**COMBUSTION MODELLING AND **

**SIMULATION OF COMBUSTION **

**EMISSIONS FOR DIESEL ENGINE **

**OPERATING ON THE BLENDS OF **

**JATROPHA BIODIESEL AND DIESEL **

DEVERSHI MOURYA

Department of Mechanical Engineering, Institute of technology – Banaras Hindu University, Varanasi, U.P, India, 221005

devershi.mourya@gmail.com

VIVEK ROY

Department of Mechanical Engineering, Institute of technology – Banaras Hindu University, Varanasi, U.P, India, 221005

vivekroy.iit@gmail.com

**Abstract: **

A combustion model is developed for the diesel engine combustion, when biodiesel and diesel are used as blended fuel. The model is applied to simulate the diesel engine combustion emissions, when different blends of jatropha biodiesel and diesel are used as fuel for diesel engine. Simulation results the variation of the mole fractions of various combustion emission products with the equivalence ratio, when different blends of jatropha biodiesel and diesel are used as fuel. The results show that the present combustion model gives reliable performance for the predictions of combustion emissions, as well as computational efficiency.

Keywords: Combustion Model; Emission; Equivalence Ratio; Simulation; Jatropha Biodiesel; Diesel

Engine.

**1. Introduction **

Fuel crisis and environmental concerns have renewed the interest of scientific community to look for alternative fuels of bio origin such as vegetable oils. Some non-edible vegetable oils such as jatropha oil, mahua oil, karanja oil, etc. are potentially effective as a diesel substitute and have reasonable energy content.

Biodiesel is an alternative to petroleum-based diesel fuel and is made from renewable resources such as vegetable oils, animal fats, or algae. It is an oxygenated, sulfur-free, biodegradable, non-toxic, and environmentally friendly fuel. Biodiesel fuels are obtained by transesterification of vegetable oils.

The present work deals with the development of a combustion model, when biodiesel and diesel are
used as blended fuel, to result the variation in the mole fractions of various combustion emission products (at
different fuel blends) with the equivalence ratio. Jatropha biodiesel is the biodiesel selected for analysis.
*Jatropha curcas is the source of jatropha biodiesel, which is a large plant and has a long productive period, *
occurring almost throughout India.

Reduction of engine emissions and a solution towards the energy crisis is a major research area since the concern of environment protection. Nabi (2006) worked towards the improvement in the engine emissions with the conventional biodiesel and diesel fuel blends. Some significant experimental work has been done by Agrawal and Agrawal (2007) for investigating the performance and emission characteristics of jatropha biodiesel. Present work analytically examines the combustion emissions for a diesel engine as a function of equivalence ratio, when different blends of jatropha biodiesel and diesel are used as fuel in diesel engine.

**2. **

**Combustion model**

The combustion model for the prediction of diesel engine (operating on blended biodiesel and diesel)
combustion emissions is as follows. Following **assumptions** are considered.

All the gases are assumed to be perfect.

Homogeneous mixing inside the cylinder.

Range of the Equivalence ratio is 0.5 to 1.5.

There is no delay time.

Effect of radiation is neglected.

No chemical changes in Diesel, Biodiesel and Air prior to the combustion.

All the properties of gases in the cylinder are only time dependent. The combustion reaction (when biodiesel and diesel are used as blended fuel) is:

*B(CaHbOc) + D(Cα*H*β*O*χ*N*δ*) + (as/Ø)(O2 + 3.76N2) z1CO2 + z2H2O + z3N2 + z4O2 + z5CO + z6H2 +

*z*7H + z8O + z9OH + z10NO (1)

Where, B: Mole of jatropha bio fuel
D: Mole of diesel fuel
as: Stoichiometric air-fuel ratio
Ø: Equivalence Ratio
zi: (i= 1-10) No. of moles of each combustion emission product
Now, including the mole fractions of the emission products for the Atomic balancing of Carbon, Hydrogen,
Oxygen and Nitrogen we obtain:
Carbon: B a D α Zt X1 X5 * (2)*

Hydrogen: B(b) + D(*β) = Zt(2X2+2X6+X7+X9) (3) *

Oxygen: B(c) + D(*χ) + (2αs/Ø) = Zt(2X1+X2+2X4+X5+X8+X9+X10) (4) *

Nitrogen: D(*δ)+ 3.76(2as/Ø) = Zt(2X3+X10) (5) *

Addition of all the mole fractions is given by, ∑ 0 (6)

Where, Xi represents the mole fraction of various combustion emission products and Ztrepresents the total
number of moles after combustion.
(Where, * and * ∑ * ) *
The dissociation reactions and the equilibrium constants associated are,
1/2H2 H,
√ _{ (7) }

1/2O2 O, √ (8)

1/2H2 + 1/2O2 OH, (9)

1/2O2 + 1/2N2 NO, (10)

CO+1/2O2 CO2, (12)

Here, *Ki *(i = 1 to 6) represents the equilibrium constants for the respective reactions and P represents the
combustion pressure. The value of the equilibrium constants can be calculated with the help of the formula,
given in equation (13)
∆ °
(13)

Here, ∆ ° represents Gibbs free energy for the particular product of the equilibrium reaction, associated with Ki. R stands for Gas constant and stands for the combustion temperature. The equilibrium constants from the equations (7-12) are then re-arranged to express the mole fractions of the emission products in terms of Xi, √ , (where, √ ) (14)

√ , (where, √ ) (15)

√ , (where, √ ) (16)

√ , (where, √ ) (17)

, (where, ) (18)

, (where, ) (19)

To obtain the value of , , and , equation (3, 4, 5 and 6) is divided by equation 2 and is arranged accordingly to result four equations with four unknowns given below. 2 2 0 (20)

2 2 0

( (21)

2 0 (22)

1 0 ( (23)

Where, , , and

.

On solving equation 20-23, the value of , , and can be obtained, and these values can be substituted in equation 14-19, to get the values of remaining mole fractions.

**3. Application Of The Combustion Model For The Simulation Of The Combustion Emissions, When **
**Jatropha Biodiesel And Diesel Are Used As Blended Fuel **

The model is applied to simulate the combustion emissions of Jatropha Biodiesel and Diesel, when used in blends as a fuel for the diesel engine.

The objective of the simulation is to obtain the mole fractions of the different combustion emission products as a function of equivalence ratio, when Jatropha biodiesel and Diesel are used in different blends.

3.1 Parameters for simulation

(Table 1) shows the different parameters for diesel and jatropha biodiesel, which has been considered for the simulation. (Table 2) shows other parameters relating to the engine combustion, which has been considered.

Table 1. Parameters for fuel.

Variable Diesel Jatropha Biodiesel

Combustion Pressure(bar) 200 105

Combustion Temperature(bar) 2600 2200

Carbon atoms 12 20

Hydrogen atoms 23 40

Oxygen atoms 0 2

Table 2. Other Parameters.

Variables Values Considered

Stoichiometric air-fuel ratio 20 Range of equivalence ratio 0.5 to 1.5

Table 3. Interpretation for Mole Fraction obtained at different fuel blends.

Blends Interpretation
*X*i*D * Mole Fraction at 100% Diesel and 0% Jatropha Biodiesel

*X*i20 Mole Fraction at 80% Diesel and 20% Jatropha Biodiesel
*X*i50 Mole Fraction at 50% Diesel and 50% Jatropha Biodiesel
*X*i80 Mole Fraction at 20% Diesel and 80% Jatropha Biodiesel

Note: *X*i (i = 1 to 10) represents the mole fraction for ith _{combustion emission product. }

Four fuel blends of diesel and jatropha biodiesel viz. [100%, 80%, 50% and 20% Diesel (remaining Jatropha Biodiesel)] has been considered. Mole fractions of different combustion emission products are calculated for all the four different fuel blends. (Table 3) gives the interpretation for mole fractions, obtained at different fuel blends.

3.2 Simulation procedure

A C program is prepared to apply the combustion model for the prediction of combustion emissions. Program
takes the inputs of: Blend of the fuel to be used (B and D), Number of Carbon, Hydrogen, Oxygen and Nitrogen
atoms in Biodiesel (a, b, c) and Diesel (*α*, *β*, *χ*, *δ*), Stoichiometric air-fuel ratio (as), equivalence ratio (Ø) as in
equation (1).Combustion temperature (Tcomb), gibbs free energy for particular product in the dissociation
reaction (∆ ° ) as in equation (13), for calculating the values of Ki (i = 1 to 6) and the Combustion pressure (P).
Respective formulations (according to the combustion model) are applied to obtain the values of

, , , , and [equation (14-19)] and , and [equation (20-23)].

Using the values of , , , , , , , and [equation (20)], equation (21), equation (22) and equation (23) are solved with the help of MATHEMATICA Software. Thus getting the values of

, , and .

Another C program is prepared for calculating the values of , , , , and [equation (14-19)] which takes , , and as the inputs. In this way all the mole fractions for the different combustion emission products to ) are obtained.

The values of the respective parameters, which have been given as the inputs for the simulation program, are given in Table 1 and Table 2. Interpolated values have been used, whenever required (During the variation of the fuel blend). In this way, results are obtained.

**4. RESULTS AND DISCUSSION: **

Fig 1. Variation of CO2 with Equivalence ratio

1 3 5 7 9 11 13

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**%C**

**O**

**2**

**ø**

Fig 2. Variation of H2O with Equivalence ratio

Fig 3. Variation of N2 with Equivalence ratio

Fig 4. Variation of O2 with Equivalence ratio

5 8 11 14

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**% H**

**2**

**O**

**Ø**

X2 D X2 20 X2 50 X2 80

55 62 69 76

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**% N**

**2**

**Ø**

X3 D

X3 20

X3 50

X3 80

0 3 6 9 12

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**% O**

**2**

**Ø**

X4 D

X4 20

X4 50

Fig 5. Variation of CO with Equivalence ratio

Fig 6. Variation of H2 with Equivalence ratio

Fig 7. Variation of H with Equivalence ratio

0 5 10 15 20

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**%C**

**O**

**Ø**

X5 D

X5 20

X5 50

X5 80

0 3 6 9 12 15

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**% H**

**2**

**Ø**

X6 D

X6 20

X6 50

X6 80

0 0.01 0.02 0.03 0.04 0.05

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**% H**

**Ø**

X7 D

X7 20

X7 50

Fig 8. Variation of O with Equivalence ratio

Fig 9. Variation of OH with Equivalence ratio

Fig 10. Variation of NO with Equivalence ratio

Figure (1-10) shows the variation in the mole fractions of the various diesel engine combustion emission products as a function of equivalence ratio, when different blends of Jatropha biodiesel and Diesel are used as a

0 0.01 0.02 0.03 0.04 0.05 0.06

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**% O**

**Ø**

X8 D

X8 20

X8 50

X8 80

0 0.1 0.2 0.3 0.4

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**%O**

**H**

**Ø**

X9 D

X9 20

X9 50

X9 80

0 0.4 0.8 1.2 1.6 2

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

**% N**

**O**

**Ø**

X10 D

X10 20

X10 50

fuel in the diesel engine. CO2, CO, and NO are the major pollutants, so they have been discussed firstly. Remaining by-products has been discussed afterwards.

Figure 1 shows the variation of CO2 with equivalence ratio at various fuel blends. It is observed that, the increase in the ratio of the jatropha biodiesel in the fuel blend increases the peak value of the exhaust CO2 emission. CO2 emission initially increases with the increase of the equivalence ratio for all the fuel blends, but all the curves tends to decline somewhere around the equivalence ratio 1, because the amount of air gets reduced for the formation of CO2.

Figure 5 shows the variation of CO with equivalence ratio at various fuel blends. Before equivalence ratio 1, due to the presence of enough air, all the carbon is converted into CO2, but after the equivalence ratio 1, due to the lack of air CO is produced. It is observed that, the increase in the ratio of the jatropha biodiesel in the fuel blend increases the exhaust CO2 emission.

Figure 10 shows the variation of NO with the equivalence ratio. Before the equivalence ratio 1, enough air is available in the engine for the reaction of the nitrogen with oxygen to form NO, but after equivalence ratio 1, the value of NO emission tends to zero for all the fuel blends due to lack of air. Combustion temperature of diesel is greater than the jatropha biodiesel, so the value of the exhaust NO is high for a higher concentration of diesel in the fuel blend.

Other emission products are H2O, O2, H2, N2, H, O and OH. Figure 2 shows the variation of H2O with the equivalence ratio at various fuel blends. The formation of CO2 in the engine results in the formation of the H2O. When CO2 formation is reduced, the formation of the H2O also gets reduced. Figure 4 shows the variation of O2 with the equivalence ratio at various fuel blends. Enough air is present in the engine, so O2 is accordingly present in the emission, but declines after equivalence ratio 1. Figure 6 shows the variation of H2 with the equivalence ratio at various fuel blends. Due to the presence of enough air before the equivalence ratio 1, there is complete burning of fuel, so no H2 is formed, but after equivalence ratio 1, lack of air results in the incomplete combustion; thus H2 is formed.

Figure (3, 7, 8 and 9) gives the details of the emissions for N2, H, O and OH respectively. Presence of these emissions products are very less as compared to the above discussed emission products.

**5. Conclusions **

1. A complete and detailed mathematical model for the combustion of diesel and biodiesel, when used as a blended fuel in a diesel engine is done.

2. The distribution of various exhaust emission products as a function of equivalence ratio can be calculated by this model.

3. The model is applied to simulate the combustion exhaust emissions, when different blends of jatropha biodiesel and diesel are used as fuel in a diesel engine.

4. Simulation results show that with the increase in equivalence ratio the concentration of exhaust NO reduces but the concentration of exhaust CO increases. NO and CO are the major pollutants and it is required to exhaust the minimum quantity of these products into air. So equivalence ratio should be selected less than and near to 1.

5. For Jatropha biodiesel (blended with diesel), the concentration of NO emission is low but the concentrations of CO and CO2 emissions is higher when compared to the 100% diesel usage after the equivalence ratio 1, because after the equivalence ratio 1, CO2 gets converted into CO due to lack of air. So for the use of jatropha biodiesel in diesel engine, a platinum catalytic converter must be used to convert the CO into CO2. So from the environment point of view, proper equivalence ratio and proper blend must be selected.

6. The results are useful for getting an optimum value of equivalence ratio and fuel blend to regulate the emissions for an eco-friendly operation.

7. The model can also be applied for other biodiesels as well.

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