Supercharging

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Supercharging : 

Supercharging Introduction: The method of increasing the inlet air density is called Supercharging. “More fuel + More air = Bigger explosion = Greater horsepower”. Supercharging can be done both for SI engines as well as in CI engines Supercharging of an engine can be done using either one of the following components; 1) Superchargers 2) Turbochargers

Why Supercharging ?? : 

Why Supercharging ?? Increases the power of an engine. Increases the torque produced. Necessary in aero-planes as they have less oxygen at high altitudes. Ensures complete combustion of the fuel. Reduces pollution to some extent.

Supercharger : 

Supercharger A supercharger is an equipment that compresses the air being delivered to an engine, allowing the combustion chamber to be overfilled without enlarging the space. The higher concentration of oxygen provided by a super- charger is matched with a larger amount of fuel from the fuel injectors thus boosting the power of the engine.

Classification of Superchargers : 

Classification of Superchargers Based on method of compression, Superchargers can be classified as:- 1. Positive-displacement type, which deliver a nearly-fixed volume of air per revolution at all speeds and a fairly constant level of boost regardless of engine speed. 2. Dynamic compressors rely on accelerating the air to high speed and then exchanging that velocity for pressure by diffusing or slowing it down & deliver increasing boost with increasing engine speed.

Commonly used Superchargers : 

Commonly used Superchargers

Roots supercharger: : 

Roots supercharger: The Roots supercharger is the oldest design. Philander and Francis Roots patented the design in 1860. In 1900, Gottleib Daimler included a Roots supercharger in a car engine Working: 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 positive pressure. Roots superchargers are usually large and sit on top of the engine. Roots superchargers are the least efficient supercharger for two reasons:- 1.)They add more weight to the vehicle. 2.)They provide air in discrete bursts instead of providing in a smooth and continuous manner.

Twin-Screw Supercharger: : 

Twin-Screw Supercharger: A twin-screw supercharger operates by pulling air through a pair of meshing lobes that resemble a set of worm gears. A twin-screw supercharger compresses 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

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Comparison of Twin-screw Supercharger with Roots Supercharger: Twin-screw superchargers are more efficient than Roots supercharger They cost more because the screw-type rotors require more precision in the manufacturing process. They also make a lot of noise. The compressed air exiting the discharge outlet creates a whine or whistle that must be checked with noise suppression techniques

Centrifugal Supercharger: : 

Centrifugal Supercharger: A centrifugal supercharger works by powering 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’. Centrifugal superchargers are the most efficient and the most common induction systems. They are small, lightweight and attachable to the front of the Engine.

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Working of Centrifugal supercharger: As the air is drawn in 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 converts the high-speed , low-pressure air to low-speed , high-pressure air. Thus pressurized air is achieved. Drives Used in Supercharger: Belt (V belt, Toothed belt & Flat belt). Gear drive. Chain drive

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Advantages of Superchargers Over a Turbocharger: Supercharger do not suffer lag which is a serious problem in Turbocharger . Modification of the exhaust system:- Installing a turbocharger requires extensive modification of the exhaust system, but superchargers can be bolted to the top or side of the engine , that makes them cheaper to install and easier to service and maintain. Shutdown procedure:-No special shutdown procedure is required with superchargers as they are not lubricated by engine oil . They can be shut down normally. Turbochargers must idle for about 30 seconds or so prior to shutdown so the lubricating oil has a chance to cool down. Disadvantages of Superchargers: It derives power from the engine itself: Crankshaft drives superchargers so they steal some of the engine's horsepower. A supercharger can consume as much as 20 percent of an engine's total power output An added strain on the engine: Supercharging puts an added strain on the engine, which needs to be strong to handle the extra boost and bigger explosions Since some of the superchargers are larger in size the efficiency of the engine decreases as it is the additional component bolted on to it. Detonation is serious problem

Turbochargers : 

Turbochargers Introduction: Turbochargers are a type of forced induction system whose function is same as that of Supercharger. In order to achieve the boost, the turbocharger uses the exhaust flow from the engine to spin a turbine, which in turn spins an air pump. The turbocharger is bolted to the exhaust manifold of the engine. The exhaust from the cylinders spins the turbine, which works like a gas turbine engine. The turbine is connected by a shaft to the compressor, which is located between the air filter and the intake manifold. The compressor pressurizes the air going into the pistons.

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Important Parts of Turbocharger: Turbine Compressor Ball bearings & Fluid Bearings Intercooler: When air is compressed, it heats up; and when air heats up, it expands. So some of the pressure increase from a turbocharger is the result of heating the air before it goes into the engine. An intercooler or charge air cooler is an additional component that looks something like a radiator, except air passes through the inside as well as the outside of the intercooler.

Methods of Turbocharging and their Advantages and Limits: : 

Methods of Turbocharging and their Advantages and Limits: Constant Pressure Turbocharging: The exhaust from various cylinders discharge into a common manifold at pressures higher than the atmospheric pressure. The exhaust gasses from all the expanded in the exhaust valves to an approximately constant pressure in common manifold from here it passes to turbine. Thus the blow-down energy, in the form of internal energy, is converted into work in the turbine. The exhaust gases are maintained at constant pressure during the whole cycle so that a pure Reaction turbine can be used. Advantages: The exhaust piping is very simple for a multi-cylinder engine as well as single-cylinder, highly efficient turbine can be used. Engine speed is not limited by the pressure waves in the exhaust pipes. Disadvantages: Scavenging is not efficient. At part load the efficiency of turbine reduces due to partial admissions to the turbine Pulse Turbocharging: Considerable part of the blow-down energy is converted into exhaust pulses as soon as the exhaust valve opens. Towards the end of exhaust the pressure in the exhaust pipe drops below the scavenging and large air pressure making scavenging quite easy. The rate of the exhaust gas at the various turbine inlet is different and variable in time. Advantages: The space required is less due to short and smaller diameter pipes. Comparatively better scavenging is obtained at low loads due to reduced pressure.   Disadvantages: With large number of cylinders complicated inlet and exhaust pipe arrangements are needed. The length of the pipe or engine speed is limited.

Two Stage Turbocharging: : 

Two Stage Turbocharging: Two –stage turbocharging is defined as use of two turbochargers of different sizes In series; for example a high-pressure stage operating on pulse system and a low-pressure stage on constant pressure operation. Advantages: Better matching of the turbochargers to engine operating conditions possible. The efficiency of two-stage turbocharger is higher than that of a single stage turbocharger having a high boost ratio. Disadvantages: The space requirement is higher. The total system is heavier.

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Limitations of Turbocharging: The use of Turbochargers requires special exhaust manifolds. Fuel injection has to be modified to inject more fuel per unit time. The efficiency of the turbine is sensitive to gas velocity so difficult to obtain good efficiency over a wide range of operations. One of the main problem with turbochargers is that they do not provide an immediate power boost. It takes a second for the turbine to get up to speed before boost is produced. This results in a lag known as ‘Turbo Lag’ Advantages of Turbochargers over Superchargers: Less weight and size. No load on the engine. Efficiency of the engine is increased.

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