Numeric Investigation of Gas Distribution in the Intake Manifold and Intake Ports of a Multi-Cylinder Diesel Engine Refined for Exhaust Gas Stratification

Abstract

In-cylinder exhaust gas recirculation (EGR) stratification, generally achieved by supplying EGR asymmetrically into intake ports on a four-valve diesel engine, is sensitive to trapped exhaust gas in the intake manifold and intake ports that is caused by the continuous supply of EGR during the valve-close periods of the intake valves. The subject of this study is to evaluate the distribution of trapped exhaust gas in the diesel intake system using commercial Star-CD software (version 4.22.018). Numeric simulations of the intake flow of fresh air and recycled exhaust in the diesel intake system were initialized following previous experiments that were conducted on a reformed six-cylinder diesel engine by supplying CO₂ instead of EGR to the tangential intake port alone to establish CO₂ stratification in the first cylinder. The distributions of the intake CO₂ in the intake manifold and intake ports under the conditions of 1330 r/min and 50% load with different mass flow rates of CO₂ are discussed. This indicates that CO₂ supplied to one intake port alone would escape to another intake port, which not only weakens the CO₂ stratification by diminishing the mass fraction disparity of the CO₂ between the two intake ports of cylinder 1, but also influences the total mass of CO₂ in the cylinder. There is 4% CO₂ by mass fraction in the intake port without CO₂ supply under the condition that the CO₂ mass flow rate is 5 kg/h during the intake process, and 10% CO₂ under the condition of 50 kg/h.

1. Introduction

The role of the emission of smoke and nitric oxide (NO_x) from diesel engines is becoming increasingly important in air pollution and continues to deteriorate since the implementation of diesel engines. Lower particle emissions, especially of nanoparticles, are urgently required to satisfy the increasingly strict engine emission regulations [1,2]. New combustion concepts, homogeneous charge compression ignition (HCCI) [3,4,5], dilute diffuse combustion [6,7], and low-temperature combustion (LTC) [8,9], achieved by introducing high exhaust gas recirculation (EGR) up to 55% [10,11], have been successfully applied to significantly reduce toxic diesel emissions. Longer fuel air mixing progress and lower combustion temperatures due to high EGR are beneficial for simultaneously reducing NO_x and smoke [12,13], However, considerable penalties of these technologies still exist in the current situation, such as the deterioration of combustion stability and engine performance due to low oxygen in the engine cylinder, and more cooling loss of high EGR [14,15,16,17]. Fortunately, the penalties caused by high EGR can be solved by in-cylinder EGR stratification, which modulate the combustion temperature and oxygen distribution to reduce NO_x and smoke emissions that are formed under high temperature and a poor oxygen environment [18,19,20].

In 2009, Rothamer and coworkers [21] investigated EGR stratification on an HCCI diesel engine with a large negative-valve overlap (NVO) using planar laser-induced fluorescence (PLIF). Significant stratification in EGR distribution owing to the large level of retained exhaust gases were found to be effective to control the HCCI combustion process and to increase the range of loads and speeds, which is one of the urgent challenges for HCCI combustion [22,23]. In 2010, exhaust gas was supplied to one of the two intake ports to organize EGR stratification of a four-valve diesel engine by Fuyuto and coworkers [24]; the radial stratification of the EGR was obtained at the end of the compression stroke, the level of residual gas trapped in the cylinder can be increased by controlling the exhaust valve timing [25]. In 2011, EGR stratification was improved via a two-step bowl piston and an offset chamber by Choi and coworkers [26], where a high-EGR region is formed at the central upper region of the combustion chamber, where combustion is initiated, and NO_x is reduced without a smoke penalty [27,28,29]. In 2012, laser-induced fluorescence diagnostics were carried out by Andre and coworkers [30] in an optical engine to provide fuel and exhaust gas recirculation distributions. It was found that the exhaust gas recirculation stratification can be maintained until late timings corresponding to the combustion event. In 2014, Shen and coworkers [31] investigated the in-cylinder EGR distribution on a reformed six-cylinder diesel engine, where lower emissions of NO_x and smoke were obtained, and EGR distribution during the intake and compression processes were also analyzed.

In summary, exhaust gas recirculation stratification is an effective and potential technique to reduce toxic emissions in diesel engines [32,33,34], However, its effect on combustion was very sensitive [35,36], and further understanding of EGR stratification is required. Generally, in-cylinder EGR stratification is achieved by supplying EGR asymmetrically to intake ports for a four-valve diesel engine. One question of this strategy for organizing EGR stratification is a continuous supply of EGR gas to one of the two intake ports while the intake valves periodically open and close, which would cause EGR to be trapped in the intake port when intake valves close and changes to the intake composition in the intake manifold and intake ports, especially for single-cylinder engines. To determine the distribution of intake compositions in the intake ports during the intake process, and the mass fraction of EGR inducted into the cylinder through each intake valve when supplying EGR asymmetrically, numeric simulations were conducted on a simplified intake system of a six-cylinder diesel engine in this paper. CO₂, instead of EGR, was selected to analyze the asymmetrical intake compositions under the influence of the continuous supply of intake gas in a four-stroke diesel engine.

2. Apparatus and Simulation

2.1. Apparatus

The studied engine was a six-cylinder diesel engine, and its specifications are listed in

Table 1. Engine specifications.

Tested Engine	Heavy Duty Diesel Engine
Valves	4
Cylinders	6
Piston bowl	ω
Supercharge	Turbocharging
EGR cooling	Intercooling
Bore × Stroke (mm × mm)	112 × 145
Displacement (L)	8.6
Compression Ratio	17.0:1
Swirl ratio	1.1
IVO (° CA BTDC)	23
IVC (° CA ABDC)	30
EVO (° CA BBDC)	60
EVC (° CA ATDC)	33
Rated power/Speed (kW/rpm)	257/2100
Intake air temperature (K)	313
Engine speed (rpm)	1330
Fuel consumption(kg/h)	22.99

. Only the first cylinder of this engine was reformed by inserting two CO₂ runners into the two intake ports, respectively, as shown in Figure 1. CO₂ was supplied to one of the two intake ports, which was manually controlled by a three-way valve. The outlets of the two runners were placed as close to the intake valves as possible to keep CO₂ separated from fresh air in intake ports during the intake process. More details of the studied engine have been documented in our previous paper [31]. The different mass fraction and distribution of CO₂ in the two intake ports, of critical importance for achieving CO₂ stratification within cylinder 1, are sensitive to trapped CO₂. In this paper, only the mass fraction and distribution of trapped CO₂ in the intake manifold and intake ports caused by a continuous supply of fresh air and CO₂ were evaluated.

2.2. Numerical Methods and Initial Conditions

The cost of modeling and calculation of a six-cylinder diesel engine are extremely large. For simplicity, the intake port configuration of the concerned engine was treated as a straight and circular pipe with the same volume and same area as the cross-section of the boundary inlets, as shown in Figure 2, with 730,000 cell grids. CO₂ pipes inserted through intake manifold and intake ports to the intake valves are shown in Figure 3. CO₂ gas is supplied by a compressed gas tank, and flows through one CO₂ pipe in one of the two intake ports to cylinder 1. The inlets of fresh air and CO₂ are kept open with continuous flow. The outlets’ boundary condition of the two intake ports of each cylinder are set as the outflow outlet when the intake valves open, or wall when the intake valves close, according to fire sequence of the diesel engine.

To determine the mass fraction and distribution of CO₂ in the outlets of the intake ports of cylinder 1, several cycles of intake, compression, expansion, and exhaust stroke were simulated until the results were stable at the same time of each cycle. When the intake valves of cylinder 1 are open, the intake valves of cylinder 4 are also open at the beginning of the intake process of cylinder 1, and the intake valves of cylinder 5 are open at the end of the intake process of cylinder 1, as a result of early opening and late closing of the intake valve timing. Therefore, taking intake the advancing and lagging of the valve timing into account, the 720° crank angle of one cycle was divided into 12 time segmentations, during which the outlets of the intake ports of each cylinder were set as wall boundaries if intake valves remain closed, or the outlets of the intake ports were set as outflow-outlets if the intake valves are open. The simulations were conducted using Star-CD 4.22.018 CD-adapco, London, England, Siemens PLM Software, 2017 Siemens Product Lifecycle Management Inc., Siemens Aktiengesellschaft, Munich, Germany. The inlets of fresh air and CO₂ are velocity-inlets, and the initial velocity of fresh air and CO₂ are calculated by mass flow measured in experiments as follows:

q_m = ρvA,

(1)

where q_m is the mass flow of fresh air or CO₂ measured by the mass flow meter during the EGR stratification experiments, ρ is the density of fresh air or CO₂ under the temperature and pressure measured in the EGR stratification experiments, A is the area of the inlet of the intake manifold or the CO₂ pipe, and v is the velocity of fresh air or CO₂.

Table 2. Simulation cases.

Conditions	Case 1	Case 2
speed	1330 r/min
load	50%
Intake temperature	313 K
Density of fresh air	1.35 kg/m3
Density of CO2	2 kg/m3
Mass flow rate of fresh air	481 kg/h
Mass flow rate of CO2	5 kg/h	50 kg/h

shows the simulation cases. CO₂ was injected into intake port 1, as shown in Figure 3. The distribution of the mass fraction of CO₂ in the intake manifold, intake ports, and the boundary outlets of the intake ports during the intake process of cylinder 1 are discussed.

2.3. Turbulence and Mass Transfer

The simulations were based on the transient time domain, the run time control was set according to 12 time segmentations of one whole cycle, and the time step is 12.6 μs (1° crank angle under the speed condition of 1330 r/min).

The mass and momentum conservation equations for compressible fluid flows in the intake manifold and intake ports were solved in Cartesian tensor notation [37,38,39]:

$$\frac{\partial \rho }{\partial t}+\frac{\partial }{\partial x_j}\left(\rho u_j\right)=s_m$$

(2)

$$\frac{\partial \rho u_i}{\partial t}+\frac{\partial }{\partial x_j}\left(\rho u_j{u}_i-{\tau }_{ij}\right)=-\frac{\partial p}{\partial x_i}+s_i$$

(3)

where t is the time, x_i is the Cartesian coordinate (i = 1, 2, 3), u_i is the absolute fluid velocity component in direction x_i, p is the piezometric pressure, ρ is density, τ_ij is stress tensor components, s_m is mass souce, and s_i are the momentum source components.

The re-normalization group (RNG) k-ε turbulence model was employed to evaluate the turbulence level in the intake ports and manifold. The additional, final term in the dissipation equation is obviously distinctive from the linear standard k-ε model, the derivation via the RNG theory is more fundamental than the standard k-ε model and produces a version that is more general and accurate.

The turbulence kinetic energy in RNG k-ε turbulence model:

$$\frac{\partial }{\partial t}\left(\rho k\right)+\frac{\partial }{\partial x_j}\left(\rho u_{j}k-\left(\mu +\frac{{\mu }_t}{{\sigma }_k}\right)\frac{\partial k}{\partial x_j}\right)={\mu }_t\left(P+P_B\right)-\rho \epsilon -\frac{2}{3}\left({\mu }_t\frac{\partial u_i}{\partial x_i}+\rho k\right)\frac{\partial u_i}{\partial x_i},$$

(4)

The turbulence dissipation rate in RNG k-ε turbulence model:

$$\frac{\partial }{\partial t}\left(\rho \epsilon \right)+\frac{\partial }{\partial x_j}\left(\rho u_j\epsilon -\left(\mu +\frac{{\mu }_t}{{\sigma }_{\epsilon }}\right)\frac{\partial \epsilon }{\partial x_j}\right)=C_{\epsilon l}\frac{\epsilon }{k}\left[{\mu }_{t}P-\frac{2}{3}({\mu }_t\frac{\partial {\mu }_i}{\partial x_i}+\rho k)\frac{\partial {\mu }_i}{\partial x_i}\right]+C_{\epsilon 3}\frac{\epsilon }{k}{\mu }_t{P}_B-C_{\epsilon 2}\rho \frac{{\epsilon }^2}{k}+C_{\epsilon 4}\rho \frac{\partial {\mu }_i}{\partial x_i}-\frac{C_{\epsilon }{\eta }^3(1-\frac{\eta }{{\eta }_0})}{1+\beta {\eta }^3}\frac{\rho {\epsilon }^2}{k},$$

(5)

where:

η ≡ k/ε

(6)

Each constituent m of a fluid mixture, whose local concentration is expressed as a mass fraction Y_m, is assumed to be governed by a species conservation equation in the form:

$$\frac{\partial \rho }{\partial t}\left(\rho Y_m\right)+\frac{\partial }{\partial x_j}\left(\rho u_j{Y}_m+F_{m,j}\right)=s_m$$

(7)

where F_m,j is the diffusional flux component, s_m is the rate of mass production or consumption (s_m = 0) turbulent flow (time averaged):

$$F_{m,j}\equiv \rho Y_m{V}_{m,j}+\overline{\rho }{\prime }_j{Y}_m^{\prime }$$

(8)

where the rightmost term, containing the mass fraction fluctuation Y_m’ represents the turbulent mass flux:

∑Y_m = 1

(9)

2.4. Independent Validaton

Figure 4 shows comparisons of the mass fraction of CO₂ at three points in the outlet of intake port 1 between two simulation models with different grids. It indicates that there is little difference in the CO₂ mass fraction at the three points in the outlet of intake port 1 between the two simulation models with 730,000 and 1,370,000 grids. The maximum difference in the CO₂ mass fraction at these three points is 0.882%. Therefore, the model with 730,000 grids was selected in this study after independent validation of grids when compared with a finer model with 1,370,000 grids.

The effects of transient time steps (0.5° CA and 1° CA) on the transient simulation results were compared in this study, as shown in Figure 5. This indicates that there is little difference in the mass fraction at the three points in the outlet of intake port 1 between time step 0.5° CA and 1° CA. The maximum difference in the CO₂ mass fraction at these three points is 0.295%. A time step of 1° CA was selected in this study to reduce the time cost.

3. Results and Discussion

Figure 6 shows the distribution contour of the mass fraction of CO₂ in the intake manifold and intake ports during the intake process. It can be seen from the figure that the mass fraction of CO₂ in intake port 1 increases rapidly after the intake valves close until the intake valves open again. The trapped CO₂ mixed with fresh air when the intake valves were closed, which impaired the in-cylinder CO₂ stratification at the beginning of the intake process of cylinder 1. However, most of the time, CO₂ and fresh air were fed into cylinder 1 separately during the intake process owing to the separate CO₂ runner. This difference in the CO₂ mass fraction between intake ports 1 and 2 has an important role on the organization of CO₂ stratification. This figure also indicates that the homogenous mixture of CO₂ and fresh air distributed in intake port 2 as a result of CO₂ escaping from intake port 1 during the intake process. Escaping CO₂ weakened the difference of the CO₂ mass fraction between the two intake ports of cylinder 1. It is fortunate that there is still an obvious difference in the CO₂ mass fraction between the two intake ports to form CO₂ stratification. To enhance CO₂ stratification, further improvements that avoid mixing of CO₂ and fresh air in the intake manifold and intake ports should be developed. As shown in Figure 6, the higher the mass flow of CO₂, the more CO₂ is trapped in the intake ports before the intake valves of cylinder 1 open. This proves that an appropriate velocity of CO₂ is necessary for good CO₂ stratification. There is no doubt that CO₂ stratification can be achieved by supplying CO₂ to one of the two intake ports.

EGR stratification is formed in the engine cylinder after two processes: one is the intake flow and mass transfer in the intake manifold and intake ports; the other is the in-cylinder flow and mass transfer before ignition. This paper mainly discusses the difference in the CO₂ mass fraction between the outlets of the two intake ports during the first process by supplying CO₂ to one of the two intake ports. The aim of supplying CO₂ asymmetrically is to create a difference in the CO₂ rate between the two entrances to cylinder 1. The average mass fractions of CO₂ at the outlets of the intake ports of cylinder 1 versus the crank angle under conditions with different CO₂ mass flow rates are shown in Figure 7. It illustrates that the average mass fractions of CO₂ increase after the intake valves close and reach a maximum at the time that the intake valves of cylinder 1 open owing to the continuous supply of CO₂ and fresh air, and then drop sharply to the target value designed for the desired CO₂ rate. Trapped CO₂ caused a higher in-cylinder CO₂ rate due to higher average mass fraction of CO₂ in the early stage of the intake process of cylinder 1 under both conditions of Case 1 and Case 2.

Figure 8 shows average CO₂ mass fraction at the outlet of intake port 1 of cylinder 1 versus the crank angle with different mass flow rates of CO₂. It can be seen that the average mass fraction of CO₂ at the outlet of intake port 1 increased sharply due to the continuous supply of CO₂ and fresh air after the intake process of cylinder 1 and reached a maximum when the intake valves of cylinder 1 open, and then decrease rapidly to the target value after trapped CO₂ flowed into cylinder 1. Under the case of a CO₂ mass flow rate of 50 kg/h, the CO₂ mass fraction at the outlet of intake port 1 is close to 1 when the intake valves of cylinder 1 open. The trend of the average mass fraction of CO₂ at the outlet of intake port 1 versus the crank angle is similar with that of the intake ports. When compared with Figure 7, it is clear that the target value of the CO₂ mass fraction at the outlets of the two intake ports of cylinder 1 is higher than that of intake port 1. It is the escaped CO₂ from intake port 1 to intake port 2 that makes this difference. Intake port 2 filled with a homogenous mixture of CO₂ and fresh air, which fed into cylinder 1 during the whole intake process in spite of supplying CO₂ to intake port 1 only.

Figure 9 shows average CO₂ mass fraction at the outlet of intake port 2 of cylinder 1 versus the crank angle with different mass flow rates of CO₂. It is shown that intake port 2 supplied a mixture of gas with a 4% mass fraction of CO₂ in Case 1 and a 10% mass fraction of CO₂ in Case 2 to cylinder 1. The average mass fraction of CO₂ in intake port 2 is almost constant during the intake process, as can be seen from Figure 6, in that the distribution of CO₂ is rather even. Escaped CO₂ not only weakened the CO₂ stratification in the cylinder, but also influenced the total mass of CO₂.

Intake gas at the outlets of the intake ports is what is directly fed into the cylinder. The difference of the CO₂ mass fraction between the two intake ports is what we designed to achieve by supplying the CO₂ asymmetrically for CO₂ stratification. Figure 10 shows the distribution of the CO₂ mass fraction at the outlets of the intake ports of cylinder 1. It is shown that the CO₂ mass fraction at the outlet of intake port 1 of cylinder 1 keeps increasing before the intake valves open. CO₂ stratification was successfully organized due to the difference in the CO₂ mass fraction between intake port 1 and intake port 2, however, the inhomogeneous distribution of CO₂ was weakened by continuously supplying CO₂ and blow-by CO₂ from intake port 1 to the other.

4. Conclusions

This paper determines the differences of the CO₂ distribution in the intake ports during the intake process and the mass fraction of CO₂ inducted into one cylinder through each intake valve when supplying CO₂ asymmetrically. Numeric simulations were conducted on a simplified intake system of a six-cylinder diesel engine in this paper. The intake port configuration of the concerned engine were treated as a straight and circular pipe with the same volume as the intake manifold and the intake ports of the tested engine. The initial conditions and boundary conditions were set up according to the CO₂ stratification experiments. The distribution of CO₂ at the outlets of the intake ports under the influence of a continuous supply of CO₂ was discussed.

The following conclusions were drawn:

• CO₂ supplied to one intake port alone escapes to the other intake port, which not only weakens the CO₂ stratification in cylinder 1, but also influences the total mass of CO₂ that is supplied to the cylinder. Fortunately, there is still an obvious difference in the CO₂ mass fraction between the two intake ports.
• The average mass fraction of CO₂ at the outlet of intake port 1 increases sharply due to the continuous supply of CO₂ and fresh air after the intake process, and reaches a maximum when the intake valves open, and then decreases to the target value after trapped CO₂ flows into cylinder 1.
• There is 4% CO₂ by mass fraction in the intake mixture that is fed into cylinder 1 through the outlet of intake port 2 under the condition of Case 1 (a CO₂ mass flow rate of 5 kg/h) during the intake process, and 10% CO₂ under the condition of 50 kg/h.