To improve the power-to-weight ratio of a marine diesel engine, the engine must produce more power without greatly increasing its size and weight. This is achieved by increasing the Mean Effective Pressure (MEP), which is the average pressure acting on the piston during the power stroke.

A higher MEP can only be obtained if a larger quantity of fuel is burnt efficiently inside the cylinder. For complete combustion of this additional fuel, more air must be supplied to the engine. This is the reason why the turbocharger compression ratio becomes very important.

The turbocharger compressor increases the pressure and density of the scavenge air supplied to the cylinders. When the compression ratio of the turbocharger is increased:

• More air enters the cylinder.
• Air density increases.
• More fuel can be injected and burnt efficiently.
• Combustion pressure increases.
• Engine power and MEP increase.

However, there is a practical limit to the pressure ratio that can be achieved by a single-stage turbocharger. At very high compression ratios:

• Compressor efficiency reduces.
• Air temperature rises excessively due to heat of compression.
• Hotter air becomes less dense.
• Thermal loading on engine components increases.

To overcome these limitations, two-stage turbocharging is introduced.

In a two-stage turbocharging system, air is compressed in two separate stages instead of one. After the first stage of compression, the air passes through an intercooler where the heat of compression is removed by cooling water.

Cooling the compressed air provides several advantages:

• Air temperature reduces close to ambient temperature.
• Air density increases.
• Less work is required in the second stage of compression.
• Overall compression efficiency improves.

The cooled dense air then enters the second-stage compressor, where it is compressed further to a much higher pressure than possible with a conventional single-stage turbocharger.

Advantages of two-stage turbocharging:

• Higher scavenge air pressure.
• Increased Mean Effective Pressure.
• Greater engine power output.
• Improved thermal efficiency.
• Lower specific fuel consumption.
• Reduced exhaust emissions.

Since intercooling reduces the temperature rise during compression, the compression process approaches nearly isothermal compression, which reduces the power required for compression.

Variable Turbine Area (VTA) or Variable Geometry Turbochargers (VGT) are designed to provide efficient turbocharger operation over the full engine load range.

In conventional turbochargers, the turbine nozzle area remains fixed. Therefore, at low engine loads, exhaust gas velocity becomes low and the turbine speed reduces, resulting in poor scavenge air delivery.

To overcome this problem, VTA turbochargers use adjustable nozzle vanes to vary the turbine inlet area according to engine load.

(i) Which Area is Varied

The area varied is the nozzle vane throat area at the turbine inlet.

Instead of a fixed nozzle ring, the turbocharger is fitted with movable guide vanes arranged around the turbine wheel. By changing the angle or pitch of these vanes, the effective flow area through which exhaust gas enters the turbine is altered.

(ii) Why the Area is Varied

The turbine area is varied to control the velocity and direction of exhaust gases striking the turbine blades.

At low engine load:

• Exhaust gas quantity and pressure are low.
• The nozzle area is reduced.
• Exhaust gas velocity increases.
• Turbine speed increases.
• Sufficient scavenge air is supplied even at low load.

At high engine load:

• Exhaust gas quantity is already high.
• The nozzle area is increased.
• Excessive turbine speed and back pressure are avoided.
• Turbocharger efficiency is maintained.

By continuously varying the turbine area:

• Air supply matches fuel injection quantity.
• Combustion improves.
• Turbocharger response becomes faster.
• Fuel consumption reduces.
• Smoke and exhaust emissions decrease.

(iii) How the Area is Varied

The nozzle vanes are connected through levers to an actuating ring surrounding the turbine casing.

This actuating ring is operated by an electric or hydraulic actuator fitted with a reduction gear arrangement.

An electronic control unit continuously receives signals such as:

• Charge air pressure,
• Engine load,
• Exhaust gas temperature before turbine,
• Exhaust gas temperature after turbine.

Based on these operating conditions, the control system automatically adjusts the vane position to obtain the optimum turbine area for efficient turbocharger operation at all engine loads.