Mumbai University > Mechanical Engineering > Sem 5 > IC Engines

Marks: 10 M

Year: May 2015

• Turbocharging employs a turbine-driven forced induction device which forces extra air into the combustion chamber and thereby increases engine’s efficiency and power output.
• The turbocharger uses exhaust flow from engine to spin a turbine which in turn drives a centrifugal compressor. Thus it does not draw upon the engine’s power.

Turbocharging employs a turbine-driven forced induction device which forces extra air into the combustion chamber and thereby increases engine’s efficiency and power output. Following are the main methods of turbocharging:

1. Constant pressure turbocharging

• The various cylinders discharge their exhaust into a common manifold at pressures higher than atmospheric pressure.
• These exhaust gases undergo expansion in exhaust valves, to an approximately constant pressure in the common manifold and then enter the turbine.
• This allows the conversion of blow-down energy (internal energy) to work in the turbine. Higher the pressure ratio of turbine, higher is the recovery of blow-down energy.

• Pulse turbocharging

• In this significant part of blow down energy is converted into exhaust pulses as soon as exhaust valve opens.

• These pulses enter the turbine through narrow exhaust pipes where this energy is utilized and so a large part of energy is recovered.
• Separate exhaust pipes are used so that the exhaust process of various cylinders do not interfere with each other.

• Pulse converter

• This method provides advantages of both constant pressure and pulse turbocharging simultaneously.

• The arrangement involves connecting the different branches of exhaust manifolds together in a specially designed venturi junction called pulse converter before the turbine.

• Two stage turbocharger

• This method involves use of two turbochargers of different sizes in series. Eg. A high pressure stage operating on pulse system and low pressure stage on constant pressure system.

• It is used for diesel engines requiring very high degree of supercharging which cannot be obtained by single stage turbocharger.

1. Miller turbocharging

• The basic idea is to increase the expansion ratio relative to the compression ratio by closing the inlet valve early as the boost pressure is increased.
• Its main disadvantage is that back up pressure of turbine becomes higher and reduces the advantage obtained in power increase.

• Hyperbar turbocharging

• It consists of low compression (7:1) diesel engine, high pressure ratio (upto 5.1) turbine, bypass control, and auxiliary combustion chamber.

• The turbocharger is started using electric starter and kept running by bypassing air and injecting first into the auxiliary combustion chamber. After reaching suitable temperatures and pressures diesel engine is started.