cost is generally not a factor.

Lag:This is perhaps the biggest advantage that the supercharger enjoys over the turbo.

Because a turbocharger is driven by exhaust gasses, the turbocharger's turbine must first spool up before it even begins to turn the compressor's impeller. This results in lag time which is the time needed for the turbine to reach its full throttle from an intermediate rotational speed state. A Supercharger, on the other hand, is connected directly to the crank, so there is no "lag". Superchargers are able to produce boost at a very low rpm, especially screw-type and roots type blowers.

Efficiency:The turbocharger is generally more economical to operate as it as it is driven

primarily by potential energy in the exhaust gasses that would otherwise be lost out the exhaust, whereas a supercharger draws power from the crank, which can be used to turn the wheels.

Heat: Because the turbocharger is mounted to the exhaust manifold (which is very hot),

turbocharger boost is subject to additional heating via the turbos hot casing. A centrifugal supercharger on the other hand creates a cooler air discharge, so an intercooler is often not necessary at boost levels below 10psi.

Surge:Because a turbocharger first spools up before the boost is delivered to the engine,

there is a surge of power that is delivered immediately when the waste gate opens (around 3000 rpm). This surge can be damaging to the engine and drivetrain, and can make the vehicle difficult to drive or lose traction.

Back pressure:Because the supercharger eliminates the need to deal with the exhaust gas interruption

created by inserting a turbocharger turbine into the exhaust flow, the supercharger creates no additional exhaust backpressure. The amount of power that is lost by a turbos turbine reduces its overall efficiency.

Noise:The turbocharger is generally quieter than the supercharger. Because the turbos

turbine is in the exhaust, the turbo can substantially reduce exhaust noise, making the engine run quieter. Some centrifugal superchargers are known to be noisy and whistley which, annoys some drivers.

Reliability:In general, superchargers enjoy a substantial reliability advantage over the

turbocharger. When a turbo is shut off (i.e. when the engine is turned off), residual oil inside the turbos bearings can be baked by stored engine heat. This, combined with the turbos extremely high rpms (up to 150,000rpm) can cause problems with the turbos internal bearings and can shorten the life of the turbocharger. In addition, many turbos require aftermarket exhaust manifolds, which are often far less reliable than stock manifolds.

Ease of installation:Superchargers are substantially easier to install than a turbos because they have far

fewer components and simpler devices. Turbos are complex and require manifold and exhaust modifications, intercoolers, extra oil lines, etc. - most of which is not needed with most superchargers. A novice home mechanic can easily install most supercharger systems, while a turbo installation should be left to a turbo expert.

Maximum power output:Turbos are known for their unique ability to spin to incredibly high rpms and make

outrages peak boost figures (25psi+). While operating a turbocharger at very high levels of boost requires major modifications to the rest of the engine, the turbo is capable of producing more peak power than superchargers.

Tunability:Turbochargers, because they are so complex and rely on exhaust pressure, are

notoriously difficult to tune. Superchargers, on the other hand, require few fuel and ignition upgrades and normally require little or no engine tuning.

Conclusion:While the supercharger is generally considered to be a better method of forced

induction for most street and race vehicles, the turbo will always have its place in a more specialized market. Superchargers generally provide a much broader powerband that most drivers are looking for with no "turbo lag". In addition, they are much easier to install and tune, making them more practical for a home or novice mechanic.

The other solution:In order to mitigate the weakness of exhaust driven turbocharger and engine driven

supercharger there is another technique called TWIN CHARGING.

Twin charger:Twincharger refers to a compound forced induction system used on some piston-type

internal combustion engines. It is a combination of an exhaust-driven turbocharger and an engine-driven supercharger, each mitigating the weaknesses of the other. A belt-driven supercharger offers exceptional response and low-rpm performance as it has no lag time between the application of throttle and pressurization of the manifold. Combined with a large turbo which would offer unacceptable lag and poor response in the low-rpm range, the proper combination of the two can offer a zero-lag powerband with high torque at lower engine speeds and increased power at the higher end. Twincharging is therefore desirable for small-displacement motors, especially those with a large operating rpm, since they can take advantage of an artificially broad torque band over a large speed range.

How it operates?

The construction of Twincharger is quite simple. It has a Roots supercharger and a turbocharger connected in series. The supercharger can be bypassed through an alternative path, or disengaged completely by an electromagnetic clutch. At low rev, the supercharger provides most of the boost pressure. The pressure it built up also helps spooling up the turbocharger so that the latter can run into operating range more quickly. At 1500 rpm, both chargers contribute about the same boost pressure, with a total of 2.5 bar. (Had the turbocharger worked alone, it could provide only 1.3 bar at the same rev).

Fig 12. Twincharger

Then the turbocharger – which is optimized for high-rev power – started taking the lead. The higher the rev, the less efficient the Roots-type supercharger becomes. Therefore a by-pass valve depressurizes the supercharger gradually. By 3500 rpm, the turbocharger contributes all the boost pressure, thus the supercharger is disconnected by the electromagnetic clutch to save energy. The Twincharger is quite an achievement. It delivers excellent power and tractability yet the package is surprisingly compact.

The only disadvantage in this system is high cost.