How can we increase power of an engine? To generate more power we require more Fuel and Air. Conventionally this was achieved by increasing length of piston stroke or number of cylinders. But what if there’s a way to achieve all without making any change in the engine. Yes! Using Turbocharger we can generate more power from the same engine at much lesser fuel as it would be required by a new engine.
You might be known to the fact that exhaust from the engines lead to air pollution; But do you know they also take away the much needed energy in the form of waste ? Exhaust gas are perfect example of waste energy in the form of heat and kinetic energy lost in the environment. A turbocharger use this waste energy to pump in more fresh air into the cylinder’s to burn faster and efficient.
Q. Why do we need Turbocharger in first place?
A non turbocharged marine diesel engine ( or perhaps any other diesel engine ) will take in same amount of fresh air for each rotation of crankshaft ( 360 degree ). This implies that we can increase the power of an engine by adding more fuel during combustion; but after a point there won’t be enough fresh air to burn it.
A turbocharger is so needed to provide that extra air required for the proper combustion. This leads to advantage in efficiency and power of the engine. The other reason that made it popular is that; you don’t have to dismantle engine parts or change anything to install. Its quite easy to install, maintain and replace with a better newer design when needed at low cost. It is a device that really makes possible to produce relatively more power at less cost by burning more fuel and recovering a major part of waste heat from exhaust gases.
Q. How Turbocharger leads to Efficient Fuel Consumption?
You might heard or just learned in the above paragraphs of this article that ” A turbocharger reduces overall fuel consumption”. Wait a minute ! Isn’t it contradictory that, while turbocharger increases power enabling more fuel consumption and waste heat recovery; How does it save fuel anyway?
The answer is; An engine installed with turbocharger for the same power output is of smaller size with less no of cylinders. Now even by law if turbocharger increase fuel consumption by 10-15%; it still reduces the overall fuel consumption than a large engine with more cylinders. This lead to much efficient fuel consumption in turbocharged engines.
Construction and Working of Individual Parts
A turbocharger is a device consists of two main parts turbine and compressor mounted on the same shaft. On basis of its design all turbocharged engines can be classified into two main types; Axial flow and radial / centrifugal turbochargers. A radial / centrifugal type is used in automobile engines to produce similar torque and power with much smaller ( lighter ) turbocharger compatible with small engines.
The exhaust blows in radially along the turbine blades to leave through the axis of the turbine shaft. On other hand; an axial flow turbine is used in large land based plant and marine ships. In these design the exhaust moves in and out of the blade along the axis of the shaft. These design have following main parts with their function:
1 ) Turbine
High velocity exhaust gas from the nozzle ring is directed on to the rotor blades. Nozzle rings are used to generate kinetic energy in exhaust gas while rotor blades are held tightly by roots of fir tree structure. They are secured from vibration by lacing wires passing through each of them.
Heat resistant nickel chromium steel is used for the construction of turbine blades, wheel nozzle and shaft. Adequate arrangement for cooling water space is also done on its casing made of cast iron.
2 ) Compressor / Blower
A compressor or blower is fitted with a set of silencer and filter at the inlet side of a turbocharger. To direct the flow of fresh air towards the center an inducer made of light alloy aluminum steel is fitted just before the impeller. The impeller takes fresh air axially while delivers it radially along the volute casing.
A volute casing is placed just after the impeller to convert all that kinetic energy of the fresh air into pressure energy. The pressurized air is then sent to the cylinder via inter cooler to cool the compressed air.
3 ) Bearings & Seals
Separate shaft bearings are fitted on both parts of the turbocharger. Generally ball and roller or plain sleeve bearings are used. Additional labyrinth seal is fitted in between the bearings and turbine ( 1st set) & in between thrust bearing and compressor / blower ( 2nd set ). Labyrinth seals are sealed by air discharge from the blower to avoid risk of lub oil contamination from exhaust gas.
How does Turbocharger Works?
The basic idea is to produce compressed air using blower and turbine mounted on the same shaft.
- Fresh air is sucked in by the impeller of the blower via a set of fitter and inducer.
- The blades of impeller ( blower ) takes the air axially while releases it radially.
- The squeezed hot air is then passed through the volute casing to produce pressure.
- This hot pressurized air is then passed through the inter cooler or so called heat exchanger.
- A heat exchanger is used to cool down the air; thus increasing its density.
- Compressed cool air helps burn more fuel thus producing more power for the shaft, piston and ( propeller / wheels / Gear ) depending upon engine application.
- More combustion leads to more exhaust which are then used to rotate turbine blades.
- Nozzle rings are used to add high kinetic energy to the exhaust gas.
- The turbine rotates turning the blower impeller along with the shaft.
- The exhaust then leaves out of the system with much lesser waste heat then it would otherwise.
How Turbocharger Bearings are Lubricated?
A turbocharger is one of the most reliable piece of equipment installed on an engine ( On ship, vehicle or any other land based plant ). They rarely fail and if they do the main reason are oil contamination or starvation. I know – I know; you would ask what is the need for oil in first place? You know what ! A turbocharger have two main parts: turbine and blower which rotates at extremely high rpm with their common shaft. Such high speed demands for protective bearings installed on the shafts.
They not only bear the load but also assist in unrestricted movement of the shaft. The only problem is; they require lubrication for maintaining proper operation without faults. This is why the quality of lubricant is of so importance for proper operation of turbocharger. A good lubricant thus must have following characteristics:
- Resistant to high temperature.
- Good viscosity index
- High demulsibility ( The ability to release or get separated with water )
- Does not react with parts of turbocharger.
- Good load caring capacity
- No foam / foaming
- Resist corrosion and rust formation
- High flash point
- Allow for quick and easy air release
- Low pour point
A ball and roller bearing use self contained gear pump for lubrication. Oil is drawn from the sump ( Independent of engine ) provided on either side of the casing to these bearings. A sight glass is provided to see and check the level of oil in the sump. On other hand a sleeve type bearing ( generally white metal ) utilize the main engine lub oil for this purpose. Lub oil is provided to these bearings from the gravity tank through a non return valve. An orifice is used for supplying oil from the main engine lub oil line to the gravity tank with a return line back to the sump to maintain oil level.
Q. What is Turbocharger Surging?
If you ever go to the engine room during bad weather; chances are you will notice a sudden loud noise with a bang and vibration. This is what is called surging. For all those who never witness it or heard from anyone its a rapid deceleration due to uneven supply and demand arose due to mass flow rate of air lower than the given pressure ratio.
In simple words it is a phenomenon where pressurized air released backward to the atmosphere through the blower. When the scavenge pressure is far above the delivered pressure from turbocharger this reversal of air flow take place with a loud noise. Although it don’t occur frequently but can be seen during bad weather ( 4 out of 10 times ).
Some of the main reason for turbocharger surging are; improper power distribution between units, dirty impeller blades or chocked inlet filter and bad weather. Surging is not a good thing and so must be avoided at all condition. Following key preventive measures are taken on board ship to avoid surging:
- Periodic cleaning of blower inlet filters
- Proper maintenance
- Shoot blowout to be carried for boiler exhaust and economizer.
- Blades of blower impeller be washed by water.
- Use indicator card to maintain all unit performance.
You can see Live surging example by clicking this link ( This is a third party content and we are not responsible for content quality, link or any other thing ).
Regular inspection and overhaul of turbochargers are done on board ship as per the planned maintenance. This generally includes checking of oil levels in sump, cleaning air inlet filters and inspecting or renewing bearings based on running hours, clearance, damage and vibration. Other than that water washing of each side is done on each such inspection.
Dust and oil sucked from the engine room may get stick to the impeller blades affecting its performance and efficiency. So regular water washing of blower / compressor side is done. To do so first fill the tank with water and close the vent. Now open the valve A to supply air into the tank; slowly open the valve B and wait for next 20 to 30 minutes for the process to complete. Now close all the valves, inspect and empty the tank.
Although water washing of turbine side is not generally advised due to high risk of corrosion and thermal stress. But to reduce shoot and dirt deposits on turbine blades its quite necessary sometimes to water wash the turbine side. Engine speed is lowered for this operation and then drains ( of turbocharger ) are opened. Slowly water is admitted from the top to come out from the drains.
Appearance of drain water is watched till clean water start to come from the drain. Then injection of water is stopped and drains are closed. Engine are run on the same speed for next 5 to 10 minutes to dry up the system.
#NOTE: I will look forward for your helpful comment and recommendations to improve this Article ( Turbocharger – Function, Construction And Working ).
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