Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines
Подождите немного. Документ загружается.
250 Fuel Injection
unit Injectors
The initial concern with the application of unit injectors is usually cylinder head
installation and the drive. Increased space demands and the need to provide
FIgure 8.11 outline of electronic unit injector (delphi bryce)
FIgure 8.12 Pump/pipe/injector system (delphi bryce)
fuel passages (and cooling passages in the case of residual fuel operation) can
conflict with cylinder head air and exhaust passages and cooling requirements.
Such issues can be resolved through close co-operation at the design stage.
A unit injector has a low trapped fuel volume, which yields several major
benefits:
l Pumping efficiency is high, allowing smaller plungers to be used and
hence fostering reduced camshaft loading, which can be important as a
means of minimizing camshaft diameters.
l Very responsive to drive input, thus reducing hydraulic delays. The
injection rate diagram more closely follows, in both time and shape, the
cam profile, making it easier to optimize injection system performance
requirements.
l Fast end of injection with limited HP wave effects; this eliminates the
need for a delivery valve and, experience has shown, leads to enhanced
durability, particularly of the nozzle.
The fuel system drive must be sufficiently stiff to counteract the high oper-
ating loads, or the hydraulic performance benefits can be lost. A typical unit
injector drive is illustrated in Figure 8.13. The responsiveness of unit injectors
can lead to greater pressure turndown at reduced speed and loads greater than
that exhibited by pump/pipe/injector systems. This can be addressed by cam
design selection. The cam design can be tailored to achieve maximum plunger
velocity when delivering low fuel quantities; Figure 8.14 shows the effect of
using a falling rate cam on the injection pressure.
Unit injectors offer many advantages, particularly as operating pressures
increase towards 2000 bar, but they have installation drawbacks. In practice, unit
injectors are limited to engines of under 300-mm bore. In larger engines, Delphi
unit injector versus pump/pipe/injector 251
Rocker arm
Push rod
Swing arm
roller
follower
Cam
FIgure 8.13 typical unit injector drive (delphi bryce)
252 Fuel Injection
Bryce suggests that they are bulky and difficult to handle, and their withdrawal
height from the cylinder head can pose a problem in some applications.
Pump/Pipe/Injector systems
Engine designs with bore sizes larger than 300 mm can benefit from the advan-
tages of pump/pipe/injector systems. The primary requirement for installation
is to minimize the trapped HP fuel volume, which is usually achieved by hav-
ing a high camshaft and the HP connection to the injector as low as possible
through the cylinder head. The resulting low trapped fuel volume maximizes
hydraulic efficiency and responsiveness.
Pressure wave effects in the HP passages will still be sufficient to require a
delivery valve to eliminate secondary injection and cavitation erosion. Pressure
unloading delivery valves have been used for many years and, with experience,
have become reliable. The need to create the fastest possible pressure collapse
at the end of injection and a trapped residual pressure, and to ensure durability
of the valve components, led to a valve design with a series of tailoring fea-
tures (Figure 8.15). Valve opening pressures, an unloading feature on the pro-
flow valve and a control orifice can all be used to tune hydraulic performance.
Pump/pipe/injector systems have hydraulic disadvantages but offer flexibility.
Elements and valves can be changed relatively easily during servicing and to offer
uprating opportunities. Pump/pipe/injector systems also facilitate water injection
and HP gas injection, which are of increasing interest to engine designers.
The nozzle is the interface between the injection system and the combus-
tion process: good nozzle performance is essential for good combustion. The
detailed design of the nozzle seat, sac and spray hole entry is vital in mini-
mizing pressure flow losses and maximizing energy in the spray (Figure 8.16).
This ensures minimum droplet size and an even spray distribution and pene-
tration. Abrasive flow machining techniques are typically exploited to secure
rounded edges and polished surfaces to improve discharge coefficients.
500 700 900
Speed (rpm)
Constant rate cam
Falling rate cam
1800
1600
1400
1200
1000
800
2000
Peak injection pressure (bar)
FIgure 8.14 eect of cam design on the injection pressure (delphi bryce)
Major durability and reliability concerns for injection equipment have been
cavitation erosion at HP spill and element seizure. Cavitation erosion is being
addressed by improved delivery valve designs and pressure decay techniques
at the barrel port/fuel gallery. To promote seizure resistance at the increased
plunger velocities required to generate higher pressures, it is necessary to
improve element geometries and control surface textures. The use of surface
coatings is also becoming widespread.
Programs are available to design fuel cams and analyse unit injector rocker
arrangements. The results can be used to predict injection pressure, period
and rate. Accurate system modelling is a considerable aid in reducing applica-
tion development time and cost; a wide range of variables can be analysed with
confidence.
eleCtronIC Fuel InjeCtIon
It was inevitable that electronic fuel injection technology, successfully intro-
duced for high-speed diesel engines, would eventually be applied to benefit
medium-speed designs.
Lucas Bryce (which became Delphi Bryce, now part of the Woodward
Governor group) first demonstrated electronic fuel injection in the late 1970s,
showing that improved fuel consumption and emissions could be achieved. The
electronic fuel injection 253
FIgure 8.15 typical pressure unloading delivery valve design (delphi bryce)
FIgure 8.16 typical nozzle ‘sac end’ design; low volume for reduced emissions
(delphi bryce)
254 Fuel Injection
market was not ready, however, the technology was new and there was then
no environmental legislation to provide the incentive to improve the perform-
ance of engine emissions. The introduction of electronic fuel injection in the
heavy truck market to meet stringent emission controls gave that impetus and
the investment to advance the technology.
The Lucas Bryce system comprised a single-cylinder, electronically control-
led plunger pump, which supplies fuel to the injector via a HP pipe, and is driven
directly from the engine camshaft; alternatively, an electronically controlled unit
injector may be used (the choice depends on the engine design). Fuel control is
achieved by solenoid valves operated from the electronic control unit, which is
called the smart drive unit (SDU) by Delphi Bryce. Lucas Electronics also devel-
oped an engine governor, called a control and protection card (CPC); in some cases,
however, the governor is supplied by specialist manufacturers or the enginebuilder.
With larger volume fuel flows to handle, Lucas Bryce’s main effort was to
achieve a suitable hydraulic performance while using the solenoid/valve cap-
sule developed for the truck engine market by its then sister company Lucas
EUI Systems. This approach avoided the potentially prohibitive cost required
for a special production line for a low volume solenoid assembly.
Fuel is introduced via a port into the space above the pump plunger. As the
plunger rises beyond the port, fuel is displaced through the open solenoid and past
the secondary valve seat. The solenoid is energized, the valve closes and pressure
builds up above the secondary valve. The pressure drops across the secondary valve
orifice, then closes it rapidly. With the spill passages closed, fuel is forced through
the delivery valve to the injector. At the computed time for end of injection, the
solenoid is de-energized and pressure collapses above the secondary valve, causing
it to open and spill fuel. The injector nozzle then closes to finish injection.
As well as limiting harmful engine emissions, Delphi Bryce claimed its
electronic fuel injection system offered enhanced reliability, diagnostic capa-
bility, optimized timing and fuel control for all load and speed conditions,
including transient operation.
Electronically controlled fuel injection equipment is now playing a key role
in meeting exhaust emissions legislation. In isolation, however, electronic sys-
tems do not provide a total solution. Other important complementary develop-
ments are higher injection rates, higher injection pressures, balanced systems,
initial rate control, optimized spray pattern, flow-controlled nozzles and low
sac volume nozzles.
The move to electronic control of fuelling brings advantages in accuracy
of delivery and timing and underwrites sophisticated scheduling of those para-
meters over the full range of engine operating conditions. Developments are
expected in many areas, Woodward Governor (Delphi Bryce) citing:
l Pilot injection/rate shaping and control.
l Skip cylinder firing and reduced cylinder operation under low loads.
l Individual cylinder balancing by end-of-line test or adaptive software
compensating for in-service wear.
l Dual-fuel engine management systems.
l Variable fuel quality compensation.
l Combustion diagnostics.
l Advanced closed loop controls with ‘smart sensors’.
l Water injection controls.
l Engine health monitoring/failure prediction for preventative
maintenance.
l Data collection for engine life history monitoring.
Electronic fuel control offers the opportunity to cut out fuel to individual
cylinders, thereby keeping cylinders equally warm and preventing other prob-
lems which can occur on unfired cylinders such as lubricating oil ‘souping’
in the exhaust passages. Cylinder cut-out is also applied on some engines to
secure more stable idling.
Generally, Woodward Governor (Delphi Bryce) explains that if a trans-
ducer can be fitted to the engine to measure a feature which can be benefi-
cial to enhancing engine control, then it can be incorporated into an electronic
control/engine management system. An obvious example is cylinder pressure,
whereby fuel delivery can be continually adjusted within agreed maximum
limits throughout the operating period to the next overhaul to compensate for
cylinder wear and injection system wear.
A schematic diagram of a typical electronically controlled fuel injection
system (Figure 8.17) shows the main transducers.
Common raIl InjeCtIon sYstems
Common rail injection systems 255
Operator’s
speed/load
controller
Turbocharger
Fuel
in
Injection
pump &
injector
Fuel
cam
Speed & cylinder
position
pickup
Cyl. ident.
Fuel air press
Engine speed
Boost air temp.
Governor
unit
Solenoid
drive
Unit
Electrical signals to fuel pumps
Coolant water temp.
Coolant water press
Luboil pressure (safety)
Luboil pressure (secondary safety)
Alternator
Application
controller
Fuel
FIgure 8.17 schematic diagram of typical electronically controlled fuel injection
system showing the main transducers (delphi bryce)
256 Fuel Injection
In mechanical fuel injection systems, the injection pressure is a function of
engine speed and load. At low load, therefore, the pressure drops, and the
result is very large fuel droplets, which survive as such until they contact the
combustion space surfaces. CR fuel injection technology offers the possibility
to maintain the same high injection pressure (and hence small droplets) at all
loads down to idling (Figure 8.18). Highly efficient and clean combustion is
thus fostered across the engine operating range, delivering economic and envi-
ronmental benefits.
An optimum injection pressure and timing can be selected for a given oper-
ating mode, irrespective of engine speed, because the generation of fuel pres-
sure and the injection of the fuel are not interconnected. In addition, pilot and
post-injection patterns can be exploited to meet differing demands: for exam-
ple, invisible exhaust at the lowest loads and NOx reductions at medium loads
without undermining fuel economy.
Successful automotive diesel applications of CR systems, in car and truck
power units, were largely driven by stricter emission regulations. These 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
features. Tougher emission curbs on shipping stimulated the transfer of CR tech-
nology to marine engines, although major developments were dictated to enable
system components to handle heavy fuel oils, working under conditions of high
temperature and high viscosity, and with fuels containing abrasive particles and
other aggressive elements.
CR systems existed in the early 1900s in mechanically activated form
and were applied for years on the Doxford opposed-piston two-stroke marine
engines. The comparatively recent proliferation of the concept in four- and
two-stroke engine applications resulted from advances in fast and reliable rail
valves and electronic controls. Developments in materials and manufacturing
technology also fostered the creation of CR systems capable of handling pres-
sures of 1500 bar and higher.
Injection pressure (bar)
1500
1000
500
Common rail
Conventional at constant speed
Conventional on
propeller curve
Engine
load
0 50 100
0
FIgure 8.18 In contrast to conventional systems, Cr fuel injection allows a high
injection pressure and small fuel droplets to be maintained down to idling (Wärtsilä)
MTU Friedrichshafen, the German high-speed engine designer, was a pio-
neer in adopting the technology in the modern era and worked on CR systems
in conjunction with specialist fuel injection partner L’Orange from 1990. Its
first-generation system was introduced on the 165-mm-bore Series 4000 engine
in 1996. Development progressed to a second generation system (on the 265-
mm-bore Series 8000 engine) in 2000, with a third-generation system benefit-
ing the 130-mm-bore Series 2000CR design in 2004.
Finland-based Wärtsilä developed a CR system suitable for application
across its heavy fuel-burning medium-speed engine programme, the first sea-
going W46 installation entering cruise ship service in 2001. MAN Diesel of
Germany followed up with a system for its 320-mm and 480-mm-bore medium-
speed designs, and other European, Japanese, Korean and US designers—
including Caterpillar Marine Power Systems (MaK)—have introduced or plan
to offer CR-equipped engines.
Whatever the detailed differences in system architectures and control mech-
anisms, CR fuel injection can be expected to mature and eventually dominate
the four-stroke engine market in the future, whether for propulsion or genset
drives. Some existing engines can also benefit from retrofitted systems.
The following advantages are cited for CR systems over conventional
mechanically and electronically controlled jerk pumps:
l Any injection pressure can be used, at any engine load or speed opera-
tion point, with the injection rate control close to the nozzle; this makes
it possible to improve performance over the full engine load range.
l Injection timing can be varied during engine running to optimize engine
performance without the timing range being limited by cam working
lengths; this is beneficial for emissions optimization as well.
l Camshaft peak torque is smaller; this allows a smaller camshaft to be
used and makes possible greater engine power density and smaller
engine outlines.
l The CR pumping principle gives higher efficiency (no helix spill), and
the potential to reduce mechanical drive losses, underwriting lower fuel
consumption.
l The design is simpler, allowing the elimination of the one pump-per-
cylinder principle, the fuel rack control shaft and the mechanical gover-
nor; this results in cost reductions for the overall fuel injection system.
Among the potential operational benefits are lower fuel consumption, lower
NOx emissions, no visible smoke at any load (and the possibility to start the
engine without visible smoke), load cycling without smoke and lower mainte-
nance costs.
mtu Common raIl sYstem For hIgh-sPeed engInes
Development work on CR fuel injection systems by MTU from 1990 benefited
the German designer’s Series 4000 high-speed model (see Chapter 30), which
mtu common rail system for high-speed engines 257
258 Fuel Injection
was introduced in 1996 as the first production engine in its class to feature such
a system (Figure 8.19). The group’s Stuttgart-based subsidiary L’Orange was
responsible for developing the system components (Figures 8.20 and 8.21).
Research and development progressed from a pilot project on single-
cylinder engines and an eight-cylinder demonstrator engine. The positive results
encouraged MTU to develop the Series 4000 engine from the outset with a CR
FIgure 8.19 l’orange Cr fuel injection system installed on an mtu series 4000 high-
speed engine, showing the engine-driven hP pumps feeding the Cr and injector pipes
FIgure 8.20 the main elements of the l’orange Cr fuel injection system are a low-
pressure fuel supply line to the hP pump, a hP accumulator (Cr), the fuel injectors
and a double-walled hP fuel line
injection system so that its benefits could be realized not just in terms of injec-
tion but in the overall design. The system embraces the following key elements:
l The injectors in the cylinder heads.
l The HP pump, driven by a gear system.
l The fuel rail (fuel supply line running along the engine length).
l The fuel lines between the fuel rail and the injectors.
One fuel rail is provided for the injectors and their respective solenoid
valves on each bank of cylinders. The fuel pressure is generated by a HP pump
driven via a gear train at the end of the crankcase.
The electronic control system controls the amount of fuel delivered to the
injectors by means of the solenoid valves, and the injection pressure is also
adjusted to the optimum level according to the engine power demand. Separate
injection pumps for each cylinder are eliminated and hence the need for a com-
plicated drive system for traditional pumps running off the camshafts (which,
due to high mechanical stresses, calls for reinforced gearing systems or even a
second system of gears on larger engines). Fewer components naturally foster
greater reliability.
Another benefit cited for the CR system is its flexible capability across
the engine power band, and the fact that it is able to deliver the same injec-
tion pressure at all engine speeds from rated speed down to idling. In the case
of the Series 4000 engine, that pressure is around 1200 bar, corresponding to
1600–1800 bar on a conventional system.
Success is reflected in the fuel consumption (below 195 g/kW h) and emis-
sion characteristics of the engine. Future fuel injection systems must be dis-
tinguished by extremely flexible quantity control, commencement of injection
mtu common rail system for high-speed engines 259
FIgure 8.21 an l’orange lead Cr injector