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

Подождите немного. Документ загружается.

particularly on starting. This feature, known as ‘injector drive turbocharging’, was

introduced on the KSZ-type engines but was replaced by a system of electrically

driven blowers for the KSZ-A and subsequent engine models. These blowers cut

in and out as a function of scavenge air pressure and usually remain in operation

until the engine reaches 50 per cent load. The blowers are placed ahead of the

turbocharger and need to be in operation before the engine will start.

Turbocharging is based on the constant pressure system and embodies low

ﬂow resistance air and exhaust piping with suitable diffusers between the ports

and the manifolds. For engine outputs of up to 8000 kW, only one turbocharger

is used. All engines above this power require two chargers, one at each end

of the engine at the cylinder head level, although turbochargers have been

mounted on special base frames together with the auxiliary blower adjacent to

the engine.

Design particulars 503

FigurE 15.15 Fuel injector pump

504 MAN Low-Speed Engines

ELECtroNiC FuEL iNjECtioN

A system developed for MAN engines—though never applied to produc-

tion models—was electronically controlled fuel injection (Figure 15.16). The

‘heart’ of the system was a microprocessor to which the actual engine speed

and crankshaft position were transmitted as output signals. When the desired

speed is compared with the actual speed, the processor automatically retrieves

the values of injection timing and pressure for any load under normal operat-

ing conditions. In the event of deviations from normal operating conditions,

additional signals are transmitted to the microprocessor, either manually or

by appropriate temperature and pressure sensors. The reported result was

an engine operating at optimum injection pressure and timing at all times.

Adjustments to the system during operation were possible.

Corrections could be made for adapting the injection system to varying

conditions such as ballast voyages and bad weather, matching the engine to

different fuels, ignition lag and cetane numbers, readjusting for operation in

the tropics or in winter, controlling the injection characteristics of individual

cylinders, such as when running in and reducing the minimum slow running

speed (e.g., during canal transits).

Fuel delivery pump

Fuel delivery pipe

Electro-hydraulically controlled

injection valve

Fuel delivery pipe

Fuel pressure

accumulator

Electric pulse cable

‘Fuel pressure’

Electric pulse cable

‘Injection’

Electric pulse cable

‘Crank angle’

Electronic control

unit

Electric pulse cable

‘Pressure

measurement’

Connection to servo-

oil circuit

Resolver

FigurE 15.16 Components of electronic injection system

injection System Components

The most noticeable feature of a KEZ-type engine with electronic fuel injection

is the absence of a camshaft and individual fuel pumps. The fuel is delivered to

a high-pressure accumulator by normal fuel injection plunger pumps driven by

gears and cams from the crankshaft. These pumps are located above the thrust

bearing and, depending on the size and cylinder number of the engine, two or

more are used. To safeguard against failure of a pump or a pressure pipe, two

independent fuel delivery pipes are placed between the high-pressure pumps

and the accumulator.

The high-pressure accumulator generates a buffer volume to prevent the

pressure in the injection valve from dropping below an acceptable limit when

the fuel is withdrawn during injection into the cylinder. For the electronically

controlled injection system metering of the fuel volume for one cylinder is

not effected by limiting the delivery volume per stroke of the injection pump

plunger, as in conventional systems, but by limiting the duration of injection.

This is initiated by an electronic controller: a low-voltage signal which con-

trols the opening and closing actions of an electro-hydraulic servo valve. The

electro-hydraulic valve is actuated by oil under pressure from two pumps and

an air cushion-type accumulator (Figure 15.17).

Sequential control ensures that, according to the engine ﬁring sequence,

the correct engine cylinder is selected in the proper crank angle range and that

Electronic fuel injection 505

Starting

control valve

Injection

valve

To the other cylinders

Starting

valve

Electronic

control

unit

Main starting

valve

Quick-acting

relief valve

Pressure switch

Resolver

Fuel

pressure

pump

Electro-

hydraulic

positioner

Tank

Pressure

transducer

Remote

pressure

gauge

Relief

valve

Flow control

servo-valve

FigurE 15.17 Schematic arrangement of electronic injection system

506 MAN Low-Speed Engines

the electronically controlled injection valve is actuated at the exact point of

injection.

For engine starting and reversing, the correct injection timing is linked with

the electronically controlled starting air valves. Because electronically control-

led starting valves have no time delay between the switching pulse and opening

of the air valve, and because there are no ﬂow losses due to the elimination of

the control air pipes, the starting air requirement is reduced in comparison with

the former conventional air start system.

The constant pressure maintained during fuel injection with the electroni-

cally controlled fuel injection system, and the fact that the injection pressure

can easily be matched to each load condition, foster lower fuel consumption

rates over the entire load range. It also means satisfactory operation at very

slow speeds, which contributes favourably to the manoeuvrability of a vessel;

a K3EZ 50/105 C/CL engine on the test bed was operated at a dead slow speed

of 30 rev/min, which had never been achieved before. The full load speed is

183 rev/min.

(Electronic fuel injection has since been applied by MAN Diesel to produc-

tion low speed engines (Chapter 10).)

507

C h a p t e r | s i x t e e n

Medium-Speed

Engines—

Introduction

New designs and upgraded versions of established models have maintained the

dominance of medium-speed four-stroke diesel engines in the propulsion of

smaller ships as well as larger specialist tonnage such as cruise vessels, car/

passenger ferries and ro-ro freight carriers. The larger bore designs can also

target the mainstream cargo ship propulsion market formed by bulk carriers,

container ships and tankers, competing against low-speed two-stroke machin-

ery, and dual-fuel versions have found a new volume market in LNG carrier

propulsion. The growth of the fast passenger and ropax ferry sector beneﬁted

those medium-speed enginebuilders who could offer designs with sufﬁciently

high power/weight and volume ratios, an ability to function reliably at full load

for sustained periods, and attractive through-life operating costs. Medium-

speed engines further enjoy supremacy in the deep-sea vessel genset drive

sector, challenged only in lower power installations by high-speed four-stroke

engines.

Signiﬁcant strides have been made in improving the reliability and dura-

bility of medium-speed engines in the past decade, both at the design stage

and through the in-service support of advanced monitoring and diagnostic

systems. Former weak points in earlier generations of medium-speed engines

have been eradicated in new models, which have beneﬁted from ﬁnite element

method calculations in designing heavily loaded components. Designers now

argue the merits of new generations of longer stroke medium-speed engines

with higher speciﬁc outputs allowing a smaller number of cylinders to satisfy a

given power demand and foster compactness, reliability, reduced maintenance

and easier servicing. Progress in fuel and lubricating oil economy is also cited,

along with enhanced pier-to-pier heavy fuel burning capability and better per-

formance ﬂexibility throughout the load range.

Completely bore-cooled cylinder units and combustion spaces formed by

liner, head and piston combine good strength and stiffness with good temperature

508 Medium-Speed Engines—Introduction

control, which are important factors in burning low-quality fuel oils. Low noise

and vibration levels achieved by modern medium-speed engines can be reduced

further by resilient mounting systems, a technology which has advanced con-

siderably in recent years.

IMO limits on nitrogen oxide emissions in the exhaust gas can generally

be met comfortably by medium-speed engines using primary measures to

inﬂuence the combustion process (in some cases, it is claimed, without com-

promising speciﬁc fuel consumption). Wärtsilä’s low NOx combustion tech-

nology, for example, embraces high fuel injection pressures (up to 2000 bar) to

reduce the duration of injection, a high compression ratio (16:1), a maximum

cylinder pressure of up to 210 bar and a stroke/bore ratio 1.2:1. Concern over

smoke emissions, particularly by cruise ship operators in sensitive environmen-

tal areas, has called for special measures from engine designers targeting that

market, notably electronically controlled common rail (CR) fuel injection and

fuel–water emulsiﬁcation.

The number of leading medium-speed enginebuilders with established CR

designs—including Caterpillar (MaK), MAN Diesel and Wärtsilä—suggests

that such systems will eventually dominate production programmes to ben-

eﬁt ship operators and the environment. Signiﬁcant advantages are delivered

in engine operational ﬂexibility, economy and environmental friendliness by a

system in which the generation of fuel pressure and the injection of fuel are not

interconnected.

In contrast to a conventional system, the injection pressure in a CR conﬁg-

uration is independent of engine speed, and full pressure is always available at

all loads down to idling. Highly efﬁcient and clean combustion is thus fostered

across the engine operating range, yielding economic and environmental mer-

its. An optimum injection pressure and timing can be selected for a given oper-

ating mode—irrespective of the engine speed—and pilot and post-injection

patterns exploited to meet differing demands: for example, invisible exhaust

at the lowest loads and NOx emission reductions at medium loads, without

undermining fuel economy.

The concept of electronically controlled CR fuel systems had been appre-

ciated for many years before the recent wave of R&D and implementation in

engine programmes. Feasible solutions, however, awaited the development of

fast and reliable rail valves and electronic controls. Advances in materials and

manufacturing technology also allowed the creation of systems capable of hand-

ling heavy fuel and pressures of 1500 bar and above.

Successful automotive diesel applications of CR layouts, in car and truck

power units, were largely driven by stricter emission regulations, which called

for ﬂexible fuel injection systems offering injection rate shaping, free adjust-

ment of injection pressure, variable start of injection and pre- and post-injection

patterns. Current and future emission curbs on shipping stimulated the trans-

fer to marine engines. Increasingly stringent regulations on NOx and smoke

in the exhaust will be difﬁcult to meet without intelligent controls and a ﬂexible

injection system if engine efﬁciency is to remain the same. Part-load operation

imposes a particular challenge in satisfying invisible smoke requirements with-

out CR technology.

Ease of inspection and overhaul—an important consideration in an era of

low manning levels and faster turnarounds in port—was addressed in the lat-

est designs by a reduced overall number of components (in some cases, 40 per

cent fewer than in the preceding engine generation) achieved by integrated and

modular assemblies using multi-functional components. Simpliﬁed (often

plug-in or clamped) connecting and quick-acting sealing arrangements also

smooth maintenance procedures. Channels for lubricating oil, cooling water,

fuel and air may be incorporated in the engine block or other component cast-

ings, leaving minimal external piping in evidence. Compact and more accessi-

ble installations are achieved by integrating ancillary support equipment (such

as pumps, ﬁlters, coolers and thermostats) on the engine. Lower production

costs are also sought from design reﬁnements and the wider exploitation of

ﬂexible manufacturing systems to produce components.

The cylinder unit concept is a feature of the modern four-stroke designs,

allowing the head, piston, liner and connecting rod to be removed together as

a complete assembly for repair, overhaul or replacement by a renovated unit

onboard or ashore. This modular approach was adopted by—amongst others—

MAN Diesel for its L16/24, L21/31 and L27/38 designs, by MTU for its Series

8000 engine, by Rolls-Royce for its Bergen C series, and by Hyundai for its

HiMSEN designs: all detailed in the subsequent chapters.

Compactness and reduced weight remain the key attractions of the medium-

speed engine, offering ship designers the opportunity to increase the cargo

capacity and lower the cost of a given newbuilding project, and the ability to

achieve the most efﬁcient propeller speed via reduction gearing. Medium-

speed enginebuilders can offer solutions ranging from single-engine plants

for small cargo vessels to multi-engine/twin-screw installations for the most

powerful passenger ships, based on mechanical (geared) or electrical transmis-

sions (see Chapter 6). Multi-engine conﬁgurations promote plant availability

and operational ﬂexibility, allowing the number of prime movers engaged at

any time to match the service schedule. The convenient direct drive of alterna-

tors and other engineroom auxiliary plant (e.g., hydraulic power packs) is also

facilitated via power take-off gearing.

Among the design innovations in recent years must be noted Sulzer’s use

of hydraulic actuation of the gas exchange valves for its ZA50S engine, the

ﬁrst time that this concept (standard on low-speed two-stroke engines for many

years) had been applied to medium-speed four-stroke engines (Figure 16.1).

In conjunction with pneumatically controlled load-dependent timing to secure

variable inlet closing, hydraulic actuation on the ZA50S engine allowed ﬂexi-

bility in valve timing, fostering lower exhaust gas emissions and improved fuel

economy.

Variable inlet closing, combined with optimized turbocharging, contrib-

uted to a very ﬂat fuel consumption characteristic across the load range of the

engine as well as a considerable reduction in smoke levels in part-load operation.

Medium-speed engines—introduction 509

510 Medium-Speed Engines—Introduction

The ZA50S engine, like the smaller bore ZA40S design it was derived from,

features Sulzer’s rotating piston, which was also exploited in GMT’s upgraded

550-mm bore medium-speed engine.

Wärtsilä’s 46 engine exploited a number of innovations in medium-speed

technology, originally including a twin fuel injection system (featuring pilot

and main injection valves), thick-pad bearings (large bearings with thick oil

ﬁlms) and pressure-lubricated piston skirts. The twin injection system was later

superseded as advances in fuel injection technology allowed a single-valve sys-

tem to be applied.

The operating ﬂexibility of MAN Diesel’s L32/40 design beneﬁts from

the provision of separate camshafts, arranged on either side of the engine, for the

fuel injection and valve actuating gear. One camshaft is dedicated to drive

the fuel injection pumps and to operate the starting air pilot valves; the other

serves the inlet and exhaust valves. Such an arrangement allows fuel injection

and air charge renewal to be controlled independently, and thus engine opera-

tion to be more conveniently optimized for either high fuel economy or low

exhaust emissions mode. Injection timing can be adjusted by turning the cam-

shaft relative to the camshaft driving gear, an optional facility (Figure 16.2).

The valve-actuating camshaft can be provided with different cams for full-load

and part-load operations, allowing valve timing to be tailored to the conditions.

A valve camshaft-shifting facility is optional, the standard engine version fea-

turing just one cam contour (Figure 16.3).

FIgurE 16.1 The rst example of the Sulzer ZA50S engine, a nine-cylinder model,

on test. The design (no longer produced) was distinguished by hydraulic actuation of

the gas exchange valves

A carbon-cutting ring is now a common feature of medium-speed engines

speciﬁed to eliminate the phenomenon of cylinder bore polishing caused by

carbon deposits and hence signiﬁcantly reduce liner wear. It also fosters a

cleaner piston ring area, low and very stable lubricating oil consumption, and

reduced blow-by.

Also termed an anti-polishing or ﬁre ring, a carbon-cutting ring comprises

a sleeve insert, which sits between the top piston ring turning point and the

top of the cylinder liner. It has a slightly smaller diameter than the bore of the

liner, this reduction being accommodated by a reduced diameter for the top

Medium-speed engines—introduction 511

FIgurE 16.2 Optimization of fuel injection timing on MAN Diesel’s L32/40 engine is

facilitated by turning the dedicated camshaft relative to its driving gear

512 Medium-Speed Engines—Introduction

land of the piston. The main effect of the ring is to prevent the build-up of

carbon around the edges of the piston crown, which causes liner polishing and

wear, with an associated rise in lubricating oil consumption.

A secondary function is a sudden compressive effect on the ring belt as

the piston and carbon-cutting ring momentarily interface. Lubricating oil is

consequently forced away from the combustion area, again helping to reduce

consumption: so effectively, in fact, that Bergen Diesel found it necessary to

redesign the ring pack to allow a desirable amount of oil consumption. The

Norwegian engine designer reports that lubricating oil consumption is cut by

FIgurE 16.3 Variable valve timing on MAN Diesel’s L32/40 engine is secured by

dierent cams for part- and full-load operations