(a) Purpose of Auxiliary Blower

In a 2-stroke turbocharged marine diesel engine, the auxiliary blower is essential for supplying scavenge air when the turbocharger cannot produce sufficient air pressure. Its functions include:

1. Low-Load Operation: At low engine speeds (e.g., maneuvering), exhaust gas energy is insufficient to drive the turbocharger efficiently. The auxiliary blower provides the required scavenge air pressure for proper scavenging and combustion.
2. Engine Starting: Before starting, it supplies fresh air to purge exhaust gases from the cylinders, ensuring clean and smooth ignition when fuel is introduced.
3. Improved Cooling: The additional air flow helps cool engine components such as pistons, liners, and cylinder heads.
4. Uniform Cylinder Conditions: Ensures even distribution of air across all cylinders for balanced combustion.
5. Emergency Backup: Provides limited air supply in case of turbocharger malfunction, allowing controlled engine operation.

(b) Safeties Provided for the Scavenge Air Receiver

The scavenge air receiver stores pressurized air for delivery to the cylinders, so several safety devices are installed:

1. Pressure Relief Valves (PRV): Protect the receiver from over-pressurization by releasing excess air.
2. Water/Oil Level Alarms or Switches: Detect water or oil accumulation in the receiver to prevent carry-over into the engine, which can cause damage or scavenge fires.
3. High Temperature Alarms: Monitor scavenge air temperature and warn of issues such as cooler malfunction, fouling, or abnormal compression.
4. Drain Cocks (Manual or Automatic): Remove condensate (water/oil) from the receiver to prevent contamination and fire hazards.

(c) Why a Scavenge Air Cooler Is Required

The scavenge air cooler is placed after the turbocharger to cool the compressed air before entering the scavenge receiver. Its purposes are:

1. Increase Air Density: Cooling compressed air raises its density, allowing more oxygen mass to enter the cylinder.
2. Increase Power and Efficiency: Denser air allows more fuel to be burnt efficiently, improving power output and reducing specific fuel consumption.
3. Reduce Engine Operating Temperatures: Lowers cylinder, piston crown, and exhaust gas temperatures, protecting components from thermal stress.
4. Improve Scavenging Efficiency: Cooler, denser air enhances the purging of exhaust gases from the cylinder, ensuring cleaner combustion.
5. Control NOx Emissions: Cooler charge air reduces peak combustion temperatures, thereby reducing NOx formation.
6. Maintain Engine Reliability: Ensures components operate within safe temperature limits, improving longevity and reducing risk of failure.