[2]Simulation Based Approach for Optimization of Intake Manifold

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Simulation Based Approach for Optimization of
Intake Manifold
2011-26-0074
Published on
19th-21st January
2011 SIAT, India
Devananda B Pai, Hari Shankar Singh and P V Fayaz Muhammed
Maruti Suzuki India Limited, India
Copyright © 2011 SAE International and Copyright © 2011 SIAT, India
ABSTRACT
INTRODUCTION
Change is only thing which is permanent in today’s world.
In a highly volatile market, one can only survive by being
fast, flexible and first mover. This requires analyzing market
situation, conceiving new models and developing them in the
minimum possible time. Simulation is a very powerful tool for
squeezing model development cycle. It significantly reduces
time required for optimization and also gives the designer
flexibility of going for wide variety of design options,
which is not feasible otherwise through experimentation.
Simulation also helps in detailed understanding of the
physical phenomenon of the respective system.
The Intake system of an engine regulates the air-charge inside
the combustion chamber.
For a gasoline engine, the five main parts of an intake
system, apart from sensors, are:
Ducts, resonators, air cleaner, the intake manifold and throttle
body.
The torque output obtained from an engine is determined by
its volumetric efficiency. The volumetric efficiency of the
engine is dependent upon the quantity of air charge present
inside the combustion chamber which in-turn is controlled
by the intake System.
Detailed Analysis was done using 1-D CFD code to optimize
various parameters of the Intake Manifold at the initial design
stage. Detailed virtual DOE was done to achieve the required
level of engine performance. The numbers of concepts were
limited for final engine bench testing resulting in reduction
of the testing time. Strong co-relation was achieved between
the simulation and actual testing data.
The design considerations that affects yield from the intake
system are:
Intake Manifold Runner length
Intake Manifold Runner diameter
Intake Manifold Plenum volume
Air Cleaner volume and
For further fine tuning 1D and 3D CFD codes were coupled
to enable better optimization of the intake manifold geometry.
This also led to better prediction of the engine performance.
Clean-Side Duct length
Dirty Side Duct
This paper describes the effective use of advanced simulation
tools for design optimization of intake manifold.
The air cleaner volume and dirty/clean side ducts help in
getting maximum air at optimum temperature. These parts
also play a vital role in the noise characteristics of the engine
intake system. System designer typically uses intake manifold
to tune the torque output as per the requirement.
Separate simulation was carried out for capturing the 3D
effects of intake manifold geometry on 3D CFD code using
standard boundary conditions.
Keywords : Simulation, Intake Manifold, 1D-CFD, DOE,
3D-CFD, Optimization, Brake Torque
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Symposium on International Automotive Technology 2011
The optimal design of the intake manifold has the objective
of getting minimum flow resistance, good air distribution
between cylinders and runners that take advantage of the ram
and tuning effects, trying to achieve the maximum cylinder
charge for a selected operating range of the Engine.
Initially, simulation base file of a 3 cylinder Spark Ignition
Engine was created in GT Power. This virtual engine consists
of the entire engine components starting from air Suction pipe
to exhaust tail pipe. The geometrical parameters including
section diameters and flow length of different components
was obtained from CAD drawings. The other parameters
required for conducting analysis are pressure drop across
engine valves and a definition of combustion inside the
cylinder.
IMPORTANCE OF SIMULATION IN
INTAKE DESIGN
Traditionally intake manifolds are designed by experimental
approach, in which various prototype intake manifolds
would be built and tested on the engine. Intake manifold
performance for each build would be assessed from the
evaluation of experimentally determined data. This “cut and
try” approach of intake manifold development can be very
costly and time consuming, and it may lead to an acceptable
but not necessarily optimal solution. Complete Intake system
simulation, on the other hand, can often be an extremely
rapid, cost effective and insightful method of getting a peak
solution and understanding of physical phenomena behind it.
SCOPE OF INTAKE
TUNING
The Pressure drop across engine valves was represented with
measured coefficient of discharge from the flow bench and
the combustion was defined by measured in-cylinder pressure
across engine cycle (P-θ) using a combustion analyzer.
The subsequent step was to simulate the base file using GT
Power and match the result (performance results) with the
actual bench testing data.
The measured and predicted engine torque and volumetric
efficiency are shown in Fig. 1 and 2 respectively. The
predicted torque and volumetric efficiency show good
correlation to data throughout the speed range. The maximum
error in torque and volumetric efficiency was about 4%.
MANIFOLD
The customers of overseas markets are quite different from
their Indian counterparts. In European countries and Japan,
the cars tend to travel at speeds greater than 100 km/h quite
frequently and the engine revs more than 4000 speed to
achieve this speed. The torque output of the engine needs
to be adequate at this speed range in order to provide a
comfortable drive. So engines are normally designed by
providing a healthy torque in higher speed band.
On the contrary, the Indian traffic condition rarely allows
driver such a luxury. The Engine’s operating speed in Indian
road condition hardly crosses 4000. In India, the customers
have to stop and accelerate very frequently. Consequently a
good low-end torque turns out to be handful.
For a naturally aspirated engine, the torque distribution
between lower and higher speeds is a trade-off. Considering
the growing Indian market and regular operating speed range
in India, it was reasonable to think of re-tuning engine to suit
the Indian road conditions, giving a better low end torque.
Figure 1. Simulation and Test Data Comparison of
Brake Torque
DESIGN OF EXPERIMENTS
After the matching of virtual output to actual bench test
data, the focus now shifted to the design of experiments
and associated iterations in order to achieve an improved
low–end torque.
Thus the functional target was set - improving the low-end
torque.
SIMULATION METHODOLOGY
The effect of varying critical parameters on Engine output
were assessed as explained below
The simulation of the Engine discussed herein was conducted
using GT Power simulation code. The GT Power code
contains comprehensive engine performance models built on
top of a 1D Computational Fluid Dynamics (CFD) code, thus
allowing the prediction of both engine performance quantities
as well as the characterization of intake system dynamics.
Manifold Runner Length
As the intake valve opens, a rarefaction zone is created
behind the intake valve and this low-pressure region starts
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Symposium on International Automotive Technology 2011
Table 1. Iterations Details with respect to Runner Length
Figure 2. Simulation and Test Data Comparison of
Volumetric Efficiency
to move away from the cylinder through the runner. When
this zone reaches the plenum - all contiguous air rushes to
this low-pressure zone and it forms a compression zone. This
compression wave moves back through the runner towards
the cylinder. As it reaches the cylinder it creates a ram
effect and maximum air enters into the cylinder. This wave
travels with a velocity of sound. At the same time piston
moves down and reaches its maximum velocity at 850 ATDC
simultaneously create maximum vacuum inside cylinder.
For a perfectly tuned manifold the ram air has to reach the
cylinder at the same time when there is maximum vacuum
inside the cylinder (at 85 degree crank-angle after TDC) so
that maximum air enters the cylinder. But as the engine speed
varies these two incidents do not happen at the same time.
Therefore peak volumetric efficiency at one particular speed
which represents a peak point in the engine torque curve.
As the intake manifold runner length changes the distance
which compression and rarefaction waves travel also changes,
and the peak torque point shifts away from its earlier peak.
Figure 3. Variation of Brake Torque with respect to
Intake Manifold Runner Length
Manifold Runner Diameter
At lower speeds a smaller runner diameter would increase
the air-flow velocity and therefore facilitate entry of more
air-charge inside the cylinder. On the contrary, at higher
operating speeds, a lower runner diameter would result
in choking effect or turbulence and consequently lower
volumetric efficiency. After conducting several simulations
it was found that a diameter reduction of 5mm gives an
improvement of output torque in the lower speeds without
much concession in the higher speeds. Table 2 shows the
iterations details with respect to runner diameter. Fig. 4
shows the variation of brake torque with respect to intake
manifold runner diameter.
When the runner length increases, it delays the arrival of
compression wave at the intake valve and this tends to shift
the peak torque point to a lower speed. In simpler terms – If
an engine is tuned for a given speed and if the runner length
is increased, the arrival of compression wave at intake valve
happens after 850 for the corresponding rated speed. As the
runner length increases peak torque point shifts to a lower
speed. So iterations done with different runner length and
observed the variation in torque and power. A runner length
increase of 120mm found to be good option by considering
lay out and performance. Table 1 shows the iterations details
with respect to runner length. Fig. 3 shows the variation of
brake torque with respect to intake manifold runner length.
Table 2. Iterations Details with respect to Runner Diameter
Plenum Volume
At higher speeds a large Plenum volume caters to the
higher mass flow requirements of the Engine giving a minor
improvement in output. On the contrary, the experimental
data shows that the throttle response for higher plenum
volumes is poor. After conducting several simulations, it
was found that a plenum volume increase of 0.2 liter gives
good performance. This factor is practically constrained by
the space available in the engine room.
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Symposium on International Automotive Technology 2011
with respect to air cleaner volume. Fig. 6 shows the variation
of brake torque with respect to air cleaner volume.
Table 4. Iterations Details with respect to Air Cleaner
Volume
Figure 4. Variation of Brake Torque with respect to
Manifold Runner Diameter
Table 3 shows the iterations details with respect to plenum
volume. Fig. 5 shows the variation of brake torque with
respect to intake manifold plenum volume.
Table 3. Iterations Details with respect to Plenum Volume
Figure 6. Variation of Brake Torque with respect to Air
Cleaner Volume
Air Cleaner Hose Length
The functioning of an intake system can be considered as
being analogous to a two-spring - two-mass system with two
separate natural frequencies. These two frequencies represent
the two peaks in torque curve. The increase or decrease in
hose-length corresponds to a change in mass of the system
resulting in a change in distance between the two peaks
in the torque curve. An increase in the hose length would
increase the distance between the two torque peaks. In test
engine, an increase in hose length results in a decrease in
the peak torque value. But as already mentioned it would
cause an improvement in the lower and upper range torques.
A hose length increase of 60 mm was found to be optimal
in terms of lay out and performance. Table 5 shows the
iterations details with respect to air cleaner hose length.
Fig. 7 shows the variation of brake torque with respect to
air cleaner hose length.
Figure 5. Variation of Brake Torque with respect to
Plenum Volume
Air Cleaner Volume
Table 5. Iterations Details with respect to Air Cleaner
Hose Length
The air cleaner volume also enhances the torque at lower
speeds but only up to a limited increase in Air Cleaner
volume. Any further increase only helps in improving the
acoustics performance as it reduces low frequency noises. A
large air cleaner avoids the usage of a resonator. Based on
constraints and targets, a volume increase of 1.5 liter worked
out to be a good solution. Table 4 shows the iterations details
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Symposium on International Automotive Technology 2011
Figure 7. Variation of Brake Torque with Respect to Air
Cleaner Hose Length
Combined Results
Figure 9. Combined Simulation: Predicted Vol Efficiency
Output vs. Speed
The optimal value for each parameter was chosen from the
above study and the results were calculated in a combined
simulation exercise. The present and suggested values of
these parameters have been shown in Table 6. Fig. 8 and 9
show the combined results for brake torque and volumetric
efficiency respectively.
After arriving at this optimized solution, a proto part was
developed and actual bench testing was conducted on the
physical part. The following results were obtained for the
bench test:
Table 6. Present and Suggested Values of Design
Parameters
The result shows an overall improvement in torque values
in line with the prediction. There is a deviation observed
between actual and simulated torque curve in the speed range
between 1000 and 3000 rpm. This is primarily accounted
due to the influence of idle air control valve, which was
not taken into consideration in the simulation because of
its complexity. Also changed knock characteristics in lower
speed in the new engine with respect to the present engine
reduced the accuracy of simulation. Fig. 10 and 11 show
the actual testing results for brake torque and volumetric
efficiency respectively.
Figure 8. Combined Simulation: Predicted Brake Torque
Output vs. Speed
Figure 10. Actual Testing results: Brake Torque vs.
Speed
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Symposium on International Automotive Technology 2011
0.2 Nm of torque increase was found due to the presence
of bell mouth. Fig. 14 shows the variation of brake torque
with respect to bell mouth.
Figure 11. Actual Testing Results: Vol Efficiency vs. Speed
Figure 12. Intake Manifold Without Bell Mouth
The better performance at the higher speed were majorly
contributed by the flow resistance. This was improved in
the new design by changing the material of manifold from
aluminium to plastic for experimentation.
T H E O P T I M I S AT I O N O F F L O W
JUNCTIONS
The 1D software GT POWER is used to decide the basic
parameters like runner length, runner diameter and the plenum
volume and its effect on engine performance parameters like
torque and power. Since GT POWER is 1D software it is
not possible to capture the effects of complex 3D geometry
of the intake manifold on the engine performance.
Standalone analysis is done in 3D CFD software STAR CD
to calculate the air flow resistance of the Intake Manifold
and to find the charge distribution in each cylinder. Standard
boundary conditions are used for the simulation against the
actual engine boundary conditions and also it is difficult to
correlate the results of 3D analysis on engine performance.
Figure 13. Intake Manifold With Bell Mouth
Coupling the 2 software’s: GT POWER and STAR CD,
with complex 3D Geometry represented in STAR CD and
engine boundary conditions from GT POWER eliminates
the disadvantages of both the software’s and also helps in
detailed understanding of the flow inside the intake manifold.
This coupled analysis is used to find out effect of different
intake manifold geometry on engine performance. Study
was done on intake manifold with different bell mouth
configurations to optimize flow junctions in order to get
a reduced pressure drop. Figs. 12 and 13 show the intake
manifold without and with bell mouth respectively.
Simulation was carried out at the rated speed (6000 rpm,
the speed at which maximum air-flow happens) and nearly
Figure 14. Simulation Results: Brake Torque vs. Speed
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Symposium on International Automotive Technology 2011
CONCLUSIONS
Some guidelines for the design of Intake system of 3
Cylinder naturally aspirated spark-ignition engine has been
reported. A 1-D analysis was utilized for a more refined
definition of its geometrical characteristics, which also take
in to account constraints imposed by layout of Engine bay
.A further refinement is done with help of 3-D CFD software
to get exact shape of flow components. The comparison with
experimental data has confirmed the robustness of the whole
design procedure.
3.
John J Silvestri, Thomas and Michael Costallo, “Study
of Intake System wave Dynamics and Accoustics by
Simulation and Experiment”, SAE Paper No. 940206,
1994
4.
John B Haywood, “Internal Combustion Engine
Fundamentals”, McGRAW HILL INTERNATIONAL
EDITIONS Automotive Technology Series
5.
GT-Power, user’s manual and tutorial, GT-SUITETM
Version 6.0, March 2003, Gamma Technologies
ACKNOWLEDGMENTS
CONTACT
To Mr. Piyush Agrawal and Mr. Prasenjit Khan for the
consistent support and encouragement received from them.
P V Fayaz Muhammed
Assistant Manager,
EN1D, ERD1
Maruti Suzuki India Ltd
Email : [email protected]
REFERENCES
1.
Carl H Wolgemuth and Donald R Alson., “Study of
Engine Breathing Charecteristics”, Pensylvania State
University. SAE Paper No. 650448, 1965
2.
Rafel Royo, Jose Corberan and Antonio Perez, “Optimal
Design of the Intake System”, Universidad Politechnica
de Valencia. SAE Paper No. 940210, 1994
The Technical Paper Review Committee (TPRC) SIAT 2011 has approved this paper for publication.
This paper is reviewed by a minimum of three (3) subject experts and follows SAE guidelines.
Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of
SIAT 2011. The author is solely responsible for the content of the paper.
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