44.Explain types of Turbocharging

Turbocharging is one kind of supercharging by using exhaust gas turbocharger. In which the energy in the exhaust gas expelled from the engine cylinder is utilized in driven in gas turbine, which is connected to a centrifugal air blower and air is supplied to scavenge air trunk.

Constant Pressure System Turbocharging

• Exhaust gas from all cylinders into a common large manifold where pulse energy is largely dissipated. 
• The gas flow will steady rather than intermittent and at a constant pressure at turbine inlet.

Turbocharger Arrangement in Constant Pressure System

• No exhaust grouping
• Exhaust gases enter into large common manifold and then to turbine
• Firing order not considered

Advantages and Disadvantages for Constant Pressure System

Advantages

1. Good performance in high load (Efficient when Bmep is above 8 bar)
2. More suitable for high output engine.
3. There is no need to group the cylinders exhaust into multiple of three. (Simple piping system)
4. No exhaust grouping 
5. High turbine efficiency due to steady flow of exhaust.
6. The work transfer at the turbine wheel is smooth.
7. Reduction in SFOC (Specific Fuel Oil Consumption) of 5% – 7%

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 require scavenge assistant (Auxiliary Blowers).
3. Poor response in changing load.

Pulse System of Turbocharging

• Makes full use of the higher pressure and temperature of the exhaust gas during the blow down period
• While rapidly opening the exhaust valves, exhaust gas leave the cylinder at high velocity as pressure energy is converted into kinetic energy to create the pressure wave or pulse in exhaust
• These pressure waves or pulses are lead directly to the turbocharger
• Exhaust pipe, so constructed in small diameter, is quickly pressurized and boosted up to form pressure pulse or wave
• Pressure waves reach to turbine nozzles and further expansion takes place.

Turbocharger Arrangement in Pulse System

• Interference exists between exhausting and scavenging among cylinders 
• To prevent this, cylinders are grouped relatively with connections to two or more exhaust pipes
• Pipes are arranged, in small diameter to boost up pressure pulse and in short, straight length to prevent energy loss 
• Number of exhaust branch depends upon firing order, number of cylinders and turbocharger design

Advantages and Disadvantages of Pulse System

Advantages

1. At low load and low speed it is more efficient (Still efficient when Bmep is < 8 bar)
2. No need assistant of scavenge pump and blower at any load change.
3. It is highly response to change engine condition giving good performance of all speed of engines.  
4. High available energy at turbine 
5. Good turbocharger acceleration

Disadvantages

1. The exhaust grouping is complicated.
2. Different sizes of exhaust pipes are needed for spare.
3. High pressure exhaust from one cylinder would pass back into another cylinder during the low pressure scavenging period thus adversely effecting the combustion efficiency.