In the previous article, we looked at how fuel gas and air travel through their separate supply systems, mix, and enter the engine.
To produce more power, a gas engine must burn more fuel while maintaining stable combustion.
Simply increasing the amount of fuel is not enough. A corresponding mass of air must also be supplied.
A turbocharger uses energy from the engine’s exhaust gas to compress the air or mixture entering the engine.
Changes in turbocharger performance can affect boost pressure and engine output as well as exhaust gas temperature and combustion stability.
In this article, we will look at the construction and operating principles of a turbocharger, the conditions to monitor during operation, and the main items checked during scheduled disassembly and inspection.
Related article｜How Do Fuel Gas and Air Enter a Gas Engine?
1. Why Does a Gas Engine Need a Turbocharger?
A naturally aspirated engine relies on the pressure difference created by piston movement to draw outside air into the cylinders.
However, there is a limit to the mass of air that atmospheric pressure and piston movement alone can supply.
A turbocharger raises the pressure in the intake system, allowing a greater mass of air to enter cylinders of the same size.
When more air is available, the engine can burn a corresponding amount of additional fuel and produce the required output.
A turbocharger uses energy from the engine’s exhaust gas instead of relying on a separate electric motor to drive the compressor.
In this way, part of the energy remaining in the exhaust gas is used to rotate the turbine and compress the intake air or mixture.
2. What Are the Main Components of a Turbocharger?
A turbocharger consists mainly of a turbine, a compressor, and a center housing.
The turbine is installed on the exhaust side, while the compressor is installed on the intake side.
The turbine wheel and compressor wheel are connected by a common shaft and rotate together around the same axis.
The center housing contains the shaft that connects the two wheels and the bearings that support the rotating assembly.
The basic power-transfer path can be summarized as follows:
Exhaust gas
→ Turbine wheel
→ Shaft
→ Compressor wheel
→ Boosted air or mixture
The detailed construction, size, bearing and sealing arrangements, lubrication method, and cooling method vary depending on the engine and turbocharger model.
3. How Does Exhaust Gas Drive the Turbine?
Exhaust gas leaving the cylinders still contains pressure and thermal energy.
After passing through the exhaust valves, the gas travels through the exhaust manifold and enters the turbine housing.
The turbine housing directs the exhaust gas toward the turbine-wheel blades, causing the turbine wheel to rotate.
The turbine wheel transmits this rotation through the shaft to the compressor wheel on the opposite side.
After passing through the turbine, the exhaust gas continues through the exhaust system. Depending on the installation, some of its remaining heat may also be recovered.
Restricted exhaust flow, deposits, or damage on the turbine side can affect turbocharger rotation and boost performance.
4. What Does the Compressor Compress?
As the compressor wheel rotates, it accelerates the air or mixture entering the intake system and raises its pressure.
The turbine and compressor handle separate gas streams. Only mechanical rotation is transmitted between them through the common shaft.
What passes through the compressor depends on the engine’s fuel-supply system and the location of the gas mixer.
In some of the premixed gas engines I have worked with, the fuel gas and air were mixed before entering the compressor.
In this arrangement, the turbocharger compressor boosts a combustible mixture of fuel gas and air.
The boosted mixture was cooled in a fuel mixture cooler and then supplied to each cylinder through the intake manifold.
The basic flow was as follows:
Fuel gas and air
→ Mixing
→ Boosting by the compressor
→ Cooling in the fuel mixture cooler
→ Intake manifold
→ Cylinders
If the mixer or fuel-supply device is installed downstream of the turbocharger, the compressor may handle air only.
Some engines supply air first and introduce fuel gas at the intake ports or closer to the cylinders.
Before inspecting the turbocharger or intake piping, technicians should determine whether each section contains air or a combustible premixed charge by checking the fuel-gas and intake-system diagrams.
Related article｜The Role of a Gas Engine Intercooler and What to Inspect (Coming Soon)
5. Why Are the Shaft, Bearings, and Lubrication Important?
The shaft connecting the turbine and compressor wheels rotates at very high speed while the engine is operating.
The bearings support the shaft and rotating assembly while controlling radial and axial movement.
Lubricating oil reduces friction between the bearings and shaft, carries away heat, and allows the rotating assembly to operate smoothly.
Insufficient oil supply or contaminated lubricating oil can cause wear or damage to the bearings and shaft.
As shaft and bearing wear increases, internal clearances and rotating conditions may change, resulting in abnormal noise or vibration.
Inspection should therefore include not only the shaft and bearings but also the oil-supply and drain lines, external oil leakage, and possible restrictions.
If the turbocharger has a separate cooling circuit, the coolant supply and possible leakage should also be checked according to the manufacturer’s procedure.
6. How Are Intake Cooling and Boost Pressure Related?
Compressing air or an air–fuel mixture increases its temperature.
At the same pressure, a higher temperature lowers the density of the intake gas, so less mass can occupy the same volume. Higher intake temperature can also affect the margin against knock and abnormal combustion.
For this reason, many turbocharged engines use an intercooler or fuel mixture cooler downstream of the turbocharger.
The cooler transfers heat from the compressed air or mixture to cooling water or ambient air.
Reducing the intake temperature increases density and helps supply the required mass of air or mixture to the cylinders.
When the turbocharger compresses the intake charge, the pressure in the intake system can rise above atmospheric pressure. This increased intake pressure is generally referred to as boost pressure.
As engine load increases, demand for fuel and air rises, and exhaust-gas flow and energy may also increase. This can change turbine and compressor speed and increase boost pressure.
However, boost pressure and the actual mass flow of air or mixture are not the same.
Even at the same boost pressure, the mass entering the cylinders may vary with intake temperature, gas composition, piping resistance, and engine operating condition.
Boost pressure may be controlled by a wastegate, bypass system, or another device, depending on the engine design.
Boost pressure should therefore not be used by itself to assess the condition of the turbocharger or the engine’s combustion.
Related article｜How Does Mixture Boost Pressure Affect a Gas Engine? (Coming Soon)
7. What Changes Can Indicate a Turbocharger Problem?
If turbocharger performance deteriorates, the engine may not receive enough boosted air or mixture.
The following changes may appear during operation:
- A change in boost pressure
- Reduced engine output
- Changes in exhaust gas temperature
- Abnormal turbocharger noise or vibration
- Lubricating-oil leakage
- Contamination in the intake or exhaust system
However, these changes do not automatically mean that the turbocharger itself has failed.
A restricted air filter or intake pipe, an intake leak, fouling or increased differential pressure across the intercooler or fuel mixture cooler, increased exhaust-system resistance, and sensor or control-device faults can produce similar symptoms.
During operation, boost pressure, intake temperature, exhaust gas temperature, engine output, and available turbocharger data should be compared with normal values at a similar load.
Technicians should also listen for unusual changes in high-frequency sound, rubbing noise, or vibration around the turbocharger.
With the engine stopped, inspect the intake and exhaust connections, clamps, flanges, cooler connections, and lubricating-oil lines for leakage, movement, or deformation.
Even when the rotating assembly can be checked without completely disassembling the turbocharger, the engine must first be stopped and secured against starting. The inspection must then follow the manufacturer’s procedure.
8. When Should a Turbocharger Be Disassembled and Inspected?
A turbocharger is not disassembled only after a fault occurs.
Even when no obvious abnormal condition is present, it should be disassembled and inspected at the operating-hour interval specified by the engine or turbocharger manufacturer.
If abnormal noise or vibration, or an unusual change in boost pressure or engine output, appears during operation, an additional inspection may be required before the scheduled interval.
The decision to disassemble the turbocharger and the required maintenance scope should consider:
- Current operating condition
- Maintenance history
- Fuel type and operating environment
- Contamination condition
- Manufacturer’s inspection criteria
After high-load operation, the turbocharger may remain at a high temperature.
If the manufacturer specifies an unloaded running period, cooldown procedure, or shutdown sequence, it should be completed before the engine is stopped. Inspection should begin only after the turbocharger has reached a safe condition.
9. What Should Be Checked After the Turbocharger Is Disassembled?
After the turbocharger has been disassembled, inspect the turbine-wheel and compressor-wheel blades for damage and abnormal wear.
The shaft should be checked for wear and damage, while the bearings, seals, and internal clearances should also be inspected.
The inspection may include:
- Damage to the turbine and compressor wheels
- Blade wear and deformation
- Shaft wear and damage
- Bearing and seal condition
- Radial and axial clearances
- Contact marks between rotating and stationary parts
- Oil and carbon deposits
- Damage to the nozzle ring or other fitted components
- Contamination or restriction in the oil-supply and drain passages
Where required, rotor balance should be checked using the method specified by the manufacturer. The wheels and other critical components may also need to be inspected for cracks.
Parts that exceed the manufacturer’s limits should be replaced.
Reusable parts should be cleaned using an approved method and inspected again before a reuse decision is made.
Because a turbocharger contains a high-speed rotating assembly, external appearance alone is not enough to determine whether a part can be reused.
Measurements, balancing, crack inspection, and assembly must follow the manufacturer’s limits, required equipment, and maintenance procedures for that turbocharger model.
10. What Should Be Checked After Maintenance?
After the turbocharger has been serviced and assembled, check the lubricating-oil supply and drain lines, intake and exhaust piping, and all related connections.
Before the engine is started or loaded, complete the startup, oil-priming, or pre-lubrication procedure specified by the manufacturer and confirm that oil can reach the bearing system.
If the turbocharger has a separate cooling circuit, also check the coolant piping and possible leakage.
After the engine is started, compare the following items with the data recorded before maintenance:
- Boost pressure
- Intake-air or mixture temperature
- Engine output
- Exhaust gas temperature
- Abnormal turbocharger noise or vibration
- Lubricating-oil supply and drainage
- External oil leakage
- Leakage at intake and exhaust connections
Rather than checking only whether each value falls within its normal range, compare pre-maintenance and post-maintenance data under similar loads and operating conditions.
If unusual noise, vibration, or lubricating-oil leakage appears, determine the cause before continuing operation.
Follow the engine and turbocharger manufacturer’s requirements for post-maintenance checks and operating confirmation.
Conclusion
A turbocharger uses energy from the engine’s exhaust gas to rotate a turbine and drive a compressor through a common shaft.
The compressor then increases the pressure of the air or mixture supplied to the engine.
Stable operation depends on the condition of the shaft and bearings, correct lubrication, and the condition of the intake, exhaust, and cooling systems.
During field inspection, boost pressure, engine output, exhaust gas temperature, noise, and vibration should be evaluated together with the air filter, piping, cooler, exhaust system, and lubricating-oil system.
A condition that appears to be a turbocharger problem may be caused by another part of the intake, exhaust, or control system.
The turbocharger should also be disassembled and inspected at the maintenance interval specified by the manufacturer. After maintenance, operating data should be compared under similar conditions to confirm that normal performance has been restored.
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