Turbocharging and Supercharging:
The process of supplying compressed air to the engine for combustion is called either supercharging or turbocharging based on the mode of operation. In supercharging we use an external source of energy to run the blower for supplying compressed air to the engine.
But in turbocharging, we use exhaust gases from the engine itself to run the blower for supplying combustion air to the engine.
As we are converting waste energy of exhaust gases to run turbochargers for providing combustion air to the engine, the turbocharging system is more efficient than the supercharging system.
Turbocharger basically divided into two parts. Turbine side and Blower side. Exhaust gases from the engine outlet hit the turbine blade via nozzle ring. This will make the turbine to rotate at higher rpm. This also makes the blower run at the same rpm, as Turbine Shaft is directly connected with blower Shaft. The blower consists of an Inducer to suck the air from outside space and a diffuser to convert K.E(Kinetic Energy) into P. E(Pressure Energy). So as a result pressurized air will be supplied to the scavenge manifold. Which supplies combustion air to the required unit.
Types of turbocharging:
1. Pulse-type turbocharging
2. Constant Pressure Turbocharging.
Pulse-type Turbocharging :
- Pulse-type turbocharging is generally used in low load engines (Generators).
- Exhaust gas outlet from each unit is like a pulse which directly fed to the turbine blades. Hence the name Pulse-type turbocharger.
- In this type of turbocharging system, we need to have a grouping of exhaust pipes to prevent Interference of exhaust pulse from one unit with another unit. Hence the exhaust grouping is done in such a way that, it should not interfere with any other units.
- Normally two inputs will be there at the turbine inlet to prevent interference of pressure pulses.
- Quick response for load change and fluctuations.
- Additional auxiliary blowers are not necessary.
- Hitting of exhaust pulses reduces the life of the turbine and bearings.
Constant pressure turbocharging:
- Normally used in high load engines (main engine)
- Exhaust gases from all units are stored and stabilized in a common manifold called Exhaust Manifold.
- The size of the manifold makes the exhaust pressure stabilized before supplying it to the turbine.
- Constant Pressure exhaust gas is then supplied to the turbine through the nozzle ring.
- Constant pressure input to the turbine increases the life of the turbine blades and bearings as compared to pulse type.
- high efficiency at higher loads.
- No need for the exhaust pipes grouping.
- Poor response at low load hence requires additional auxiliary blowers during starting or running at low load.
What is Turbocharger Surging?
In simple words, change of direction of airflow from Scavenge manifold to Blower side instead of Blower side to Scavenge Manifold. This makes a huge noise and vibration in the turbocharger which is called turbocharger surging.
Normally air is supplied from the Blower side to scavenge manifold because of high pressure at the blower side. If in any case, the pressure at the manifold becomes more than the pressure at the blower side, the air starts coming towards the blower side which causes the surging of the turbocharger.
Reasons for turbocharger Surging:
There are various reasons but the major reasons are,
- Sudden load changes – Either it may be due to weather conditions or manual decrease of rpm. The engine will be running at certain rpm so does the turbocharger. But if we reduce the rpm suddenly the engine will come to that rpm but turbocharger will be still running at higher rpm supplying more than the required quantity of air to the engine. This will slowly start building up pressure in Scavenge manifold and results in the air coming out from the blower side.
- Unbalanced Engine – Due to faulty injectors or any other reason some units may be producing higher power compared to others. This makes the turbocharger surge due to improper inlet to each unit.
- Scavenge fire.
- Exhaust Trunk Fire.
- Poor maintenance of Turbocharger – may cause blocking of nozzle ring or improper distribution of deposits on turbine blades.
- Imbalance in rotating parts of the turbine after overhaul.
- Clogged or dirty air intake filter
- Damaged blades.
Prevention of turbocharger surging
- Water wash or dry wash the turbocharger regularly.
- Soot blow Economise or Exhaust boiler regularly if fitted.
- Carrying out periodic maintenance of turbocharger
- Taking and assessing engine performance cards.
- Periodic overhaul of Engine.
How to do Turbocharger Cleaning?
There are two ways to do turbine side cleaning.
- Water wash
- Dry wash
Water washing of Turbocharger-
- Read the instructions give by the manufacturer carefully before performing the water wash.
- Inform the bridge and slowly reduce the rpm to the required value.
- Wait for some time to stabilize the rpm of the turbocharger.
- Check temperature value is up to the range or not. Mentioned by the manufacturer. (if not you’ll end up cracking the turbine blade due to quenching effect)
- After all the above conditions are satisfied, open water inlet and drain.
- 90% of the water will get evaporated due to high temperature and only 10% will come out from the drain.
- Check the condition of water coming out from the drain. If clear and clean water is coming out from the drain means turbocharger is cleaned properly.
- Now you can close drain and water inlet.
- Wait for some time and slowly increase rpm by monitoring the temperature and physical condition of the turbocharger.
Dry-washing is done by using Grit. It’s should be done on a regular basis. Procedure and required conditions such as rpm, interval, etc will be clearly written near the turbocharger or you can refer manual.
Water washing is more effective than dry washing. But it’s riskier if we don’t carry out properly.
What is Lambda Control Of Turbocharger.?
If we use constant pressure turbocharger in low load engines it won’t response quickly at staring or fluctuating loads. So to prevent this problem we have a 7 Bar compressed air connection to the blower blades. During starting or load fluctuations this 7 bar air is supplied to run the turbocharger. This air connection is operated by a solenoid valve which continuously monitors the speed and load variations of the engine. THIS IS CALLED LAMBDA CONTROL OF TURBOCHARGER.
What is the reason For high Exhaust Gas temperature at the turbine inlet compared to the engine outlet(which is supposed to be the same or less)?
- Adiabatic Compression: It means compressing a gas before allowing it to expand. In the turbocharging system exhaust gas is compressed at the turbine inlet and later it allowed to expand in turbine blades. Hence the temperature at the turbine inlet is higher than at the engine outlet.
What is TCA and TCR turbocharger?
- It represents the direction of exhaust input to the turbine blades and air input to the blower side. If the direction is axial (parallel to the turbine Shaft) is called TCA. and if the direction is radial(Perpendicular to the turbine Shaft) is called TCR.
- Normally we use TCA type in Main Engines due to high load and TCR in Generator Engines due to low load. If we use TCR there should be a header tank for lubrication of bearings. Because in case of blackout lubrication should be continuous until the turbine comes to rest.
- But in the case of TCA type, there will be a separate L. O pump driven by the turbine Shaft. So it will keep supplying the L.O until the turbine is running.
What is the matching of a turbocharger?
Matching of the turbocharger means supplying enough quantity of air to the required load change.
What are VTA and VTG?
VTA- Variable Turbocharger Area
VTG- Variable Turbocharger Geometry.
To supply a sufficient quantity of air to the engine we need to run the turbocharger at the required speed. To do that we need to adjust the design nozzle ring as it’s the main part that controls the inlet of exhaust gases to the turbocharger. In older designs, the nozzle ring was fixed but in new technologies, the nozzle ring angle changes automatically depending upon the load. So I case of high load the nozzle ring wide open and allowing more quantity of exhaust inlet to the turbocharger. And in case of low load the nozzle ring angle reduces to limit the exhaust inlet to the turbocharger. Hence maintaining rpm of turbocharger for required load.