Answer MEP Question 29

Q. Using sketches explain the difference between Pulse and constant pressure turbocharger systems.

A. In the event of a Pulse turbocharger becoming inoperative due to mechanical breakdown explain the modifications required to allow the engine to operate safely.

B. State the instruction you as second engineer would issue regarding the additional engine monitoring requirements following the steps taken in (a).

Pulse vs constant pressure turbocharger systems

A turbocharging increases power of an engine as more air is available to burn the fuel. It enhances the thermodynamic efficiency thus lower fuel consumption for the same power. For the same power, the engine can be made of a more compact design with savings in engine room space and weight of the engine. the cost of engine per horsepower is less.

Constant pressure turbocharging system:

    In the constant pressure, the turbocharging system shown above each cylinder exhaust gas is piped to a common exhaust gas manifold. From the manifold, a single pipe is led to the turbine. One turbocharger is normally sufficient but for a higher number of cylinders, two turbochargers may be provided.

Advantages

a. Later opening of exhaust, more expansive use of combustion gas

b. Smooth entry of gas, steady load

c. High-efficiency

d. Simpler pipe connections, more flexibility in the location of turbocharger

Disadvantages

a. Poor response on starting

b. Poor acceleration

c. Poor performance at low loads

d. Requires auxiliary blower at low loads

Pulse turbocharging system:

In the pulse turbocharging system shown above each cylinder exhaust gas is piped individually to the turbine. In a 2 stroke 6·cylinder engine, normally 2 turbochargers would be fitted. This is in order to avoid higher pressure exhaust gas from a cylinder blowing back and interfering with the scavenging process of another cylinder. Ideally, there should be a gap of at least 120 deg between the consecutive exhaust openings. For a higher number of cylinders, more turbochargers may be necessary.

Pulse turbocharging was common at earlier times when the combustion pressures and power of the diesel engine were low. In modern high power engines with mean effective pressures of 18 bar or more, the constant pressure system is much more efficient and common.

Advantages:

a. Higher energy of gas at turbine entry

b. Quick response on starting

c. Good acceleration

d. Good performance at part load

e. No need for an auxiliary blower. although often provided

Disadvantages

a. Need more number of turbochargers

b. Complex pipe connections

c. Owing to varying pressure, cannot be optimized for best efficiency

d. At higher mean effective pressures less quantity of air delivered.

A. In the event of a Pulse turbocharger becoming inoperative due to mechanical breakdown explain the modifications required to allow the engine to operate safely.
It is a common practice to install a Pulse system with a four stroke engine. In the event of failure of turbocharger the engine can be run as a naturally aspirated engine with suitable attaintion to be paid to operating parameters. The tubocharger shaft to be removed and blank plate to be fitted. Compensator between the compressor outlet and scvange inlet to be removed to reduce to air inlet resistance. An external air blower may be used at the scvange inlet.

B. State the instruction you as second engineer would issue regarding the additional engine monitoring requirements following the steps taken in (a).
The watchkeeper must closely monitor:
The operating temperature of the engine.
The exhasut gas condition, black smoke, and temperature of each unit.
Check of any abnormal noise and Vibration.
Check if the engine is within the operating limit recommended by the maufecturer for such operation.
Check for jacket cooling temperure and lube oil supply is proeprly maintained.