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

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A diesel–electric tanker can exploit the abundance of electrical power to

drive a low-noise cargo pumping outﬁt, perhaps reducing time in port; and

(particularly in the case of a dynamically positioned shuttle carrier) to serve

powerful bow and stern thrusters during sensitive manoeuvring at the loading

buoy (Figure 6.7).

Load diversity: Certain types of tonnage have a requirement for substan-

tial amounts of power for ship services when the demands of the propulsion

system are low: for example, tankers and any other ship with a signiﬁcant

cargo discharging load. A large auxiliary power-generating plant for cruise

ships and passenger ferries is dictated by the hotel services load (air condition-

ing, heating and lighting) and the heavy demands of transverse thrusters dur-

ing manoeuvring. The overall electrical demand may be 30–40 per cent of the

installed propulsion power and considerable standby capacity is also called for

to secure system redundancy for safety. These factors have helped to promote

the popularity of the medium-speed diesel–electric ‘power station’ concept for

meeting all propulsion, manoeuvring and hotel energy demands in large pas-

senger ships.

Economical part-load running: This is best achieved when there is a central

power station feeding propulsion and ship services. A typical medium-speed

diesel–electric installation features four main gensets (although plants with up

to nine sets are in service) and, with parallel operation of all the sets, it is easy

to match the available generating capacity to the load demand. In a four-engine

plant, for example, increasing the number of sets in operation from two that are

fully loaded to three partially loaded will result in three sets operating at 67 per

cent load: not ideal but not a serious operating condition.

It is not necessary to operate gensets at part load to provide the spare capac-

ity for the sudden loss of a set: propulsion load reduction may be available

instantaneously and in most ships a short-term reduction in propulsive power

Diesel–electric drive 169

Diesel generators

(LV system)

Cargo pump

COP

(LV system)

Cargo pumps

Thrusters

Propulsion motor

Thrusters

Cyclo

COP

COPBT

Main switchboard

FigurE 6.7 Diesel–electric ‘power station’ meeting the load demands of propulsion,

cargo pumping, thrusters and hotel services in a tanker

170 Engine and Plant Selection

does not represent a hazard. The propulsion plant controls continuously

monitor-generating capability, any generator overload immediately resulting in

the controls adjusting the input to the propulsion motors. During manoeuvring,

propulsion plant’s requirements are below system’s capacity and the failure of

one generator is unlikely to present a hazardous situation.

Ease of control: The widespread use of CP propellers means that control

facilities once readily associated with electric drives are no longer able to

command the same premium. It is worth noting, however, that electric drives

are capable of meeting the most exacting demands with regard to dynamic

performance, exceeding by a very wide margin anything that is required of a

propulsion system.

Low noise: An electric motor can provide a drive with very low-vibration

characteristics, a quality valued for warships, research vessels and cruise ships

where, for different reasons, a low-noise signature is required. In the case of

warships and research vessels it is noise into the water which is the critical

factor; in cruise ships it is structure-borne noise and vibration to passenger

spaces that are desirably minimized. Electric transmission allows the prime

movers to be mounted in such a way that the vibration transmitted to the

seatings, and hence to the water or passenger accommodation, is minimized.

The use of double-resilient mounting may be necessary for very low-noise

installations.

Environmental protection and ship safety: Controls to curb noxious exhaust

gas emissions—tightening nationally, regionally and globally—also favour the

speciﬁcation of electric transmission since the constant speed running of die-

sel prime movers optimized to the load is conducive to lower NOx emission

levels. An increasing focus on higher ship safety through redundancy of pro-

pulsion plant elements is another positive factor. A twin-propeller installation

is not the only method of achieving redundancy. Improving the redundancy

of a single-screw ship can also be secured by separating two or more propul-

sion motors in different compartments and coupling them to a reduction gear

located in a separate compartment. The input shafts are led through a water-

tight bulkhead.

Podded Propulsors

Signiﬁcant economic and technical beneﬁts in ship design, construction

and operation are promised by electric podded propulsors, whose appeal has

extended since the early 1990s from icebreakers and cruise liners to offshore

vessels, ferries and tankers.

A podded propulsor (or pod) incorporates its electric drive motor in a hydro-

dynamically optimized submerged housing which can be fully rotated with the

propeller(s) to secure 360° azimuthing and thrusting capability (Figure 6.8).

The motor is directly coupled to the ﬁxed pitch propeller(s) mounted at

either or both ends of the pod. Pusher and tractor or tandem versions can be

speciﬁed, with input powers from 400 kW to 30 000 kW supplied by a shipboard

generating plant. The merits cited for pods over traditional diesel–electric

installations with shaftlines highlight:

l Space within the hull otherwise reserved for conventional propulsion

motors can be released and exploited for other purposes.

l More creative freedom for ship designers since the propulsors and the

prime movers require no direct physical connection.

l Steering capability is signiﬁcantly better than with any conventional

rudder system; stern thruster(s) can be eliminated, along with rudder(s)

and shaftline(s).

l Excellent reversing capability and steering during astern navigation, and

enhanced crash stop performance.

l Low-noise and vibration characteristics associated with electric drive

systems are enhanced by the motor’s underwater location; and hull exci-

tations induced by the propeller are very low, thanks to operation in an

excellent wake ﬁeld.

l Propulsor unit deliveries can be late in the shipbuilding process, reduc-

ing ‘dead time’ investment costs; savings in overall weight and ship con-

struction hours are also promised.

Combined Systems

A combination of geared diesel and diesel–electric drive is exploited in some

offshore support vessels whose deployment proﬁle embraces two main roles:

(1) to transport materials from shore bases to rigs and platforms and (2) to

standby at the offshore structures for cargo and anchor handling, rescue and

Diesel–electric drive 171

FigurE 6.8 three 14 000 kW azipod propulsors power royal Caribbean Cruises’

voyager-class liners

172 Engine and Plant Selection

other support operations. In the ﬁrst role the propulsive power required is that

necessary to maintain the free running service speed. In the second role, little

power is required for main propulsion but sufﬁcient electrical power is vital to

serve winches, thrusters and cargo-handling gear.

The hybrid solution for these dual roles is to arrange for the main propel-

lers to be driven by medium-speed engines through a reduction gear which is

also conﬁgured to drive powerful shaft alternators. By using one shaft alterna-

tor as a motor and the other to generate electrical power, it is possible to secure

a diesel–electric main propulsion system of low horsepower and at the same

time provide sufﬁcient power for the thrusters. Such a twin-screw plant could

simultaneously have one main engine driving the alternator and the propeller,

while the other main engine is shut down and its associated propeller driven by

the shaft alternator in propulsion motor mode.

A further option is to ﬁt dedicated propulsion motors as well as the shaft

alternators, yielding greater plant ﬂexibility and fuel-saving potential but at the

expense of extra cost and complexity. For the above power plants to operate

effectively CP main and thruster propellers are essential.

Combined medium- or high-speed diesel engine and gas turbine (CODAG)

systems are now common for large fast ferry propulsion, while some large

diesel–electric cruise liners feature medium-speed engines and gas turbines

driving generators in a CODLAG or CODEG arrangement.

173

C h a p t e r | s e v e n

Pressure Charging

A naturally aspirated engine draws air of the same density as the ambient

atmosphere. Since this air density determines the maximum weight of fuel

that can be effectively burned per working stroke in the cylinder, it also deter-

mines the maximum power that can be developed by the engine. Increasing

the density of the charge air by applying a suitable compressor between the air

intake and the cylinder increases the weight of air induced per working stroke,

thereby allowing a greater weight of fuel to be burned with a consequent rise

in speciﬁc power output.

This boost in charge air density is accomplished in most modern diesel

engine types by exhaust gas turbocharging in which a turbine wheel driven by

exhaust gases from the engine is rigidly coupled to a centrifugal type air com-

pressor. This self-governing process does not require an external governor.

The power expended in driving the compressor has an important inﬂuence

on the operating efﬁciency of the engine. It is relatively uneconomical to drive

the compressor direct from the engine by chain or gear drive because some of

the additional power is thereby absorbed and there is an increase in speciﬁc

fuel consumption for the extra power obtained. About 35 per cent of the total

heat energy in the fuel is wasted to the exhaust gases, so by using the energy in

these gases to drive the compressor, an increase in power is obtained in propor-

tion to the increase in the charge air density.

A turbocharger comprises a gas turbine driven by the engine exhaust gases

mounted on the same spindle as a blower, with the power generated in the tur-

bine equal to that required by the compressor.

There are a number of advantages of pressure charging by means of an

exhaust gas turbocharger system:

l A substantial increase in engine power output for any stated size and

piston speed, or conversely a substantial reduction in engine dimensions

and weight for any stated horsepower

l An appreciable reduction in the speciﬁc fuel consumption rate at all

engine loads

174 Pressure Charging

l A reduction in initial engine cost

l Increased reliability and reduced maintenance costs, resulting from less

exacting conditions in the cylinders

l Cleaner emissions (see p. 188)

l Enhanced engine operating ﬂexibility (Figure 7.1).

Larger two-stroke engines may be equipped with up to four turbochargers,

each unit serving between three and ﬁve cylinders.

Turbocharging had been applied to four-stroke diesel engines in the 1930s,

the 20-year gap before general adoption in two-stroke engines in the 1950s

reﬂecting the fundamental differences between the cycles. In the four-stroke

cycle, the gas exchange process works by ﬁlling and emptying with little valve

overlap. Early turbochargers thus had adequate efﬁciency to deliver the neces-

sary quantities of charge air required by contemporary four-stroke engines.

In contrast, the two-stroke cycle is a gas ﬂow process. The working proc-

esses of intake, compression, combustion, expansion and scavenging must all

be completed in one engine revolution. In the two-stroke cycle, the gases must

be blown through by the scavenging air during the overlap between scavenge

and exhaust openings instead of clearing the combustion gases by piston dis-

placement, as in a four-stroke cycle. There needs to be a clear pressure drop

between the scavenge air and exhaust manifolds to secure the necessary ﬂow

Figure 7.1 Basic arrangement of exhaust turbocharging. M, diesel engine; Q, charge

air cooler; T, turbocharger exhaust turbine; V, turbocharger compressor

through the cylinder for good scavenging. Two-stroke engines therefore call

for greater efﬁciency from the turbochargers than four-stroke engines to deliver

the required larger air quantity for scavenging, while having comparatively less

exhaust energy to drive the turbine.

It took some years of development before turbochargers were available

with sufﬁcient efﬁciency for large two-stroke engines, and for engine design-

ers/builders to evolve techniques for matching engine and turbocharger.

Four-sTroke engines

Exhaust gas turbocharged four-stroke marine engines can deliver three times

or more power than naturally aspirated engines of the same speed and dimen-

sions. Even higher power output ratios are achieved on engine types featur-

ing two-stage turbocharging, where turbochargers are arranged in series. At

one time almost all four-stroke engines operated on the pulse system, though

constant pressure turbocharging has since become more common as it pro-

vides greater fuel economy while considerably simplifying the arrangement of

exhaust piping.

In matching the turboblower to the engine, a free air quantity in excess of

the swept volume is required to allow for the increased density of the charge

air and to provide sufﬁcient air for through-scavenging of the cylinders after

combustion. For example, an engine with a full-load brake mean effective pres-

sure (bmep) of 10.4 bar would need about 100 per cent of excess free air, about

60 per cent of which is retained in the cylinders, with the remaining 40 per

cent being used for scavenging. Modern engines carry bmeps of up to 28 bar in

some cases, requiring greater proportions of excess air, which is made possible

by the latest designs of turbocharger with pressure ratios as high as 5:1.

In a typical case, to ensure adequate scavenging and cooling of the cylin-

ders, a valve overlap of approximately 140° is normal with the air inlet valve

opening at 80° before top dead centre (TDC) and closing at 40° after bottom

dead centre (BDC). The exhaust valve opens at 50° before BDC and closes at

60° after TDC.

For a low-rated engine, with a bmep of 10.4 bar compared with 5.5 bar for

a naturally aspirated engine, a boost pressure of some 0.5 bar is required, cor-

responding to a compressor pressure ratio of 1.5:1. As average pressure ratios

today are between 2.5 and 4, considerable boost pressures are being carried on

modern high bmep medium-speed four-stroke engines.

Optimum values of power output and speciﬁc fuel consumption can be

achieved only by the utilization of the high-energy engine exhaust pulses. The

engine exhaust system should be so designed that it is impossible for gases

from one cylinder to contaminate the charge air in another cylinder, either by

blowing back through the exhaust valve or by interfering with the discharge of

gases from the cylinder. During the period of valve overlap, it follows that the

exhaust pressure must be less than the air charging pressure to ensure effec-

tive scavenging of the cylinder to remove residual gases and cooling purposes.

Four-stroke engines 175

176 Pressure Charging

It has been found in practice that if the period between the discharges of suc-

cessive cylinders into a common manifold is less than about 240°, then inter-

ference will take place between the scavenging of one cylinder and the exhaust

of the next. This means that engines with more than three cylinders must have

more than one turbocharger or, as is more common, separate exhaust gas pas-

sages leading to the turbine nozzles.

The exhaust manifold system should be as small as possible in terms of

pipe length and bores. The shorter the pipe length, the less likelihood there is

of pulse reﬂections occurring during the scavenge period. The smaller the pipe

bore, the greater the preservation of exhaust pulse energy, though too small a

bore may increase the frictional losses of the high-velocity exhaust gas to more

than offset the increased pulse energy. Sharp bends or sudden changes in pipe

cross-sectional area should be avoided wherever possible.

Two-sTroke engines

Turbocharging and scavenging arrangements for two-stroke engines are now

fairly standardized. All such engines are today uniﬂow scavenged, with air

entering the cylinder through ports around the full circumference of the cyl-

inder liner at the bottom of the piston stroke and exhausting via a single pop-

pet-type valve in the centre of the cylinder cover. Up to four turbochargers

operating on the constant pressure system may be installed, depending on the

engine size, all mounted high on the engine side, outboard and beneath the

exhaust manifold.

This fosters an efﬁcient conﬁguration, with the scavenge air cooler and its

associated water separator unit located immediately below the turbocharger

and adjacent to the scavenge air space on the piston underside. (In some spe-

cial cases, however, with smaller engines having a single turbocharger, the tur-

bocharger and air cooler can be arranged at the end of the engine.)

Compared with four-stroke engines, the application of pressure charging to

two-stroke engines is more complicated because, until a certain level of speed

and power is reached, the turboblower is not self-supporting. At low engine

loads there is insufﬁcient energy in the exhaust gases to drive the turboblower

at the speed required for the necessary air-mass ﬂow. In addition, the small pis-

ton movement during the through-scavenge period does nothing to assist the

ﬂow of air, as in the four-stroke engine. Accordingly, starting is made very dif-

ﬁcult and off-load running can be very inefﬁcient; below certain loads it may

even be impossible.

A solution was found by having mechanical scavenge pumps driven from the

engine arranged to operate in series with the turboblowers. However, electrically

driven auxiliary blowers are standard on modern engines to supplement the scav-

enge air delivery when the engine is operating below around 30 per cent load.

Two-stroke engine turbocharging is achieved by two distinct methods,

respectively, termed the ‘constant pressure’ and ‘pulse’ systems. It is the con-

stant pressure system that is now used by all low-speed two-stroke engines.

For constant pressure operation, all cylinders exhaust into a common

receiver, which tends to dampen out all the gas pulses to maintain an almost

constant pressure. The advantage of this system is that it eliminates compli-

cated multiple exhaust pipe arrangements and leads to higher turbine efﬁcien-

cies and hence lower speciﬁc fuel consumptions. An additional advantage is

that the lack of restriction, within reasonable limits, on exhaust pipe length

permits greater ﬂexibility in positioning the turboblower relative to the engine.

Typical positions are at either or both ends of the engine, at one side above the

air manifold or on a ﬂat adjacent to the engine.

The main disadvantage of the constant pressure system is the poor per-

formance at part-load conditions, and, owing to the relatively large exhaust

manifold, the system is insensitive to changes in engine operating conditions.

The resultant delay in turboblower acceleration, or deceleration, results in poor

combustion during transition periods.

For operation under the pulse system, the acceptable minimum ﬁring order

separation for cylinders exhausting to a common manifold is about 120°. The

sudden drop in manifold pressure, which follows each successive exhaust

pulse, results in a greater pressure differential across the cylinder during the

scavenge period than that obtained with the constant pressure system. This is a

factor which makes for better scavenging.

Figure 7.2 shows the variation in rotational speed of a turboblower during

each working cycle of the engine; that is, one revolution. This diagram clearly

shows the reality of the impulses given to the turbine wheel. The ﬂuctuation in

speed is about 5 per cent. Each blower is coupled to two cylinders, with crank

spacing of 135° and 225°. It is now standard practice to ﬁt charge air coolers to

turbocharged two-stroke engines, the coolers being located between the turbo-

chargers and the cylinders (Figure 7.3).

Two-stroke engines 177

1 Rev.

8350 rpm

8000 rpm

8300 rpm

7900 rpm

8400 rpm

8000 rpm

No. 2 Blower

No. 1 Blower

No. 3 Blower

Figure 7.2 Cyclic variations in turbocharger revolutions

178 Pressure Charging

Charge air Cooling

The increased weight or density of air introduced into the cylinder by pressure

charging enables a greater weight of fuel to be burned, and this in turn brings about

an increase in power output. The increase in air density is, however, fractionally

offset by the increase of air temperature resulting from adiabatic compression in

the turboblower, the amount of which is dependent on compressor efﬁciency.

This reduction of air density due to increased temperature implies a loss

of potential power for a stated amount of pressure charging. For example,

at a charge air pressure of, say, 0.35 bar, the temperature rise is of the order

of 33°C—equivalent to a 10 per cent reduction in charge air density. As the

amount of pressure charging is increased, the effect of turboblower tempera-

ture rise becomes more pronounced. Thus, for a charge air pressure of 0.7 bar,

the temperature rise is some 60°C, which is equivalent to a reduction of 17 per

cent in the charge air density.

Much of this potential loss can be recovered by the use of charge air cool-

ers. For moderate amounts of pressure charging, cooling of the charge air is

not worthwhile, but especially for two-stroke engines it is an advantage to ﬁt

charge air coolers, which are standard on all makes of two-stroke and most

medium-speed four-stroke engines.

Charge air cooling has a double effect on engine performance. By increas-

ing the charge air density, it thereby increases the weight of the air ﬂowing

into the cylinders, and by lowering the air temperature it reduces the maximum

cylinder pressure, the exhaust temperature and the engine thermal loading. The

increased power is obtained without loss and, in fact, with an improvement in

fuel economy. It is important that charge air coolers should be designed for low

pressure drop on the air side; otherwise, to obtain the required air pressure the

turboblower speed must be increased.

The most common type of cooler is the water-cooled design with ﬁnned

tubes in a casing carrying sea water over which the air passes. To ensure

Emergency blower

Feed

2-Cycle engine

To funnel

Exhaust gas

Exhaust valve

Air to manifold

Air inlet

Turbine

Turbo-compressor

Exhaust boiler

Blower

Charge air cooler

Water circulating

Figure 7.3 exhaust gas turbocharging system with charge air cooling