ABSTRACT:

The design geometry of exhaust manifold plays a vital role on smooth
combustion and emission reduction of spark ignition engine. By measuring the
exhaust gas back pressures and its velocities at different operating load
condition for different type of manifold chosen, such as long bend side exit
with reducer, long bend centre exit with reducer, short bend side exit, short
bend centre exit etc. the optimal design geometry of short bend centre exit
manifold is obtained. The analysis is done with virtual model of manifold.
Modelling and analysis of exhaust manifold are carried out by CATIA v5 and
ANSYS software.

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Keyword: Exhaust manifold; exhaust gas back pressures; Emission; SI Engine;
ANSYS

INTRODUCTION:

Exhaust Manifold: An exhaust manifold collects the exhaust gases from
multiple cylinders and collects it into one pipe and throws it out to
atmosphere.

Fig. Engine Manifold

Engine Back
Pressure:  Engine exhaust back pressure is defined as
the exhaust gas pressure that is produced by the engine to overcome the
hydraulic resistance of the exhaust system in order to discharge the gases into
the atmosphere.

This pressure
is a scalar quantity, not a vector quantity, and has no direction. The flow of
gas is driven by pressure gradient with the only possible direction of flow
being that from a higher to a lower pressure.

Gas cannot
flow against increasing pressure .It is the engine that pumps the gas by
compressing it to a sufficiently high pressure to overcome the flow
obstructions in the exhaust system.

Effects of
Increased Back Pressure:

Ø The engine has to compress the exhaust gases to a higher
pressure which involves additional mechanical work and/or less energy extracted
by the exhaust turbine which can affect intake manifold boost pressure. This
can lead to an increase in fuel consumption. The increased exhaust temperature
can result in overheating of exhaust valves.

Ø It affects the performance of the turbocharger, causing
changes in the air-to-fuel ratio-usually enrichment, which may be a source of
emissions and engine performance problems.

Ø It increases the likelihood of failure of turbocharger
seals, resulting in oil leakage into the exhaust system. In systems with
catalytic DPFs or other catalysts, such oil leak can also result in the
catalyst deactivation by phosphorus and/or other catalyst poisons present in
the oil.

All engines have a maximum
allowable engine back pressure specified by the engine manufacturer. Operating
the engine at excessive back pressure might invalidate the engine warranty.

Exhaust
Velocity:
Exhaust system is designed to evacuate gases from the combustion chamber
quickly and efficiently. Exhaust gases are not produced in a smooth stream;
exhaust gases originate in pulses.

A 4-cylinder
motor will have 4 distinct pulses per complete engine cycle. More the pulses
produced, the more continuous the exhaust flow. Back pressure is the resistance
to positive flow of the exhaust stream.

Many
researchers have done their research works on this field to reduce the emission
from exhaust manifold like PL.S Muthaiah1, has analysed and then modified the
exhaust manifold using CFD by varying the size of conical manifold with the
help of grid mesh and so it kept a check on backpressure.
Likewise K.S.Umesh, K.Rajagopal and V.K.Pravin2 researched and designed 8
different manifold designs and classified it as SBCE(Short Bend Centre
Exit),SBSE,LBSE(Long Bend Side Exit),etc.And after analysis found out that
LBCER(LBCE with reducer) gives highest overall performance.

Siddaveer
Sangamad and Vivekananda Navadagi3 analysed the flow of exhaust gas from two
different modified exhaust manifold with the help of CFD. To achieve the
optimal geometry for the low backpressure they have analysed two different
exhaust manifolds, base geometry and the modified geometry exhaust manifold. In
the base model of the exhaust manifold the outlet is at side of the first inlet
where as in the modified model of the exhaust manifold the outlet is at the
centre of the exhaust manifold. Analysis has been done for the two different
exhaust manifolds. The results were compared for the two models and it is found
that the modified gives low backpressure in comparison with other base model
which ensures the improvement in the efficiency of the engine.

DESIGNED MODELS OF EXHAUST MANIFOLD:

Large numbers
of design and analysis software are available in the market for designing and
analysis of parts. Some of those are:

·
PTC Creo

·
CATIA

·
ANSYS

·
Hypermesh

·
Inventor

From above software’s for
my convenience i had chosen CATIA and ANSYS for design and
analysis of exhaust manifold.

And below are designed
models of manifold in CATIA v5:

Fig.A-Shape of the
inlet has
been
modified
from
straight inlet to convergent
inlet

Fig.B-Outlet
of exhaust
manifold
is modified  from converging   outlet   to   divergent-straight-convergent
outlet.

Fig.C-The divergence length of
the outlet is increased
and convergence length
is decreased.

Fig.D-The divergence length of
the outlet is decreased
and
the convergence
length
is

Increased.

Fig.E-The divergent area and convergent
area
of the outlet are kept
equal and straight
area is decreased.

ANSYS ANALYSIS:

The ANSYS program allows
engineers to construct computer models or transfer CAD models of structures,
products, components, or systems, apply loads or other design performance
conditions and study physical responses such as stress levels, temperature
distribution or the impact of lector magnetic fields.

Competitive companies look
for ways to produce the highest quality product at the lowest cost. ANSYS (FEA)
can help significantly by reducing the design and manufacturing costs and by
giving engineers added confidence in the products they design. FEA is most
effective when used at the conceptual design stage. It is also useful when used
later in manufacturing process to verify the final design before prototyping

Material Fluid Properties: Exhaust gas is considered as an
incompressible fluid operating at 230? 280°c.
The material properties under these conditions are:

Table 1.Material Fluid Properties

Material

Air + Gasoline

Density (kg/m3)

1.0685

Viscosity (Pa-s)

3.0927 x 10?5

Specific heat (J/kg-K)

1056.6434

Thermal conductivity

0.0250

Boundary Conditions: The inlet mass flow rates
for different models at six different loading conditions are given below using these
mass flow rates the pressure and velocity contours were obtained.

Table 2. Inlet Mass Flow Rate

Inlet 1

Inlet 2

Inlet 3

Inlet 4

2
KG

0.00188kg/s

0.00188kg/s

0.00188kg/s

0.00188kg/s

Fig.1-Model :1

From the fig.1 it is evident that due to the convergent shape of the
inlet, velocities are found to be lower at the connecting area and outlet. The
low velocity results in high backpressure. It is observed that exhaust
velocities are considerably decreases by designing the manifold using the
convergent inlet. Low turbulence kinetic energy is observed throughout the
flow.

Fig.2-Model 2

From
the fig.2 it is seen that the velocities are found to be slightly higher at the
outlet in comparisons with model 1. It is observed that the exhaust velocities
are considerably increased by designing the exhaust manifold using this outlet
in comparison with model 1. It is evident that higher turbulence kinetic energy
is observed at the outlet. At the inlets the turbulence energy is minimum.

It is observed that the velocity is
slightly high at the outlet in comparison with the model 1. Pressure is higher
at the outlet in comparison with model 1. It is evident that the pressure is
high at the inlets of model 2. Pressure is high at the middle of the exhaust
manifold in comparison with the exit of the outlet.

Fig.3-Model 3

Fig.3 gives the velocity contour of the model 3. It is seen that the
velocity is higher at the outlet incomparison with the model 1 and 2. It is
observed that exhaust velocities are considerably increases by designing the
manifold using the divergent-convergent outlet. The results gives the
turbulence kinetic energy contour. It is observed that the turbulence increases
by decreasing the convergent length. Higher turbulence kinetic energy is
observed at the connecting area and the outlet.

Fig.4-Model 4

Fig.4
gives the velocity contour of model 4. It is observed that due to the sudden
expansion of the area of outlet velocities are found to be higher at the
outlet. Velocity at the outlet is higher in comparison with the model 1, 2 and
3. It is observed that the exhaust velocities are considerably increased by
designing the exhaust manifold by reducing the divergent length of the outlet
in comparison with model 1, 2 and 3. This results gives the turbulence kinetic
energy contour of model 4. It is evident that higher turbulence kinetic energy
is observed at the outlet in comparisons with the models 1, 2 and 3.

Fig.5-Model 5

Fig.5 gives the velocity contour of the model 5. It is observed that due
to the divergent convergent shape of the outlet velocity is higher at the
outlet in comparison with the other models. It is observed that the exhaust
velocities are considerably increased by designing the exhaust manifold by
reducing the straight length of the outlet in comparison with other models. It
gives the turbulence kinetic energy contour of the model 5. It is observed that
the turbulence is almost same in comparison with the other models.

Results and Discussion:

Five
different models are designed and results were analysed through CFD Post
processing. The use of different shapes of exhaust manifold helps in easy flow
of exhaust gases.

Ø Model 5 facilitates easy flow of exhaust without recirculation and low
backpressure at the exhaust outlet in comparisons with all other models.

Ø
Turbulence kinetic energy is almost
zero in the model 5 and hence the exhaust flows easily.

Ø Velocity at the outlet of model 5 is more and hence the backpressure
reduces considerably.

Ø The optimum design for an exhaust manifold is Model 5 with 0.845 bar
back pressure and outlet velocity 12.5m/s.

Ø The minimum backpressure and higher exhaust velocities are achieved by
using exhaust manifolds with reducers, thus also reducing emissions.

Conclusions:

Thus out of 5
different exhaust manifold designs designed and analysed, 5th Model
is considered as the best of all because it has lesser turbulence energy, back
pressure and also higher exhaust velocity and volumetric efficiency.