Turbocharger Construction and Design for Marine Engines
1. Turbocharger Construction (Axial Flow Design)
A turbocharger in a marine engine consists of two primary components mounted on the same shaft: a single-stage impulse reaction turbine and a centrifugal compressor. Here’s a breakdown of its components and operation:
Turbine Construction
- Gas Inlet and Outlet Casings: The turbine features a gas inlet casing with a nozzle ring and an exhaust gas outlet casing.
- Turbine Wheel and Blades: The turbine wheel is forged integrally with the shaft, with side-entry blades fitted to the wheel. Modern designs typically do not require water cooling for the turbine casing, unlike older models.
Compressor Construction
- Volute Casing: Houses key components like the impeller, inducer, and diffuser. The inducer ensures a smooth air flow into the impeller, where the impeller uses centrifugal force to accelerate the air outward.
- Diffuser: Located at the compressor’s discharge end, the diffuser converts the kinetic energy of the air into pressure energy, which is then further enhanced as it enters the volute-shaped casing.
Bearings and Lubrication
- Bearing Types: Turbochargers typically use a combination of ball, roller, or journal sleeve bearings. These bearings are housed in resilient casings with laminar springs that provide both axial and radial damping. This design prevents bearing chatter or flutter when the engine stops.
- Bearing Lubrication: The system may be integral or separate and plays a crucial role in heat transfer.
Types of Bearings
- Roller Bearings: Offer less friction and more precise alignment but are more expensive and prone to brinelling (localized surface indentation). They require higher-grade lubrication.
- Sleeve Bearings: While they perform better at higher temperatures, they generate more friction at low loads.
Seals and Blade Components
- Labyrinth Seals: These seals prevent exhaust gases from leaking into the air side and the bearing housing, with air from the air side used to cool and seal the shaft.
- Binding Wire and Fir-Tree Blade Root: Binding wire segments pass through the turbine blades, working on centrifugal force to dampen vibrations. The fir-tree blade root design evenly distributes stress and provides easier blade replacement.
2. Materials Used in Turbocharger Construction
- Turbine Wheel, Nozzle Ring, Rotor Shaft, and Blades:
Made from Nimonic 90 (Nickel-Chrome alloy) containing 75% Nickel, 18% Cobalt, and smaller amounts of Titanium, Aluminum, and Chromium. These materials provide impact resistance, strength, and thermal stability, ideal for continuous operation at high temperatures (up to 650°C). - Turbine Casing:
Constructed from cast iron, often with corrosion-preventive plastic coatings in older water-cooled turbochargers. - Compressor Components (Impeller, Volute Casing, Diffuser, Inducer):
These components are made from aluminum alloy for a lightweight and smooth surface finish.
3. Uncooled Turbochargers: A Modern Marine Engine Innovation
Modern marine engines now utilize uncooled turbochargers due to their superior performance in maintaining lower exhaust gas temperatures. Here’s why uncooled systems are more efficient:
- Heat Recovery: Instead of losing heat through water-cooled casings, the heat energy is now directed to the exhaust gas economizer, improving the plant’s overall thermal efficiency.
- Higher Exhaust Gas Temperatures: The exhaust gas temperatures at Point B (in uncooled systems) are much higher than those at Point A (in water-cooled systems). This results in more heat being available at the economizer inlet.
- Corrosion Reduction: Water-cooled systems often suffered from corrosion due to sulfuric acid formation at low loads. By eliminating the need for water cooling, uncooled turbochargers minimize corrosion risk and further enhance the engine’s longevity.