MAN B&W
ME-series engines, introduced to the market in2001, dispense with the camshaft
and exploit hydraulic-mechanical systems supported by electronic hardware and software
for activating fuel injection and the exhaust valves.
Electronically-controlled
fuel injection and exhaust valve actuation allows individual and continuous
adjustment of the timing for each cylinder, securing these key benefits:
1.
Reduced
fuel consumption
Fuel injection characteristicscan be optimized at many
different load conditions, while a conventional engine is optimized forthe
guarantee load, typically at 90–100 per cent maximumcontinuous rating
Constant Pmax inthe upper load range can be achieved by
acombination of fuel injection timing and variation of thecompression ratio
(the latter by varying the closing of the exhaustvalve). As a result, the
maximum pressure can be kept constantover a wider load range without
overloading the engine, leading to significant reductions in specific fuel
consumption at part load.
On-line monitoringof the cylinder process ensures that the load distribution among the cylinders
and the individual cylinders firing pressure can be maintained at as new standard
over the lifetime of the engine.
2.
Operational
safety and flexibility
·
Swifter acceleration of the engine, since the scavenge air pressure can
be increased faster than normal by opening the exhaust valve earlier during
acceleration.
·
Dead slow running is improved significantly: the minimum rev/ min is
substantially lower than for a conventional engine, dead slow running is much
more regular, and combustion is improved thanks to the electronic control of
fuel injection.
·
The engines crash stop and reverse running performance is improved
because the timing of the exhaust valves and fuel injection can be optimized
for these manoeuvres as well.
·
Engine braking may be obtained, reducing the ships stopping distance.
·
Electronic monitoring of the engine (based on MAN
B&Ws CoCoS-EDS system: see MAN
B&W Medium Speed Engines chapter) identifies running conditions that could
lead to performance problems. Damage due to poor ignition-quality fuel can be
prevented by injection control (pre-injection).
The
engine control system incorporates MAN B&Ws on-line Overload Protection System (OPS) feature, which ensures the
engine complies with the load diagram and is not overloaded (often the case in
shallow waters and with heavy propelleroperation).
·
Maintenance costs will be lower (and maintenance easier) as a result of
the protection against general overloading as well as overloading of individual
cylinders; and also because of the as newrunning conditions for the engine,
further enhanced by the ability of the diagnosis system to give early warning
of faults and thus enable proper countermeasures to be taken in good time.
3.
Exhaust
gas emissions flexibility
·
The engine can be changed over to various low emissionmodes, its NOx
exhaust emissions reduced below the IMO limits if dictated by local
regulations.
The following components of the conventional MC
engine are eliminated in the ME engine: chain drive for camshaft; camshaft with
fuel cams, exhaust cams and indicator cams; fuel pump actuating gear, including
roller guides and reversing mechanism; conventional fuel injection pumps;
exhaust valve actuating gear and roller guides; engine-driven starting air
distributor; electronic governor with actuator; regulating shaft; mechanical
engine-driven cylinder lubricators; and engine side control console.
These elements
are replaced on the ME engine by an
electro-hydraulic platform comprising: a hydraulic power supply (HPS); a
hydraulic cylinder unit (HCU) with electronic fuel injection (ELFI) and
electronic exhaust valve actuation (ELVA); an electronic Alpha cylinder
lubricator (see above and Fuels and Lubes chapter); an
electronically-controlled starting valve; a local control panel; a control
system with governor; and a condition monitoring system.
ME engine systems
Valuable
experience was gained by MAN B&W Diesel from its 4T50MX research engine at
Copenhagen, operated from 1993 to 1997 with a first-generation Intelligent
Engine (IE) system. Second-generation IE systems fitted to the engine in 1997
aimed for simplified design, production and installation of the key
electronically-controlled fuel injection and exhaust valve actuation systems.
Subsequent R&D focused on transforming the electronic elements into a
modular system, whereby some of the individual modules could also be applied to
conventional engines. This called for the development of a new computer unit
and large software packages, both of which had to comply with the demands of
classification societies for marine applications.
The
second-generation IE system is based on an engine-driven high pressure servo
oil system which provides the power for the hydraulically operated fuel
injection and exhaust valve actuation units on each cylinder. Before the engine
is started the hydraulic power system (or servo oil system) is pressurized by a
small electrically-driven high pressure pump. Fine-filtered main system lube
oil is used as the actuating medium supplied by engine-driven multi-piston
pumps at around 200 bar (Figure 10.37).
Fuel injection system
A common rail
servo oil system applies this cool, clean and pressurized lube oil to power the
fuel injection pump of each cylinder. Each cylinder unit is provided with a
servo oil accumulator to ensure sufficiently swift delivery of oil in
accordance with the requirements of the injection system, and to avoid heavy
pressure oscillations in the associated servo oil pipe system. The movement of
the pump plunger is controlled by a fast-acting proportional control valve (a
so-called NC valve) which, in turn, is controlled by an electric linear motor
that receives its control input from a cylinder control unit. The fuel
injection pump features well proven technology and the fuel valves are of a
standard design.
Second- and
third-generation fuel injection pumps are much simpler than the
first-generation design and their components are smaller and easier to manufacture
(Figure 10.38). A major feature of the third-generation pump is its ability to
operate on heavy fuel oil; the pump plunger is equipped with a modified
umbrella design to prevent heavy fuel from entering the lube oil system. The
driving piston and injection plunger are simple and kept in contact by the fuel
pressure acting on the plunger and the hydraulic oil pressure acting on the
driving piston. The beginning and end of the plunger stroke are both controlled
solely by the fast-acting NC valve, which is computer controlled.
Optimum
combustion (and thus optimum thermal efficiency) calls for an optimized fuel
injection pattern, which in a conventional engine is generated by the fuel
injection cam shape. Large two-stroke engines are designed for a specified
maximum firing pressure and the fuel injection timing is controlled so as to
reach that pressure with the given fuel injection system (cams, pumps,
injection nozzles).
For modern
engines, the optimum injection duration is around 18–20 degrees crank angle at
full load, and the maximum firing pressure is reached in the second half of
that period. To secure the best thermal
efficiency, fuel injected after the maximum firing pressure is reached must be
injected (and burned) as quickly as possible in order to obtain the highest
expansion ratio for that part of the heat released. From this it can be deduced
that the optimum rate shaping of the
fuel injection is one showing an increasing injection rate towards the end of
injection, thus supplying the remaining fuel as quickly as possible. The
camshaft of the conventional engine is designed accordingly, as is the fuel
injection system of the ME engine. In contrast to the camshaft based injection
system, however, the ME system can be optimized at a large number of load
conditions.
MAN B&W
Diesel claims the fuel injection system for the ME engines can execute any
sensible injection pattern needed to operate the engine. It can perform as a
single-injection system as well as a pre injection system with a high degree of
freedom to modulate the injection in terms of injection rate, timing, duration,
pressure or single/double injection. In practice, a number of injection
patterns are stored in the computer and selected by the control system for
operating the engine with optimum injection characteristics from dead slow to
overload, as well as during astern running and crash stop. Changeover from one
to another of the stored injection characteristics may be effected from one
injection cycle to the next.
Exhaust valve actuation system
The exhaust
valve is driven by the same servo oil system as that for the fuel injection
system, using cool pressurized lube oil as the working medium. The necessary functionality
of the exhaust valve is less complex than fuel injection, however, calling only
for control of the timing of its opening and closing. This is arranged by a
simple fast-acting on/ off control valve. Well proven technology from the
established MC engine series is retained. The actuator for the exhaust valve
system is of a simple, twostage design. The first-stage actuator piston is
equipped with a collar for damping in both directions of movement. The
second-stage actuator piston has no damper of its own and is in direct contact
with a gear oil piston transforming the hydraulic system oil pressure into oil
pressure in the oil push rod. The gear oil piston includes a damper collar that
becomes active at the end of the opening sequence, when the exhaust valve
movement will be stopped by the standard air spring.
No comments:
Post a Comment