Scavenging and Turbocharging

Scavenging and turbocharging

Constant Pressure System :

In the constant-pressure system, exhaust gases from all cylinders pass into a common large diameter exhaust manifold.

This is of sufficient capacity so that the intermittent exhaust from the different cylinders does not cause pressure fluctuations.

Because of the fairly constant gas pressure and temperature at the turbine inlet; it operates at about its optimum efficiency.

This system is most suitable for high output engines, and there is no need to group the cylinder exhausts into multiples of three.

A major disadvantage of the constant-pressure system is that when running at reduced speed and especially when starting up, the pressure energy available at the turbine inlet is insufficient to drive it fast enough to supply the quantity of air at the scavenging pressure necessary for efficient scavenging and combustion.

Therefore, it is necessary to be made an additional source of air compression, such as reciprocating pump or turbo blower.

Turbocharger Arrangement in Constant Pressure System:

• No exhaust grouping required and pipe distribution is simple.

• Exhaust gases enter into large common manifold and then to turbine

• Firing order not considered

Advantages:

1. Good performance in high load.

2. More suitable for high output engine.

3. There is no need to group the cylinders exhaust into multiple of three.

4. High turbine efficiency due to steady flow of exhaust.

5. Reduction in SFOC (Specific Fuel Oil Consumption)

Disadvantages:

1. When running at reduced speed and starting up low available energy at turbine. Thus it supplies inadequately air quantity of the scavenge pressure necessary for efficient scavenging and combustion.

2. It requires Auxiliary Blowers.

3. Poor response in changing load.

Turbocharger Surging:

• It is the aerodynamic phenomena which produces a back flow of air from discharge to suction side (impeller and diffuser) when the discharge volute pressure exceeds the pressure build up in the suction side and it causes heavy noise and vibration of turbocharger.