Background
Marine turbocharging emerged in the 1920s with the work of Buchi, became routine on larger engines through the mid-20th century, and reached high pressure ratios (over 4:1, with two-stage arrangements approaching 10:1) in the late 20th and early 21st centuries. Modern turbochargers operate at rotational speeds of 6,000 to 30,000 rpm depending on size, with peripheral velocities approaching the speed of sound at the compressor wheel tip. The mechanical and aerodynamic engineering required is at the forefront of marine machinery design.
This article describes turbocharger fundamentals, the principal manufacturers (MAN, ABB, MET, Mitsubishi), the differences between two-stroke and four-stroke engine turbocharging, turbo matching to engine characteristics, the scavenge air system, the charge air cooler, the operational issues of surge and choke and turbo lag, the recent development of variable geometry turbines, water-side turbo washing, and the bearing arrangements that support the high-speed rotor.
Turbocharger Fundamentals: Axial vs Radial
A turbocharger has two principal stages: a turbine driven by the engine exhaust gas, and a compressor that compresses the engine intake air. The turbine and compressor share a common shaft, with the turbine power directly driving the compressor. There is no mechanical or electrical connection to the engine other than the gas paths.
The turbine can be of two configurations:
Axial turbines have flow entering and leaving the turbine in the axial direction (parallel to the shaft). The blades are arrayed radially around the disc and the flow expands through one or more stages of stator-rotor blade pairs. Axial turbines are dominant on large turbochargers (typical above 5 to 6 megawatts engine power) because they handle the large gas flow rates more efficiently than radial designs.
Radial turbines have flow entering radially and leaving axially. They use a single-stage configuration with the flow expanding from the periphery to the centre of a radial wheel. Radial turbines are dominant on smaller turbochargers (engine power below about 5 megawatts) because of their simpler manufacture and adequate efficiency at small scale.
The compressor is almost always radial (centrifugal). Air enters axially at the eye of the wheel and is accelerated centrifugally through the impeller blades, then decelerated in the diffuser to convert kinetic energy into pressure. Two-stage compressor arrangements (used in some high-pressure-ratio applications) place two centrifugal stages in series.
The choice of axial versus radial turbine is therefore dictated by the engine size, with the cross-over around 5 to 6 MW. The compressor is uniformly radial across all marine applications.
MAN, ABB, MET, and Mitsubishi Turbochargers
Four manufacturers dominate the marine turbocharger market:
MAN Energy Solutions produces the TCA, TCR, and TCT series turbochargers in-house, used on its own engines and supplied to other engine builders. The TCA is a large axial-turbine unit for slow-speed two-stroke engines; the TCR is a radial-turbine unit for medium-speed engines; the TCT is a small radial unit for smaller engines.
ABB Turbo Systems (formerly BBC) is a long-established supplier with broad coverage of medium- and slow-speed engine applications. The A-series and TPL series have been industry standards for decades. ABB’s A100 series is the modern axial-turbine product line for slow-speed engines, with multiple frame sizes covering the full range of engine power.
MET (Mitsubishi Heavy Industries Marine Engineering) produces the MET series turbochargers, with MET-MA and MET-MB variants for axial-turbine slow-speed applications and MET-SE for medium-speed radial. MET turbochargers are factory-fitted on Mitsubishi-built engines and are available for retrofit on engines from other builders.
Mitsubishi Heavy Industries (separate from MET, which is a subsidiary line) also produces marine turbochargers under various designations.
In practice, the engine builder specifies the turbocharger model at engine design, and the matching of turbocharger to engine is integrated into the engine type test under IACS UR M51. Operators rarely change turbocharger model in service, although replacement units of the same model are routinely fitted at major overhauls.
Two-Stroke vs Four-Stroke Turbocharging
The turbocharging arrangement differs significantly between two-stroke and four-stroke engines:
Two-stroke turbocharging is characterised by the absence of a separate intake stroke. The gas exchange between cylinders is performed by ports in the cylinder liner uncovered by the piston, with exhaust valves in the cylinder head completing the exhaust path. The turbocharger must supply scavenge air at a pressure sufficient to push out the exhaust gases and fill the cylinder for the next compression stroke. Modern slow-speed two-stroke engines run with scavenge air pressures of 3 to 4 bar absolute (roughly 2 to 3 bar gauge).
The exhaust gas path on a two-stroke engine is a continuous flow from the cylinder via the exhaust manifold (a “constant pressure” manifold buffering the exhaust pulses) to the turbine. The relatively constant pressure manifold matches well with the constant-flow characteristic of axial turbines.
Two-stroke engines often have electrically driven auxiliary blowers that supply scavenge air at low engine speed (when turbocharger output is insufficient), starting up automatically below a threshold engine speed.
Four-stroke turbocharging has a discrete intake stroke during which the cylinder is filled. The turbocharger provides charge air to the intake manifold, with intake valves on each cylinder controlling the actual flow. Pressure ratios are often higher than on two-stroke engines (because the intake valve restriction adds to the pressure drop the turbocharger must overcome).
The exhaust gas path on a four-stroke engine can be either constant-pressure (with a manifold buffering the pulses) or pulse-converter (using the pressure pulses themselves to drive the turbine). Pulse-converter systems are favoured at lower engine power where the small turbine flow rate makes constant-pressure operation inefficient.
Turbo Matching
Turbo matching is the engineering exercise of selecting the turbocharger to optimise engine performance across the operating envelope. The compressor map (showing pressure ratio versus mass flow at various speeds) and the turbine map are matched to the engine’s air consumption curve.
The matching point at maximum continuous rating (MCR) determines the design pressure ratio and air flow. The matching at part load determines low-load efficiency, with the compressor operating point migrating along its surge boundary at low loads.
Modern engine designs use multiple turbochargers (typically two for medium-speed engines, two or three for large slow-speed engines) with sequential cut-out: at low loads, one turbocharger is bypassed or shut off, concentrating the exhaust energy on the remaining unit and driving it to higher speed and efficiency.
Two-stage turbocharging, with a low-pressure compressor in series with a high-pressure compressor and corresponding turbine arrangement, is used on the most highly rated engines to achieve pressure ratios of 8:1 or more.
Scavenge Air System
The scavenge air system on a marine engine includes the turbocharger, the charge air cooler, the air receiver (a manifold supplying air to the cylinders), the auxiliary blowers (on slow-speed two-strokes), and various drains and instrumentation.
The air receiver is a substantial volume that buffers the air supply between the steady output of the turbocharger and the pulsing demand of the cylinders. It is typically located between the engine and the charge air cooler outlet.
Drains at the bottom of the air receiver and after the charge air cooler collect water that condenses out of the air when it is cooled. Drain operation is usually automatic, with float-controlled valves or solenoid valves cycling on a timer.
Instrumentation on the scavenge air system includes pressure transducers (scavenge air pressure is a key combustion analysis parameter), temperature sensors at multiple points (particularly after the charge air cooler), and air flow sensors on some installations.
Scavenge port inspection on slow-speed two-strokes is performed during operation by opening hand-hole covers on the air receiver and visually inspecting the cylinder liner through the scavenge ports. This is a critical maintenance check, looking for scuffing, ring damage, and combustion-related deposits.
Charge Air Cooler
The charge air cooler (intercooler) is a heat exchanger between the compressor outlet and the air receiver. It cools the compressed air, which has been heated significantly by the compression process (an ideal gas at 3 bar pressure ratio reaches over 200 degrees Celsius from 25 degrees inlet through compression alone). The cooling is essential because cooler air is denser, allowing more mass to enter the cylinder per cycle.
Marine charge air coolers are typically tube-type heat exchangers with the air flowing on the shell side and freshwater (in a closed circuit) on the tube side. The freshwater is in turn cooled by seawater in a separate heat exchanger.
The cooler reduces the air temperature from compressor outlet to typically 40 to 50 degrees Celsius (a few degrees above the freshwater inlet temperature). The pressure drop through the cooler is typically 100 to 200 millibars; excessive pressure drop indicates fouling.
Air-side fouling on the cooler tubes is a frequent issue, accelerated by oil mist in the compressed air (typically from leaking turbocharger seals) and from particulates in the engine room air. Cooler tube cleaning is performed during major maintenance intervals using mechanical brushing or chemical descaling.
Water-side fouling is generally less severe in the closed freshwater circuit but can be a concern in the seawater secondary loop. See marine sea water cooling systems for the seawater system management.
Surge and Choke
Surge is an aerodynamic instability of the centrifugal compressor that occurs when the mass flow drops below the surge boundary on the compressor map. The flow temporarily reverses through the compressor, the pressure drops, the engine momentarily loses scavenge air, and the flow re-establishes. The cycle repeats at audible frequency, typically 10 to 100 Hz.
Surge is destructive: the impulsive flow reversals cause high mechanical loads on compressor blades, thrust bearings, and the housing. Repeated surge events shorten the turbocharger’s service life and can cause blade fracture.
Surge is normally avoided by ensuring the operating point remains to the right of the surge boundary. It can be triggered by sudden engine load reduction (the air flow demand drops while the turbocharger speed is still high), by exhaust manifold backpressure increase (after a fire in the exhaust gas economiser, for example), or by compressor fouling that shifts the surge boundary.
Choke is the opposite condition: at very high mass flow, the compressor reaches an aerodynamic limit where additional flow is impossible regardless of pressure ratio. Choke is normally not a service issue because the engine cannot demand more flow than the compressor can supply at full speed.
The compressor map plots the operating envelope between surge (left boundary) and choke (right boundary), with the engine load line traversing the map from idle to MCR.
Turbo Lag and Variable Geometry
Turbo lag is the delay between commanded engine load increase and the turbocharger reaching the new operating speed. The turbocharger rotor has substantial inertia, and the increase in exhaust energy must accelerate it through several seconds. During this period the engine is briefly air-starved relative to the fuel demand, producing visible smoke and reduced power.
Turbo lag is most noticeable on smaller engines and in transient conditions (manoeuvring, dynamic positioning, ice operation). It is mitigated by:
Auxiliary blowers on slow-speed two-strokes provide air at low speed and during transients.
Sequential turbocharging brings additional turbochargers online progressively, smoothing the response.
Variable geometry turbine (VGT) uses adjustable vanes at the turbine inlet to vary the flow area. By closing the vanes during acceleration, the flow is concentrated and the turbine accelerates more quickly. By opening the vanes at full load, the flow capacity matches the engine demand.
Electrically assisted turbocharging (eTurbo, hybrid turbocharger) combines an electric motor on the turbocharger shaft to provide additional acceleration during transients. This is a recent development on medium-speed engines and naval applications.
Turbo Washing
Turbo washing is the periodic cleaning of internal turbocharger surfaces to remove deposits that accumulate during operation, particularly when running on residual fuel. Two types of washing are performed:
Compressor washing uses water injection at the compressor inlet to remove oil and dust deposits from the impeller. The water is finely atomised and is partially evaporated and partially flung outward by the impeller, mechanically removing soft deposits. Compressor washing is performed with the engine at reduced load (typically 25 to 50% MCR) and follows a procedure specified by the turbocharger manufacturer.
Turbine washing is more involved, since the turbine operates at high gas temperature where free water injection would cause thermal shock. Modern systems inject water mixed with compressed air to provide a controlled wet mist on the turbine inlet. The mist evaporates partially, scavenging deposits from the turbine blades and stator vanes. Turbine washing is performed at low load (typically 30% MCR) with the exhaust gas temperature reduced sufficiently that the water injection does not cause damaging thermal stress.
The frequency of washing varies. Operators following recent guidance from MAN and ABB perform compressor washing every 100 to 250 hours and turbine washing every 250 to 500 hours, with adjustments based on observed deposits and turbo performance trends. Deferring washing leads to fouling, reduced turbo efficiency, and ultimately damaging deposits that may fall off and unbalance the rotor.
Bearing Types
The turbocharger rotor is supported in journal bearings and one or more thrust bearings. The bearings must operate at high speed (up to 30,000 rpm), high temperature (the turbine end is exposed to bearing temperatures of 100 degrees Celsius or more), and accommodate the asymmetric loading from the unbalanced rotor.
Plain journal bearings lubricated by the engine’s lubricating oil are the most common arrangement. They are similar in principle to the engine main bearings but smaller in diameter and at much higher speed. The oil supply is at relatively low pressure (3 to 5 bar) and continuous, with separate drain back to the crankcase.
Rolling element bearings (ball or roller) are used on some smaller turbochargers. They have lower friction and faster start-up response but limited service life relative to plain bearings. The rolling element design has gained ground in recent years as bearing technology has improved.
Magnetic bearings are used on a small number of advanced installations where the elimination of mechanical contact is justified. They are uncommon in mainstream marine practice.
The bearing oil is filtered (a fine filter is fitted in the supply line to each turbocharger) because contamination is the principal cause of bearing failure. The bearing temperatures are monitored and trended through the engine room automation system.
Related Wiki Articles
- Marine Diesel Engine
- Marine Engine Combustion Analysis
- Marine Engine Camshaft and Valve Train
- Marine Engine Cylinder Liners and Pistons
- Marine Engine Fuel Injection Systems
- Marine Engine Common Rail Technology
- Marine Engine Crankshaft and Main Bearings
- Marine Engine Performance Monitoring
- Marine Engine Room Ventilation and Uptakes
- Marine Engine Room Automation and Monitoring
- Marine Sea Water Cooling Systems
- Marine Auxiliary Engines and Generators
- Marine Lubricating Oil Systems
- Continuous Survey of Hull and Machinery
References
- IACS Unified Requirement M51, Type Testing of Diesel Engines
- IACS Unified Requirement M73, Type Testing of Turbochargers
- ISO 3046-1, Reciprocating Internal Combustion Engines - Performance
- ISO 6976, Natural Gas - Calculation of Calorific Values, Density and Wobbe Index
- IMO MARPOL Annex VI, Regulations for the Prevention of Air Pollution from Ships
- MAN Energy Solutions, TCA Turbocharger Technical Description
- MAN Energy Solutions Service Letter SL2017-642, Turbocharger Compressor Washing
- MAN Energy Solutions Service Letter SL2018-665, Turbine Washing
- ABB Turbo Systems, A100 Series Operation Manual
- ABB Service Bulletin TPL-A, Turbine Side Cleaning
- Mitsubishi Heavy Industries, MET Turbocharger Series Manual
- Wartsila Service Bulletin RT-67, Turbocharger Maintenance
- DNV Class Guideline CG-0341, Diesel Engine Type Approval
- CIMAC Recommendation No. 22, Turbocharger Application Guide