Effect of Hydrogen Addition on Diesel Engine Operation and NOx Emission: A Thermodynamic Study

Problem statement: The worldwide increasing energy demand and the env iro mental problem due to greenhouse gas emission, especially produced from fossil fuel combustion, have promoted research work to solve these crises. Diese l engine has proven to be one of the most effective energy conversion systems. It is widely used for po wer generation, land vehicles and marine power plant. To reduce diesel fuel consumption, an altern ative energy sources, such as Hydrogen (H 2), is promoted to use as dual-fuel system. H 2 is considered as a fuel for future because it is m ore environmental friendly compared to carbon-based fue l. However, the most exiting diesel engines were designed for using diesel fuel. Feeding H 2-diesel dual fuel to the engine, it is required to s udy its effect on engine operation parameters. Moreover, it is also an interesting point to observe the engine emission when H2-diesel dual fuel is used. Approach: The thermodynamic modeling was used to simulate the operating parameters, i.e., cylinder p r ssure and gas temperature. Finite different metho d was employed to find the solution. The H 2 supply and EGR were varied. The pressure and tempe ratur were observed. For NO x emission, which is a major problem for use of dies el engine, the thermodynamic equilibrium calculation was conducted to find the mole fraction of gas species in the exhaust gas. The mole fraction of NO and NO 2 were combined to present as the mole fraction of N Ox. Results: The simulation showed that at 5% EGR, increase of H2 caused increasing of cylinder pressure and temperature. It also increased NO x in exhaust gas. However, when H 2 was fixed at 10%, increasing EGR led reducing of cylinder pressure and temperatu re. The mole fraction of NO x decreased with increasing EGR. Conclusion: The H2 supplied to the engine provided positive effect on the engine power indicated by increasing pressure and temperat ur . However, it showed the negative effect on NOx emission. Use of EGR was recommended for controlli ng NOx emission when H2 is supplied.


INTRODUCTION
Due to energy crisis and environmental problem, more efficient and cleaner engines have been developed. Diesel engines are widely used, especially for transportation and power generation because of their higher thermal efficiency. However, it is well known that NO x and smoke emissions are the important problems for use of diesel engine. Therefore, many researches have been done in order to improve diesel engines efficiency and lower their emission. One of the most frequently used methods to control NO x is supplying Exhaust Gas Recirculation (EGR) into the intake manifold of engine. Maiboom et al. (2008) investigated the effect of EGR on the diesel engine emission. They found that EGR was more effective way to control NO x emission. The simulation of NO formation in diesel engine has been done. The NO emission at different equivalence ratio, which was predestined by the single zone zero dimensional model, agreed with the experimental results.
Hydrogen is one of the most promising energy carriers fulfilling energy, environment and sustainable development needs. Since hydrogen is a carbon-free fuel, hydrogen combustion does not generate CO 2 and smoke (Miyamoto et al., 2011). Using hydrogen and diesel fuel in diesel engine, called dual-fuel diesel engine, it has been interested by many researchers. Varde and Frame (1983) studies the effect of hydrogen added in the intake of a diesel engine. The result showed that smoke decreased with the increase in hydrogen addition. Dual-fuel operation of biodiesel with hydrogen was studied by Geo et al. (2008). They found that NO x emission increased with increase in hydrogen. The diesel engine combustion process and knocking behavior was investigated by Szwaja and Grab-Rogalinski (2009) when the proportion of hydrogen and diesel fuel was varied. They reported that the hydrogen addition affected the ignition delay. The simulation of exhaust emission for diesel engine using diesel blended with hydrogen was conducted by Masood and Ishart (2008). The conclusion of their study showed that NO x emission was depended on the equivalent ratio.
In this study, the thermodynamic simulation for diesel engine was developed. The amount of H 2 supply was varied while the fraction of Exhaust Gas Recirculation (EGR) was fixed in order to study the effect of H 2 on the engine operation parameters i.e., cylinder pressure and temperature and NO x emission. To investigate the effect of EGR on hydrogen-diesel dual fuel engine, the amount of EGR was changed and H 2 supply was fixed. The cylinder pressure and gas temperature were found. The effect o f EGR at constant H 2 supply was also observed in this study.

MATERIALS AND METHODS
To find the Pressure (P) and the Temperature (T) of working fluid in the engine cylinder, the first law of thermodynamics for closed system was applied and it can be expressed as: Considering the equation of state, a differentiation of gas temperature can be obtained as: where, R is the gas constant. Substituting Eq. 3 into Eq. 2, it leads to the following equation: By substituting Eq. 4 into Eq. 1, the following equation is obtained.
Using chain rule of differentiation, Eq. 5 can be rewritten as: where, θ is the crank angle and dθ = ω dt is the crank angle angular velocity which is related to the engine speed. Rearranging Eq. 6, it is finally obtained an equation describing the relationship between the cylinder pressure and the crank angle as Eq. 7: where, γ is the specific heat ratio (γ = c p /c v ). The heat release rate due to combustion of fuel, in dQ dθ , can be calculated by using Eq. 8 (Gogio and Baruah, 2010): x b is the mass fraction burned obtained from the Weibe function which is defined as: where, θ sc is the start of combustion and ∆θ is the combustion duration. a and m are the parameters that characterizes the combustion process in the engine cylinder. In Eq. 9, the coefficients a and m are 5.0 and 2.0, respectively (Ferguson and Kirkpatrick, 2001). For rate of heat transfer from gas in cylinder to cylinder wall, out Q ω ɺ , it can be estimated by using Eq. 10: The convective heat transfer coefficient, h g , is expressed as shown in Eq. 11 (Heywood, 1988):  (11) where, w is the velocity of the burned gas given by Eq. 12 : The coefficient c 1 = 2.28 whereas c 2 = 0 during the compression process and c 2 = 0.00324 during the combustion and the expansion processes. S p is the piston speed. V d is the displacement volume. The quantities V r , T r and P r are reference state properties at closing of inlet valve and P m is the pressure value in cranking.
The instantaneous cylinder volume, area and displacement are given as Eqs. 13-15, respectively: where, a is crank radius, S is stoke and L is connecting rod length. For the working fluid temperature, the calculation was done by using Eq. 16, which is derived from the ideal gas equation of state.
In this study, the interested result is not only the thermodynamic state, represented by pressure and temperature, but also the chemical compositions of exhaust gas. To show the effect of added hydrogen and EGR on the engine emission, the thermodynamic equilibrium method based on the minimization of Gibbs free energy described in Ref. (Jarungthammachote, 2011) was used. This method is based on that at equilibrium state, the total Gibbs free energy of the system is minimized. The total Gibbs free energy of system is defined in Eq. 17: where, n i and µ i are the number of moles and the chemical potential of species i, respectively. G i represents the partial molar Gibbs free energy of species i. If all gases are assumed as ideal gas, the chemical potential of species i can be obtained from Eq. 18: where, R and T are the universal gas constant and temperature, respectively. y i is The mole fraction of gas species i and it is the ratio of n i and the total number of moles in the reaction mixture. where, a ij is the number of atom of the j-th element in a mole of the i-th species. A j is defined as the total number of atom of j-th element in the reaction mixture. The solutions n i have to be real numbers in the boundary such that 0 ≤n i ≤n tot . In this study, there are the number of mole of CH 4 , CO, CO 2 , H 2 , H 2 O, O 2 , N 2 , NO and NO 2 . The summation of NO and NO 2 is presented in terms of NO x . The Newton-Raphson method is used to find the solution. The data from Jarungthammachote (2011) is employed to calculate all thermodynamic properties in this model.
The engine and operational conditions used in this simulation are presented in Table 1. The thermal properties of diesel fuel, ignition delay and duration of combustion were assumed following the information obtained from (Heywood, 1988).

RESULTS
The simulation results were obtained by solving Eq. 7 which is differential equation. The numerical method called finite difference technique was employed. The mole fraction of fed hydrogen was varied while the EGR was fixed. Then, the results were observed. To investigate the effect of EGR on the engine operation and emission, the amount of hydrogen was fixed and the mole fraction of EGR was changed. The results of simulation are given in Fig. 1-6. Figure 1 and 2 present the effect of hydrogen supply on cylinder pressure and gas temperature, respectively. For the effect of EGR, Fig. 3 and 4 shows the cylinder pressure and gas temperature, respectively at different EGR rates. The last two figures elucidate the variation of NO x due to the change of hydrogen supply and EGR.

DISCUSSION
In the first case, EGR was fixed at 5% and H 2 supply was varied. From the simulation results, Fig. 1 and 2 clearly show that increasing H 2 supply causes increase of cylinder pressure and temperature. As shown in Fig. 1 the peak of cylinder pressure is 6.4 MPa for 5% H 2 supply while 20% H 2 supply raises the peak of pressure up to 8.0 MPa. As observed from the simulation results, for each 5% increase of H 2 supply, the peak of cylinder pressure gains about 490 kPa. For the gas temperature, it can be increased with increasing H 2 supply. The peak of gas temperature for 5% H 2 supply is about 1700 K and it reaches 2120 K when H 2 is fed with 20%. 5% H 2 fed into engine can increase the peak of gas temperature about 140 K. From the results, it can be explained that more H 2 induced into the cylinder increases releasing energy from combustion process. Thus, combustion gas has higher pressure and temperature. The relationship between cylinder pressure and EGR is expressed in Fig. 3. The H 2 supply was fixed at 10% while EGR was varied from 5-20%. The result shows that the cylinder pressure decreases with increasing EGR. The same effect can be observed for the gas temperature. Reduction of gas temperature is found when EGR fraction is increased, as presented in Fig. 4. From the results, it can be implied that EGR acts as combustion dilutor. Most of gas species in EGR do not react with H 2 and diesel fuel.
To study NO x emission, chemical equilibrium calculation was simultaneously done with pressure and temperature simulation. Figure 5 indicates that NO x fraction in exhaust gas increases with increasing H 2 supply. At 20% H 2 supply, the NO x emission is higher than that at 5% H 2 supply with 2500%. This is the effect of rising temperature due to increasing H 2 supply. The same effect was observed by Varde and Frame (1983). Miyamoto et al. (2011) demonstrated that at the energy per cycle of 0.9 kW/cycle, NO first decreased, attained minimum at 4% of H 2 supply and then NO increased with increasing H 2 supply. In contrast, increase of EGR reduces the NO x emission, as shown in Fig. 6, because EGR lower the gas temperature. Comparing with NO x emission at 20% H 2 supply, it is 1400% lower than that at %5 H 2 supply.

CONCLUSION
In this study, the thermodynamic model for diesel engine was developed to simulate the effects of H 2 addition and EGR on the operation condition and NO x emission. The chemical equilibrium method was used to find the mole fraction of NO x in the exhaust. The result showed that increasing H 2 caused increases of cylinder pressure and temperature. Therefore, the NO x emission was grown due to increasing temperature. Therefore, it should be make sure that the engine structure can handle the increasing pressure and the engine cooling system can control the temperature to protect the overheat damage. In contract, EGR could reduce the cylinder pressure and temperature. To control NO x emission, use of higher EGR was recommended to diesel engine added H 2 to the intake air.

ACKNOWLEDGEMENT
The study is a part of research project "The application of hydrogen with multi-cylinder diesel engine for energy saving and pollution reduction" which was financially supported by The Kasetsart University Si Racha Campus Research Committee.