1 Introduction

An exhaust manifold is a part of the internal engine that collects exhaust gases from multiple cylinders into a single pipe. The manifold is referred to as a branching of pipe into various openings. Both exhaust and manifold fit a complementing surface on the exhaust ports region in the cylinder head. The exhaust passages from each port in the manifold join into a common single route before they reach the flange [1,2,3]. The exhaust manifold plays an important role in receiving the exhaust gases from the combustion chamber and letting them out into the atmosphere and hence has an impact on the performance of an engine system. The emission and fuel consumption efficiencies are closely related to the exhaust manifold [4, 5].

Few researchers examined exhaust manifolds using five different models and determined their performance under various conditions [6,7,8]. For a multi-cylinder engine, FE analysis of three exhaust manifold models with varying pipe diameters and speeds was performed [9]. In another study, various manifold materials were used to design the exhaust manifold. It demonstrates that redesigning the exhaust manifold occurs under different operating conditions with varying materials and temperatures [10]. Other researchers have emphasized numerical analysis for two different exhaust manifolds for a four-cylinder engine running at a constant speed [11]. Similarly, researchers performed FE analysis on various exhaust manifold shapes to achieve an optimal geometry for low back pressure and high exhaust speed.

The exhaust manifold is a critical component of any automotive vehicle exhaust system. Manifolds are used in diesel and gasoline engines of Light Commercial Vehicles (LCV), Medium and Heavy Commercial Vehicles (M&HCV) across the globe. These manifolds must be designed and manufactured with the utmost quality and precision to avoid mechanical and thermal disasters during engine operation, which otherwise can endanger human life and vehicle. The demand for the high performance exhaust manifolds is increasing with increase in vehicle production and the enhancement of safety features.

The automotive exhaust manifold market has a lot of room for research, as new approaches and technologies for manufacturing upright and reliable engine parts and components are being discovered. Further, newer manufacturing technologies have made it possible to produce complex shaped parts and components such as cylinder heads and tubular exhaust manifolds in a very short time.

Modern users are will be fascinated by performance-oriented vehicles with comfortable driving. Over the forecast period, this could be the most important driver for the exhaust system, which includes the exhaust manifold. Furthermore, a high standard of living has enabled consumers to use more powerful vehicles, with full engine capability and high driving performance. As a result, using a lightweight stainless-steel exhaust manifold may help reduce overall vehicle weight in the future.

The failure of the exhaust manifold is mainly attributed to material with which the components are made of up that can withstand the excessive temperature and forces acting on it during the service. The most common failures in exhaust manifolds are in the form of breaking and spillage [12, 13]. These are closely related to the manifold’s shape and various other parameters. In the present scenario, the engine running temperatures of the vehicle have been increasing due to environmental restrictions on the emission and the need to improve the engine performance [14]. As the motor industry is highly competitive and operates on small margins with large volumes, modification of the design plays a major role in increasing efficiency and gaining an upper hand in competition [15, 16].

Variation in mechanical and thermal loadings has a significant effect on the failure of the exhaust manifold. A lot of research is being carried out in improving the performance of exhaust manifolds. As experimental tests require close attention to the low-cycle thermal fatigue parameters and strength of the material, finite element modelling methods have been employed to evaluate the life of the exhaust manifold for various mechanical and thermal loadings [17,18,19]. The focus is also on using better materials to reduce weight to provide automotive industries with an edge in achieving optimal engine design. Many researchers are trying to determine the fatigue life of structural and thermal loading on the different design conditions of the manifold [20,21,22,23]. In designing the exhaust manifold, the thermal and mechanical issues such as deformation behaviour under thermal shock, thermal fatigue durability, and mechanical stresses induced by vibration and heat in the exhaust system need to be critically addressed [24,25,26,27].

The design of an exhaust manifold is a complex structure that is affected by several parameters such as back pressure, exhaust speed, and thermal and mechanical efficiency. As a result, numerical analysis is an important method for developing a new design and analysing its performance using commercially available FE software. The FEM technique is one of the methods that can be used to estimate thermal and structural behaviour under varying load and heat conditions [28]. The exhaust manifold can be modelled and simulated by changing different parameters such as material properties, loading conditions, and heating effect, leading to a significant difference in manifold life and subsequent performance. In addition, the design of the model is another crucial parameter. By optimising the shape of the manifold, a better performance can be achieved without changing the material. FEM technique allows such things to take place with less time and with suitable meshing and boundary conditions giving accurate results [29,30,31].

The present study is aimed at modelling an exhaust manifold and analysing the structural and thermal changes occur due to different materials and changes in design. The study focuses on finding the optimal design and material for the exhaust manifold to minimize thermal and mechanical stresses.

2 Materials and method

2.1 Structure of exhaust manifold

The exhaust manifold’s inlet and outlet structures were created on the commercially available SolidWorks workbench. The extruded geometry of the inlet plate is 8 mm thick and 68 mm in diameter. The multiple intake object, i.e. four plates, was created using the linear pattern feature, and the centre-to-centre distance of the inlet plate was 120 mm. The exhaust manifold outlet plate is triangular. The exhaust manifold pipe structure was added using a sweep and connecting the two edges. Finally, by removing the edges and adding thickness to the pipes, the solid object is transformed into a hollow structure using the shell feature. Figure 1 depicts the complete geometry of a two- and three-dimensional exhaust manifold. Similarly, multiport exhaust manifolds with the same dimensions but having different structures were created.

Fig.1
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2D and 3D views of a single port exhaust manifold

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2.2 CAD model

The exhaust manifold consists of an inlet flange and exhaust pipes, runners, and an outlet flange. The exhaust pipe is assumed to be a curved beamline, and the path of the beam follows the centreline of the pipe. Most common exhaust manifolds are of 4-to-1 type where in the four exhaust pipes merge into one outlet pipe. Hence, this design is considered the single port model. Another model is created where there is a multi-ended central pipe to exhaust the air.

Figure 2 shows two CAD models of exhaust manifolds varying in design where the first one is designed as a single exhaust manifold (Fig. 2a) and the other one is designed as a multi-exhaust manifold (Fig. 2b).

Fig. 2
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Images showing CAD models of a single-ended and b multi-ended exhaust manifold

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2.3 Material

Materials are commonly used in the manufacture of exhaust pipes, mufflers, and other exhaust system components. The exhaust system is subjected to a variety of chemical and physical environments, which must be considered when selecting materials and designing the system. Also, the material selected for manufacturing of exhaust system must possess not only sufficient strength and fatigue resistance but also excellent corrosion resistance against varieties of operational conditions [32, 33].

Stainless steel appears to be a better material for most exhaust system components during the material selection process. Because of its high corrosion resistance, stiffness, and resistance to high temperatures in the automotive exhaust system [34]. In addition, some other grades of steel and alloy material are used in the manufacture of exhaust manifolds [35].

Several factors influence material selection for exhaust systems, including performance, cost, durability and reliability, and customer satisfaction. Mild carbon steel has been the material of choice for producing automotive exhaust components for several decades due to its resistance to atmospheric corrosion. Stainless steel is a popular material for exhaust manifolds because it can withstand high temperatures, protect against prolonged oxidation, and has good strength and performance [36]. According to recent research, several alloy materials are used in stainless steel to experience good performance, cost savings, and a good finished product, among other benefits.

In the present study, to study the behaviour of the exhaust manifold, four different materials were imported into the CAD model. The materials chosen for the model include mild steel, cast iron, stainless steel (440C), and medium carbon steel (EN9). Materials chosen for the model and their respective properties are provided in Table 1.

Table 1 Material used for the exhaust manifold and their properties
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2.4 FE analysis

FE analysis was carried out to analyse the thermal and structural behaviour of the exhaust manifold using commercially available Ansys software. The FE analysis was used to find the critical location under defined boundary conditions such as design parameters, loading area, and type of material. Figure 3 shows the detailed steps involved in the FE analysis.

Fig. 3
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Image showing steps involved in FE analysis of exhaust manifold

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The single port and multiport exhaust manifold models were imported into the Ansys software, and the meshing was done meticulously using tetrahedral meshing elements for the analysis. Tetrahedral mesh has 12,300 elements and 18,567 nodes for single port exhaust manifolds and 13,856 elements and 19,876 nodes for multi-port exhaust manifolds. Figure 4 depicts the meshed exhaust manifold models of single port and multi-port exhaust manifolds using tetrahedral meshing elements. Figure 5 depicts the boundary conditions used in the structural and thermal analyses of single and multi-port exhaust manifold models.

Fig. 4
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Meshed model of a single-port and b multi-port exhaust manifold

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Fig. 5
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Boundary conditions applied for structural and thermal analysis of a, b single-port and c, d multi-port exhaust manifold

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The exhaust manifold is secured at one end, and 0.5 MPa pressure is applied along the cylinder. The steady state structural and steady state thermal analysis models are set up separately in the exhaust manifold. Pressure is passed through the cylinder as load in the structure analysis, and heat flux is passed through for the thermal analysis. The maximum stress and deformation of the exhaust manifolds are determined using static analysis. Similarly, thermal mapping is done for temperature and heat flux. For thermal analysis, the inlet temperature is taken to be 500 °C that is coming out of cylinders and the outlet temperature is taken as 25 °C which is the outer atmospheric temperature.

3 Results and discussion

Two models with different geometries are analysed for their mechanical and thermal behaviours along with various materials. The following sections discuss the mechanical and thermal analysis carried out on the exhaust manifolds.

3.1 Mechanical analysis

The post-processing result of the deformation pattern of the exhaust manifold is shown in Fig. 6. Left side images show the single exhaust manifold model with different models whereas the right-side images show respective multi-exhaust manifolds. The deformation behaviour of single and multi-exhaust manifolds of four materials is observed. The deformation observed for the single port exhaust manifold is higher in comparison to the multi-port exhaust manifold. For mild steel material, the maximum deformation of a single port manifold is 0.5508 mm and that of a multi-port manifold is 0.4832 mm. In the same way, all other material deformation is shown in Fig. 6 for single and multi-port exhaust manifolds. Also, the analysis indicates that multi-port exhaust manifolds are flexible and adjustable based on applicability.

Fig. 6
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Deformation pattern of the single and multi-port exhaust manifold taken at different time steps

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Along with deformation, stress distribution also plays a critical role in designing the exhaust manifold. The deformation behaviour of the exhaust manifold with varied materials is shown in Fig. 7a. It is observed that magnitude of stress observed is higher for the single exhaust manifold is higher in comparison to the multi-exhaust manifold with respect to all kinds of materials. This confirms that stress-relieving is better in multi-exhaust manifolds. It is important to note that the stress value is decreased by 20 MPa suggesting a 12.5% reduction in stress in multi-exhaust manifolds. However, changes in materials have resulted in marginal variation in stress distribution indicating that change in material does not have much effect with respect to stress distribution (Fig. 7b).

Fig. 7
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Graphs showing a deformation and b stress behaviour of the exhaust manifold with different materials

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3.2 Thermal analysis

Thermal analysis is carried out for single and multi-exhaust manifolds with different materials to study the thermal behaviour and performance. The exhaust manifold’s heat flux and temperature parameters were analysed for different materials. Figure 8 shows the simulation results of the total temperature behaviour of single and multi-exhaust manifolds with different materials. The concentrated temperature behaviour is noticed in the single exhaust manifold whereas distributed temperature is observed in the multi exhaust manifold. It is to be noted that a single exhaust manifold has a concentrated temperature at the edge of the manifold, which may cause an increase in the failure rate of the exhaust manifolds.

Fig. 8
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Images showing simulation results of the total temperature of the single and multi-exhaust manifold

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Heat flux is another important parameter in the exhaust manifold, which depends on the material's conductivity. The heat-flux and overall temperature distribution on the single and multi-exhaust manifold are shown in Fig. 9a, b. Heat flux generated in the single exhaust manifold is lesser than multi exhaust. This indicates that creating more multi-exhaust manifolds leads to a decrease in the thermal performance of the manifold. Mild steel material has a higher heat flux than the other materials, which may help to use for the design of the exhaust manifold, but it may lead to higher costs than cast iron.

Fig. 9
figure 9

Graphs showing bar charts of heat flux and total temperature of the exhaust manifold

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The temperature distribution of the multi-exhaust manifold is higher than the single exhaust manifold. The multi-point exhaust can efficiently distribute the temperature in the manifold. The bar chart shows that medium carbon steel (EN9) has higher temperature distribution than other materials. Thus, the optimum way of choosing the material for the exhaust manifold may depend on the geometry of the manifold.

The mechanical and thermal behaviour of single and multi-design exhaust manifolds are the essential parameters for the automotive industry. Hence, optimization of the thermal and mechanical properties of the manifold becomes crucial. Optimizing the mechanical and thermal features may lead to higher costs rather than maximum performance. Thus, experimental performance still needs to be analysed for a better understanding of the mechanical and thermal behaviour of the exhaust manifold.

4 Conclusions

In the present study, modelling and simulation of two different (single and multi) exhaust manifolds are studied. Four different materials are used for the exhaust manifold to analyse the respective mechanical and thermal behaviour.

The magnitude of the stress experienced by a multi-point exhaust manifold is 20 MPa lesser than a single exhaust manifold. Medium carbon steel has higher deformation compared to other materials. Thermal behaviour is an essential parameter in the exhaust manifold. The multi-exhaust manifold has a higher heat transfer rate and temperature distribution than the single exhaust manifold. The study confirms that by modifying/optimizing the geometry and material selection for the exhaust manifold, the efficiency of the exhaust manifold can be improved, thus improving the overall efficiency of an engine.