SUPERCHARGERS AND TURBOCHARGERS

UNIT-3Superchargers &

TurbochargersNaturally aspirated engines, Forced Induction, Types of Superchargers, Turbocharger construction and operation, Intercooler, Turbocharger lag

SuperchargingIntroduction: 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) Superchargers2) Turbochargers

Why Supercharging ??

Increases the power of an engine.Increases the torque produced.Necessary in aero-planes as they have less

oxygen at higher altitudes.Ensures complete combustion of the fuel.Reduces pollution to some extent.

Advantages of supercharging (engine performance) Power output of the engine can be increased More quantity of charge can be inducted in to

engine cylinder Better atomization of fuel is possible Better mixing of air and fuel can be obtained Better scavenging of exhaust gases is possible Torque is improved for whole speed range and

better range and better torque at low speeds Faster acceleration of the engine is possible The specific fuel consumption is lowered slightly a better mechanical efficiency and efficient

combustion is possible In CI engines, exhaust smoke is reduced

Disadvantages of superchargingDetonation tendency increases in SI

enginesHeat losses due to turbulence and thermal

stresses are moreThe valve overlap period increases up to

1600 of crank angleBetter lubrication is requiredBetter cooling of piston and valves is

required It increases cost of the engine

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

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.

Methods of superchargingSupercharging of engine by

compressorSupercharging with turbine driven

by engine exhaustSupercharging arrangement in

which engine, turbine and compressed air coupled

Supercharging method in which engine runs compressor and turbine develops power

Commonly used Superchargers

Superchargers

Roots Supercharg

ers

Twin-Screw Supercharg

ers

Centrifugal Supercharg

ers

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.

ROOTS 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

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:• 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, light weight and attachable to the front of the Engine.

CENTRIFUGAL SUPERCHARGER

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

Vane type supercharger

Working of Vane type Supercharger The number of vanes are mounted on the drum in such a manner that

they can slide in or out against some spring force , so that all the time they are in contact with inner surface of the supercharger body .

The vanes are usually made from laminates of linen impregnated with phenolic resin. However Tufnol because of similar qualities has also been used.

The properties required for vane material are quite running, low friction, low coefficient of thermal expansion and resistance to continuous exposure to oil and petrol.

It is seen that space between body and the drum goes on decreasing from inlet to outlet as the drum rotates. Thus the mixture entrapped between any two vanes at the inlet will experience decrease of volume and hence increase of pressure as it reaches the outlet.

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

TurbochargersIntroduction: 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.

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.

Exhaust Turbocharging for single cylinder engine

Working of turbochargerCOOLER COMPRESSE

D AIR

I.C. ENGINE

EXHAUST GAS

COMPRESSOR

TURBINE

TURBO-CHARGER

Advantages of Turbochargers over Superchargers:

Less weight and size.

No load on the engine.

Efficiency of the engine is increased.

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.

‘Turbo Lag’

Methods of Turbocharging and their Advantages and Limits:1. 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 cylinder’s exhaust valves which have an approximately constant pressure in common manifold passes through turbine.

Thus the blow-down energy is converted into mechanical work by 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 reduced exhaust gases to the turbine

2. Pulse Turbo charging:

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.The pressure of exhaust vary with 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 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.

Turbo charger LagTurbocharger 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’ It is the time period required for the

exhaust gases to accelerate the turbine and compressor.

It represents short delay period before the boost pressure increases

Lag reduction By reducing the rotational inertia of the turbine by using

lighter parts By changing the aspect ratio of the turbine by reducing

its diameter and increasing the length of the gas-flow path

By using a precision bearing, which reduces friction, thereby leading to faster acceleration of the turbine rotor

By using two small turbos, one always active and the other one operating only at higher speeds

Comparison b/n Turbocharging and Supercharging

TURBOCHARGING SUPERCHARGING The energy of exhaust gases is used to run super charger

The mechanical energy of prime mover is used to run super charger

It needs a waste gate control

It does not require a waste gate control

It requires special exhaust manifolds

It does not require special exhaust manifolds

In CI engine it reduces smoke

In CI engines it reduces knocking tendency

Blade erosion takes place due to entry of dust particles

No blade erosion problem

Larger pumping element or nozzles are needed. This over loads cams

Fuel injection modification is not required and no cam over loading

Pressure ratio is high Comparatively pressure ratio is low

It is bulky and heavy It is light and compactEasy scavenging Scavenging is difficultPoor response to load change

Better response to load change

TURBOCHARGING SUPERCHARGING

Considerations During Supercharging an engine

It increases power output of the engineEngine should be designed to with stand these

higher forcesThe power required for air compression has to

be drawn from engine itself. But net output will be more than power output without supercharging for the same capacity

The higher pressure and temperature may lead to detonation. So the fuel with better anti-knock characteristics is required