Structure optimization for manifold catalytic converter based on 1D-3D coupled simulations

Rui QU, Jian-peng YUE, Yu WANG, Ying-jian LI

(1School of Energy & Power Engineering, Wuhan University of Technology, Wuhan 430063, China)(2School of Civil Aerospace and Mechanical Engineering, University of Bristol, Bristol BS81TH,UK)

Abstract: In this paper, BOOST and FIRE software were respectively used to establish thermodynamic model of an engine’s intake, an exhaust system and internal CFD steady-state model of the exhaust manifold. Then initial calibration and boundary condition fusion were conducted through experimental data. The pressure loss and transient flow field distribution of the exhaust manifold in low and high-speed conditions were analyzed by one-dimensional(1D) and three-dimensional(3D) coupling methods. According to the evaluation test results, the optimized structure has significantly improved internal flow field fluency and the exhaust uniformity of each manifold, reduced back pressure, shortened ignition time, and improved the maximum torque of the engine to a certain extent.

Key words: Manifold catalytic converter,1D-3D coupling, Structural optimization

1 Introduction

Manifold catalytic converter plays a great role in purifying the exhaust gas discharged from the cylinder. Due to the complexity of its structure and the particularity of its location, the internal flow characteristics of the manifold catalytic converter have a great influence on its performance. In addition, the flow characteristics of exhaust system not only affect the catalytic conversion, but also have a great impact on the engine’s power performance and economy.

In the past, the traditional design method is to constantly modify the structure through experiments or designers’ experience, which not only consumes a lot of manpower and material resources, but also increases the cost. At the same time, the design cycle time is too long, and even the structure has potential dangers, which are not conducive to the development of new products. This paper takes a certain type of manifold catalytic converter as an example. According to the 1D-3D coupling analysis theory, to solve the flow characteristics of the whole intake and exhaust system, not only take advantage of the high efficiency of one-dimensional simulation calculation, but also analyze the influence of the shape of key components in the intake and exhaust system (such as manifold, air filter, catalytic converter, etc.) on the engine performance. Relevant software is used to analyze the gas flow process inside the manifold and optimize the structure[1].

The basic premise of using CAE software for simulation analysis is that the mathematical model followed by the software calculation must be clearly understood, which is very critical for the correct use of the analysis software and to ensure the reliability of the analysis results. Take CFD analysis as an example, it boils down to this: the basic idea of the original in time domain and space domain quantities of the straight, such as the velocity field and pressure field, with a series of a finite number of discrete points on the set instead of a variable’s value, and through certain principle and the way to establish the relationship of these discrete points out variable algebra equations, and then to solve the algebraic equations for variable approximation.

The one-dimensional CFD model is mainly used to analyze and calculate the components in the whole system and investigate the overall influence of the subsystem, such as the influence of the exhaust manifold branch length, diameter and curvature on the engine performance. The influence of exhaust manifold structure and shape on exhaust flowability and uniformity is analyzed in 3D model. In order to accurately analyze the influence of gas flow in the manifold on engine and exhaust gas purification effect, this paper adopts the latest 1D-3D coupling calculation method. The main analysis process is shown in Fig.1.

Fig.1 flow chart of 1D-3D analysis of manifold catalytic converter

2 Mathematical model

The gas flow in the manifold catalytic converter is unsteady and viscous, and the flow process satisfies the conservation of mass, momentum and energy as shown in formula (2-1), (2-2) and (2-3). Considering compressible flow, the governing equation of instantaneous turbulence is written[2].

Equation of conservation of mass (equation of continuity):

(1)

Momentum conservation equation (navier-stokes equation):

(2)

Where,Sirepresents the resistance of the purifier carrier;τijis the stress term.

Energy conservation equation:

(3)

Where,cpis the specific heat capacity,Tis the temperature,kis the heat transfer coefficient of the fluid,STis the internal heat source of the fluid and the part where the mechanical energy of the fluid is converted into heat due to the viscous action.

The internal working process of the manifold catalytic converter is very complicated, including the turbulent phenomenon of gas flow, the integrated heat transfer phenomenon of natural convection with external air and internal forced convection, and the pressure loss of pipeline. Therefore, the following sub-models need to be established.

(1) Turbulence model

Thek-εmodel is applied in the study.k-εmodel assumes that the field is totally turbulent and the viscosity between molecules can be ignored. As a result, the standardk-εmodel can be valid in the field appearing as complete turbulence[3].

Turbulent kinetic energyktransport equation:

Gk+Gb-ρε-YM+Sk

(4)

Turbulent dissipation rateεtransport equations:

(5)

Gk,Gbare generation term due to the average velocity gradient and buoyant respectively,YMis contribution of ripple expansion in compressible turbulence,C1ε,C2ε,C3εrepresents the empirical constant,σk,σεis Prandtl number, andSk,Sεare the user-defined source term.

These constants are derived from experiments, including the basic turbulence of air and water. Although this set of coefficients has a wide range of applicability, its applicability cannot be overestimated. In the specific problems of flow field analysis of catalytic converters, reasonable modifications should be made based on the test results and different exhaust conditions.

(2) Heat transfer model

On the condition of mainly considering the natural convection between the converter and the external air and the forced internal convection, the external heat transfer coefficient is set as 20n, ignoring the influence of internal radiation heat.

(3) Pressure loss model

The pressure loss in the manifold catalytic converter can be divided into the following two types: the overall loss and the local loss. The overall loss can be calculated by Darcy formula[4]:

(6)

(4) Porous medium model

Owing to the carrier structure of catalysts mostly appears as the shape of honeycomb and shows a high complexity, it is impossible to simulate the structure of the carrier by only utilizing the mesh generation. Therefore, it is necessary to establish the porous medium model.

(7)

3 1D-3D coupling analysis of manifold catalytic converter

3.1 BOOST model

The gas flow characteristics of the engine intake and exhaust system are complex, which affects the aeration efficiency and air exchange loss of the engine, as well as the power and economy of the engine. Currently, computational fluid dynamics (CFD) simulation is available in the following models: one-dimensional model, three-dimensional model and hybrid model. The hybrid model combines the two to solve the flow characteristics of the whole intake and exhaust system. It not only takes advantage of the high efficiency of 1D simulation calculation, but also realizes the analysis of the influence of the shape of the key components in the intake and exhaust system (such as intake and exhaust manifold, air filter, catalytic converter, etc.) on the engine performance[5].

The thermodynamic model of the intake and exhaust system of 4-cylinder (PFI) gasoline engine was established as shown in Fig.2 (the thin solid wire frame is the coupling part). The model is compared with the experimental results to verify the validity of the model. The exhaust system of the engine was optimized by 1D simulation calculation to meet the design requirements of improving power performance and reducing fuel consumption of the engine. Based on the results, 3D model was established to investigate the flow loss and uniformity of the exhaust manifold. Finally, 3D model of the exhaust manifold is coupled in 1D exhaust system model of the whole engine, and the dynamic simulation of the 1D and 3D coupling engine exhaust system is carried out, and the flow characteristics in the exhaust manifold are analyzed in detail.

Fig.2 Engine intake and exhaust system BOOST model (1D)

The structural dimensions of the engine’s intake and exhaust system and combustion system are given according to the actual measured values and set values, as shown in Table 1. The validity of the model is verified through experiments. The calculation results are more reliable after considering the actual working conditions and the actual structure[6].

Table 1 Engine parameter setting

3.2 3D model and boundary conditions

In this paper, UG software is used for three-dimensional solid modeling of the engine intake system. The prototype exhaust manifold of FUTIAN 486 that meet Euro V emission standards, and the 1- 4 inlet ports are connected to the engine box through the flange.

The change of engine exhaust flow and temperature with crankshaft Angle was calculated by 1D model, as shown in Fig.3. The flow and mass flow curves at the crankshaft corner of the exhaust pipe inlet at 2000r/min was negative. The outlet static pressure (atmospheric pressure) and wall convection heat transfer were set by mass inlet boundary conditions[7]

Fig.3 The change in the exhaust flow at the inlet of the manifold with the change of crankshaft angles

3.3 Transient calculation results and analysis

Transient calculation can show the flow status of gas in the pipeline at any time. Here, the distribution of flow field at the maximum lift of exhaust valve at 2 000 r/min was analyzed. Fig.4 show the velocity vector distribution of cylinder 1 to 4.

Fig.4 The distribution of flow velocity at the end face of the carriers for the purifier

The study establishes the criterion for evaluating the flow distribution characteristic of carriers by applying the uniformity coefficient defined by Weltens et al[8]. The velocity distribution cloud chart and calculation results of parameter “ΔP” “γ” are listed below as Table 2 in the steady state of each cylinder outlet profile.

Table 2 The pressure loss of the inlets for each cylinder and uniformity coefficients of front-end of the carriers

4 Structure optimization

The structural optimization of exhaust manifold mainly refers to the following aspects:

(a) Minimize the length of the manifold to shorten the working ignition time and improve the catalytic conversion effect.

(b) The design of the manifold shape shall take into account the actual space in the engine compartment and the chassis structure.

(c) Ensure smooth gas flow and improve the uniformity of front end velocity of the catalytic carrier.

(d) Reduce exhaust resistance, flow loss and system noise to improve engine performance.

According to the results of 1D-3D coupling calculation, it can be clearly seen that the original manifold catalytic converter structure design is unreasonable. In order to realize the European V emission standards to the VI improve, after 4 modifications to adjust the length, diameter, curvature and shape of the manifold, the optimized structure scheme (5) shown in Fig.5 is obtained. The length of the manifold axis was shortened from 427 to 395 mm and the manifold was increased to 52 mm by 48 mm.

Fig.5 Structural scheme (1) to (5)

Fig.6 show the change of crankshaft catalyst cross section gas uniformity when the engine rotates at a steady speed of 4 500 r/min in the optimized model. Without changing the boundary conditions, the calculation results showed that the average value of uniformity coefficient in the whole cycle is 0.860 7, which meets the design requirements.

Fig.6 The gas uniformity of catalyst section varies with the change of crank angle

Meanwhile, the uniformity coefficient of the front face of the carrier in the new scheme increased by more than 16% on average, the pressure loss decreased by at least 8KPa, and the temperature change gradient of the manifold was relatively uniform, reducing the fracture risk caused by local thermal stress.

5 Test evaluation

With reference to the test method of vehicle catalytic converter bench evaluation, the original model and the optimized model were tested by using the performance evaluation test device of manifold catalytic converter. Fig.7 and Fig.8 show engine bench test and vehicle test respectively [9].

Fig.7 Engine bench test

Fig.8 Vehicle test

In the case of cCA, the back pressure of the original model is epa, and the maximum back pressure of the optimized model is opa, and the pressure fluctuation range of the original model is larger than that of the optimized model. The pressure loss of the optimized model is also relatively small, which has a significant effect on improving the performance of the engine.

In addition, the engine emission test system prescribed by the national standard is adopted to monitor the performance indexes of the engine and the whole vehicle installed with the catalytic converter. Test data shows that: the optimization scheme, emissions standards from the European V to VI, maximum torque from 142 to 148, power has obviously improved, and through the actual fuel consumption test, effective maximum 6.4% lower fuel consumption, in meet the requirement and the utilization ratio of catalyst life at the same time improve the engine performance and fuel economy requirements.

6 Conclusions

In this paper, a numerical model of one dimensional and three-dimensional coupling of manifold catalytic converter is established. The main conclusions are as follows:

(1) The flow characteristics of the catalytic converter were studied, the governing equation of fluid motion was simplified reasonably, and the mathematical model and parameter calculation model for specific problems were established, which provided theoretical support for the reliability of CFD software application. The simulation results show that the process convergence is good and the result accuracy is high.

(2) 1D engine inlet and exhaust system model was established to obtain the transient crankshaft Angle and mass flow data at the cylinder outlet under different working conditions. Through the 3D model, the transient flow field distribution data of the exhaust manifold were obtained, the gas uniformity and flowability in the exhaust manifold were well evaluated.

(3) The original exhaust manifold was optimized based on the 1D-3D coupling model and carried out the experiment on engine test bench. The tests indicate that the optimized model can greatly improve gas flow characteristics, improve catalytic conversion efficiency, shorten ignition time, and improve engine performance.