KNOW YOUR ENGINE

Supercharging

Purpose

1) To increase power output
2) To maintain power output of aircraft
3) To reduce weight of an engine per unit power developed such as aircraft & racing cars
4) To reduce bulk of an engine to fit in limited space in marine & locomotive

Factors increasing power output

1) Amount of air taken in cylinder
2) Utilization of this air
3) Speed of engine (limit of friction)
4) Quantity of fuel taken
5) Combustion
6) Thermal efficiency of engine

Supercharging – forced convection – due to higher pressure more air or A/F mixture will be forced in cylinder.

Application

1) Racing cars
2) Marine engines
3) Automotive engines
4) Engines at high altitude

Types

1) Centrifugal type
2) Axial flow
3) Roots blower
4) Vane blower

Basic types

1) Mechanically driven (crankshaft driven)
2) Turbocharger




Effects of supercharging

Positive

1) Higher power output
2) Intake mass of charge increases
3) Better atomization of fuel
4) Better mixing of fuel & air
5) Better scavenging
6) Better torque characteristics
7) Quicker acceleration
8) Complete & smoother combustion
9) Poor fuel can be used
10) Reduction in diesel knock
11) Improved cold starting
12) Reduced exhaust smoke
13) Reduced SFC in turbocharging
14) Mechanical efficiency increased

Negative

1) Detonation increased in S I
2) Increase in thermal stresses
3) Increase in heat losses due to turbulence
4) Gas loading will increase
5) Valve overlap will increase
6) Increased cooling requirements of piston & valves

Limitations

1) increases in thermal load (stresses)
2) higher stresses will develop
3) require heavier engine components
4) increase in heat generation
5) increase in heat transfer
6) exhaust valve seat damages piston crown & exhaust valve *

Arrangements

1) Compressor driven by engine
2) Compressor & turbine driven by exhaust
3) Compressor & turbine driven by exhaust as well as by engine
4) Compressor driven by full power of engine




Methods of turbocharging

1) constant pressure turbocharging

Advantages

1) At higher pressure ratio efficient
2) Better fuel consumption
3) At constant pressure & temperature turbine runs at high efficiency
4) Exhaust piping is simple
5) Engine speed is not limited by the pressure waves in exhaust pipes

Disadvantages

1) pressure requires to keep constant at exhaust
2) due to proper scavenging hence more diameter pipe requires
3) response to load change is poor
4) scavenging is little difficult
5) at part load efficiency of turbine reduces

2) Pulse turbocharging

Advantages

1) recovery of exhaust blowdown is efficient
2) rapid acceleration at higher speed
3) space is less
4) better scavenging

Disadvantages

1) complicated inlet & outlet exhaust pipe
2) poor efficiency *