Turbochargers in Diesel Engines – Marine Engineering

Supercharging is the process of the increasing of the weight of the charged air by increasing the density of the charged air.

Turbocharging is one kind of supercharging by using exhaust gas turbocharger. In which the energy in the exhaust gas expelled from the engine cylinder is utilized in driven in gas turbine, which is connected to a centrifugal air blower and air is supplied to scavenge air trunk.

Constant Pressure System Turbocharging

  • Exhaust gas from all cylinders into a common large manifold where pulse energy is largely dissipated. 
  • The gas flow will steady rather than intermittent and at a constant pressure at turbine inlet.

Turbocharger Arrangement in Constant Pressure System

  • No exhaust grouping
  • Exhaust gases enter into large common manifold and then to turbine
  • Firing order not considered

Advantages and Disadvantages for Constant Pressure System

Advantages

  1. Good performance in high load (Efficient when Bmep is above 8 bar)
  2. More suitable for high output engine.
  3. There is no need to group the cylinders exhaust into multiple of three. (Simple piping system)
  4. No exhaust grouping 
  5. High turbine efficiency due to steady flow of exhaust.
  6. The work transfer at the turbine wheel is smooth.
  7. Reduction in SFOC (Specific Fuel Oil Consumption) of 5% – 7%

Disadvantages

  1. When running at reduced speed and starting up low available energy at turbine. Thus it supplies inadequately air quantity of the scavenge pressure necessary for efficient scavenging and combustion.
  2. It require scavenge assistant (Auxiliary Blowers).
  3. Poor response in changing load.

Pulse System of Turbocharging

  • Makes full use of the higher pressure and temperature of the exhaust gas during the blow down period
  • While rapidly opening the exhaust valves, exhaust gas leave the cylinder at high velocity as pressure energy is converted into kinetic energy to create the pressure wave or pulse in exhaust
  • These pressure waves or pulses are lead directly to the turbocharger
  • Exhaust pipe, so constructed in small diameter, is quickly pressurized and boosted up to form pressure pulse or wave
  • Pressure waves reach to turbine nozzles and further expansion takes place.

Turbocharger Arrangement in Pulse System

  • Interference exists between exhausting and scavenging among cylinders 
  • To prevent this, cylinders are grouped relatively with connections to two or more exhaust pipes
  • Pipes are arranged, in small diameter to boost up pressure pulse and in short, straight length to prevent energy loss 
  • Number of exhaust branch depends upon firing order, number of cylinders and turbocharger design

Advantages and Disadvantages of Pulse System

Advantages

  1. At low load and low speed it is more efficient (Still efficient when Bmep is < 8 bar)
  2. No need assistant of scavenge pump and blower at any load change.
  3. It is highly response to change engine condition giving good performance of all speed of engines.  
  4. High available energy at turbine 
  5. Good turbocharger acceleration

Disadvantages

  1. The exhaust grouping is complicated.
  2. Different sizes of exhaust pipes are needed for spare.
  3. High pressure exhaust from one cylinder would pass back into another cylinder during the low pressure scavenging period thus adversely effecting the combustion efficiency.

Under Piston Pressure Supercharging

  • It is a type of constant pressure charging system
  • Air charged by turbocharger is passed through CAC into first stage manifold, and then through non-return valves into second stage and under piston space
  • In down stroke, piston under side compress further the scavenge air
  • Differential pressure shuts the inlet non-return valves as scavenge ports are uncovered, and a pulse effect is given to cylinder

Advantages

  • Assist tangential swirl and ensure complete evacuation of remaining exhaust gas
  • No auxiliary blower may be required, during manoeuvring

Turbocharger Surging

  • When the discharge volute pressure exceed the pressure built up in the diffuser and the impeller, it produces a back flow of air from discharge to suction and it is characterized by noise and vibration of turbocharger.

Causes of Turbocharger Surging

  1. Suddenly load change by heavy sea
  2. Scavenge space fire / Exhaust trunking fire
  3. Poor power balance
  4. Dirty nozzle and blades
  5. Individual cylinder misfire
  6. Chocked scavenge /exhaust ports
  7. Incorrect matching of turbocharger to engine.
  8. Poor scavenging or leaky exhaust valve

What is exhaust tuning ?

  • Exhaust tuning means arranging the exhaust pipes with suitable length and suitable valve timing to exhaust into the same pipe without disturbance.
  • When the exhaust valve of a diesel engine opens, the cylinder rapidly expands, and gain velocity and kinetic energy as they pass into the exhaust pipe. 
  • The kinetic energy of the mass of exhaust gas carries it along the exhaust pipe, and causes a pressure build up ahead of the mass of gas and a partial vacuum behind it.
  • This principle is used in a tuned exhaust system. The partial vacuum created by exhaust from one cylinder is used to help exhaust expulsion from the following cylinder.
  • Grouping of exhaust pipes depends upon the firing order, length & diameter of exhaust pipe.

Advantages of Inter Cooling the Charged Air

  • The effect of cooling reduces the scavenge air temperature and increase the density of air delivered to the cylinders, thus increasing the power out put delivered by the engine.
  • It can increase the output by about 10%.
  • Cooled scavenge air reduces cylinder and exhaust gas temperature at a given power level and these temperatures thus remain within the acceptable limits.

Types of Turbocharger Lube Oil System

  1. Own sump
  2. External lube oil supply system

Types of Turbocharger Bearings

Ball type bearing (rolling type)

  • These are used in turbocharger lube oil from the sumps on both air and exhaust sides is sprayed onto the bearing by means of attached rotor driven gear type pumps.

Sleeve type bearings

  • These are lubricated by external L.O supply system.
  • By gravity from independent header tank situated about six meters above the bearings. The tank capacity must supply for about 15 minutes after engine is stopped or
  • From a main L.O pump lead to the bearings with a separate L.O pipe line. It has also gravity tank incase of failure of L.O supply. (Under main engine L.O system)

Function of Labyrinth Seals

  • The bearings are separated from the blower and turbine by labyrinth seals.
  • These seals are sealed by air supply from the discharge of blower to prevent oil entering the blower and to prevent contamination of the oil by the exhaust gas.
  • In case of defective labyrinth seals on blower side, oil is leaked into the air system the cooling surfaces are covered with oil which are insulators and the cooling efficiency will be reduced. Also there will be deposits on the blower and the blower efficiency will be reduced.
  • In case of defective labyrinth seals on turbine side, the oil is leaked into the exhaust side and there will be carbon deposits on the nozzle and turbine blades. Reduce blower efficiency and fire in the exhaust piping. It will cause turbocharger surging. Oil will contaminate with exhaust  gas.

How will you know turbocharger air filter chocked ?

  • It can be known by comparing the manometer difference. If manometer difference is greater than normal, turbocharger air filter may be choke.
  • It can cause reduced engine power
  • Black smoke will emit from the funnel
  • Scavenge pressure will reduce

Function of nozzle ring in turbocharger

  • To change pressure energy to velocity (kinetic energy)

Function of shroud ring in turbocharger

  • Shroud ring is a component in turbochargers which forms a part of the inner exhaust gas casing adjacent to the turbine.
  • The area around the turbine is always in contact with highly corrosive exhaust gases. If no shroud ring is fitted, the turbine inner casing will be slowly wasted and complete housing to be renewed for repair. Otherwise only shroud ring can be replaced with new one.
  • Minimize maintenance cost.
  • The radial clearance between turbine blades and shroud ring to be maintained within manufacture tolerance limits. This is very difficult in absence of shroud ring since casing can go for uneven wear.

shroud ring in turbochargers

Purpose of inducer in turbochargers ?

  • To feed the shock less air flow to the impeller or
  • To guide the air smoothly into the eye of impeller

Function of Diffuser in turbochargers ?

  • To direct the air smoothly into volute casing 
  • Convert kinetic energy to pressure energy inlet air

What is K value in turbochargers ?

  • It is a distance between the rotor shaft end and the flange of bearing cover measured by blower side.

Purpose of  K value in turbochargers ?

  • To ensure that rotating impeller does not to touch the stationary blower casing cover in case of thrust bearing worn out.

How will you run engine in case of turbocharger failure ?

  • Rotor to be blocked
  • Exhaust gas to be by-passed the turbocharger
  • Run engine with reduced speed with remaining turbocharger
  • Use Auxiliary Blower
  • Maintain all temperature and pressure of fuel, cooling water and lubrication within limit

How to Cut off Damaged Turbochargers for Engine Operation

Engines with one turbocharger (Engines with exhaust by-pass)

  1. Stop the engine.
  2. Lock the turbocharger rotor.
  3. Remove the blanking plate from the exhaust by-pass pipe.
  4. Remove the compensator between the compressor outlet and the scavenge air duct. This reduces the suction resistance.
  5. Run engine with 15% of MCR load and 53% speed.

Engines with one turbocharger (Engines without exhaust by-pass)

  1. Stop the engine.
  2. Remove the rotor and nozzle ring of the turbocharger.
  3. Insert blanking plates.
  4. Remove the compensator between the compressor outlet and the scavenge air duct. This reduces the suction resistance.
  5. Run engine with 15% of MCR load and 53% speed.

Engines with two or more turbochargers

  1. Stop the engine.
  2. Lock the rotor of the defective turbocharger.
  3. Insert orifice plates in the compressor outlet and the turbine inlet. (A small air flow is required to cool the impeller, and a small gas flow is desirable to prevent corrosion)
  4. Run engine with 20% of MCR load and 58% speed.

One Thought on “Turbochargers in Diesel Engines – Marine Engineering

  1. meenachi on May 22, 2017 at 8:13 pm said:

    Good

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