TURBOCHARGING SYSTEMS

Waste gate or air bypass

Exhaust waste gates or air bypasses are usually envisaged for improving the part-load

performance of turbocharging systems.

However, such devices are not needed with large two-stroke engines owing to their general

operating regime. In ship propulsion these engines operate mainly at around 85 per cent

CMCR (contracted maximum continuous rating) power. In addition they have few transients

during ocean passages, with long periods spent running at steady loads. In any case, auxiliary

blowers are needed for operation at loads below about 30 per cent.

In fact, exhaust waste gates can be a definite disadvantage for large two-stroke engines

because the loss of exhaust gas through the waste gate reduces the turbocharging efficiency

which may lead to higher thermal loads in combustion chamber components.

Surging

Turbocharging surging is a common hazard as it can be brought on by many events which

result in a shifting of the turbocharger operating point towards the surge line (Fig. 8). The

possible reasons for turbocharger surging include:

• Insufficient engine room ventilation

• Clogging of turbocharger intake silencer, compressor, scavenge air cooler, turbine gas

inlet protection grid, turbine nozzle ring, and exhaust gas economiser and silencer, and

deposits on turbocharger turbine blades

• Damage of scavenge air flaps

• Damage or wear of fuel injection valves

• Excessive lubricating oil feed rate

• Wear of turbocharger components such as nozzle ring, turbine blades, cover ring or shroud ring.

Thus it can be seen that the best prevention against surging is careful attention to engine

maintenance with an important aspect being the cleaning of the turbocharger.

Variable turbine geometry (VTG)

Variable turbine geometry has potential for improving part-load performance of the

Turbocharging system. It involves mechanical linkage to vary the angle of incidence of the

turbine inlet guide vanes. However, at present, it is not considered to be a practical or

economic proposition for engines burning heavy fuel oil.

The main problem is the fouling of the adjustable guide vanes by unburned fuel components

and cylinder lubricating oil. Such fouling can readily accumulate during the long periods in

which marine engines run at steady loads on long ocean passages.

As the great majority of ships run on heavy fuel oil, VTG is not practical with large marine

two-stroke engines. However, fouling might be reduced if advanced cylinder lubrication

systems are developed which would allow greatly reduced cylinder oil feed rates.

Another drawback of VTG is that the extra mechanism adds to the cost of the turbochargers.

On a two-stroke engine, the system would be further complicated by a need to provide means

of adjusting the compressor, perhaps by a variable orifice (VCG, variable compressor

geometry), to avoid surging of the compressor while changing the angle of the VTG guide

vanes.

Two-stage Turbocharging

Two-stage Turbocharging is another concept which has often been mooted in the past when

available turbochargers appear to be reaching their limits in efficiency and pressure ratio.

Higher overall turbocharger efficiencies can be reached with two stages because it is possible

to have intercooling between the two stages thereby reducing the compression work needed in

second turbocharger stage.

However, the low-load efficiency of two stages of today’s turbochargers is worse than is

obtained with single-stage operation.

The major drawback of two-stage turbocharging is the complex arrangement of air and

exhaust ducts (six ducts per turbocharging unit). It requires much more space which is, in any

case, restricted in ships’ engine rooms. Even the space required by today’s single-stage

systems is becoming critical.

The new turbocharger development combines a radial-flow compressor wheel with axial-flow

turbine blading. The first production turbocharger from this series is the TCA77.

The TCA requires neither cooling-water nor lock-air connections, let

alone an external luboil supply. In order to ensure that it can continue to operate in an

emergency, the unit can be supplied with a special emergency operation facility integrated into

the turbocharger. The TCA series represents the consistent continuation of the "pipeless engine"

principle.

Oil mist instead of water cooling

None of the TCA turbocharger bearing casings is water-cooled - not even in the largest framesizes,

the TCA88 and TCA99. The heat brought in by the compressor and the turbine is

dissipated in the luboil flung off the shaft of the rotating assembly. The oil mist thus generated

can drop down the walls of the very generously-dimensioned interior of the bearing casing,

thereby evenly absorbing the heat which is to be dissipated. The bearing casing boasts its own

air vent, which can likewise be connected to the left or right. This air vent ensures that the

leakage air which the compressor inevitably forces into the bearing casing through the shaft seal

of the rotating assembly does not increase crankcase pressure in the engine, but instead is

dissipated directly.

1. latest development in turbochargers

Ans; Adjustable nozzle ring

The intake air is compressed by the compressor wheel, which is driven by the turbine. Both the compressor wheel and the turbine can be adjusted to engine requirements by choosing from a range of meridians and blading configurations. Diffusers and nozzle rings which are very finely stepped in their mass-flow areas allow the turbochargers to be fine-tuned to the engine. For maximum variability an optional nozzle ring capable of adjustment during operation will be available in the near future.

Oil mist instead of water cooling

None of the TCA turbocharger bearing casings is water-cooled - not even in the largest frame sizes, the TCA88 and TCA99. The heat brought in by the compressor and the turbine is dissipated in the lube oil flung off the shaft of the rotating assembly. The oil mist thus generated can drop down the walls of the very generously-dimensioned interior of the bearing casing, thereby evenly absorbing the heat which is to be dissipated. The bearing casing boasts its own air vent, which can likewise be connected to the left or right. This air vent ensures that the leakage air which the compressor inevitably forces into the bearing casing through the shaft seal of the rotating assembly does not increase crankcase pressure in the engine, but instead is dissipated directly. The result is a turbine with 41 so-called "wide-chord" blades arranged in a fir-tree root in the turbine disc.

The characteristic feature of wide-chord blades is their very high chord-to-height ratio. This produces a compact-looking, very stiff and hard-wearing turbine blade. For engine matching the turbine blades can be of varying angles and lengths. With the aid of leading-edge design tools it is now possible to dispense with lacing wire to dampen exhaust-generated vibrations, even in Four-stroke engine applications. Apart from improving the blade profile, this has also been an immense boost to efficiency.

A new design of compressor volute and new designs of nozzle ring ensure optimum turbocharger matching and contribute to the high efficiency of the TCA turbochargers.

MAN Diesel debuts VTA turbocharger on low-speed marine diesel

The company reports that its new variable turbine area (VTA) technology will be developed for all the models in its TCR series radial and TCA series axial turbochargers,

Using our VTA system, we can more precisely match the volume of charge air to the quantity of injected fuel at all points on an engine’s load profile.

The result is reduced specific fuel consumption in combination with reduced HC and CO emissions and improved dynamic behaviour of the engine-turbocharger system.

In detail, the VTA system consists of a nozzle ring, equipped with adjustable vanes, which replaces the fixed-vane nozzle rings fitted in MAN Diesel ’s standard TCA turbochargers. By adjusting the vanes’ pitch, the pressure of the exhaust gases can be regulated and the compressor output optimised at all points on the engine’s performance map. In order to minimise thermal hysteresis and improve adjustment accuracy, each vane has a lever which is directly connected to a control ring. The control ring is actuated by two electric, positional motors with integrated reduction gear

The adjustable vanes are manufactured in heat- and erosion-resistant steel alloy.

Control of vane position is fully electronic with feedback or open loop control with mapped vane adjustment. A comprehensive range of control signals can be used, including charge-air pressure after the compressor and exhaust-gas temperature before and after the turbocharger. The inclusion of VTA technology on the axial TCA55 turbocharger allows up to 0.5 bars in variation in compressor output pressure at part-load,”

As a further benefit, it has also been established in relation to the two-stroke engine that the additional charge-air pressure at part load from the VTA turbocharger allows the electrically driven auxiliary blowers to be switched off at lower loads.

vane-adjustment angles range from 15% below the smallest, fixed-nozzle ring to 20% above the largest. The vane-adjustment range is matched to the charge-air requirements of the engine via the software of the electronic control system.

overall results from the test bench showed expected improvements at part-load in terms of fuel consumption, as well as considerable reductions in emissions of soot and unburnt hydrocarbons. Additionally, there was potential for improved engine response under load changes

All TCA turbochargers come equipped with a minimum of external connections. On the engine

side and in the plant system itself the only additional connections, apart from the air and exhaustgas connections already mentioned, is an oil inlet and drain pipe, together with a breather

connection for the bracket.

The TCA requires neither cooling-water nor lock-air connections, let alone an external luboil supply. In order to ensure that it can continue to operate in an emergency, the unit can be supplied with a special emergency operation facility integrated into the turbocharger. The TCA series represents the consistent continuation of the "pipeless engine" principle.

Inlet-side noise emission was reduced by optimising the fluid mechanics of the compressor

wheel. The aim was to find the best compromise between high compressor efficiency and noise

emission. Additionally, the silencer was designed with radially -arranged curved plate elements.

With this innovative silencer concept noise emissions can be dampened to a very high degree

whilst intake pressure losses are kept low.

Comparison of the charge-air delivery characteristics of MAN Diesel turbochargers with fixed

nozzle rings and VTA technology