Marine Turbochargers: Aerodynamics, Bearings, Washing & Surge Mitigation

1. Thermodynamics of Exhaust Gas Turbocharging & Scavenging Principles

In two-stroke crosshead marine diesel engines, there are no dedicated suction and exhaust piston strokes. The entire process of expelling high-temperature combustion products and charging the cylinder with fresh, dense air must take place during a brief angular window of approximately 110° to 130° of crank rotation while the piston traverses the Bottom Dead Centre (BDC) zone. This gas exchange process is termed scavenging.

The efficiency of scavenging directly dictates engine thermal efficiency, fuel consumption, and component thermal loading:

Scavenging Air Parameters & Definitions

Pulse vs. Constant-Pressure Turbocharging Architectures

2. Turbocharger Mechanical Architecture & High-Temperature Metallurgy

Modern marine turbochargers (such as the MAN TCA/TCR, ABB A100/TPL, and Mitsubishi MET series) operate at rotational speeds between 8,000 and 22,000 RPM with turbine inlet exhaust temperatures exceeding 450°C to 550°C. The rotor must withstand severe centrifugal hoop stresses, aerodynamic flutter, and cyclic thermal shock.

Primary Mechanical Subassemblies

• Turbine Casing & Nozzle Ring: Manufactured from nodular cast iron or cast alloy steel. The stationary nozzle ring comprises precision aerodynamic aerofoil vanes (cast from heat-resistant austenitic chrome-nickel steel). The nozzle ring converts exhaust gas pressure energy into high-velocity kinetic jets directed onto the turbine rotor at optimum impingement angles (approx. 18°–25°).
• Turbine Rotor & Blading Metallurgy: The turbine wheel disc is forged from heat-resisting molybdenum-chromium-nickel steel. Individual turbine blades are precision-forged from nickel-based superalloys (such as Nimonic 80A or vacuum-melted Inconel 713C), exhibiting superior high-temperature creep rupture strength and hot corrosion resistance against vanadium and sulfur pentoxide attack.
• Blade Fixing & Vibration Damping: Blades are secured into the rotor disc rim via fir-tree serrated roots, which distribute immense centrifugal tensile forces evenly across multiple shear shoulders. A circular damping lacing wire (or shroud band) passes through precision holes near the blade tips to dampen torsional and bending resonance vibrations excited by nozzle passage frequencies.
• Centrifugal Compressor Impeller: Forged from high-strength aerospace aluminum alloy (Al-Cu-Mg) or titanium alloy (for extreme pressure ratios > 4.5:1). Features backward-curved 3D aerofoil impellers and splitter blades that accelerate intake air radially outward to supersonic velocities.
• Vaned / Vaneless Diffuser Ring: Surrounds the compressor impeller discharge. As high-velocity air passes through the diverging passages of the diffuser, kinetic velocity is smoothly converted into static pressure energy (governed by Bernoulli's theorem) before collecting in the volute scroll.

3. Bearing Tribology, Resilient Damping & Labyrinth Gland Seals

Because turbocharger rotors spin at peripheral velocities exceeding 400 to 550 m/s, the shaft bearing and sealing arrangement is the most critical mechanical reliability subsystem.

Bearing Design: Sleeve Bushings vs. Ceramic Hybrid Ball Bearings

Labyrinth Gland Seals & Pressurized Barrier Air

To prevent dirty exhaust gas and abrasive soot from blowing along the shaft into the lubricating oil spaces, and simultaneously prevent lube oil from being sucked into the compressor intake vacuum, turbochargers utilize multi-fin labyrinth gland seals backed by an active air seal:

• Labyrinth Sealing Mechanism: A series of sharp brass or stainless steel annular fins separated by expansion pockets. High-velocity leakage gas undergoes throttling across each fin tip followed by turbulent kinetic dissipation in the pocket, dropping gas pressure exponentially across the gland.
• Pressurized Barrier Air Tapping: Clean, pressurized air is bled from the compressor impeller discharge scroll and piped to an intermediate annular chamber in the labyrinth gland. Because barrier air pressure is maintained higher than both the turbine exhaust casing backpressure and the bearing oil drain pressure, air continuously blows in two directions:
  • Outward into the turbine casing, preventing exhaust gas soot and acidic sulfuric vapors from reaching the bearings.
  • Inward toward the oil deflector flinger, preventing lube oil droplets from escaping along the shaft.

4. Compressor Aerodynamics, Choke & Surging Physics

The most violent and destructive aerodynamic instability in a marine turbocharger is surging. Surging is not a mechanical gear collision; it is a complete aerodynamic breakdown and cyclic reversal of airflow through the compressor.

The Aerodynamics of Surging

A centrifugal compressor operates by accelerating air radially through impeller vanes and decelerating it through a diffuser to build pressure. Stable compressor operation is governed by its performance map (Pressure Ratio vs Mass Flow Rate):

The Surge Line & Engine Operating Discipline

Under normal conditions following the Propeller Law (engine operating line), the compressor runs inside high-efficiency islands (80%–85%). However, if system backpressure rises or mass flow is throttled while turbocharger RPM remains elevated, the operating point shifts leftward across the Surge Line:

1. Aerodynamic Stall: The diffuser vanes stall completely, losing their ability to support the high discharge pressure.
2. Flow Reversal (The Back-Fire): The pressurized air stored in the scavenge air receiver and charge air cooler (at 2.0 to 3.5 bar) suddenly rushes backward through the stalled compressor impeller toward the air intake silencer.
3. Pressure Collapse & Recovery: The reverse rush of air empties the scavenge receiver, dropping receiver backpressure instantly. With backpressure relieved, the spinning impeller re-establishes forward flow and begins rebuilding receiver pressure.
4. Cyclic Oscillation: As receiver pressure climbs back up, if the underlying downstream resistance remains high, the compressor stalls again. This creates a violent, self-sustaining oscillation occurring at frequencies between 1 Hz and 3 Hz.

Diagnostic Symptoms of Turbocharger Surging

• Loud Rhythmic Acoustic Thuds: Deep, explosive 'barking' or hammering thuds (like cannon shots) emanating from the turbocharger intake filter as air repeatedly slams backward.
• Scavenge Pressure Fluctuations: The scavenge air pressure gauge needle sweeps violently back and forth across 0.5 to 1.5 bar.
• Exhaust Gas Temperature Spikes: Because cylinder scavenging air is interrupted during each backflow pulse, in-cylinder combustion degrades, sending raw unburnt fuel into the exhaust manifold and causing exhaust temperatures to skyrocket.
• Mechanical Destruction Risk: Sustained surging subjects the compressor wheel to extreme alternating axial thrust reversals, destroying thrust bearings and causing compressor impeller blade tip fracture.

Root Causes & Operational Corrective Actions

5. Operational Washing Protocols: Wet vs Dry Cleaning

Burning Heavy Fuel Oil (HFO) or Very Low Sulfur Fuel Oil (VLSFO) inevitably coats turbine blading and nozzle rings with hard deposits of sodium-vanadium complexes, unburnt carbon, and sulfur ash. Simultaneously, engine room airborne oily dust coats compressor impeller vanes. Accumulation of only 0.5 mm of soot reduces turbocharger efficiency by 5% to 10%, directly causing exhaust temperature rise, loss of scavenge pressure, and fuel penalty.

1. Turbine Wet Washing Procedure (Water Washing)

Turbine water washing relies on thermal softening and mechanical impingement of high-velocity water droplets to break brittle carbon scale off turbine blades. Because cast iron casings and superalloy blades will crack under thermal shock if cold water strikes them at full load, wet washing requires strict speed reduction:

1. Reduce Engine Load to Idling: Lower main engine speed to the lowest possible load point (Dead Slow Ahead / Idling, typically ≤ 10%–15% MCR).
2. Wait for Exhaust Cooldown: Monitor turbine inlet exhaust temperature until it falls below 200°C to 250°C (manufacturer specification) to eliminate thermal shock risks.
3. Open Turbine Casing Drains: Physically open the drain cock on the turbine gas outlet casing and verify clear drainage. Crucial: If the drain is choked, water will accumulate in the turbine casing and hydro-lock the rotor wheel, destroying the blades!
4. Connect Water Supply & Adjust Pressure: Connect fresh water (ambient or warm water at 40°C–60°C). Adjust water supply pressure to 1.5 to 2.0 bar via the pressure gauge.
5. Inject Water: Turn the 3-way cock to inject water for 10 to 20 minutes. Observe the casing drain discharge: it will initial exit as thick, jet-black sooty water, gradually turning clear as blading deposits are dissolved.
6. Shut Off Water & Dry Out: Close the water supply cock. Leave the casing drain open. Run the engine at approx. 20%–25% load for at least 10 to 15 minutes to thoroughly evaporate all moisture and prevent acidic sulfuric corrosion.
7. Close Drains & Return to Service: Close the casing drain and gradually increase engine speed to normal service load while monitoring for any abnormal vibration or rubbing noise.

2. Turbine Dry Washing Procedure (Organic Granules)

3. Compressor Air-Side Washing

Compressor impeller blades and diffuser passages are cleaned using fresh water injected directly into the air intake while the engine operates at normal service load (full RPM):

• Clean fresh water (typically 0.5 to 1.5 litres, without chemical detergents) is poured into a pressurized container mounted on the silencer casing.
• Compressed air (2 to 3 bar) or atmospheric depression injects the water slug in 4 to 10 seconds.
• The water droplets strike the spinning impeller at 15,000+ RPM; mechanical impact scrubs oily dust films off the blade leading edges and diffuser vanes.
• The water droplets atomize instantly into aerosol mist and pass harmlessly through the charge air cooler into the water mist catcher drains.

6. Auxiliary Blower Integration & Scavenge Receiver Dynamics

Because constant-pressure turbochargers exhibit lower exhaust enthalpy at low loads, the turbocharger cannot deliver sufficient scavenge air pressure to support clean combustion when the engine operates below 35% to 40% MCR.

Auxiliary Blower Architecture & Automatic Control

• Electrically Driven Centrifugal Blowers: Modern two-stroke engines are fitted with two redundant, electric motor-driven auxiliary blowers mounted on the scavenge air receiver.
• Automatic Pressure Switch Control: Pressure transmitters monitor scavenge receiver pressure. During engine starting and low-speed maneuvering:
  • When scavenge pressure falls below a pre-set threshold (typically P_scav ≤ 0.7 to 0.8 bar), the auxiliary blowers start automatically in consecutive sequence (staggered by 5 seconds to prevent generator switchboard blackout).
  • When engine load accelerates above 40% MCR and turbocharger boost exceeds the upper limit (typically P_scav ≥ 0.9 to 1.1 bar), the auxiliary blowers automatically shut down.
• Non-Return Flap Valves: Installed in the scavenge receiver partition. When the auxiliary blowers run, their suction pulls air from the cooler outlet, while the non-return flaps close to prevent air recirculation. Once the turbocharger delivers higher pressure than the blowers, the flap valves are forced wide open by forward airflow, allowing unrestricted gas delivery.
• Water Mist Catcher (Demister): Located immediately downstream of the charge air cooler. As humid ambient air is cooled below its dew point, large quantities of water vapor condense into liquid droplets. The demister's cyclonic chevron vanes separate liquid droplets, draining water out of the receiver and preventing cold corrosion acid attack on cylinder liner oil films.