CFD Analysis of Exhaust Manifold of Multi- Cylinder Petrol Engine for Optimal Geometry to Reduce Back Pressure

CFD Analysis of Exhaust Manifold of Multi- Cylinder Petrol Engine for Optimal Geometry to Reduce Back Pressure

1Vivekanand Navadagi, 2Siddaveer Sangamad

1Assistant professor, dept of Mechanical Engineering SVERISCE Pandharpur, Maharastra, India

2Assistant professor, dept of Mechanical Engineering SVERISCE Pandharpur, Maharastra, India

Abstract Exhaust manifold is one of the critical components of IC engine for improving the volumetric efficiency. The volumetric efficiency of the engine can be increased by reducing the backpressure in the exhaust manifold. This work analyzes the flow through two different models of exhaust manifold using CFD. The design of exhaust manifold is modified to get optimal geometry. The analysis results of two models are compared for back pressure. By comparing the results of two models the decrease in back pressure is found which ensure improvement in volumetric efficiency of the engine.

Keywords Exhaust Manifold, CFD, Multi-Cylinder SI Engine, Back Pressure

1. INTRODUCTION

   An exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. It is attached downstream of the engine and is major relevance in multi cylinder engines where there are multiple exhaust streams that have to be collected into a single pipe. When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. When the exhaust valve opens, the high pressure exhaust gas escapes into the exhaust manifold or header, creating an exhaust pulse comprising three main parts: The high pressure head is created by the large pressure difference between the exhaust in the combustion chamber and the atmospheric pressure outside of the exhaust system. As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the exhaust velocity decreases. This forms the mediumpressure body component of exhaust pulse. The r e ma i n i n g exhaust gas fo r ms the l o w pressure tail component. This tail component may initially match ambient atmospheric pressure, but the momentum of the high and medium pressure components reduces the pressure in the combustion chamber to a lower than atmospheric le vel. This relatively low pressure helps to extract all the combustion products from the

   cylinder and induct the intake charge during the overlap period when both intake and exhaust valves are partially open. T he effect is known as scavenging. Length, crosssectional area, and shaping of the exhaust ports and pipe works influences the degree of scavenging effect. Seenikannan et al. [1] analyzed a Y section exhaust manifold system experimentally to improve engine performance. This paper investigates the effect of using various models of exhaust manifold on CI engine performance and exhaust emission. Yasar Deger et.al [2] had done CFD-FE-Analysis for the Exhaust Manifold of a Diesel Engine aiming to determine specific temperature and pressure distributions. The fluid flow and the heat transfer through the exhaust manifold were computed correspondingly by CFD analyses including the conjugate heat transfer. Dr. Kutaiba et.al [3] made an approach to estimate of flow characteristic in inlet and exhaust manifolds of internal combustion engines using a four-stroke variable compression ratio single cylinder gasoline engine. In the experimental work, the compression ratio was varied from 7 to 11 at variable speed with constant throttle opening, where engine performance was obtained. Scheeringa [4] studied analysis of Liquid cooled exhaust manifold using CFD. Detailed information of flow property distributions and heat transfer were obtained to improve the fundamental understandings of manifold operation. A number of computations were performed to investigate the parametric effects of operating conditions and geometry on the performance of manifolds. Gopal et al. [5] has conducted experimental analysis of flow through the exhaust manifold of a multi cylinder Petrol engine of a contessa engine of 20 hp at maximum speed of 2000 rpm and then analyzed using FLUENT [1].

2. BACK PRESSURE

   Back pressure usually refers to the pressure exerted on a moving fluid by obstructions against its direction of flow. Back pressure caused by the exhaust system of an automotive

   engine has a negative effect on engines performance as it will restrict the flow rate of the exhaust gasses. The result would be the engine not being able to expel the spent exhaust gasses fast enough to prevent spent exhaust gasses from contaminating the fresh air/fuel mixture that is drawn into the engine on the next intake stroke. Ultimately, this will result in reduced engine power.

3. CONSTRUCTION

   The exhaust manifold system considered in the present case has 4 inlets connected to the exhaust port of the engines and a single outlet from where the flow is passed on to the exhaust system before ejection into the ambient. Due to lack of experimental data, an industrially available manifold geometry has been considered for the present analysis. It consists of the pipe diameter of 42 mm and total span of the manifold to be 0.6 m. The base model has the outlet placed besides the first port having smaller length of the curved pipe from the individual engine exhausts. The modified geometry has the outlet port placed in the middle with longer curved pipes from the individual engine exhausts, as shown in Figure.

   on ANSYS Workbench. A steady state singlespecies simulation will be carried out under isothermal conditions for exhaust gas. Turbulence will be modeled by k RNG turbulence model appropriate to account for high velocities and strong streamline curvature in the flow domain. The reference pressure will be set at 1 atm and all pressure inputs and outputs will be obtained as gauge values with respect to this.

   1. MATERIAL

      Air will be the working fluid considered to be operating at 350 0C and 1.35 bar. The material properties under these conditions are:

      Table-1: Properties of material

      Property	Air
      Density (kg/m3)	0.7534
      Viscosity (Pa.s)	3.0927 x 105
      Specific heat (J/kgK)	1056.6434
      Thermal conductivity (W/mK) (not	0.0242

   2. BOUNDARY CONDITIONS

   The two models will be tested for 4 different engine loads corresponding to different massflow rates at each inlet:

   Fig-1: Base geometry of the exhaust manifold

   Fig-2: Modified geometry of the exhaust manifold

4. METHODOLOGY

   A steady state single phase singlespecies simulation with exhaust gas as the working fluid will be carried out for the two geometries at 4 different mass flow rates from each inlet

   to determine the pressure drop.

   The geometry will be created using ANSYS ICEM CFD and a multiblock structured mesh will be created. ANSYS CFX will be used for doing the flow simulation under isothermal conditions. The overall analysis will be performed

   Table-2: Boundary conditions

   Cases	Engine speed(rpm)	Total exhaust gas flow rate (m3/s)
   i)	1300	3.143
   ii)	1700	4.516
   iii)	1800	4.981
   iv)	1900	5.627

RESULTS AND DISCUSSIONS

The models are analyzed using CFX and the results obtained are shown in the color Contours

1. Pressure contours for the existing manifold

   The high pressure at the head is due to high pressure difference from exhaust in combustion chamber and atmosphere pressure. Fig shows, the pressure was not uniformly distributed in the existing manifold 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.

   1900rpm

   1900rpm

   1300rpm

   1700rpm

   1800rpm

   1900rpm

   5.2 Pressure contours for the modified manifold

   1300rpm

   1700rpm

   1800rpm

   1900rpm

   1. Velocity contours for the modified manifold

   2. Pressure graphs

      1300rpm

      1700rpm

      1800rpm

      1900rpm

      Base Model (1300 rpm)	Modified Model (1300 rpm)
      Base Model (1700 rpm)	Modified Model (1700 rpm)
      Base Model (1800 rpm)	Modified Model (1800 rpm)

      Base Model (1900 rpm)	Modified Model (1900 rpm)

      Modified Model (1800 rpm)

   3. Velocity graphs

      Base Model (1900 rpm)

      Modified Model (1900 rpm)

      Base Model (1300 rpm)	Modified Model (1300 rpm)
      Base Model (1700 rpm)	Modified Model (1700 rpm)

   4. Back Pressure Comparison

   The graph shows that the back pressure is more in base model but it is less in case of modified model.

   150

   130

   110

   Pressure Drop

   90

   70

   50

   30

   2 4 6

   Base Model

   Modified model

   Mass flow rate

   Fig-5.7: Comparison of Back Pressure

5. CONCLUSION

The flow analysis of exhaust manifold was performed. The existing manifold is modified by changing its geometry to get optimal geometry. Both old and new models are analyzed under same boundary conditions. The results of new model are compared with the existing model. Pressure and velocity graphs are drawn for the new model and are compared with existing model. The decrease in back pressure is shown by using contour and vector diagrams. The flow is made efficient by decreasing the exhaust gas back pressure in the newly modified model thus increasing the volumetric efficiency of the engine.

REFERENCES

1. Seenikannan P., Periyasamy V.M. and Nagaraj P. An experimental analysis of a Y section exhaust manifold system with improved engine performance.

2. Yasar Deger, Burkhard Simperl, and Luis P. Jimenez Coupled CFD- FE-Analysis for the Exhaust Manifold of a Diesel Engine Sulzer Innotec, Sulzer Markets and Technology Ltd, Winterthur, Switzerland.

3. Dr. Kutaiba J.M. AL-Khishali, Dr. Mahmoud A. Mashkour & Ehsan Shamil Omaraa Analysis of Flow Characteristics In Inlet And Exhaust Manifolds of Experimental Gasoline Combustion In A VCR Engine.

4. Scheeringa K. Analysis of Liquid cooled exhaust manifold using CFD.

5. Gopal P., Senthil Kumar T. and Kumaragurubaran B., Analysis of Flow Through The Exhaust Manifold of a Multi Cylinder Petrol Engine for Improved Volumetric Efficiency International Journal of Dynamics of Fluids ISSN 0973-1784 Volume 5 Number 1 (2009)

6. K.S. Umesh, V.K. Pravin and K. Rajagopal CFD Analysis and Experimental Verification of Effect of Manifold Geometry on Volumetric efficiency and Back Pressure for Multi-cylinder SI Engine International Journal of Engineering & Science Research IJESR/July 2013/ Vol-3/Issue-7/342-353