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

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48 Dual-Fuel and Gas Engines

and automatically (e.g. in the event of gas supply interruption). A correct air–

fuel ratio under any operating conditions is essential for optimum performance

and emissions—a status addressed by an exhaust gas wastegate valve. Part of

the exhaust gases bypass the turbocharger through this valve, which adjusts the

air–fuel ratio to the correct value independently of the varying conditions under

high engine loads. The wastegate is actuated electro-pneumatically.

A ﬂexible cooling system, optimized for different cooling applications, has

two separate circuits: high temperature and low temperature. The HT circuit

controls the cylinder liner and cylinder head temperatures, while the LT circuit

serves the lubricating oil cooler. Both circuits are also connected to the respec-

tive parts of the two-stage charge air cooler; and both HT and LT water pumps

are engine driven as standard.

All engine functions are controlled by the WECS, a microprocessor-based

distributed control system mounted on the engine. The various WECS mod-

ules are dedicated and optimized for certain functions and communicate with

each other via a databus. Diagnostics are built in for easy trouble shooting.

The core of the WECS is the main control module, which is responsible for

keeping the engine at optimum performance in all operating conditions such

as varying ambient temperature and methane number. The main module reads

the information sent by all the other modules and applies it in adjusting the

engine speed and load control by determining reference values for the main

gas admission, air–fuel ratio and pilot fuel amount and timing. It also automat-

ically controls the start and stop sequences of the engine and the safety system,

and communicates with the plant control system.

Each cylinder control module is arranged to monitor and control three cylin-

ders; it controls the cylinder-speciﬁc air–fuel ratio by adjusting the gas admission

individually for each cylinder. The module measures the knock intensity (uncon-

trolled combustion in the cylinder), this information being applied in adjusting

the cylinder-speciﬁc pilot fuel timing and gas admission. Light knocking leads to

automatic adjustment of the pilot fuel timing and cylinder-speciﬁc air–fuel ratio;

heavy knocking leads to load reduction or a gas trip.

MAn 51/60DF EnGinE

MAN Diesel’s 510-mm-bore 51/60DF medium-speed engine (Figure 2.6) was

designed for LNG carrier propulsion with the capability to operate in both gase-

ous and liquid fuel modes. In the former mode, the engine runs according to

the lean-burn Otto combustion process: the pre-mixed lean gas–air mixture is

ignited by the compression ignition of a small quantity of marine diesel pilot

fuel injected into the main combustion space. A pilot injection of 1 per cent

of the normal liquid fuel quantity ensures very low NOx emissions in gas mode.

In liquid fuel operating mode, the fuel is injected via conventional main pumps

adopted from MAN Diesel’s 48/60B heavy fuel engine, and the thermodynamic

working process is the diesel combustion cycle. Fuels ranging from marine die-

sel oil to HFO can be used, with swift and smooth switching between liquid and

gaseous fuels with the engine running.

Development was mainly inﬂuenced by the need to challenge steam turbine

and gas turbine propulsion in the LNG carrier market by offering: high power

output and high thermal efﬁciency; fuel ﬂexibility (vaporized LNG or DMA,

DMB and HFOs); high availability in liquid and gaseous fuel modes; short

start-up and loading times; low emission levels and easy maintenance.

MAN 51/60DF engines are offered in 6-to-9 in-line and V12 to 18-cylinder

models covering a power range up to 18 000 kW at 500/514 rev/min with a mean

effective pressure rating of 19 bar. The design was based on the proven 48/60B

diesel engine, the following main features being adapted to facilitate operation

on gas:

l Gas admission to each cylinder by individual valves

l Charge air and exhaust manifolds equipped with pressure relief valves to

avoid any gas operation risks

MAn 51/60DF engine 49

FiGurE 2.6 Cross-section of MAn Diesel 51/60DF medium-speed engine

50 Dual-Fuel and Gas Engines

l A CR system for pilot fuel oil injection integrated in the cylinder head;

although well below 1 per cent of the main fuel supply, the pilot fuel

quantity is controllable by the injection system

l Separate conventional fuel system for the liquid fuel mode

l SaCoS DF engine management system for the whole operating range.

The cylinder bore of the base engine was enlarged from 480 mm to 510 mm

to secure a higher power output, but the moderate brake mean effective pressure

of 19 bar—more favourable in gas mode than in liquid fuel operation—was

maintained. Design work mainly involved adapting gas-related components to

the 48/60B engine, the key elements being the air–fuel ratio control, individual

cylinder gas admission valves, knock detection system and CR pilot fuel injec-

tion system.

A compressor bypass controls the air–fuel ratio of the cylinders, the optimum

ratio fostering stable operation in gas mode with high efﬁciency and low emis-

sions. The electronically driven gas valves feed each cylinder individually with

its required amount of gas; and an electrical actuator controls the main injection

pumps. An additional phase pick-up is necessary to deliver the required signals

of crank angle position in the liquid fuel operating mode.

Knocking control plays an important role in reliable gas operation, a

system detecting knock behaviour for each cylinder unit individually via

an acoustic sensor mounted on the cylinder head. The independent acoustic

signals are fed into a main sensor unit where the knocking level is detected.

The overall engine control system reacts to the knocking level and takes appro-

priate measures by adjusting the engine operating parameters.

Pilot fuel injection is executed by an electrically driven CR pump and a

solenoid-activated pilot fuel oil injector. For safety reasons, the pressure

relief valves are mounted on the charge air manifold and exhaust gas mani-

fold. Installing the solenoid-driven CR pilot fuel injector separately from the

existing main fuel injection system eases changing of the injectors (which have

different times-between-overhauls) compared with combining them in a central

injection system, MAN Diesel asserts.

In gas operating mode the required amount of gas is metered for each cyl-

inder individually by the gas admission valves integrated in the rocker arm

housing. A ﬂow control pipe ensures a homogeneous mixture of air and gas

entering the combustion space. Double-wall piping with insulation and venti-

lation of the main gas valve itself follows the gas-free engineroom concept,

which allows the installation of several engines in a single machinery space.

MAN Diesel’s SaCoS DF engine management system was evolved from

the group’s established SaCoS safety and control system to cover DF engine

operation. It undertakes control in both liquid and gas operating modes, with

the air–fuel ratio controlled by a compressor bypass installed after the charge

air cooler. The SaCoS system controls the ratio via the compressor bypass ﬂap.

SaCoS system architecture is dominated by two independent governors:

one controls the main injection fuel oil pumps and the individual fuel gas

admission valves for each cylinder unit; the other controls the CR pilot fuel

injectors. The individual knocking levels, collected from each cylinder by the

knocking detection unit, are processed in the SaCoS system. In combination

with the individual cylinder control of pilot injection and gas admission, the

system control secures stable operation in gas mode with a sufﬁcient margin

for knocking and misﬁring. In addition, the control system establishes connec-

tions with the ship’s overall alarm and safety networks.

GAS–DiESEL EnGinES

Wärtsilä GD technology differs from the DF diesel concept in using direct,

high-pressure injection of gas and 3 per cent pilot fuel for its ignition. GD

medium-speed engines can be operated on heavy fuel and diesel oils, crude oil

directly from the well or natural gas; supply can be switched instantly and auto-

matically from one fuel to another without shutdown. Conversions of existing

engines from normal heavy fuel mode to natural gas/diesel oil operation can be

executed with small modiﬁcations. Containerized engineroom packages have

been delivered along with the associated gas compressors and other ancillaries.

The GD engine was developed in the late 1980s mainly as a prime mover

for the offshore industry. High-pressure direct injection for the gaseous fuel

maintains the diesel process, making the concept insensitive to the methane

content of the fuel. Such a feature makes the GD engine especially suitable

for mobile oil production processing plants, where the gas composition may

change with the location of the vessel and the stage in the production process.

Initially, the gas injection was controlled by a mechanical/hydraulic sys-

tem using a cam-actuated jerk pump to generate the control pressure for the gas

needle. Although reliable, Wärtsilä explains, this system did not offer the

required ﬂexibility and was later replaced by an electro-hydraulic system based

on a constant rail pressure to control the gas injection via solenoid valves.

Gas-speciﬁc monitoring and safety functions were all integrated, and the sys-

tem allowed adjustment of individual cylinders. This represented the ﬁrst step

towards fully computerized systems for engine control, and valuable experience

was gained in both software and hardware requirements.

GAS EnGinES

Rolls-Royce is a specialist in marine gas engine technology, its Norwegian

subsidiary Bergen Diesel having started development of a lean-burn gas-

fuelled version of the 250 mm bore K-series medium-speed diesel engine in the

mid-1980s. More than 400 engines of this type have been sold for land power and

co-generation installations, reportedly demonstrating high efﬁciency and very low

emissions. The company has also been keen to demonstrate the potential in natu-

ral gas-fuelled ferries in an era of concern over atmospheric coastal pollution and

the increasing availability of gas from domestic pipeline terminals in Norway.

Rolls-Royce Bergen spark-ignition lean-burn gas engine technology has

matured through several generations of engines and thousands of operating

Gas engines 51

52 Dual-Fuel and Gas Engines

hours in shoreside power installations, running on various gas fuels including

natural gas and biogas derived from landﬁll. Its lean-burn gas engine operates

on the Otto cycle with mixture compression and external ignition. By providing

a strong ignition source, the pre-chamber and the gas–air mixture in the cylin-

der can be leaned out, giving much improved engine performance. Efﬁciency

is increased, emissions reduced (particularly NOx) and the speciﬁc power of

the engine can be signiﬁcantly raised because the knock limit is extended.

Combustion air is supplied by the turbocharger to the individual cylinders

via the intercooler and the charge air manifold, and a timed mechanical valve

injects gas into the inlet air stream. A special inlet port design and engine

control system ensures a homogeneous and lean mixture of air and gas.

During compression, the lean mixture in the cylinder is partially pushed

into the pre-chamber, where it mixes with pure gas to form a rich mixture

that is easily ignited by the spark plug. Fast and complete combustion of

the main mixture in the cylinder is then fostered by a powerful ignition

discharge from the pre-chamber and an optimized combustion chamber con-

ﬁguration. A responsive and ﬂexible ABB turbocharger with variable turbine

geometry reduces ‘turbo lag’, while Lamda-control avoids ‘over-fuelling’ dur-

ing hard manoeuvring.

An advanced electronic control system ensures that the operating para-

meters of the engine are adjusted and optimized in relation to each other. The

system sets the optimum main and pre-chamber gas pressures, the air–fuel

ratio, the fuel rack position, the ignition timing and air throttle position.

Extensive testbed running on various load proﬁles has reportedly demon-

strated an ability to handle propeller law operation and all the load conditions

imposed by dynamic interactions between the hull, mechanical transmission

and the prime mover itself. No limits are placed on low load running; optimum

combinatory proﬁles can be used with CP propellers, and crash stops and rap-

idly changing loads are said to present no problems.

Bergen’s KV-G4 gas engine was derived from the company’s long-

established 250 mm-bore K-series medium-speed diesel design. Introduced

for genset applications in 2002, it was released in 2004 as a variable speed

engine for direct mechanical propulsion. Electric propulsion installations with

the engine were pioneered in 2007 with V12- and V16-cylinder LNG-burning

versions driving the main gensets of Norwegian double-ended coastal ferries.

Design work on the spark-ignited 350 mm-bore B-gas (B35:40) engine

began in spring 2002, tapping experience with the 320 mm-bore B-series

medium-speed diesel engine launched in the late 1980s. A V12-cylinder

prototype developing 5 100 kW made its commercial debut in a Danish com-

bined heat and power station in 2003 after a comprehensive test programme in

Bergen.

The B-gas engine is also offered in V16- and V20-cylinder versions, tak-

ing the power band up to 9 000 kW. Complementing the K-series and B-series

portfolios, Rolls-Royce also planned a gas variant of Bergen’s 250 mm-bore

C-series (C25/33) diesel engine in six-, eight- and nine-cylinder models.

MAn DiESEL ME-Gi EnGinE

Medium- and low-speed gas-burning engine designs are offered by the MAN

Diesel group, which cites successful experience with a Holeby 16V28/32-GI

(gas injection) four-stroke model (Figure 2.7), and a 6L35MC two-stroke

model.

A high-pressure gas-injection system for its MC low-speed engines was

jointly developed by MAN Diesel and its key Japanese licensee Mitsui, which

delivered a 41 000 kW 12-cylinder K80MC-GI engine for power generation

in an environmentally sensitive region (Figures 2.8 and 2.9). Extensive land-

based experience from the early 1990s is reported from this installation in peak

load service on high-pressure gas.

A new generation of large LNG carriers stimulated reﬁnement of the

ME-GI engine design to burn cargo boil-off gas in almost any ratio with fuel

oil. Different natural gas qualities, depending on the source ﬁeld, may be trans-

ported as LNG, but the ME-GI system is claimed to be easily adaptable to a

new shipping route with a different gas composition. An ME-GI system can

be retroﬁtted to existing ME/ME-C HFO-burning engines should rising HFO

prices versus LNG prices make such projects economically desirable.

The working principle of the GI engine is similar to that of its traditional

two-stroke diesel counterpart, but with the combustion process based on a high

air surplus and a high-pressure gas injection system. There is no derating or

knocking due to gas pressure or air ﬂow, and the exhaust gas boiler design is

also unchanged. Power, exhaust gas volume and temperature values are similar

MAn diesel ME-Gi engine 53

Combi injector

Pilot fuel Control oil

Puncture

valve

Gas

inlet valve

High-pressure

gas supply

High-pressure

injection pump

Camshaft Fuel cam

Control oil

actuator pump

Twin pump

FiGurE 2.7 Fuel injection system of the MAn B&W 16 V 28/32-Gi high-pressure gas-

injection engine

54 Dual-Fuel and Gas Engines

for GI and HFO-burning engines; and performance conditions, choice of cylin-

der liner, cover, piston and cooling system are the same for the two types. The

cylinder cover is slightly changed for the GI engine, however, to accommodate

the gas valves and a block for handling the fuel gas. Pressurized fuel gas (at

a maximum of 250–300 bar) is required but the thermodynamic condition in

the combustion chamber is similar to that associated with heavy fuel burning.

There is one limitation: condensation in the system is not allowed.

GI engine control and safety is handled by an add-on unit to the proven ME

engine control system. The main modiﬁed parts in a GI engine are double-walled

high-pressure gas pipes; a gas valve control block with internal accumulator on

the (slightly modiﬁed) cylinder cover; gas injection valves and electronic gas

injection (ELGI) valves to control the amount of gas injected. Small modiﬁca-

tions to the exhaust gas receiver and the control and manoeuvring system were

Exhaust receiver

Cylinder cover

Valve block

High-pressure pipes

Pumps

Camshaft

arrangement

Gas pipes

FiGurE 2.8 MAn Diesel’s MC-Gi low-speed DF engine, indicating new or modied

components compared with the standard diesel version

also dictated. The new units required were a ventilation system for the space

between the inner and outer pipes of the double-wall piping; a sealing oil sys-

tem, delivering oil to the gas valves separating the control oil and the gas: and an

inert gas system to enable purging of the engine’s gas arrangements.

Operational safety in DF mode is addressed by incorporating a control and

safety system with an analyser checking the hydrocarbon content of the air in

the gas pipes. The scavenge air pressure, exhaust gas temperature and com-

pression, combustion and expansion pressures are continuously monitored. The

GI control and safety system is designed to ‘fail to safe’ condition. All failures

detected during gas fuel operation (including failures of the control system

itself) will result in a gas fuel stop/shutdown and changeover to HFO opera-

tion. Changeover from LNG to HFO—which will reportedly never trigger any

engine power loss—is followed by blowout and purging of the high-pressure

gas pipes and the complete gas supply system.

Since the gas supply piping is of CR conﬁguration, the gas injection valve

must be controlled by an auxiliary control oil system. In principle, this con-

sists of the ME engine hydraulic control oil system and an ELGI valve. High-

pressure control oil is supplied to the gas injection valve, thereby controlling

the timing and opening of the valve. DF operation requires the injection of both

MAn diesel ME-Gi engine 55

Gas

valve

Pilot oil

valve

Puncture

valve

Safety

valve

Pilot oil pump Control oil pump

FiGurE 2.9 Gas injection is stopped immediately on the rst failure to inject the

pilot oil by the patented safety valve incorporated in the control oil pump of the

MAn B&W MC-Gi engine

56 Dual-Fuel and Gas Engines

pilot and gas fuels into the combustion chamber for which different types of

valves are used; two are ﬁtted for gas injection and two for pilot fuel. The aux-

iliary media for both fuel and gas operation call for a high-pressure gas supply;

a fuel oil supply (pilot oil); control of oil supply for activating the gas injection

valves and sealing oil supply.

Under normal operation, with no malfunctioning of the fuel oil valve, the

fuel gas valve is opened at the correct crank angle position and gas is injected.

The gas is fed directly into an ongoing combustion process, so the chance of

having unburnt gas eventually slipping past the piston rings and into the scav-

enge air receiver is considered very low. Monitoring the scavenge air receiver

pressure safeguards against such a situation; and in the event of high pressure

the gas mode is stopped and the engine returned to burning only fuel oil.

Gas ﬂow to each cylinder during one cycle is detected by measuring the pres-

sure drop in the accumulator. Any abnormal gas ﬂow, whether due to seized gas

injection valves or blocked valves, will be detected immediately. The gas supply

will then be discontinued and the gas lines purged with inert gas. Also in such an

event, the engine will continue running on fuel oil alone without any power loss.

The prime marine industry target for large gas-fuelled engines is LNG

carrier propulsion, where the cargo boil-off gas can be tapped, but a modiﬁed

high-pressure gas-injection version of the MAN B&W MC low-speed engine

is at the heart of MAN Diesel’s system for burning volatile organic compound

(VOC) discharges from offshore shuttle tankers. Oil vapours or VOCs are light

components of crude oil evaporated mainly during tanker loading and unload-

ing but also during a voyage when cargo splashes around the tanks.

VOC discharge represents a signiﬁcant loss of energy as well as an envi-

ronmental problem (the non-methane part of the VOC released to the atmos-

phere reacts in sunlight with NOx and may create a toxic ground-level ozone

and smog layer). The VOC-burning system was ﬁrst applied at sea to the twin

6L55GUCA main engines of the 125 000 dwt shuttle tanker Navion Viking in

2000—an onboard recovery system collecting the discharges and liquefying

the product by compressing and cooling. The resulting condensate is stored

and burned as fuel for propulsion, yielding substantial savings in HFO con-

sumption (Figure 2.10).

Engines for such applications must be able to operate on ordinary HFO if

VOC fuel is unavailable, and they must also accept any possible composition of

VOC condensate since it is not possible to apply any fuel speciﬁcation: what-

ever is collected must be burned. Shoreside tests by MAN Diesel conﬁrmed that

its DF MC engines could use any VOC/HFO ratio between 92/8 per cent and

0/100 per cent, as well as any relevant VOC quality.

PErFOrMAnCE GAS injECTiOn EnGinE

The dominant systems for igniting the air–gas mixture in non-compression

ignition gas engines (those using the Otto working principle) are high energy

electrical ignition systems exploiting spark plugs. These industrial-grade spark

plugs, however, are subject to increasing wear from the higher levels of energy

at their electrodes resulting from higher speciﬁc outputs. With such ignition

systems, the amount of ignition energy that can be created to ignite the very

lean air–fuel mixtures employed is limited because the more powerful the

spark, the more rapid the erosion of the spark plug electrodes. Such a situation

gets worse when higher compression ratios are used to increase engine efﬁ-

ciency. High engine performance and extended service levels are thus mutually

exclusive in spark-ignited gas engines, according to MAN Diesel.

An alternative mode of mixture ignition is pilot injection, a small quan-

tity of diesel fuel being introduced into the combustion chamber via a separate

injection system at close to TDC to ignite the lean gas–air mixture. The low

spontaneous ignition temperature of the liquid fuel leads ﬁrst to ignition of the

diesel fuel and then to ignition of the air–gas mixture.

MAN Diesel’s performance gas injection (PGI) four-stroke engine is report-

edly capable of igniting considerably leaner mixtures than those typically used

in gas engines with electrical ignition systems. This effectively counters the ten-

dency of the gas–air mixture to pre-ignite during the compression stroke (causing

combustion knock or misﬁring) and possible damage to the piston and combus-

tion chamber components.

A PGI engine can consequently operate at much higher compression ratios

than spark-ignited designs and at mean effective pressures comparable to the

latest diesel engines, resulting in efﬁciency levels of 46 per cent or more. High

levels of efﬁciency are achieved while complying with exacting NOx emission

limits. Another claimed advantage is that the PGI ignition system is not subject

to the erosive wear processes that affect spark plugs. A much longer effective

life yields savings in consumables (the spark plugs themselves) and enhanced

engine availability, since the PGI system avoids the regular interruptions of

power generation needed for spark plug replacement.

Performance gas injection engine 57

2. VOC gas

condensa-

tion system

1. VOC gas

cleaning

system

VOC treatment

and collection

system on deck

Vent to atmosphere

(Mostly nitrogen)

Exhaust gas low

on SO

, NO

, and

particulates

Fuel oil Air

VOC

injection

system on

engine

5. VOC

pre-

heating

system

3. VOC

storage

tank (Con-

densed

VOC gas)

4. High-pressure

VOC supply pump

Tanker

Crude

oil

VOC

gas

Crude oil

supply

FiGurE 2.10 Schematic representation of system for burning VOC discharges in the

main engines of a shuttle tanker (MAn Diesel)