Analysis of flow through the exhaust manifold of a multi cylinder petrol engine for improved volumetric efficiency.

Gopal, P. ; Kumar, T. Senthil ; Kumaragurubaran, B. 等

Introduction

In internal combustion engines, exhaust system plays a vital role in the improvement of the engine efficiency. A good conditioned exhaust system increase the performance of the engine. Resonator Exhaust valves are commonly mushroom shaped poppet type. They are provided either with a cylinder head or ion the side of the cylinder for discharging the products of combustion from the cylinder These are madder as integral parts of the camshaft and are designed in such a way to open the valves at the correct timing and to keep them open the necessary duration. The pipe which connects the exhaust system to the exhaust valve of the engine and through which the products of combustion escape into the atmosphere is called the exhaust manifold. The exhaust manifold contains an exhaust port for each exhaust port in the cylinder head, a flat machined surface on this manifold fits against a matching surface on the exhaust port area in the cylinder head. In other applications, the machined surface fits directly against the matching surface on the cylinder head. A pulse to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The body of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient.

This differential keeps the pulse moving. The low-pressure tail end of an exhaust pulse will most definitely attract the high-pressure bow of the following pulse, effectively sucking it along. The runners on a header are specifically tuned to allow the exhaust pulses to line up and suck each other along. This brings up a few more issues, since engine revolves at various speeds, the exhaust pulses don't always exactly line up. Turbo introduce a bit of back pressure to the exhaust system, making it quieter. Catalytic converter is usually a stainless steel container mounted somewhere along the exhaust pipe of the engine. Inside the container is a porous ceramic structure through which the exhaust gas flows. Tin most converters, thee ceramic is a single honey-comb structure with many flow passages. So converters use loose granular ceramic with the gas passing between the packed spheres. Volume of the ceramic structure of a converter is generally about half the displacement volume of the engine. These results in a volumetric flow rate of the exhaust gas such that there are 5 to 30 changeovers of gas each second, through the converter. Catalytic converters for CI engines need larger flow passages because of the solid soot in the exhaust gases. Mufflers are used for silencing chores by three major methods: Absorption, Restriction, and Reflection. Mufflers can use one method or all the three method, to attenuate sound that is not so pleasing to the ears of the highway patrol. Computational fluid dynamics is used nowadays for a variety of engineering applications such as automobiles, aircrafts, dam constructions, blood pumps used to play the role of heart, in open-heart surgery etc. CFD is attractive to industry since it is more cost-effective than physical testing. Gambit is used to draw designs, mesh the model and specify the boundary layer for the model.

The boundary layers are applied to the volume. Boundary layers are a meshing tool used to grow the first layers of cells on those boundaries of flow domains where precise placement of high quality cells is critical for accurate computation of near wall phenomena, such as heat transfer and turbulence. Fluent is used for analyzing the models. The various models are read, scaled and then the problem is defined. The defining of the model is the setting the solver for the flow, applying the material properties and the boundary conditions for the problem. Then the problem is set for first or second order equation and then the problem is analyzed. The results obtained can also be compared and the meshed volume can also be changed can further differentiated into fine mesh in fluent the result are obtained for the new model, thus reducing the complexity of the model and the error is reduced.

Methodology

During exhaust stroke, piston moves up the cylinder bore, decreasing the total chamber volume. At some point the exhaust valve will open. The high pressure exhaust gas escapes into the exhaust headers, creating exhaust pulse.

The high pressure 'head' is due to high pressure difference from exhaust in combustion chamber and atmospheric pressure. As the exhaust gases equalize between the combustion chamber and the atmospheric pressure outside of the exhaust system, the difference in pressure decreases and velocity of leaving exhaust decreases. Thus medium pressure 'body' component of exhaust pulse is formed. The remaining exhaust gases form the 'tail' component. The tail component may initially match in pressure to that of atmosphere. It is further reduced by siphoning effect created by the momentum of high and medium pressure components. This results in less pressure at the low end of the exhaust pulse than atmospheric pressure.

This creates high pressure difference between the intake manifold and combustion chamber, which increases the velocity in which air is brought into the engine. This increase in intake air velocity leads to an increase in amount of air in combustion chamber which allows the engine to add more fuel and thus make more power. Headers help to increase exhaust velocity.

Turbulance Modeling

There are two radically different states of flows that are easily identified and distinguished, laminar flow and turbulent flow. Laminar flows are characterized by smoothly varying fields in space and time in which individual "laminae" (sheets) move past one another without generating cross currents. These flows arise when the fluid viscosity is sufficiently large to damp out any perturbations to the flow that may occur due to boundary imperfections or other irregularities. These flows occur at low-to-moderate values of the Reynolds number. In contrast, turbulent flows are characterized by large, nearly random fluctuations in velocity and pressure in both space and time. These fluctuations arise from instabilities that grow until nonlinear interactions cause them to break down into finer and finer whirls that eventually are dissipated (into hear) by the action of viscosity. Turbulent flows occur in the opposite limit of high Reynolds numbers.

The equations governing a turbulent flow are precisely the same as for a laminar flow. The solution is clearly much more complicated in this regime. The approaches to solving the flow equations for a turbulent flow field can be roughly divided into two classes. Direct numerical simulations (DNS) use equations, resolving all of the spatial and temporal fluctuations, without resorting to modeling. In essence, the solution procedure is the same as for laminar flow except the numerics must contend with resolving all of the fluctuations in the velocity and pressure. DNS remains limited to very simple geometries and is extremely expensive to run, the alternative to DNS found in most CFD packages including FLUENT is to solve the Reynolds Averaged Navier Stokes (RANS) equations. RANS equations govern the mean velocity and pressure.

Experimental Set Up

The compact and single engine test rig consisting of a four stroke four cylinder, water cooled, variable speed, diesel engine coupled with hydraulic water pump. The hydraulic water pump assembly is mounted on the shaft unit, continuous water supply arrangement is provided for cooling the engine. The engine is started by varying the water pump. The engine is started and multipoint digital temperature indicator and burette with three way lock is provided. There is thermocouple provided to measure the temperature of exhaust gas. An air tank with baffles is connected to the air inlet of the engine.

Engine Specifications

The specification of the engine is as follows.

i. Stroke length L: 90 mm

ii. Bore B: 84 mm

iii. No. of cylinders: 4

iv. Fuel: H.S Diesel

A. Specific gravity of fuel = 0.83 g/sec

B. Calorific Vale of fuel = 45627 kJ/kg

v. Speed: power present at 20 BHP at 2000 rpm

vi. Displacement cc: 1995 cc

vii. Loading: Hydraulic Loading

viii. Air-fuel ratio: 14.4:1

ix. Compression ratio: 21:1

Volumetric Efficiency

Volumetric efficiency is used as an overall measure of the effectiveness of a four-stroke cycle engine and its intake and exhaust systems as an air pumping device.

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The air density can be evaluated at atmospheric conditions and then the overall volumetric efficiency is calculated. It can also be evaluated by inlet manifold flow conditions. Volumetric efficiency measures the pumping performance of the cylinder, inlet port and valve alone. Volumetric efficiency is affected by the fuel, engine design and engine operating variables

The effects of several of the above groups of variables are essentially quasi steady in nature; i.e., their impact is either independent of speed or can be described adequately in terms of mean engine speed. However, many of these variables have effects that depend on the unsteady flow and pressure wave phenomena that accompany the time-varying nature of the gas exchange processes.

Effect of Inlet and Exhaust Pressure Ratio and Compression Ratio.

As the pressure ratio ([p.sub.e]/[p.sub.i]) and the compression ratio are varied, the fraction of the cylinder volume occupied by the residual gas at the intake pressure varies. As this volume increase so volumetric efficiency decreases. The effects on ideal-cycle volumetric efficiency are given by the {} term in equation.

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The experimental results are used as input load conditions for fluent validation. The values are compared in form of graph. The graph values are more agreeable and the fluent graph for the existing manifold is drawn.

Meshing of the Model

The results thus obtained from experiment are tabulated and other values are calculated. Then the required graphs are drawn. The existing exhaust manifold is drawn using gambit. Then the model is analyzed using fluent. The manifold is meshed using tetra hedral, T-grid so that fine mesh is obtained.

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Results and Discussion

The models are analyzed using fluent and the results obtained are shown in the color contours.

Pressure contour for the existing manifold and new design

The high pressure at the head is due to high pressure difference from exhaust in combustion chamber and atmosphere pressure. In the existing manifold the pressure was not uniformly distributed and the effect of non-uniformly distributed pressure gives an impact on the velocity of the flow of exhaust gases. The pressure flow through the outlet of the exhaust manifold should be uniformly distributed and so the new manifold design is drawn.

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The exhaust manifold undergoes a uniform pressure distribution along the flow. The pressure distribution is regular along the path and so the outlet velocity will be high for the flow. The exhaust gases equalize between the combustion chamber and the atmospheric pressure outside of the exhaust system. The difference in pressure increases and velocity of leaving exhaust increases. Thus the magnitude of exhaust scavenging effect is a direct function of velocity of high and medium pressure components of exhaust pulse.

Velocity flow contour,velocity vector for existing manifold

The velocity of exhaust gas is very important factor. The velocity of the exhaust gas determines the time taken for the exhaust gases to pass to the atmosphere.

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There is restriction in the flow and the particle flow velocity decreases for the pipe at the far end of outlet. The velocity is not uniformly distributed. The result is not in favor for the increase in combustion efficiency of the engine, as it takes longer time for the exhaust gases to push out to the atmosphere. So during the exhaust stroke there is a chance for the exhaust gases to accumulate inside the engine which decreases the volumetric efficiency of the engine.

Velocity contour, Velocity vector for newly designed manifold

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The red contour area shows the uniform velocity distribution through the manifold. The velocity helps to determine the flow of the exhaust. The result is in favor for the increase in combustion efficiency of the engine, as exhaust gas takes less time to flow out to the atmosphere. Thus during the exhaust stroke there is a chance for the exhaust gases to escape from the combustion chamber which increases the volumetric efficiency of the engine.

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The vector notation for the new design helps to identiy the problem, and the resultant vector is clear that the flow is maximum at the exhaust outlet.

Turbulent Kinetic Energy

Turbulent kinetic energy show the swirl movement of the exhaust gases during the flow through the exhaust manifold. The moment was set for second order linear equation. The turbulent kinetic energy is set for second order kinetic energy. The turbulent intensity is set for maximum of 5% for the inlet of the exhaust gas and 4% for the outlet of the exhaust gas.

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Conclusion

The flow through the exhaust manifold was analyzed and the problem regarding the flow was studied. The problem was rectified in the proposed new design and flow is made efficient to decrease the exhaust gas back pressure, thus increasing the combustion efficiency and volumetric efficiency of the engine.

The flow in a real exhaust port can easily be sonic with choked flow occurring and even supersonic flow in areas. The very high temperature causes the viscosity of the gas to increase, all at which alters the Reynolds number drastically.

Added to the above is the profound effect that downstream elements have on the flow of the exhaust port far more than upstream elements found on the intake side. It is for these reasons that most published information on exhaust port flow is vague. Particularly when concerning how much flow would be required in any given situation. Often quoted is that the exhaust should flow 60% of intake flow but this is only crude guess work.

APPENDIX 1

Table 5.1 Time taken for fuel consumption

S.No   Applied Load   Speed   T1    T2    Tavg
             (kgf)    (rpm)   (s)   (s)   (s)

1                0     986      9    10    9.5
2              3.5    1200      5     6    5.5
3                6    1640    4.5     4   4.25
4                9    1842    3.6   3.2    3.4
5               12    1933      3   2.9   2.95

Table 5.2 Performance variables of the engine

Sl.No   Torque (N-m)   B.P(kW)   I.P (kW)   T.F.C(kg/hr)        S.F.C
                                                           (kg/kW hr)

1                 0         0       0.65    1.572631579             0
2            5.4936     0.690     1.3406    2.716363636    3.933201911
3            9.4176     1.618     2.2680    3.515294118    2.172572074
4            14.126     2.725     3.3759    4.394117647    1.611933678
5            18.835     3.814     4.4642     5.06440678    1.327770775

Table 5.3 Efficiency and mean effective pressure of the engine

Sl.No   [eta]mech(%)   [eta]bth (%)   Bmep (Pa)

1                 0              0           0
2           51.5151        2.00602    34603.17
3           71.3408        3.63167    59319.73
4           80.7464        4.89478    88979.59
5           85.4398        5.94234    118639.5

Sl.No   Imep (Pa)   [[eta].sub.f](%)

1        39636.2                  0
2       67170.92             0.0001
3       83149.78             0.0002
4       110196.4             0.0002
5       138857.4             0.0003

Table 5.4 Mass flow rate of exhaust gas

Sl.No      [m.sub.a]    [m.sub.a] per    [m.sub.f]    m exhaust
        ([m.sup.3]/s)       cylinder        (kg/s)      (kg/s)
                        ([m.sup.3]/s)

1          9.6581818     2.414545455    0.754545455   3.1690909
2          12.498823     3.124705882    0.976470588   4.1011764
3          15.623529     3.905882353    1.220588235   5.1264705
4          18.006779     4.501694915    1.406779661   5.9084745

Table 5.5: Temperature of the exhaust gas

Sl.No   Applied Load (kgf)   Temperature of exhaust gas (K)

1                       0                              503
2                     3.5                              535
3                       6                              548
4                       9                              550
5                      12                              553

APPENDIX 2

GRAPHICAL REPRESENTATION

Comparison is done for the fluent graphs between the existing manifold and newly designed exhaust manifold for various parameters.

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References

[1] 'FSAE: Engine Simulation with Wave' journal author Mario Farrugia, Oakland University Formula SAE team

[2] 1-D Simulation of Turbocharged SI Engines - Focusing on a New Gas Exchange System and Knock Prediction author 'Christel Elmqvist-Moller' Published on December 2006 Kungliga Tekniska Hogskolan, Brinellvagen.

[3] Turbocharger System Exhaust Bypass Valve, Final report, Me450, Design and Manufacturing lll, Winter 2007, Department of Mechanical Engineering, University of Michigan

P. Gopal, T. Senthil Kumar and B. Kumaragurubaran

Department of Automobile Engineering,

Anna University Tiruchirappalli,

Tiruchirappalli, Tamil Nadu, India

Email: gopalpp@gmail.com