On the optimization of the air-LPG mixing system of a Diesel bus engine converted to gas

Mohamed Ali Jemni 1, Gueorgui Kantchev1 and Mohamed Salah Abid 1

1University of Sfax, National School of Engineers of Sfax (ENIS), Laboratory of Electro-Mechanic Systems (LASEM),

B.P. 1173, km 3.5 Soukra, 3038 Sfax, TUNISIA. Email: jemni_med_ali@yahoo.fr,

Gueorgui.Kantchev@enis.rnu.tn, MohamedSalah.Abid@enis.rnu.tn.

ABSTRACT

This paper investigates the air-LPG (Liquefied petroleum gas) engine mixer geometry effect on performance of a six-cylinder, heavy duty, IVECO engine, which is used to power urban buses. This engine was retrofitted from its Diesel version into bi-fuel gaseous fueling. Investigations were performed by numerical and experimental methods. A three-dimensional numerical modeling of the turbulent flow through two mixer geometries (17 holes mixer with a premixing room and 8 holes mixer) was undertaken using the 3D CFD code FloWorks. This modeling made it possible to provide a fine knowledge of the flow structures through the mixers, and to predict its operating feature fields. This model allowed the selection of the optimum mixer. The results indicated that mixer with 17 holes can produce a better homogenous mixture than those of 8 holes mixer. Experimental measurements are also carried out to validate this mixer by measuring the important engine performances. Comparative analysis of the experimental results showed that the 17 holes mixer presence into LPG engine has improved brake power (BP) and brake torque (BT) by 6.25 % and 3 % respectively.

Index Terms—Alternative fuels, converted engine, LPG mixing system, CFD, experiment

1. INTRODUCTION

Due to air pollution and restricted petroleum

reserves, many fuels like CNG, LPG, hydrogen, biodiesel and biogas, were regarded as promising alternatives for urban transport. Several professional researchers in the internal combustion engine (ICE) field have emerged to respond these requirements. Furthermore, to reduce traffic in many countries, including Tunisia, the use of public transportation is encouraged. Urban buses are generally equipped with Diesel engines whose emissions take place in densely populated areas, contributing significantly to the exposure of the general population. In the last decade, the idea of alternative fuels for ICE applications had emerged to replace the traditional fuels such as gasoline and Diesel. Among these alternative fuels, natural gas and liquefied petroleum gas (LPG) have remarkable impact on reducing toxic emissions. Using these gases, promising results were obtained in terms of fuel economy and exhaust emissions reduction using gaseous fueled engine [1-2]. There are countries that. Japan, Italy, and Canada have used this mode of fuel for public transportation, as much as 7% of the buses are powered by LPG, and some European countries are planning to employ LPG vehicles, due to pollution considerations [3]. Actually, LPG supply exceeds the demand in most

petroleum-refining countries, so the price is lower than other hydrocarbons [4]. Numerous reliable researches on LPG fuelled engines have been done to enhance the benefits of this gas. Ozcan et al. [5] have examined the use of LPG under a newly modified engine design called the variable stroke engine. M. Gumus [6] has studied the effects of variation in volumetric efficiency on the engine emissions characteristics with different LPG usage levels (25%, 50%, 75%, and 100%), on an engine operated with new generation closed loop, multipoint, and sequential gas injection system. Experimental results showed that the volumetric efficiency decreased considerably at the use of 25% LPG level. As for the 50%, 75% and 100% LPG usage, volumetric efficiency decreased in proportion to LPG usage level. To convert Diesel engines to LPG-gasoline bi-fuel engine, many systems are changed, including the intake and air-fuel mixing systems. Consequently, a reliable design of this system leads to improve the combustion process, therefore to optimize engine performances. Gas mixer is an apparatus used to determine the amount of LPG mixed with air before entering the engine and able to supply the engine with the appropriate mixture of gas and air according to various engine regimes and operations. The perfect blend of gas and air mainly helps to raise gaseous engine performance and produces much cleaner exhaust emissions [7]. Various studies are performed to analyze mixer design effect on the engine behavior, and especially the engines converted from

its Diesel originality [8-9]. This study is interested in the inlet system optimization and especially the gas mixer to ensure the correct operation of the IVECO Diesel retrofitted engine into LPG. Two possible designs are presented of this mixer. The first design is consisted of 17 gas outlet holes mixer. The second is consisted of 8 gas outlet holes mixer (figure 1). The comparison of the inlet flow structures of the two mixers informs us of the optimized mixer effectiveness compared to the initial one. The simulation objective is to examine the LPG mixer optimal geometry and, thus, making it possible to provide a fine knowledge of the mixture aerodynamics nature (velocity, turbulent characteristics and air-gas friction). Hence, the three-dimensional resolution of Navies-Stokes equations in conjunction with the standard k-ε turbulence model is undertaken using the FloWorks CFD code. The numerical modeling is realized considering the aspiration of one cylinder. The second objective of this paper is to identify the effects of fueling type and mixer influence on engine performances using experimental method.

a. 17 holes mixer

b. 8 holes mixer

Figure 1. 3D-Model of air fuel mixers

2. ENGINE CONFIGURATION

An IVECO engine, six-cylinder, 13.8 liter displacement, water-cooled, heavy duty, direct injection (DI), installed at the authors’ laboratory was modified to bi-fuel spark ignition (SI) engine gasoline and gas fuelling (LPG and CNG). It is used to power the urban Diesel buses engines in Sfax.

3. CFD MODELING OF AIR-LPG MIXER

CFD is largely used to simulate the non stationary engine flows. In this study, the commonly available CFD tool FloWorks is applied. FloWorks has the advantages of importing geometry directly from CAD software such as SolidWorks. At the beginning, 3D geometry of the two mixers is built using the SolidWorks (SW) software. The files created by SW are imported in FloWorks to build the

grid for the final calculation of simulation.

3.1. Governing equations Equations governing the flow model, including the

conservation equation of mass, conservation of momentum, and the conservation equation of energy, are summarized in conservative form of the Navier-Stokes equations:

( ) ( ) ( )

( )( )( )

( ) 0 (1)

; 1, 2,3 (2)

(3)

1 (4)

ρ ρ

ρ ρ τ τ

ρρρ τ τ τ ρε

ρ γ ρ

∂ ∂+ =∂ ∂∂ ∂ ∂ ∂+ + = + + =∂ ∂ ∂ ∂

⎛ ⎞∂ +⎜ ⎟ ∂∂ ∂⎝ ⎠+ = + + − + + +∂ ∂ ∂ ∂

= = −

ii

Ri i j ij ij i

i i i

iR R i

j ij ij i ij i i Hi i j

ut x

pu u u S it x x x

pu EuE u q S u Q

t x x x

p RT e

3.2. Boundary conditions

An air ideal mass fraction equal to 1 and a pressure equal to 1.1 bars are considered as the air inlet initial conditions. The initial boundary conditions for fuel inlet are the gas mass fraction as 1 and 1.5 bars for propane pressure. The boundary condition for mixer outlet is an output volumetric flow equal to 0.06 m3/s determined from the engine filing parameters. To simplify the calculation, the mixers walls are assumed adiabatic: no transfer of heat with outside. The outlet volumetric flow is taken for the engine speed corresponding to the maximum engine torque (n=1500 rpm). In this speed, the engine has minimum specific fuel consumption. The LPG fuel simulated in the present study is C3H8, (almost 100% propane). 3.3. Results and discussion The purpose of numerical simulation tests is the selection of the mixer that would give optimum results in engine performance and required to give optimum mixing quality of fuel and air. The flow structures through the two mixer models are studied using CFD. Velocity and propane mass concentration fields are determined.

a. 8 holes mixer

LPG inlet

Air inlet

Mixture outlet

b. 17 holes mixer

Figure 2. Velocity field in (x-z) plan Velocity field is presented during the intake stroke.

The control of the velocity distribution in the mixer improves quality of cylinder filing. Figure 2 show the velocity field distribution in the x-z plan, for 8 and 17 holes designs respectively. From this figure, we can observe that the air is drawn by the downward motion of the piston and forms with propane an annular jet. The propane flow direction is created by the pressure difference. The annular jet expands in mixers body, interacts with these walls. We notice that the field is divided into three main parts: the first is red in color fades to yellow, the second is green and the third is blue. It can be seen that the largest velocity value decreases heading towards the mixers exit. In the mixers input, the propane flow accelerates then the gas undergoes a brief deceleration when approaching the holes. Towards the mixers outlet, and after blending with air, the mixture is accelerates again and forms a potent jet with a maximum speed of around 128 m/s (17 holes mixer) and 124 m/s (8 holes mixer). Velocity starts to slow down as the mixture flow through the venturi to the mixer outlet with the maximum value of 160 m/s in the inlet air section.

a. 8 holes mixer

b. 17 holes mixer

Figure 3. 3D propane mass fraction

Figure 3 shows the 3D propane mass fraction, meaning air-fuel ratio, for the two mixers. As shown in this figure, the mixture is basically lean in mixers inlet. When the air molecules meet the gas, the mixture becomes increasingly near the stoichiometric ratio (SR).

In this study, our fuel is LPG. The SR air-fuel, which provides favorable conditions for combustion process, is equal to 15.5, that is to say an equivalence propane-air (PA) ratio equal to 0.065. At the mixers output, we note that the average PA ratio is more important in the 17 HM compared to 8 HM, and brings more to the SR. The mixture at the exit of 8 HM appears lean, which influences on the combustion nature. Furthermore, the poor PA ratio as compared to mean equivalence ratio was caused by the reduction of gas surface passage and low turbulence region between the jet shear layers. As a general conclusion, we specify:

• The distributions of the velocity fields, and propane concentration, strongly depend on air-gas mixer geometry.

• From this numerical prediction of air-propane mixture structure, it can be concluded that the optimal mixture homogeneity is tend to be occurred in 17 holes mixer.

Therefore, the 17 holes mixer was selected and manufacturing as an air-fuel mixing system.

4. Experimental study

To validate our modeling work, we used an engine test bench installed at the authors’ laboratory. The brake power (BP) and brake torque (BT), of the engine are measured after conversion with and without selected air-LPG mixer (figure 4).

LPG engine with mixer LPG engine without mixer

Figure 4. Engine test bench

Figure 5 presents the engine characteristics (brake power and brake torque). The experimental results showed that LPG version with 17 multiple holes mixer has the highest power rise compared to LPG without mixer (almost 6.25%). This outcome is explained by two processes. 1- The turbulent flow, generated by using the mixer gas pre-chamber increased the homogenous mixture that affected the better combustion performance. 2- The control of fuel flow promotes mixing fuel and closer to its stoichiometric reports. So we do not have the problem of rich mixtures.

In the torque LPG profiles, the 17 holes mixer operation produce, in the average, 3 % compared to the engine operation without mixer. According to the power and torque curves, it is clear that the use of this mixer has advantages over the engine performances. It improves these performances since these are reduced after the engine conversion from Diesel into gas fueling.

a- Variation of brake power with engine speed

b- Variation of brake torque with engine speed

Figure 5. Engine characteristics

5. Conclusion

The investigation and the analysis of the air-gas

intake flow characteristics, during intake stroke, are numerically carried out using a CFD code. The effectiveness of a liquefied petroleum gas mixer is determined in order to provide a good air fuel mixing quality. Through the numerical approach, two different types of LPG mixer are compared and one of the mixers was able to give more engine efficiency. The results showed that 17 HM is more efficient in mixing both air and fuel before the mixture is forced to enter the engine’s combustion chamber than 8 HM. Therefore, this result shows that the air-fuel mixing system cheek an important role on the gaseous fueling engine performance. Thereafter, experiment tests were performed, on urban bus Diesel engine converted into bi-fuel LPG fuelled engine, in order to study the fueling type influence, as well as LPG mixer effect, on the engine performance. The gaseous fueling engine shows an average of 6.25 % and 3 %, 9 % higher of BP and BT respectively in the presence of the 17 holes mixer within intake system. The numerical and experimental results affirm well the optimal design efficiency of the selected gas mixer checking the good cylinder filling.

Nomenclature n (rpm) Engine speed

p (Pa) QH (J) qi (W/m-2) Si T (°K) u (m.s-1) ρ (kg/m-3) τij (kg m-1 s-2)

Pressure Heat source or sink per unit volume Diffusive heat flux Mass-distributed external force per unit mass Temperature Fluid velocity Density Viscous shear stress tensor

6. REFERENCES

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