Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines

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Turbo Compound systems

The high efﬁciency of some modern large turbochargers is comfortably in

excess of that required to pressure charge engines, allowing some of the

exhaust gas to be diverted to drive a power turbine. A number of these turbo

compound installations have entered service, mainly in conjunction with MAN

B&W and Wärtsilä (Sulzer) low-speed engines, the investment particularly

worthwhile in an era of high fuel prices.

The power gas turbine is arranged either to supplement directly the propul-

sive effort of the engine via power take-in (PTI) gearing to the crankshaft or to

drive a generator via power take-out/power take-in (PTO/PTI) gearing. Sulzer’s

efﬁciency booster system—based on a concept of ABB Turbo Systems—prom-

ised fuel savings of up to 5.5 g/kW h, raising the maximum engine efﬁciency to

kBB turbochargers 229

25 50 75 % 100

3.5

bar

3.0

1.5

550

°C

500

450

400

350

300

2.5

2.0

1.0

110

105

100

Rec

Figure 7.30 Turbocharger retrot benets exhaust-gas temperatures and specic

fuel consumption of an aBB VTr..1 turbocharger exhibiting low eciency (dashed

lines) compared with a replacement VTr..4 turbocharger operating with a markedly

higher eciency (continuous lines)

230 Pressure Charging

54 per cent with virtually no effect on the exhaust gas energy available for con-

ventional waste heat recovery (Figure 7.31).

Apart from yielding worthwhile fuel savings throughout the engine load

range, the efﬁciency booster system can be an attractive alternative to engine

derating for an equivalent brake speciﬁc fuel consumption. Compared with a

derated engine, the system may allow the saving of one cylinder (depending on

the type of engine and its power/speed rating), with consequent beneﬁts in both

a lower ﬁrst cost and a shorter engineroom.

The efﬁciency booster is described as a straightforward on/off system,

which can be exploited above 40–50 per cent engine load without undermining

engine reliability. Exhaust energy is recovered simply by diverting a proportion

Emergency

bypass

line with

orifice

Schematic arrangement

of the Sulzer Efficiency-

Booster system

Flap

Power take-off gear

Hydraulic coupling

BBC Epicyclic gear

Reduction in brake

specific fuel

consumption using

the Sulzer Efficiency-

Booster system

- Booster

Off on

BSFC

(g/kWh)

(g/bhph)

BSFC of engine without-Booster

BSFC of engine with-Booster

30 40 50 60 70 80 90 100

Power [%]

Turbocharger

BBC VTR-4A

Exhaust gas receiver

BBC NTC-4

Power turbine

Engine

Exhaust Air

gas

Scavenge air receiver

–5

Figure 7.31 sulzer eciency booster system layout and performance

of the exhaust gas from the exhaust manifold to a standard ABB turbocharger-

type power turbine arranged in parallel with the engine turbochargers. The

power turbine—forming a compact module with its integrated epicyclic gear—

is mechanically coupled to the Wärtsilä (Sulzer) RTA low-speed engine’s

integral PTO gear. The recovered energy is thus directly fed to the engine

crankshaft, allowing the engine to run at a correspondingly reduced load to

deliver the same total power to the propeller shaft—but with a correspondingly

lower fuel consumption. The result depends on the engine’s contract-maximum

continuous rating since fuel savings increase with bmep.

The simple mechanical arrangement, based on standard components, under-

writes a low ﬁrst cost and offers a high transmission efﬁciency. It also allows

the efﬁciency booster to be shut off at engine part load and when manoeuvring.

Similar beneﬁts are reported for MAN Diesel’s turbo compound system/

power take-in (TCS/PTI) arrangement, which can be ﬁtted to its MC low-speed

engines with bore sizes from 600 mm to 900 mm (Figure 7.32). The system is

not applicable to smaller models because of their size and the lower efﬁciency

of the smaller turbochargers used. MAN Diesel and ABB power turbines are

used, respectively, in association with the MAN Diesel and ABB turbochargers

serving the engine. At engine loads above 50 per cent of the optimized power,

the power turbine is connected to the crankshaft via a hydraulic clutch and

ﬂexible coupling.

kBB turbochargers 231

Power turbine

Epicyclic gear

Hydraulic

coupling

Bevel gear

Crankshaft

Flexible coupling

Toothed coupling

Crankshaft gear

Figure 7.32 arrangement of Man TCs/PTi system on the main engine and the

power turbine drive to the crankshaft

232 Pressure Charging

The amounts of exhaust gas actually available for the TCS depends on the

turbocharger efﬁciency and turbocharger matching chosen for the individual

case. TCS power falls off at lower loads owing to the smaller gas amounts:

at 50 per cent of the optimized engine power the output of the TCS/PTI

unit is around 25 per cent, returning negligible savings in the speciﬁc fuel

consumption of the engine. The maximum power turbine output obtainable is

4.2 per cent of the engine’s optimized power, the yield depending on the tur-

bocharger selected.

MAN Diesel’s PTO/PTI system combines a PTO serving a generator and

a TCS/PTI unit (power turbine coupled to the crankshaft gear). The combined

beneﬁts of both systems are achieved without incurring the full ﬁrst cost asso-

ciated with each as some of the most costly elements (e.g., the crankshaft gear)

are common.

Engines equipped with high-efﬁciency turbochargers can operate without

auxiliary blowers at lower loads (down to around 25 per cent) than engines

with standard turbochargers (around 35 per cent).

Based on its MET-MA turbocharger’s turbine design, Mitsubishi has devel-

oped the MPT-A series power turbine for application in advanced waste heat

recovery turbo compound systems.

Turbocharger generator/Motors

Advances in high-speed generator technology as well as in high-capacity elec-

tronic switching devices have fostered the practical development of compact,

high-output generators and frequency converters suitable for use in combination

with turbochargers. Such so-called hybrid turbochargers—in which a high-

speed generator is attached directly to the turbocharger rotor shaft—offer these

advantages over conventional turbo compound systems based on a power gas

turbine, according to Mitsubishi:

l Elimination of piping and valves associated with feeding exhaust gas to

a power turbine

l Control of turbocharger performance by raising and lowering generator

output

l Use of the generator as a motor as a substitute for the auxiliary blower

of two-stroke engines; motor mode can also be applied to accelerate the

turbocharger rotor and improve turbocharger performance, particularly

at the low load range of the engine

l High efﬁciency due to energy conversion by the turbocharger turbine.

Working in conjunction with a specialist generator manufacturer, Mitsubishi

has developed a suitable high-speed/high-capacity generator for use with hybrid

turbochargers. The four-pole permanent magnet synchronous machine has a rated

speed of 18 700 rev/min and a maximum permissible speed of 22 300 rev/min,

with a maximum output rating of 252 kW.

Mitsubishi summarizes that hybrid turbochargers incorporating a genera-

tor/motor enable the combustion air to be adjusted optimally in response to

engine operating conditions by raising and lowering the amount of output

power generated, or—in motor mode—the amount of electrical power used

to assist the turbocharger (representing a possible new mode of controlling

diesel engines).

See also Chapter 30 for sequential turbocharging (MTU and SEMT-Pielstick)

and two-stage turbocharging systems (Niigata and Paxman); Chapter 27 for

Wärtsilä SPEX system and Chapter 11 for Mitsubishi two-stage turbocharging

system.

kBB turbochargers 233

235

C h a p t e r | e i g h t

Fuel Injection

The emphasis in this chapter is on four-stroke medium- and high-speed engines

with camshaft-actuated individual jerk pumps for each cylinder or common

rail (CR) fuel injection systems and does not apply, except in a general way,

to low-speed two-stroke engines whose fuel injection systems are discussed in

later chapters under individual designs.

InjeCtIon and CombustIon

The essence of a diesel engine is the introduction of ﬁnely atomized fuel into

the air compressed in the cylinder during the piston’s inward stroke. It is, of

course, the heat generated by this compression, which is normally nearly adi-

abatic, that is crucial in achieving ignition. Although the pressure in the cylin-

der at this point is likely to be anything up to 230 bar, the fuel pressure at the

atomizer will be of the order of 1300–1800 bar.

High injection pressures at full load are beneﬁcial in terms of fuel econ-

omy, emissions and the ability to accept inferior fuel. Most modern medium-

speed engines attain 1200–1800 bar in the high-pressure (HP) injection pipe,

although some recent engine designs achieve as much as 2300 bar when pump-

ing heavy fuel. For reasons of available technology, the earliest diesel engines

had to use compressed air to achieve atomization of the fuel as it entered the

cylinder (air blast injection), and while airless (or solid) injection delivered a

signiﬁcant reduction in parasitic loads, it also presented considerable problems

in the need for high precision manufacture, and the containment of very high

and complex stresses.

The very high standard of reliability and lifetime now attained by modern

fuel injection systems is the result of considerable investment in R&D and pro-

duction technology by designers and equipment manufacturers. Ultra-high pre-

cision machining facilities for ﬂat and curved surfaces are dictated for injection

systems operating at pressures of up to 2000 bar and beyond, calling for mini-

mal manufacturing tolerances down to 0.001 mm and the highest cleanliness

in all processes. A particularly challenging component is the injector needle,

236 Fuel Injection

which, in some systems, requires microscopic helical grooves to be machined

into the pin shaft to hydraulically absorb lateral forces with the aid of the fuel.

In the early days of airless injection, many ingenious varieties of combus-

tion chamber were used, sometimes mainly to reduce noise or smoke, or to

ease starting, but often in part to reduce or to use modest injection and com-

bustion pressures. A growing emphasis on economy and speciﬁc output, cou-

pled with materials development and advances in calculation methods allowing

greater loads to be carried safely, has left the direct injection principle domi-

nant in modern medium- and high-speed engine practice.

In direct injection systems the fuel is delivered directly into a single com-

bustion chamber formed in the cylinder space (Figure 8.1), atomization being

achieved as the fuel issues from small drillings in the nozzle tip.

For complete combustion of the fuel to take place, every droplet of fuel must

be exposed to the correct proportion of air to achieve complete oxidation, or to

an excess of air. In the direct injection engine, the fuel/air mixing is achieved

by the energy in the fuel spray propelling the droplets into the hot, dense air.

Additional mixing may be achieved by the orderly movement of the air in the

combustion chamber, which is called ‘air swirl’. Naturally aspirated engines usu-

ally have a degree of swirl and an injection pressure of around 800 bar. Highly

turbocharged engines with four-valve heads have virtually no swirl, but typically

have an injection pressure of 1200–1800 bar to provide the mixing energy.

Where indirect injection is exploited, some high-speed engines retain a pre-

chamber in the cylinder head into which fuel is injected as a relatively coarse

spray at low pressure, sometimes using a single hole. Combustion is initiated

in the pre-chamber, the burning gases issuing through the throat of the chamber

to act on the piston (Figure 8.2).

FIgure 8.1 Cross-section of direct injection combustion chamber

FIgure 8.2 Indirect injection: the ricardo pre-chamber

Fuel/air mixing is achieved by a very high air velocity in the chamber, the

air movement scouring the walls of the chamber and promoting good heat

transfer. Thus the wall can be very hot—requiring heat-resistant materials—

but it can also absorb too much heat from the air in the initial compression

strokes during starting and prevent ignition. It is these heat losses that lead to

poor starting and inferior economy. Further forms of assistance, such as glow

plugs, have therefore sometimes been necessary to achieve starting when ambi-

ent pressures are low. The throttling loss entailed by the restricting throat also

imposes an additional fuel consumption penalty.

One engine designer, SEMT-Pielstick, achieved an ingenious combination

of the two systems by dividing the pre-chamber between cylinder head and

piston crown. At top dead centre a stud on the piston enters the pre-chamber

to provide a restricted outlet. On the expansion stroke, the restriction is auto-

matically removed and fuel economy comparable with normal direct injection

engines is attainable (Figure 8.3).

Direct injection, too, has variants, which reﬂect the fact that, despite

considerable expenditure on research into its mechanism, the detail of how

combustion develops after ignition is achieved is still largely empirical. The

essentials are as follows:

1. At least some of the fuel injected is atomized sufﬁciently ﬁnely to initi-

ate combustion. Ignition cannot take place until a droplet of fuel has

reached the temperature for spontaneous ignition. Since heat is taken

up as a function of surface area (proportional to the square of the dia-

meter) and the quantity of heat needed to achieve a temperature rise is

a function of volume (varying as the cube of the diameter), only a small

number of ﬁne droplets are needed to initiate combustion. High-speed

photography of combustion does indeed show that ignition takes place

in a random manner near the injector tip and usually outside the main

core of the spray.

2. The fuel should mix with the air in order to burn. Since most of the

air in a roughly cylindrical space is, for geometrical reasons, near the

periphery, most of the fuel must penetrate there, and this is easier with

large droplets; hence, also, the use of wide core angles and multiple

spray holes.

Injection and combustion 237

FIgure 8.3 the variable geometry combustion chamber of semt-Pielstick

238 Fuel Injection

3. Under no circumstances must fuel reach the liner walls, or it will con-

taminate the lubricating oil. An advantage of combustion spaces formed

in piston crowns is that the walls of the chamber form a safe target at

which spray may be directed. This type of combustion chamber in the

piston has the further advantage that, during the piston descent, air

above the piston periphery is drawn into the combustion process in a

progressive manner.

4. The injection period should be reasonably short and must end sharply.

Dribble and secondary injection are frequent causes of smoke, and also

of lubricating oil becoming diluted with fuel or loaded with insoluble

residues. Dribble is the condition where fuel continues to emerge from

the nozzle at pressures too low to atomize properly; it is caused by bad

seating faces or slow closure.

Secondary injection is what happens when the pressure wave caused by the

end of the main injection is reﬂected back to the pump and then again to the

injector, reaching it with sufﬁcient pressure to reopen the injector at a relatively

late stage in combustion. Any unburned or partly burned fuel may ﬁnd its way

onto the cylinder walls and be drawn down by the piston rings to the sump.

InjeCtor

Working backwards from the desired result to the means to achieve it, the

injector has to snap open when the timed HP wave from the pump travelling

along the HP pipe has reached the injector needle valve. Needle lift is limited

by the gap between its upper shoulder and the main body of the holder. Needle

lift is opposed by a spring, set to keep the needle seated until the ‘blow-off

pressure’ or ‘release pressure’ of the injector is reached by the fuel as the pres-

sure wave arrives from the pump. This pressure is chosen by the enginebuilder

to ensure that there is no tendency for the needle to reopen as the closing pres-

sure waves are reﬂected back and forth along the HP pipe from the pump. The

setting also has some effect on injection delay and the quality of injection; it is

usually chosen to be between 200 bar and 300 bar.

The needle valve is invariably provided with an outer diameter on which

the fuel pressure acts to overcome the spring pressure and cause the initial lift.

This brings into play the central diameter of the needle so that it snaps open to

the lift permitted.

The needle and the seat cones are usually ground to a differential angle

so that contact is made at the larger diameter (Figure 8.4). When the needle is

only slightly open, the greater restriction to ﬂow is at the outer rather than the

inner diameter of the seat. This ensures that as the needle lifts, there is a sud-

den change of pressurized area and the needle force against the spring changes

correspondingly, giving a very rapid lift to fully open; and conversely when

closing. (This is why it is bad practice to lap the needle to its seat.)

Control of the temperature of the injector, particularly of the sensitive

region round the needle seat and the sac, is very important. This is especially so

when heavy fuels are used, both because the fuels themselves have to be heated

and because they tend to burn with a more luminous ﬂame that increases the

heat input to all the metal surfaces. If the tip temperature rises too high, the fuel

remaining in the sac after injection boils and spits from the spray holes: it is

not properly burned and forms a carbon deposit around each spray hole. Such

Injector 239

FIgure 8.4 detail of needle and seat showing dierential angle (exaggerated)

(a) (b) (c) (d)

FIgure 8.5 Comparison of traditional and low inertia injectors: (a) conventional

injector, (b) and (c) lightweight spindles, (d) low-inertia injector (delphi bryce)