EXHAUST GAS TURBO-CHARGING

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Advantages of Exhaust Turbo-charging:

1. As no separate power is required to run the turbine, a substantial increase in power output for the same engine size and piston speed is obtained.
2. Substantial reduction in engine dimensions and weight for any stated horsepower.
3. Appreciable reduction in specific fuel consumption at all loads.
4. Less weight/power ratio.
5. Increased reliability.
6. Reduced maintenance.

Exhaust Turbo-charging:

Types:

In general, the manner in which the energy of exhaust gases is utilised to drive the turbocharger may be ascribed to:

1. Pulse system of operation.
2. Constant pressure system of operation

Pulse operation:

This makes use of the higher pressures and temperatures of the exhaust gas during the blow down period. As soon as the exhaust valve or port is opened (rapidly) high-pressure energy is converted into kinetic energy to create a pressure wave or pulse in the exhaust pipe leading to the turbocharger. Hence the name ‘pulse turbo-charging’.    The exhaust pipes lead from cylinders to the Turbocharger individually. For pulse operation to be efficient following conditions should be fulfilled.

1. Exhaust leads should be short and without unnecessary bends so that the volume is kept to minimum, and losses in pipe are minimum.
2. Exhaust ducting leading to the same turbocharger from different units are to be arranged such that there is no interference of the pulses of these cylinders, which will otherwise affect scavenging.
3. The period between discharge of successive cylinders into a common manifold should be more than 240* for a four stroke engine and 120* for a two stroke engine. It gives at least 10% better utilisation of available energy add interference is avoided, if a group of 3 (or multiples) cylinders is led to one Turbocharger.

Diagrammatic arrangement of ‘Buchi System’ of supercharging for four stroke slow speed engine.

Advantages:

1. Due to the small volume of the exhaust ducting and direct leading of exhaust to turbine inlet, the pulse system is highly responsive to changing engine conditions giving good performance at all speeds.
2. Theoretically, pulse system does not require any form of scavenge assistance at low speeds or when starting. However, in practice, the use of an auxiliary blower or some other means of assistance is employed to ensure optimum conditions and good acceleration from rest.
3. Sudden drop in manifold pressure, which follows each successive exhaust pulse, results in greater pressure difference causing better scavenging.

Constant Pressure Operation:

In this system the exhaust gases are discharged from the engine into a common manifold or receiver where the pulse energy is largely dissipated. Gas supply to turbine is almost at constant pressure and is steady rather than intermittent.  So optimum design conditions prevail.

For higher powers, this system becomes more efficient as the pulse energy loss in large volume receiver is of less consequence, and the air delivery rate becomes more with slight reduction in fuel rate.

Advantages:

1. Easily adaptable for any normally aspirated engine, as multi pipe exhaust arrangement is not needed.
2. Number of Turbochargers required as compared to pulse system can be reduced.
3. Higher Turbine efficiencies.
4. Lack of restriction, within reasonable limits, on exhaust pipe length permits greater flexibility in positioning the turbo-blower relative to the engine.
5. As receiver volume is larger exhaust blow down is rapid.  So exhaust can be opened a slightly later, giving more expansive work. So it gives better fuel efficiency. 
   Disadvantage:
   1. Poor performance at part loads, as energy is not sufficient to run turbine efficiently.
   2. Systems are insensitive to engine operating conditions due to large volume of exhaust manifold, so, resultant delay in turbo-blower acceleration and deceleration results in poor combustion during transition periods.
   3. To offset part load inefficiency, some scavenge assistance is essential.
   
   
   Generally, 4-stroke engines use pulse system and two stroke engines use pulse and constant pressure systems.