Turbocharger System

In any diesel engine to get complete combustion, 14.5 kg of air is required for every one Kg of fuel to burnt. In order to evacuate the cylinder entirely of exhaust gases, fresh clean air at low pressure must be supplied for scavenging and as new  charge for commencement of compression for the next cycle

If weight of air is raised, more weight of fuel can be burnt and corresponding power output will be increased

Various methods can be adopted by equipped:

Driven mechanically blower (valuable power needed)

Utilization of heat energy from exhaust gases to drive a single stage impulse turbine directly coupled to a simple blower (free running unit) called exhaust gas turbocharger

Advantages

Increased power for an engine of the same size OR reduction in size for an engine with the same power output.

Reduced specific fuel oil consumption - mechanical, thermal and scavenge efficiencies are improved due to less cylinders, greater air supply and use of exhaust gasses.

Thermal loading is reduced due to shorter more efficient burning period for the fuel leading to less exacting cylinder conditions.

Turbocharging Methods

Constant pressure system

Here all cylinders exhaust into a common receiver large enough to convert pulses to a steady pressure having one connected to the turbine through a common nozzle ring

This system is slower in its build-up of pressure when starting, and insufficient air is available for the engines during manoeuvring or operating at low speed. To overcome this difficulty modern engines have electric driven centrifugal compressors work in series and under piston compression is used to augment the air pressure in the scavenge trunk when necessary.

Pulse system

The main energy to power the turbine in this system is derived from the pulse or impulse energy in the pressure wave formed during blowdown from each cylinder. These waves travel through the manifold to the turbine nozzles where they are converted to kinetic energy at high velocity to rotate the turbine blades. This system gives a rapid build-up of turbine speed when an engine is started or manoeuvring. 

To maintain the pulses,

exhaust connections are of limited diameter

no sharp bends are used

the turbocharger is fitted close to the engine.

The peak pressure in pulse waves may be higher than the pressure in other cylinder whose exhaust valves are also open.

To prevent a back-flow into these, in multi-cylinder engines

subdivided the exhaust between a number of manifolds to a separate nozzle box at the turbine up to  three cylinders however reducing turbine efficiency and may require more that one turbocharger per engine.

Used pulse converters allows individual cylinders to exhaust to a common manifold, giving higher turbine efficiency and a more compact system which allows rapid acceleration when manoeuvring

Charge air cooler is required between the turbocharger and the engine inlet manifold to reducing air temperature with the same time increasing its density at constant pressure, before supplying it for compression in the engine cylinders.

The engine is maintained at safe working temperatures and the lower compression temperature reduces stress on piston rings, piston and liner.

    Increased density will raise scavenge efficiency and allow a greater mass of      air per cycle to be compressed thus more fuel may now be burned to give a corresponding increase in engine output and thermal efficiency.

Lubrication oil system

Lubrication of the bearings may be by various means. Ball and roller bearings may be lubricated by self-contained gear type pumps operated from the shaft and drawing oil directly from the independent bearing sump. Oil level must be maintained in these sumps and the oil should be renewed at stated intervals.

Alternatively, the bearings may be lubricated by external systems, either by connections from the engine lubricating system, through a fine filter, or by an independent system of pumps, cooler, filters, oil sump and alarms. Independent systems allow a choice of turbine oils to be used. 

Usually the sleeve type bearings of the T/C are lubricated by gravity tank situated about six metres above it

Sight glasses is fitted to observed the oil flowing returns back to drain tank

Pump capability to supplied to header tank more than the capacity flowed into the bearings to maintain the oil pressure on the bearing by overflowing at a predetermined level

An alarm system incorporated to ensure oil supply will not fail for about 15 minutes after engine stopped

Two stage turbocharging

Currently only used in high speed four-stroke engines. Two turbochargers are fitted in series, each acting as one stage of the charge air compressor. They are matched to avoid surge at all speeds. They are of normal construction and there is no mechanical linkage.

The first stage compressor is drive by the second stage exhaust gas turbine. The air then passes through a first stage charge cooler before continuing to the second stage compressor, which is driven by the first stage gas turbine. The air proceeds through a second stage charge cooler and then to the engine inlet.