Q2 2009 - Ricardo

Software&CAE
The powertrain design software and CAE newsletter of Ricardo
Q2 2009
PISDYN helps Mahle Metal
Leve avoid liner cavitation
Cavitation of heavy duty diesel
cylinder liners is a major and longidentified
problem, yet the underlying
mechanisms causing it remain
little understood. Using PISDYN to
analyse an engine affected by this
issue, Mahle Metal Leve SA of
Brazil has identified both the cause
and a cost-effective solution.
Premature corrosion of the replaceable cast-iron
cylinder liners in heavy duty diesel engines is a
major reliability and durability issue for designers.
This corrosion is typically caused by cavitation
from high frequency pressure changes at the
interface between the metal surface of the liner
and the coolant. In 2008 Dr Estela Bueno of
Mahle Metal Leve S.A. and Luis Raminelli of
MWM International Motores undertook a major
research programme to better understand the
causes of and possible solutions to the problem
using Ricardo’s PISDYN software.
The issue became apparent during tests with a
heavy duty diesel engine which began showing
significant cylinder liner cavitation problems.
The engine had a cast iron block with chrome
plated steel liners and aluminium alloy pistons.
Despite having the same cylinder pressures
and similar cylinder block, piston, liner and
gudgeon pin design, the power output of the
new engine was much higher; this led the
researchers to believe that the increased load
might be a factor in causing cavitation.
When coolant is near its boiling point, vapour
bubbles form and then collapse against the
vibrating metal surface on the coolant side
of the liner, eroding its steel surface. Several
Piston-liner clearances at bottom dead centre
on the firing stroke (above); close-up of cavitation
erosion (below/left).
strategies for preventing this were considered,
such as changes in the liner material, the
surface coating and even the additive
properties of the coolant. But manufacturing
changes to the liner add cost, while changes to
the coolant may not be sustained by operators
over the life of the engine.
Secondary motion
Dr Bueno suspected the real problem was
shock caused by the ‘secondary motion’ of
the piston impacting the cylinder wall as
combustion takes place. Factors such as the
In this Issue
Pages 1-3 Cavitation erosion
Cavitation erosion of cylinder liners
and parent bores is a well known if
little understood problem for heavy
duty diesel engines. In this article
we describe work carried out by
engineers at Mahle Metal Leve
S.A. and MWM International
Motores of Brazil, who have used
PISDYN to successfully investigate
and resolve this issue.
Pages 3-4 Code upgrades
The past twelve months has seen
a large number of improvements to
Ricardo’s mechanical simulation suite
with the introduction of VALDYN 4.3,
SABR 2.1, PISDYN 5.1 and RINGPAK
5.1, all of which have significantly
enhanced functionality to users.
These include improved functionality,
new mathematical models and user
interface enhancements.
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PISDYN helps Mahle Metal Leve avoid liner cavitation
gudgeon pin offset and piston profile are known
to have a potentially significant influence on
liner cavitation. It was decided to investigate
three different potential solutions to the
problem. The first was to change the viscosity
of the coolant by the use of additives to inhibit
the formation of the vapour bubbles. The second
was to change the liner material to a harder
steel with a nitrate surface treatment, and the
third was to investigate the effect of vibration
introduced to the liner by the secondary motion
of the piston.
Neither the change in liner material nor the
coolant additives made any difference, and
the issue persisted. As a result, the focus
of investigation was turned to the source
of vibration responsible for the formation of
bubbles in the coolant. This was done by making
a detailed analysis of the piston’s behaviour
in relation to the cylinder liner, using Ricardo’s
PISDYN software.
The investigation was divided into three parts.
The first phase was to create a baseline model
capable of representing the piston secondary
motion instability during the thermal shock test
at the engine’s rated power output, something
which proved very effective in revealing
cavitation. The engine was run at rated power
with high temperature and load, then low
temperature coolant was suddenly introduced
into the engine with power maintained. The
cycle was repeated for 100 hours in a test aimed
at exposing the liner to aggressive conditions
likely to accelerate the cavitation process. The
second phase was to optimise the offset of
the piston pin and the third sought to perform
a vertical profile optimisation of the piston in
order to achieve the best piston secondary
motion behaviour.
In creating the baseline model and to ensure the
success of the simulation, several key factors
were considered. These were the cylinder
pressure diagram, the shape of the liner when
the engine was running, and any thermal
deformation of the piston. Cylinder pressures
were measured during the physical engine
testing at rated power for use in the simulation
work. The liner shape was evaluated using finite
element analysis, taking into account thermal
expansion using temperature data harvested
during physical testing. Other factors considered
included the piston profile, thermal expansion
and compliance.
Simulation using PISDYN revealed a significant
rate of change of lateral forces in the piston
as well as substantial, though not unusual,
deformation of the piston skirt. These results
suggested considerable excitation of the
cylinder liner but in order to quantify the extent
of the condition, the analysis was made on the
earlier, less powerful, cavitation-free engine for
comparison. This engine produced 20 per cent
less power and lateral forces in the piston were
30 per cent less – despite cylinder pressures on
a par with the more powerful engine.
Piston impacts cause liner excitation
These results strongly indicated that high
excitation of the liner was being caused by the
piston striking the cylinder surface. At the same
time, it was also important to evaluate both the
magnitude and the rate of change of this lateral
force as well as any tendency to deform the
components involved.
Distorted hot shape of liner at rated power (left).
The next phase was to solve the problem by
optimising the gudgeon pin offset, something
which has a marked effect on piston secondary
motion. In standard form, the pin sat on the
centreline of the piston with no offset at all.
PISDYN was used to analyse the effects of
moving the gudgeon pin towards both the thrust
and non-thrust side of the piston in steps of
0.25mm. The piston lateral stability was found
to be at its worst between 0mm and 0.5mm on
the non-thrust side.
Moving it to the thrust side reduced the
cavitation but created unwanted secondary
effects due to lateral loads and the rocking
angle. At bottom dead centre, for example,
there was a high rate of change of secondary
kinetic energy. This was not expected to
cause cavitation because the energy levels
were lower, but it was likely to increase wear
to unacceptable levels. With the pin moved
towards the non-thrust side, these conditions
were improved but lateral loads increased,
especially at the largest offset of 1.5mm.
Combined with a baseline liner material (steel
with a chromo coating) and using pure water
as the coolant, testing focused on a pin offset
to either the thrust or non-thrust side of 1.0mm
and a 0.75mm offset to the non-thrust side.
After testing, the piston with the 1.0mm thrust
side offset suffered heavy marking to the middle
of the skirt and the second land. Similarly, the
piston with the offset to the non-thrust side
showed heavy marking to the skirt thrust side.
However, the offset of 0.75mm resulted in no
marking and also solved the cavitation problem.
Sensitivity to offset
Through simulation using PISDYN, it soon
became apparent that secondary motion of
the piston is very sensitive to the gudgeon pin
2 Ricardo Software & CAE • Q2 2009

Mechanical simulation upgrades marked with four important releases
offset and a simple and economical solution
was found to a serious problem. The process
also highlighted the potential for optimising
the design of the piston profile for improved
dynamic performance and stability in secondary
motion, where both skirt and land profiles play
a major role.
One other important factor is that any dynamic
simulation of piston behaviour must account
for several different variables simultaneously.
Piston secondary motion depends on factors
including the piston mass centre position,
gudgeon pin offset, gas pressure centres, and
the shape of the liner at high temperatures.
The investigation also explains why some
engine designs with the piston mass aligned
with the pin centre (usually considered to be
unfavourable for dynamic reasons) produce
lower cavitation effects than those in which
the centre of piston mass and pin centrelines
are offset.
Though the analysis proved to be a complete
success, it is not possible to establish a
default value applicable to all engines because
cavitation depends on such a large number of
factors, and includes cooling liquid composition
and flow behaviour, liner material and surface
finish. However, the project demonstrated how
powerful PISDYN can be when the results are
quantified and validated using analysis of a
similar engine where no cavitation occurs.
Mechanical simulation upgrades
marked with four important releases
The last 12 months has seen a host of improvements
to Ricardo’s mechanical suite with the introduction of
VALDYN 4.3, SABR 2.1, PISDYN 5.1 and RINGPAK 5.1,
all of which have significantly enhanced functionality
to users. VALDYN is Ricardo’s multi-body dynamic
and kinematic simulation package which has been
specifically developed for valvetrain and drive system
analysis and cam and spring design. VALDYN 4.3
was released in the summer of 2008 and includes the
major enhancement of run distribution, which makes
this powerful application accessible to many more
customers than before.
With VALDYN, it is now possible to use multiple
CPUs on a network to run a single model with
multiple cases or perform parametric studies with
varying parameters in one session. Put simply,
this involves spreading the analysis over many
CPUs across a network. Previously, however, this
kind of operation was effectively limited only
to large OEMs because of cost and resources.
In version 4.3, instead of being implemented
for an infinite number of CPUs, a customer can
order VALDYN with a run
distribution licence for 16,
8 or 4 runs per job, greatly
reducing cost. Thanks to
the new licenses, the full
potential of VALDYN has
become accessible to smaller
companies during the last
nine months, with distribution
from a local machine making
it easy to manage the whole
process.
In the spring of 2009,
PISDYN, Ricardo’s advanced
simulation package for
predicting the dynamics
of the piston and connecting rod assembly,
has undergone significant enhancement with
the release of version 5.1. PISDYN primarily
considers the secondary motion of the piston
and the interactions between the skirt, liner and
oil film. In this version, the Reynolds equation
solver options have been removed and replaced
with the faster and more robust method of finite
volume discretization. There are now three
options for the applied boundary conditions
for the Reynolds equation: (1) Reynolds &
Jakobsson-Floberg-Olsson boundary conditions
using the Elrod algorithm; (2) Someya boundary
conditions and algorithm, and; (3) the Half-
Sommerfield boundary condition.
The third option allows the simulation to run more
quickly than it would in option 1 with the proviso
that the solution is not mass conserving. Option 3 is
Lubricating oil temperature predictions in PISDYN
(above); VALDYN drive system animation (left).
Q2 2009 • Ricardo Software & CAE 3

Mechanical simulation upgrades marked with four important releases
equivalent to the v3.3 solver or to selecting the finite
difference method in PISDYN 5.0.
PISDYN’s sister programme, RINGPAK, has
also undergone an upgrade to version 5.1 and
there are some significant improvements. Three
main models are used in RINGPAK for simulating
the lubrication between ring and liner, gas flow
characteristics between the ring, liner and groove
and the dynamics of the ring within the groove.
RINGPAK 5.1 now handles oil flows in the
crown land and groove surfaces. The physical
phenomenon captured by oil transport models is
extremely complex: oil flow is driven by piston
acceleration and surrounding gas flow; oil
thickness on each land and groove is impacted
by the groove/land oil mass balance, and crown
oil transport is a function of ring/liner transport.
RINGPAK 5.1 captures this phenomenon,
and offers further accuracy gains if users can
supply oil film thickness boundary conditions.
In this mode RINGPAK users can run very fast
parametric studies.
In previous versions, the oil entrainment model
was based mainly on blow-back and the
geometry of the top land. Although effective,
there were limitations when it came to
entrained oil sources and droplet distributions.
This model has also been enhanced significantly
so that flow between the crown, rings, and
liner are now considered in detail and the
entrainment oil flows are included in the
oil mass balance. The prediction of oil film
thickness in the crown grooves enables accurate
prediction of the passage of oil droplets into gas
flowing around the piston. By calculating the
system of oil droplet masses over each crown
groove and land, it is possible to accurately
trace the path of oil droplets in the flowing gas
and predict the oil entrainment into both the
cylinder and the crankcase.
The combination of these improvements brings
RINGPAK to a new level of sophistication. In
the past, though good at simulating blow-by
and blow-back, modelling oil consumption was
more difficult. RINGPAK 5.1 not only makes that
SABR, Ricardo
Software’s shaft,
gear and bearing
concept and design
package
possible but allows accurate simulation of how
oil interacts and passes through the physical
ring pack.
A further major enhancement of RINGPAK, also
catered for in PISDYN 5.1, is the capability to
model ring and skirt scuffing using high fidelity
techniques to capture thermal behaviour.
Scuffing is triggered when total contact
temperature reaches a critical level, with
bulk temperature of the components and the
instantaneous increase in temperature of the
surface, known as flash temperature, also
playing major roles. RINGPAK and PISDYN
can now calculate the heat generated and the
temperature of the ring or skirt respectively,
where it makes contact with the liner. At each
integration step, the heat transfer is solved
for the system that includes ring/skirt, oil and
liner. Predicting flash temperature provides an
indication of when scuffing will occur.
New faster version of SABR
SABR, Ricardo Software’s shaft, gear and bearing
concept and design package was also upgraded
to version 2.1 in the last few months. Of major
interest is that solver performance has been
increased by a factor of between 10 and 50,
depending on the job. It also embraces multi-core
solving so when run on any PC with more than
one core (which most modern PCs have) the
speed is increased to between 20 and 100 times.
Previous releases of SABR based bearing life
prediction on the ISO 281 1990 standards;
this has some limitations, particularly
where shaft deflections cause significant
bearing misalignment. SABR 2.1 is based on
ISO/TS 16281, which specifically allows for
misalignment and internal clearance in the
bearings, a feature enabled in the new revision.
SABR 2.1 is also able to show the distribution of
load and stress within a bearing caused by the
gearbox operating loads. This allows the user
to investigate in detail the causes of bearing
damage and the impact of changes in load and
misalignment. The roller bearing model in SABR
has been enhanced to use Hertzian contact
analysis in order to calculate bearing stiffness.
Thanks to enhancements to the Shaft Load
3D view, it is now easier to identify loads and
reactions by clicking on an arrow, highlighting
load values in a table. As part of the analysis
of gear mesh misalignment, SABR also now
displays the line-of-action misalignment as well
as skew and scope. This is more directly related
to counter measures which may be employed,
such as micro-geometry corrections.
Users engaged in reverse engineering of gear
components and working from measured gear
geometry will find that the extra metrology
information for doing this can now be accessed
from the top level of the GUI via the main
geometry tab. 'Specific Slide Ratio' has also
been added to the Gear Geometry Tab making
it easier to design gears while avoiding scuffing
and scoring issues.
For further information about Ricardo
Software products, support services and
CAE applications please contact:
CAE applications (Europe): Mark.Gregory@ricardo.com
CAE applications (US): Steve.Strepek@ricardo.com
Software Sales:
RS_Sales@ricardo.com
Software Support: RS_Support@ricardo.com
Or visit
www.ricardo.com
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