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Fundamental of MAN B&W Common Rail Fuel Injection


Fundamental of MAN B&W Common Rail Fuel Injection

The general design of the MAN B&W common rail fuel injection system is shown in
Fig. 2.10 [2.3]. A common rail servo oil system using pressurized cool, clean lube oil as the
working medium drives the fuel injection pump. Each cylinder unit is provided with a servo
oil accumulator to ensure sufficiently fast delivery of servo oil in accordance with the
requirements of the injection system and in order to avoid heavy pressure oscillations in the
associated servo oil pipe system.

The movement of the plunger is controlled by a fast-acting proportional control valve
(NC valve). The NC valve is, in turn, controlled by an electric linear motor that gets its
control input from the cylinder control unit (Fig. 2.10). This design concept has been chosen
in order to maximize reliability and functionality, the fuel injection system is the heart of the
engine, and is crucial for fuel economy, emissions and general engine performance. An
example of the flexibility of the system will be given below.
The key components have a proven reliability record: the NC valves have been in serial
production for some ten years and are based on high-performance valves for such purposes as
machine tools and sheet metal machines in car production-applications where high reliability
is crucial. The fuel injection pump features well-proven fuel injection equipment technology,
and the fuel valves are of our well-proven and simple standard design.
As can be seen in Fig. 2.11, the 2nd and 3rd generations of pump design are
substantially simpler than the 1st generation design, the components are smaller, and they are
very easy to manufacture. By mid-2000, the 2nd generation pump had been in operation on
the 4T50MX research engine for more than 1400 hours, whereas the 3rd generation is starting
service testing on the 6L60MC (see below).

The major new design feature for the 3rd 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 oil from entering the lube oil system. The driving piston and the injection plunger
are simple and are 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 hydraulic valve (NC valve), which is
computer controlled.
Fuel injection system, rate-shaping capability
The optimum combustion (thus also the optimum thermal efficiency) requires an
optimized fuel injection pattern that is generated by the fuel injection cam shape in a
conventional engine. Large two-stroke engines are designed for a specified maximum firing
pressure, and the fuel injection timing is controlled so as to reach that firing pressure with the
given fuel injection system (cams, pumps, injection nozzles, etc.).
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. In order to obtain the best thermal efficiency, fuel to be injected after reaching the
maximum firing pressure must be injected (and burnt) 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 increasing injection rate towards the end of injection, thus supplying the remaining
fuel as quickly as possible. This has been proven over many years of fuel injection system development for our two-stroke marine diesel engines, and the contemporary camshaft is designed accordingly. The fuel
injection system for the Intelligent Engine is designed to do the same but in contrast to the camshaft-based injection system, the IE system can be optimized at a large number of load conditions.
Comparison between the fuel injection characteristics of the ME engine and a Staged Common Rail system in terms of injection pressure, mass flow rate and flow distribution [2.3] Common Rail injection systems with
on/off control valves are becoming standard in many modern diesel engines at present.
Such systems are relatively simple and will provide larger flexibility than the contemporary camshaft based injection systems. We do apply such systems for controlling the high-pressure gas-injection in
the dual-fuel version of our MC engines, where the (two-circuit) common rail system
provides the necessary flexibility to allow for varying HFO/gas-ratios, please refer to [2.3].
However, by nature the common rail system provides another rate shaping than what is
optimum for the engine combustion process. The pressure in the rail will be at the
set-pressure at the start of injection and will decrease during injection because the flow out of the rail (to the fuel injectors) is much faster than the supply of fuel into the rail (from
high-pressure pumps supplying the average fuel flow).
As an example, an 8-cylinder engine will have a total “injection duration” per engine
revolution of 160 deg. CA (8 x 20 degrees CA) during which the injectors supply the same
mass flow as the high-pressure supply pumps do during 360 deg. CA. Thus, the outflow
during injection is some 360/160 = 2.25 times the inflow during the same period of time.
Consequently, the rail pressure must drop during injection, which is the opposite of the
optimum rate shape. To counteract this, it has been proposed to use “Staged Common Rail”
whereby the fuel flow during the initial injection period is reduced by opening the fuel valves
one by one.
The Rate Shaping with the IE system (using proportional control valves) and the
“Staged Common Rail” are illustrated in Fig. 2.12. This shows the injection pressure, the
mass flow and the total mass injected for each fuel valve by the two systems, calculated by
means of our advanced dynamic fuel injection simulation computer code for a large bore
engine (K98MC) with three fuel valves per cylinder. In the diagram, the IE system is
designated ME (this being the engine designation, like 7S60ME-C). As can be seen, the
Staged Common Rail system supplies a significantly different injection amount to each of the
three fuel valves.

Though the Staged Common Rail system will provide a fuel injection rate close to the
optimum injection rate, combustion will not be optimal because the fuel is very unevenly
distributed in the combustion chamber whereas the combustion air is evenly distributed. This
is illustrated (somewhat overexaggerated to underline the point) in Fig. 2.13: the valve
opening first will inject the largest amount of fuel and this will penetrate too much and reach
the next fuel valve nozzle. Experience from older engine types indicates that this may cause a
reliability problem with the fuel nozzles (hot corrosion of the nozzle tip).
The uneven fuel injection amount means that there will be insufficient air for the fuel
from the first nozzle, the correct amount for the next and too much air for the third fuel valve.
The average may be correct but the result cannot be optimal for thermal efficiency and
emissions. Uneven heat load on the combustion chamber components can also be foreseen -
though changing the task of injecting first among the three valves may ameliorate this.
Thus, the IE injection system is superior to any Common Rail system - be it staged or
simple. Extensive testing has fully confirmed that the IE fuel injection system can perform
any sensible injection pattern needed for operating the diesel engine. The system 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, single/double
injection, etc.
In practical terms, a number of injection patterns will be stored in the computer and
selected by the control system so as to operate the engine with optimal injection
characteristics from dead slow to overload, as well as during astern running and crash stop.
Change-over from one to another of the stored injection characteristics may be effected from
one injection cycle to the next.
Some examples of the capability of the fuel injection system are shown in Fig. 2.14 [2.3].
For each of the four injection patterns, the pressure in the fuel valve and the needle-lifting
curve are shown. Tests on the research engine with such patterns (see Fig. 2.15) have
confirmed that the “progressive injection” type (which corresponds to the injection pattern
with our optimised camshaft driven injection system) is superior in terms of fuel consumption.
The “double injection” type gives slightly higher fuel consumption, but some 20% lower
NOx emission - with a very attractive trade-off between NOx reduction and SFOC increase.

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