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.

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

Load

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.