Turbocharging of a Two-Stroke Main Engine

Turbocharging is essential in a two-stroke engine because there is no dedicated suction stroke to draw in fresh air. Instead, air must be supplied under pressure to both scavenge exhaust gases and provide sufficient oxygen for combustion.

The most widely used arrangement is the constant pressure turbocharging system.

1. Constant Pressure Turbocharging System

In this system, exhaust gases from all cylinders are led into a large exhaust gas receiver. This receiver smooths out pressure fluctuations from individual cylinders and supplies a steady flow of exhaust gas to the turbine.

Arrangement (for sketch reference)

Working Principle

1. Exhaust Phase: High-temperature exhaust gases from each cylinder enter the exhaust manifold, where pulsations are dampened.
2. Energy Conversion (Turbine): The steady exhaust gas flow drives the turbine wheel. The turbine is mounted on a common shaft with the compressor.
3. Air Compression: The compressor draws in ambient air and compresses it, increasing its pressure and temperature.
4. Cooling of Air: The compressed air passes through a charge air cooler (intercooler), reducing its temperature and increasing air density.
5. Scavenging Process: The cooled, dense air enters the scavenge air receiver. When the piston uncovers the scavenge ports, this air:
   • Forces out remaining exhaust gases
   • Fills the cylinder with fresh air for the next cycle

2. Main Components of a Turbocharger

A modern turbocharger consists of the following main parts:

• Turbine Side: Made of heat-resistant alloys; extracts energy from exhaust gases.
• Compressor Side: Usually an aluminium alloy impeller; draws in and compresses fresh air.
• Bearing Assembly: Supports the high-speed rotor (often 15,000 RPM or more); lubricated either by engine lube oil or a dedicated system.
• Air Filter/Silencer: Prevents foreign particles from entering and reduces intake noise.

3. Routine Attention and Maintenance

Since turbochargers operate at very high speeds and temperatures, regular monitoring and maintenance are essential.

• Monitor turbocharger RPM, exhaust temperatures, and boost pressure
• Check lubricating oil pressure, level, and condition
• Observe for abnormal noise, vibration, or surging, which may indicate fouling or imbalance

• Clean or replace air intake filters to ensure proper airflow
• Drain charge air cooler and scavenge receiver to remove water and oil deposits

• Compressor Washing (Water Washing): Fresh water is injected at suitable low load to remove dirt, salt, and oil deposits from the compressor
• Turbine Cleaning (Dry Washing): Soft materials such as crushed walnut shells are used to remove carbon deposits from turbine blades

• Measure axial and radial clearances (K, L, M values)
• Replace bearings at specified running hours
• Conduct non-destructive testing (e.g., dye penetrant test) on turbine blades to detect cracks