## Numerical Investigation of Biogas Fuelled HCCI Engine

Abstract- This study investigates the effects of biogas composition, equivalence ratio and intake charge temperature on combustion phasing, combustion stability and emissions characteristics for a biogas-fueled homogeneous charge compression ignition (HCCI) engine. A numerical model is validated against the experimental results. It is noticed that with increase in the methane (CH4) concentration in biogas composition, the fuel burning rate increased. Higher concentration of carbon dioxide (CO2) in biogas composition delayed the combustion process. At high equivalence ratios, an increase in the CH4 concentration in biogas composition increased the rate of pressure rise and for a CH4 concentration greater than 50 percent in biogas composition, the rate of pressure rise is more than the allowed limit so which may lead to knocking While at low equivalence ratios, an increase in the CH4 concentration in biogas composition showed a nominal increase in the rate of pressure rise. The oxides of nitrogen emissions (NOX) are not much affected by the change in biogas composition as the in-cylinder temperature is below the threshold value for peak NOX emissions for all biogas composition. The level of carbon monoxide (CO) emissions are underpredicted due to an increase in combustion efficiency.  The unburned hydrocarbon (UHC) emissions increased with the increase in CO2 contents in biogas due to low in-cylinder temperature. In addition, 50 percent of methane in biogas composition showed good combustion phasing over other biogas compositions. Hence, at high equivalence ratios lower contents of CH4 in biogas composition and at low equivalence ratios higher contents of CH4 in biogas composition can allow more stabilized combustion in a biogas-fueled HCCI engine.

Keywords- Alternate fuel; biogas; combustion phasing; homogeneous charge compression ignition engine (HCCI); nitric oxide (NOX) emissions; renewable energy

Introduction

There is a growing need to run vehicles using renewable energy sources to control global warming, meet stringent emission standards and to replace depleting fossil fuels. Because of the clean burning nature of biogas, it can be effectively used as an engine fuel [19]. Biogas is produced by the anaerobic breakdown of an organic material with the help of bacteria. Biogas is mainly produced from the environmental waste like cow dung, waste food, and other manures. Biogas is mainly composed of methane, carbon dioxide, hydrogen, nitrogen and may have a small amount of H2S (Hydrogen sulfide) traces. The CO2 and other gases in the biogas do not take part in combustion and hence the upgraded biogas containing 80 percent methane has calorific value over 25 MJ/m3 (Mega-joules per cubic meter). This high calorific value of biogas makes it be used as a fuel for automotive engines, generators and other power applications [11].

HCCI Engines

In HCCI engines, the homogeneous charge is made to auto-ignite at several points inside the combustion chamber due to increase in temperature during the compression stroke.  HCCI engines promise several advantages over conventional engines. HCCI engines operate at a very low equivalence ratio (0.2-0.5) with a highly homogeneous charge due to which the in-cylinder temperature, NOX emissions, and soot emissions are low compared with DICI (Direct injection compression ignition) engines. The absence of fuel-rich regions and low NOX emissions makes the HCCI engine the best alternative for CI (Compression ignition) engines. The main drawback of HCCI engines is there is no direct control over combustion. SI (Spark ignition) engines have spark plug while CI engines have fuel injector to control the combustion. Whereas combustion in HCCI engines is primarily controlled by the auto-ignition characteristics of the charge which gets influenced by various parameters like charge temperature, pressure and chemical kinetics of the charge. HCCI engines produce a high amount of CO and UHC emissions at low loads and high NOX emissions at peak loads. The power density is low because of its ultra-lean operating conditions and undergoes knocking at higher equivalence ratios [10]. Scott B et al [12] used natural gas-fueled HCCI engine operating with higher compression ratios (approximately 17-20:1) at an equivalence ratio of 0.2 to 0.5. They reported indicated thermal efficiency close to 45 % and NOX emissions between 75 to 200 ppm. Burning rates should be controlled to avoid a high rate of pressure rise otherwise it could lead to knocking. The higher amount of CO2 in biogas composition retards the rate of pressure rise [5]. The presence of CO2 in biogas not only reduces NOX emissions but also helps achieve higher compressions ratios without knocking [2]. Diluents and fuel blends are used to control combustion in HCCI engines [13]. Low auto-ignition temperature of diethyl-ether makes it suitable to be used as an ignition improver in biogas fueled engines and hence avoiding high intake charge temperature requirements which could possibly increase the volumetric efficiency and the load range [14].

CFD Models for HCCI Engine Simulation

Generally, thermodynamic models such as Single-zone, multi-zone, probability-based and multi-dimensional CFD models are used to simulate HCCI engines. Single zone models can accurately predict the crank angle for auto-ignition of the charge, fuel blend characteristics and NOx emissions but overpredicts peak pressure and underpredict UHC emissions. Though multi-zone models can reasonably predict the maximum in-cylinder pressure and emissions it doesn’t include the spatial temperature stratification. Three dimensional CFD simulations with detailed chemical kinetics can accurately predict the engine performance and emissions characteristics, as it accurately defines the charge motion within the combustion chamber including turbulence and spatial temperature stratification [8].

Numerical Methodology

The aim of this project is to study the effect of different biogas composition on combustion phasing, combustion stability and emission characteristics of a biogas-fueled HCCI engine. To achieve this objective, a three-dimensional CFD (Computational fluid dynamics) model with detailed chemical kinetics is used. CONVERGE 2.3 is the CFD tool used to model the simulation. The GRI-Mech 3.0 is the reaction mechanism used for the combustion simulation. The simulations were executed for two equivalence ratios (0.2 and 0.3). The effect of intake charge temperature is studied for a higher and a lower methane concentration in biogas composition. The simulation is executed from IVC (Inlet valve closing) to EVO (Exhaust valve opening). A 60° sector geometry is created from the “make engine sector surface” utility in CONVERGE. Figure 1 shows the front view of the engine model. The top and side view of the engine model is depicted in Figure 2 and 3.

Figure 1.  Engine model (Front view)

Figure 2.  Engine model (Top view)

Figure 3. Engine model (Side view)

The engine used for simulation studies is a water cooled, single cylinder four stroke diesel engine with a compression ratio of 17. The specifications of the engine are given in Table 1.

Specifications of the engine [4]

Water cooled four stroke diesel- engineEngine

bore79.5mm

Stroke length95.5mm

Compression ratio17:1

Engine speed1800

Rated power60 kW

Flagging the Boundaries and Setting Up of Compression Ratio

Engine liner, engine head, piston, front and back face boundaries are flagged accordingly. The desired compression ratio has been set up for the geometry.

Simulation Parameters and Thermodynamic Files

The GRI-Mech 3.0 gas data and reaction mechanism files are imported and the simulation time parameters are fixed with values suggested by the CONVERGE. The compressible flow solver is used for internal combustion engine simulations. The simulation is executed from IVC to EVO. The variable time step algorithm is used to change the time step as per the solver conditions.

Boundaries and Regions Initialization

The head and liner are made as a wall boundary, while the piston is made as a translating boundary. The front and back boundaries are made as symmetric boundaries with a sector angle of 60 degrees. The initial conditions such as temperature and boost pressure at IVC for the in-cylinder simulation are taken as 473 K and 2 bar from the literature [5]. The turbulence initialization is done with the recommended values from CONVERGE. The mass fraction of the air-fuel mixture is calculated with help of the combustion equation of biogas and air. The stoichiometric air-fuel ratio and lower heating value for various biogas compositions are shown in Table 2. As seen from the Table 2, the increase in methane concentration in the biogas increases the lower heating value of the fuel.

Properties of biogas [4]

Biogas composition                  Stoichiometric air to fuel ratio          Low heating value (MJ/kg)

30% CH4 + 70% CO2                               10.1                                            6.7

40% CH4 + 60% CO2                               7.90                                            9.8

50% CH4 + 50% CO2                               6.05                                            13.3

60% CH4 + 40% CO2                               4.6                                              17.6

70% CH4 + 30% CO2                               3.3                                              23.0

80% CH4 + 20% CO2                               2.3                                              29.6

Combustion and Turbulence Modeling

The SAGE detailed chemical kinetics solver is used for the combustion modeling. Converge includes the multizone model which accelerates the chemistry calculations by grouping together similar computational cells and then invoking the chemistry solver once per group rather than once per cell. In the multizone model, at any given time t, each cell is at some thermodynamic state. Based on the thermodynamic states of the cells, CONVERGE groups the cells into zones. The thermodynamic state of each zone is based on the average temperature and composition of all the cells in that zone. Converge invokes the chemistry solver once on each zone. The RNG-K-EPSILON model was used for turbulence modeling. The various turbulence constants values are used based on the recommended values from CONVERGE.

Base Grid Control

Autonomous meshing is carried out during the runtime in CONVERGE which ease up the complex meshing process. Here, the cut cell technique is used to merge the cells to avoid cell interference and the grid independent study is carried out to ensure that the simulation results are not affected by the variation in grid size. The adaptive mesh refinement technique is used to refine the boundaries based on the fluctuating physical variables such as temperature and velocity. The fixed embedding option is used to improve the cell counts in regions which are very sensitive such as engine combustion chamber and cylinder head regions.

Results and Discussion

Grid independence studyThe grid independence study is carried out to ensure that the solution is not affected by the change in the mesh size. The simulation is executed for two cases: 1,00,000 and 1,50,000 cells. As seen in Fig 4, the pressure curves for both the cases matched perfectly.

Figure 4. Variation of in-cylinder pressure with grid size

Validation with Experimental Results

Figure 5 shows the variation of in-cylinder pressure with crank angle. It is noticed that the peak in-cylinder pressure at simulated conditions is over-predicted than that of the experimental conditions. This is due to either an over-prediction in combustion efficiency or an under-prediction in burning duration. However, this error decreases with the decrease in the power density of the engine. The same effect is reported by Scott B et al [12]. However, the motored pressure traces and crank angle for peak in-cylinder pressure at simulated conditions matched the experimental data.

Figure 5. Variation of experimental and simulated pressure traces with the crank angle

Effect of Varying Biogas Composition on combustion phasing

In this section, the effect of varying biogas composition on the combustion phasing of an HCCI engine is discussed in detail. In HCCI engines there is no direct control over the combustion phasing and the rate of heat release is also very high and hence, external parameters like fuel additives and varying fuel compositions are used to control the combustion [10].

Figure 6. Variation of in-cylinder temperature with the crank angle

The above figure depicts the variation of in-cylinder temperature with crank angle. The in-cylinder temperature increases with the increase in CH4 contents in biogas composition due to the increase in the heating value of the fuel. The crank angle for peak in-cylinder temperature advanced with the increase in methane contents in biogas composition due to higher burning rates as shown in Fig.7. Because of the high sensitivity of the auto-ignition process with intake charge temperature, slight changes in the intake charge temperature can lead to the large change in the combustion initiation and further in combustion development [4].

Figure 7. Variation of in-cylinder pressure with the crank angle

The above figure shows the variation of peak in-cylinder pressure with crank angle. The crank angle for peak in-cylinder pressure advanced with an increase in methane contents in the biogas composition. Higher contents of CO2 in biogas composition reduces the peak in-cylinder pressure.

Figure 8. Variation of heat capacity ratio with the crank angle

From the above figure, the variation of heat capacity ratio with crank angle is noticed. The heat capacity ratio decreases with an increase in CO2 contents in biogas composition. With higher CO2 concentration, N2 and O2 mass fraction in the biogas-air mixture decreased due to which the overall heat capacity ratio decreased. The decrease in the in-cylinder temperature with higher CO2 concentration is due to the decrease in the heat capacity ratio (CP/CV).

Figure 9. Variation of CA10, CA50, CA90 with varying biogas composition

The above figure depicts the variation of crank angle duration for CA10, CA50, and CA90 with varying biogas composition. The calculated values for combustion initiation (CA 10), combustion phasing (CA 50), and combustion duration (10-90% cumulative heat release) related with biogas composition is shown above. CA 10 is advanced with the increase in the contents of CH4. CA 50 is advanced with the increase in CH4 contents due to the increase in CA 10. CA 90 is increased with the increase in CO2 concentration in biogas composition due to lower burning rates.

Figure 10. Variation of NOX emission with the varying CH4 concentration

Figure 11. Variation of SOOT with the varying CH4 concentration

Figure 12. Variation of HC emission with the varying CH4 concentration

Figure 13. Variation of CO emission with the varying CH4 concentration

The CO emissions are formed due to incomplete combustion in the temperature zone between 700 and 900 K. It can be seen from the above graph that the CO emissions increase with the increase in the CH4 concentration. However, CO emissions are unpredicted due to high combustion efficiency. Due to the homogeneous mixing of the air-fuel mixture and ultra-lean equivalence ratios, the soot emissions are extremely low in HCCI engines. With the previous research on HCCI engines, high in-cylinder wall temperature and smaller crevices can significantly reduce HC and CO emissions [20]. The above figures show the variation of emissions with varying methane contents in biogas composition. The NOX emissions are not much affected by the varying biogas composition. The NOX emissions are low for all biogas composition as the in-cylinder temperature is below the threshold value for peak NOX formation. The HC emissions are quite high in HCCI engines due to the low in-cylinder temperature. However, the HC emission decreases with increase in the CH4 concentration in the biogas due to increase in in-cylinder temperature. As seen in Fig 11, the soot emission is extremely low due to the highly homogeneous charge in the HCCI engine. As we move towards higher methane concentration the soot emission extremely decreases due to the increase in the heating value of the fuel which increases the in-cylinder temperature.

Effect of Equivalence Ratio on Combustion Stability

From the knowledge gained in the literature review, biogas fueled HCCI engines are usually operated in the equivalence ratio range of (0.2<⏀>0.4) to avoid misfiring at low equivalence ratios and knocking at high equivalence ratios. The results are discussed for two equivalence ratios (0.2 and 0.3).

Figure 14. Variation of in-cylinder pressure with the crank angle for ⏀=0.2

Figure 15. Variation of in-cylinder pressure with the crank angle for ⏀=0.3

The above figures depict the variation of in-cylinder pressure for two equivalence ratios 0.2 and 0.3. The crank angle for peak in-cylinder pressure for 0.3 equivalence ratio advanced with the increase in CH4 contents in biogas composition. However, for a CH4 concentration greater than 50 percent in biogas composition, the rate of pressure rise is more than the allowed limit for safe engine operation. Whereas for 0.2 equivalence ratio increase in the CH4 concentration in biogas composition showed the nominal increase in the rate of pressure rise. Iván D at al [4] showed that the Combustion is advanced with higher equivalence ratios due to the increase in the wall temperature and decrease in the heat loss of the charge during intake and compression stroke.

Figure 16. Variation of the rate of pressure rise with the crank angle

It is noticed from the above figure that the variation of the maximum rate of pressure rise with varying methane concentration in the biogas composition. The rate of pressure rise is reduced for higher CO2 concentration in the biogas composition mainly due to delayed combustion. Hence it can be concluded that for higher equivalence ratios, lower methane concentration in biogas composition can significantly reduce knocking whereas for lower equivalence ratios higher methane concentration can significantly reduce engine misfire. Effect of Inlet Charge Temperature on CombustionThe effect of inlet charge temperature for a lower and higher methane contents in biogas compositions is investigated.

Figure 17. Variation of in-cylinder pressure with the crank angle for 30 % methane                        concentration at different intake charge temperature

The above figure shows the variation of in-cylinder pressure with crank angle at different intake charge temperature. The effect of inlet charge temperature on ignition timing shows that at the lower temperature the combustion gets retarded while at higher temperature the combustion gets advanced.  The combustion efficiency increases with the increase in intake charge temperature due to earlier ignition [20]. By reducing the intake temperature gradually, the misfiring region can be predicted for a given equivalence ratio.

Figure 18. Variation of in-cylinder pressure with the crank angle for 70 % methane                  concentration at different intake charge temperature

From the knowledge gained, all biogas compositions are sensitive to the change in temperature of 5 Kelvin. It can be seen from fig.18 and fig.19 that the requirement of high intake charge temperature decreases with an increase in the methane contents in biogas composition due to the increase in the lower calorific value of the fuel. The control of intake charge temperature has been shown as one of the effective strategies in controlling the combustion timing for different equivalence ratios, engine speeds [20].

4. Conclusion

Generally, HCCI engines produce low NOX emissions and zero soot emissions. Hence HCCI engines can be effectively used to replace CI engines. Biogas fueled HCCI engines can be effectively used for low power and constant load applications due to the low calorific value of biogas and high intake charge temperature requirement at higher loads. The presence of CO2 in biogas reduces the thermal efficiency of the engine when operated in SI and CI mode [10]. However, in HCCI mode thermal efficiency of the engine is not highly affected due to low burning duration. High in-cylinder temperature and smaller crevices can reduce HC emissions significantly. In this paper, three-dimensional CFD simulation is carried out to study the effects of varying biogas composition on combustion parameters. It is found that higher CH4 contents in biogas composition increase the burning rate while higher CO2 contents in biogas composition decrease the burning rate. The HC emissions are high due to the low in-cylinder temperature that prevails in the cylinder. It is identified that at lower equivalence ratios higher methane contents in biogas composition advanced the combustion and avoids misfiring whereas at higher equivalence ratios lower methane contents in the biogas avoid knocking. The requirement for high intake charge temperature decreases with the increase in methane contents in biogas composition.

References

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