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features a fuel reservoir integrated in its body to store the very high
pressures required to further improve the combustion process, as
well as a flow limiter valve for preventing engine overfuelling in
case of injector failure. Multi-shot capability is also mandatory for
providing maximum flexibility in shaping the heat release curve.
The paper presents how this was achieved by designing a control
stage able to operate with high temperature HFO and placed very
close to the injector needle for minimising injector response time
and dwell time between injections. This concept required the custom
design of a solenoid able to meet demanding specifications
in terms of short switching time, high force with limited size and
high working temperatures. A multidimensional numerical model
of the injector was developed inhouse, using a proprietary code
that takes into account fuel compressibility and cavitation phenomena.
The model couples the results of a 3D-CFD analyses of
the parts of the design that require a 3D study of fluid flow to be
properly characterised, with the lumped parameter model of the
mechanical parts of the injector and the one dimensional model
of the fuel channels required to correctly predict pressure wave
propagation. Rig tests were carried out to confirm the functional
performance and durability of the new injector design, as well as
to validate the numerical model predictions. This enabled the use
of the model for studying the effect of design modifications on
injector performance, thus speeding up the development process.
The paper presents a comparison between experimental and numerical
results of injector performance measured on the test rig at
the beginning of its life as well as after one thousand hours of operation
on an endurance test rig. The results obtained demonstrate
the benefits of the technical solutions adopted and why they were
needed to reach the good level of performance shown by the injector.
In particular, the minimum controllable injected quantity was
found to be very low and showing high repeatability, making this
design ideal for operating in multi-shot mode and also for dualfuel
engines, where the customer requires to use the minimum
possible amount of liquid fuel for performing gas ignition. The
injector was undergoing prototype tests on a customer engine at
the time of writing this abstract.
Fatigue design of autofrettaged diesel engine
injection parts
Rayk Thumser, MAN Diesel & Turbo SE formerly with MFPA Weimar, Germany
Wolfgang Wagner, MAN Diesel & Turbo SE, Germany
Joachim W. Bergmann, TWIN GmbH Weimar, Germany, formerly with MFPA
Weimar, now Senior Scientist of MFPA, Germany
Increasing economic and ecological demands on Diesel engines
have raised the injection pressures up to 3,000 bar. One of the
favourite ways to increase the fatigue resistance of injection parts
is autofrettage, where an initial overload near the plastic limit load
and following unloading generate favourable compressive stress
fields around the notches. In the endurance regime, autofrettage
can yield a tremendous increase of fatigue limit with crack arrest in
the compressive stress field. In the finite life regime the crack initiation
life benefits from the compressive mean stress at the notch
surface, and the crack propagation from crack growth retardation
in the compressive stress field. Focus is put on endurance and fatigue
strength under variable amplitude loading. Test results on
tubes and notched specimens representing typical design features
in pumps, rails, injectors etc. demonstrate the advantages of autofrettage
and serve as basis for the validation of design methods.
For variable amplitude loading the standardised load sequence
CORAL (common rail loading) is shown, representing the life of
an injection system of a Diesel passenger car as well as ’CORAL
extended’ with a large amount of additional small cycles. A variety
of different life prediction approaches have been investigated. The
following two design methods are discussed in the paper as they
are of practical use:
• Nominal approach using Miner’s rule,
• Local approach for crack initiation life and explicit 3D finite element
simulation for crack propagation using LEFM, with crack
closure and automatic remeshing fatigue analyses using design
method 2 could predict these experimental results with a high
accuracy, proving the significance of advanced design methods.
For practical application, the second design method can be recommended.
The first method can only be applied with a high
level of experience usually not yet available in design groups of
the industry. A considerable advantage of autofrettage is the very
small scatter in life or in fatigue strength, respectively, resulting
in remarkably small safety factors for the allowable pressures at
small failure probabilities. The methods developed in research for
life and endurance prediction have been successfully transferred
to the industry, where it has been implemented and adapted to
practical needs. MAN Diesel & Turbo SE has accompanied the research
work and has used the know-how from the very beginning.
The know-how has also been applied in the development of the
new common rail injection system CR 2.2 for four-stroke engines
operating at 2,200 bar to fulfil future requirements for fuel consumption
and emissions. An outlook is given on unsolved design
problems concerning high frequency of small cycles, temperatures
up to 200°C and load spectra with maximum pressures below endurance.
New low-cost common rail system with zero static
leakage
Raphael Fuechslin, Lafei, Switzerland
Fuel economy is more and more important for diesel engines.
Emission levels become more and more stringent. To meet these
limits, a common rail system is mandatory. Fuel economy yields
for a zero static leakage common rail system. Additional to these
two requirements, manufacturing costs should be as low as possible.
According to the requirements, a common rail system, mainly
consisting of a fuel injector and a high pressure pump, was designed,
built and tested. Because of costs, the injector uses a solenoid
actuator. As usual, the solenoid is about 20mm in diameter.
For smaller engines the slim body of the injector does not allow to
place the solenoid near the injection nozzle. To avoid static leakage,
the injector has a central fuel bore only. This means the injector
needle is completely surrounded by pressurised fuel. The needle
is very long to reach the solenoid that is placed on the top. This
length can cause friction problems due to misalignment caused
by manufacturing imperfections. To enhance the acceptance of
manufacturing imperfections, a flexible needle was invented. This
invention keeps manufacturing costs low and minimises injector
to injector deviations. To keep the costs of a high pressure fuel
pump as low as possible, the pump is fuel lubricated. Fuel lubrication
additionally minimises the overall costs of the whole engine.
The pump has a very simple crank drive with a race ring. This race
ring sits on the eccentric part of the crankshaft. The plunger feet
are rolling directly on this race ring. Except for the plunger spring,
there is no need for an additional part transforming revolutions
into strokes. The pump has two plungers. This is the minimum
number of plunger to make sure the pump is capable of delivering
fuel all the time. The pump is designed for high speed, e. g.
4,000 rpm. If the engine builder uses this potential, the pump can
be designed smaller and with less weight, which also improves
fuel economy of the whole engine. High speed pumps also cause
smaller pressure fluctuations, which is a further advantage. The
tests and measurements of the components showed good injection
behaviour for the injector and a high efficiency regarding high
pressure fuel pump.
May 2013 | Schiff&Hafen | Ship&Offshore SPECIAL 13