Performance of Diesel-Compressed Natural Gas

Performance of Diesel-Compressed Natural Gas (CNG) Dual Fuel (DDF) Engine via CNG-Air Venturi Mixjector Application A. Supee, M. S. Shafeez, R. Mohsin & Z. A. Majid

Arabian Journal for Science and Engineering ISSN 1319-8025 Arab J Sci Eng DOI 10.1007/s13369-014-1313-2

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Author's personal copy Arab J Sci Eng DOI 10.1007/s13369-014-1313-2

RESEARCH ARTICLE - MECHANICAL ENGINEERING

Performance of Diesel-Compressed Natural Gas (CNG) Dual Fuel (DDF) Engine via CNG-Air Venturi Mixjector Application A. Supee · M. S. Shafeez · R. Mohsin · Z. A. Majid

Received: 30 September 2013 / Accepted: 31 January 2014 © King Fahd University of Petroleum and Minerals 2014

Abstract The performance of the diesel engines can be improved by integrating the existing diesel fuel system with another type of fuel which is compressed natural gas (CNG) and this system is called as diesel-CNG dual fuel (DDF). In DDF system, the performance of the engines are greatly influenced by mixing quality of CNG and air before combustion chamber. There are many possible designs of CNG and air mixer available in the market; however, there are limited design using a combination of CNG injector and air mixer which is named as CNG-air venturi mixjector. Hence, this research is conducted to design new type of CNG-air venturi mixjector and to determine the effects of injector numbers in mixjector toward diesel engine performance. CNG-air venturi mixjector was installed at the entrance of the intake manifold, and only three different numbers of injectors were tested. The engine performance tests under full load condition were conducted over the engine speed range from 1,000 to 3,000 rpm for power output and for exhaust gas concentrations, the range was set from 1,000 to 2,800 rpm. Exhaust gas concentrations under various loading conditions were analyzed from light to heavy loading using rated power speed that can produce higher power output. The concentrations tested under both conditions included nitric oxides (NOx ), carbon dioxide (CO2 ) and carbon monoxide (CO) concentrations. Meanwhile, unburned hydrocarbons concentration tests were only conducted at full load condition. DDF system with four injectors in CNG-air venturi mixjector shows

higher power output at rated power speed of 2,800 rpm and less exhaust gas concentrations under maximum loading condition (100 % load) compared to the conventional diesel engine, DDF system without CNG-air venturi mixjector and DDF system with two and six injectors in CNG-air venturi mixjector. CFD simulator was used to study the possible mechanism for an optimum numbers of injectors that resulted in better engine performance focused in flow behavior of CNG and air in the CNG-air venturi mixjector connected to the intake manifold. CNG-air venturi mixjector with four injectors results in homogeneous mixture and thus lead to the high combustion efficiency. Based on the findings, the new design of CNG-air venturi mixjector can be very valuable if opted to apply this technology on existing diesel engine. Keywords Diesel-CNG dual fuel (DDF) system · CNG-air venturi mixjector · Power output · Exhaust gas concentrations

A. Supee (B) · M. S. Shafeez Faculty of Petroleum and Renewable Energy Engineering (FPREE), Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia e-mail: [email protected] R. Mohsin · Z. A. Majid UTM-MPRC Institute for Oil and Gas (formerly known as Gas Technology Centre), Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia

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1 Introduction Due to the rising cost of the diesel liquid fuel and also the environmental pollution produced by conventional diesel engines, many research has been done to find the best solution to overcome these issues. The evolution in automotive technologies indicates that diesel-compressed natural gas (CNG) dual fuel (DDF) engines can provide a short-medium solution in terms of the reduction in fuel consumption and pollutant emissions [1–4]. CNG is a clean burning fuel and have high octane number which is suitable for relatively high compression ratio diesel engines. Moreover, it mixes uniformly with air, produce an efficient combustion and results in substantial reduction in exhaust gas emissions [5–8]. Two type of methods available for CNG injection in DDF system which are directly injected in the cylinder or in most applications to date, it is inducted in the intake manifold [9,10]. In the second method, CNG and air are mixed and compressed as in a conventional diesel engine. The mixture of air and gaseous fuel does not auto-ignite themselves due to their high auto-ignition temperature. Hence, the combustion of CNG-air mixture is initiate by the injection of “pilot” amount of liquid diesel fuel near the end of compression stroke. Diesel fuel auto-ignite and acts as ignition sources for the surrounding air-gaseous mixture [11–14]. Performance of the DDF engines in terms of power output and exhaust gas emissions are related to the combustion efficiency. Better performance of the DDF engines can be obtained if the mixture of air and CNG is homogeneously mixed prior to entry to the combustion chamber [15]. Previous works proved that by an application of mixing chamber

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such as a CNG-air mixer, there is no losses occur in terms of power output at higher engine speeds due to the CNG fuel starvation [16,17]. Besides that, power output and efficiency of the DDF engines are also affected by the amount of air induced in the combustion chamber. The reduction of air quantity induced will lead to a drop in power and efficiency. Therefore, DDF engines should not to be throttled on the air side [10]. The main objectives of this research are to design new combination of CNG injector and air venturi mixer which is named as CNG-air venturi mixjector and to determine the effects of injector numbers in mixjector toward diesel engine performance in terms of power output and exhaust gas concentrations. The ratio for the diesel and CNG fuels consumption for the tested engine was set to 30:70, respectively, via diesel and CNG Electronic Control Unit (ECU) setting, whereas for the air, it was admitted to the intake manifold without controlled or un-throttled. In the first part of this research, the engine was tested solely with diesel fuel (conventional diesel engine), DDF system without CNG-air venturi mixjector and DDF system with two, four and six injectors in CNG-air venturi mixjector. Computational fluid dynamics (CFD) analysis software (FLUENT Version 6.2.16) was used for the second part. Analysis was mainly focused on flow behavior of methane and air in the CNG-air venturi mixjector connected to the intake manifold. Possible mechanism for an optimum numbers of injectors which resulted in better engine performance was investigated by using this flow behavior characterization. The results for both parts are presented in the form of graphs and visual illustration of CNG and air concentration, respectively.

2 Experimental Description 2.1 Type of Fuels 2.1.1 Compressed Natural Gas (CNG) The CNG used in this research was supplied by Gas Malaysia Sdn. Bhd. (GMSB). The composition of CNG consists of 92.73 % methane, 4.07 % propane and 3.20 % of other hydrocarbons. Table 1 shows physical and chemical properties of natural gas used together with testing method [18]. 2.1.2 Diesel The diesel fuel used in this research was supplied by Petroliam Nasional Berhad (Petronas). The physical and chemical properties of the diesel fuel used together with testing method are stated in Table 2 [19].

Author's personal copy Arab J Sci Eng Table 1 Physical and chemical properties of CNG [18]

Properties

Description

Testing method

Appearance

Colorless gas

Not mentioned

Odor

Pungent odor—Mercaptan mixture added

ASTM D1988

Boiling point

−162 ◦ C

Not mentioned

kg/Sm3

Not mentioned

Vapor density at 760 mmHg

0.747

Specific gravity at 760 mmHg

0.61

Flash point

−187 ◦ C

Not mentioned

Auto-ignition temperature

537 ◦ C

Not mentioned

Gross calorific value

9,530 kcal/Sm3

ISO 6976

Burning velocity

0.3 m/s

Not mentioned

Flammability limit

15.4 % (upper limit)

Not mentioned

ASTM D3588

4.5 % (lower limit)

Table 2 Physical and chemical properties of diesel [19]

Properties

Description

Testing method

Color (ASTM)

2.5 (maximum)

MS 2010/ASTM D1500/D6045

Ash, mass %

0.01 (maximum)

MS 2013/ASTM D482

Cloud point, ◦ C

19.0 (maximum)

ASTM D2500/D5772

Gross calorific value, kJ/kg

41,800 (minimum)

ASTM D4868/D240

Kinematic

1.5 mm2 /s (minimum)

MS 1831/ASTM D445

Viscosity at 40

◦C

5.8 mm2 /s (maximum)

Lubricity

460 µm (maximum)

MS 1892/ASTM D6079

Total sulfur

500 mg/kg (maximum)

MS 1891/ASTM D4294/D2622/ D5453/D7039/D5950

Cetane number

49 (minimum)

MS 1895/ASTM D613/D6890/IP498

Flash point

60 ◦ C (minimum)

MS 686/ASTM D93

0.810 kg/L (minimum)

MS 1893/ASTM D1298/D4052

Density at 15

◦C

0.870 kg/L (maximum) Carbon residue on 10 % bottoms, wt%

0.20 (maximum)

MS 962/ASTM D189/D4530

Water by distillation, vol%

0.05 (maximum)

MS 1800/ASTM D6304/D95

Sediment by extraction, wt%

0.01 (maximum)

MS 790/ASTM D473

2.2 CNG-Air Venturi Mixjector Design CNG-air venturi mixjector design exhibit that air enters the mixjector through the inlet, whereas CNG injected at the throat at 90◦ angle to the main flow via injectors. The schematic design of the CNG-air venturi mixjector is shown in Fig. 1. The CNG injectors, each with diameter of 0.5 cm, were distributed evenly at the throat. The circular distance of each injectors in CNG-air venturi mixjector are 180◦ apart for two injectors, 90◦ for four injectors and 60◦ for six injectors. 2.3 Laboratory Works HINO diesel engine was used and the detailed specifications of the engine are listed in Table 3. The engine was tested under different condition named as solely diesel fuel, DDF system without CNG-air venturi mixjector application and

DDF system with three different numbers of injectors in CNG-air venturi mixjector. During tests, the inlet water pressure of dynamometer was kept constant at three bars meanwhile outlet temperature of cooling water was kept constant at 75 ◦ C. The schematic layout of the test installation is depicted in Fig. 2. CNG at 200 bars was supplied from CNG tank via high pressure stainless steel pipe to the CNG fuel system. In CNG fuel system, three stage pressure regulator was used to reduce the pressure from 200 to 4 bars at first stage and for second stage, the pressure was dropped from 4 to 1.5 bars. The third stage was volume or fuel consumption set as previously mentioned. Hot water recirculation from radiator of the engine also connected to the pressure regulator to avoid icing problem due to the expansion of the CNG (Joule Thomson effect). Connection from outlet of pressure regulator to CNG injector at CNG-air venturi mixjector was done using low

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Author's personal copy Arab J Sci Eng Fig. 1 CNG-air venturi mixjector schematic design

100 cm

Inlet (air)

Table 3 Engine specifications Model

HO7C

Ignition system

Compression ignition

Displacement (cc)

6,728

Bore × Stroke

110 mm × 118 mm

Cylinder number

6

Firing order

1-4-2-6-3-5

Horse power (HP)

180 HP @ 3,000 rpm

Maximum rated HP

160 HP @ 3,000 rpm

Maximum rated torque

304 Nm @ 1,800 rpm

Cooling type

Water cooling

Fuel supply system

Direct injection (DI)

Fuel injection system (controlled by diesel electronic control unit)

Injection pump

Valve clearance (intake)

0.30 mm

Valve clearance (exhaust)

0.40 mm

Injection timing (BTDC)

14◦

Throat (70 cm)

120 cm

Outlet (mixture of CNG and air before intake manifold)

Injectors (0.5 cm)

deliver diesel fuel to the diesel fuel injector nozzles which have opening pressure of 201–209 bars. Under full load condition, engine speeds range from 1,000 to 3,000 rpm with increment of 200 rpm were used for power output. Meanwhile, for exhaust gas concentrations under full load condition, the range was set from 1,000 to 2,800 rpm. For various loading conditions, exhaust gas concentrations were analyzed from 0 to 100 % loading using rated power speed that can produce higher power output. The data for power output (smallest reading scale is 0.1), loading percentages and engine speeds (smallest reading scale is 1) were recorded from DYNOmite DYNAMOMETER (software integrated) by LANDSea, and exhaust gas concentrations data such as NOx and unburned hydrocarbons (UHC) in ppm (smallest reading scale is 1), percentages of CO (smallest reading scale is 0.01) and CO2 (smallest reading scale is 0.1) were recorded from EMS Exhaust Gas Analyzer (model 5002). 2.4 Simulation Works

pressure gas hose. For the air, it was admitted into mixjector without throttled. Diesel fuel was supplied from diesel tank to the diesel fuel system. Diesel injection pump was used to Fig. 2 Schematic layout of the test installation

Three different cases were considered which are CNG-air venturi mixjector with two, four and six injectors. All of them were designed using GAMBIT Version 2.2.30 software. CNG Tank CNG Fuel System

Air Filter

CNG-air venturi mixjector

Exhaust System

Diesel Engine (HINO HO7C)

Air

NOX Sensor

O2 Sensor

Exhaust Gas Analyzer

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Diesel Fuel System Diesel Tank

CNG-air venturi mixjector is fixed at the entrance of the intake manifold Un-throttled air intake Not used in solely diesel fuel and DDF without mixjector testing

Data Acquisition System

Dynamometer Pressure Sensor

Personal Computer (PC)

Temperature Sensor

Water Tank (Cooling Water)

Author's personal copy Arab J Sci Eng 200

Table 4 Defined parameters in CFD simulator Parameter

Definition

Solver

Segregated 2D Space Absolute velocity formulation Cell-based gradient option Superficial velocity porous formulation

Energy Viscous model

Species transport model Mixture material was defined as methane and air

Air inlet defined as velocity inlet which equal to 15 m/s Air density was 1.165 kg/m3 CNG inlet defined as velocity inlet which equal to 30 m/s CNG density was 0.711 kg/m3

Control solution

40 1000

1500

2000

2500

3000

speed (rpm)

Fig. 3 Power characteristics of the engine at different speed (rpm) under full load condition

Initial operating pressure of the air was defined as ambient pressure (101,325 Pa) Final operating pressure of CNG and air mixture was assumed to be the same with initial operating pressure of CNG

Boundary condition

diesel ddf ddf (mixjector 4) ddf (mixjector 2) ddf (mixjector 6)

80

k-Epsilon (two equation) Near wall treatment was defined as standard wall function

Operating condition

120

Energy equation was applied Standard k-epsilon model

Species model

power (Hp)

160

Momentum, turbulence kinetic energy and turbulence dissipation rate were controlled using power law

Computational fluid dynamics analysis software (FLUENT Version 6.2.16) was used to investigate the possible mechanism for an optimum numbers of injectors that resulted in better engine performance focused in flow behavior of methane and air in the CNG-air venturi mixjector connected to the intake manifold. Methane was selected because it is major composition in CNG. In the present study, same parameters and mesh ratio used for all CNG-air venturi mixjector design. The ratio was 1 mm: 2 point of line mesh which generates total of 229,249 numbers of cells and parameters involved were listed in Table 4.

3 Results and Discussions 3.1 Power Characteristic of the Engine Under Full Load Condition Figure 3 shows that DDF system with four injectors in CNGair venturi mixjector produced higher power output compared to the conventional diesel engine, DDF system without CNG-air venturi mixjector and DDF system with two

and six injectors in CNG-air venturi mixjector. As the speed of the engine increased, the power output also increased and maximum power output obtained by DDF system with four injectors in CNG-air venturi mixjector was slightly over 160 HP at 2,800 rpm. Beyond this rpm, the power output start to declined and this trend also observed for DDF system with six injectors in CNG-air venturi mixjector. The trend might be due to knocking which encountered at very high engine speed. This knocking phenomenon is caused by shorter duration of CNG-air combustion after pilot fuel ignition compared to the normal diesel combustion. Similar trend also encountered by previous works regarding to knocking phenomenon at high engine speed [20,21]. In addition, power output for DDF system with four injectors in CNG-air venturi mixjector also exhibits sudden increment in the range of 2,500–2,800 rpm engine speeds. The possible reasons for this trend to occur are the speed ranges approaching rated power speed at 2,800 rpm which cause maximum power output and improvement in combustion process efficiency as described in UHC concentrations section. This improvement was shown by lower amount of UHC at the mentioned engine speed ranges. However, there is no knocking phenomenon observed for the conventional diesel engine, DDF system without CNGair venturi mixjector and DDF system with two injectors in CNG-air venturi mixjector in the range of engine speeds tested. Based on the results obtained, it proved that the method of CNG induction into the conventional diesel engine has significant effects on engine performance at different conditions. DDF system with four injectors in CNG-air venturi mixjector produced homogeneous mixture of air and CNG prior to entry to the combustion chamber and thus lead to the better engine performance either at low or high engine speeds. The results obtained also in agreement with previous works done [5,7,15].

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3.2 Exhaust Gas Concentrations of the Engine 3.2.1 Nitric Oxides (NOx ) Concentrations NOx concentrations in ppm unit released by the engine at different engine speeds under full load condition are depicted in Fig. 4. Overall, NOx concentrations are directly proportional to the engine speed. As the engine speed increase, concentrations also increase. Less NOx concentrations produced by DDF system with four injectors in CNG-air venturi mixjector compared to others throughout the engine speeds range and conventional diesel engine results in highest NOx concentrations. Similar results also produced by previous researchers when normal conventional diesel engine converted to DDF system [2,10,12,16,17,22]. Almost 40–50 % of NOx concentrations reduced by DDF system with four injectors in CNG-air venturi mixjector compared to the conventional diesel engine at the engine speed range from 1,000 to 2,800 rpm. The reduction is mainly due to lower combustion temperatures in the engine resulting from the slower flame speed of CNG. Figure 5 illustrates NOx concentrations at 2,800 rpm for different loading conditions. As the loading percentages increased, NOx concentrations also increased and this trend was similar with previous works conducted [16,17,23]. During the light load operating condition, NOx concentrations were higher for DDF system with and without CNG-air venturi mixjector compared to the diesel operation. This phenomenon occurs because high concentrations of air were inducted into intake manifold compared to the CNG concentrations. Higher air concentration means that higher oxygen concentrations produced and lead to the faster rate of combustion. As the rate of combustion increased, peak cycle temperature within combustion chamber become higher and thus promotes greater NOx concentrations.

NOx (ppm)

500

400


300

200 0

20

40

60

80

100

% load

Fig. 5 Nitric oxides (NOx ) concentrations at 2,800 rpm for different loading conditions

However, at greater than 80 % load condition, DDF system with and without CNG-air venturi mixjector exhibits lower NOx concentrations compared to the conventional diesel engine. The reason for this difference is mainly due to the low concentration of oxygen within combustion chamber of the engine. An equal amount of air was replaced by CNG. As the oxygen concentrations reduced, slower rate of combustion produced and peak cycle temperature become lower. As a result, NOx emissions were reduced. CNG-air venturi mixjector with four injectors in DDF system exhibits better NOx concentrations compared to other system starting around 30–100 % load condition. It shows that the design of CNG-air venturi mixjector with four injectors can generate more homogeneous CNG-air mixture and results in efficient combustion process.

3.2.2 Carbon Monoxide (CO) Concentrations

900

NOx (ppm)

800 700 600 diesel ddf ddf (mixjector 4) ddf (mixjector 6) ddf (mixjector 2)

500 400 300 800

1200

1600

2000

2400

2800

speed (rpm)

Fig. 4 Nitric oxides (NOx ) concentrations at different engine speeds under full load condition

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Figure 6 shows the variation of CO concentrations under full load condition with engine speeds. CO concentrations were significantly lower for DDF system with and without CNG-air venturi mixjector compared to the conventional diesel engine at the speeds tested throughout the speed range. The rate of CO formation is a function of the air-fuel ratio, unburned gaseous fuel availability and mixture temperature, which control the rate of fuel oxidation and decomposition [24–26]. CNG-air venturi mixjector with four injectors in DDF system shows lower CO concentrations which is below 0.2 % for the tested speed range. This may be attributed mainly to the improvement of the gaseous fuel combustion quality, particularly during premixed controlled combustion phase, contributes to lower unburned gaseous fuel availability and higher CNG-air mixture temperature, thus contributing to lower formed CO concentrations.

Author's personal copy Arab J Sci Eng diesel ddf ddf (mixjector 4) ddf (mixjector 6) ddf (mixjector 2)

% CO

0.6

0.4

0.2

0.0 800

1200

1600

2000

2400

2800

speed (rpm)

Fig. 6 Carbon monoxide (CO) concentrations at different engine speeds under full load condition

Conventional diesel engine produced highest CO concentrations for the tested speed range, however, starting from 1,600 to 2,800 rpm, the CO concentrations flat at around 0.6 %. This phenomenon occurs possibly due to the less time available for combustion. It can also be concluded that the percentages of CO concentrations were influenced by the engine speed. As the speed increased, the CO concentrations increased. Except for the different trends mentioned above, this relationship was true for conventional diesel engine, DDF system without CNG-air venturi mixjector and DDF system with two, four and six injectors in CNG-air venturi mixjector. CO concentrations at 2,800 rpm engine speed for different loading condition is depicted in Fig. 7. Conventional diesel engine produced less CO concentrations than other systems at low and intermediate load which is represented by 0 % to around 50 % load. However, when the load exceed 50 %, CNG-air venturi mixjector with four injectors in DDF system shows less CO concentrations than conventional diesel system. Meanwhile, other systems generate less CO concen5

3.2.3 Carbon Dioxide (CO2 ) Concentrations CO2 concentrations under full load condition for the tested systems at various engine speeds are shown in Fig. 8. Overall, conventional diesel engine generates more CO2 concentrations compared to the DDF system with and without CNGair venturi mixjector. This is caused by CNG having a higher hydrogen-to-carbon ratio than diesel. By stoichiometric combustion in air, 1 g of methane produces about 2.8 g of CO2 , whereas 1 g of diesel produces about 3.2 g of CO2 [9]. Figure 9 indicates that under light to heavy loading condition, DDF system using CNG-air venturi mixjector with four injectors results in better CO2 concentrations compared to other systems. It was peaked at around 3.5% at 100 % load condition. For the conventional diesel engine, it was peaked at around 6.5 % at maximum load meanwhile for the other systems at 100 % load, their values were within the above


4

10

% CO2

3

% CO

trations compared to the conventional diesel system starting from approximately 60–100 % load. An explanation for the low CO concentrations produced by conventional diesel engine at low and intermediate load compared to others possibly due to the incomplete combustion of gaseous fuel. At high load, the trend was reversed. All DDF system generate less CO concentrations than conventional diesel system with CNG-air venturi mixjector with four injectors was the lowest one. Previous works done show different results produced when they considered high engine speed at high load [16]. They found that at high engine speed, DDF system with increasing engine load has no effect on CO concentrations. The differences might be related to the type, condition and setting of the engine used. CNG-air venturi mixjector application especially with four injectors improved the combustion process for the gaseous fuel. Combination of less time available with this improvement, it affect adversely the CO formation mechanism.

2

8 diesel ddf ddf (mixjector 4) ddf (mixjector 6) ddf (mixjector 2)

6

1

4 0 0

20

40

60

80

100

% load

Fig. 7 Carbon monoxide (CO) concentrations at 2,800 rpm for different loading conditions

800

1200

1600

2000

2400

2800

speed (rpm)

Fig. 8 Carbon dioxide (CO2 ) concentrations at different engine speeds under full load condition

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6

% CO2

5 4 3 2 1 0

20

40

60

80

100

% load

Fig. 9 Carbon dioxide (CO2 ) concentrations at 2,800 rpm for different loading conditions

mentioned. The possible explanations for these are proper diesel-gaseous fuel ratio obtained by an application of CNGair venturi mixjector with four injectors, no excess in term of diesel and gaseous fuel consumption and thus lead to the efficient combustion. 3.2.4 Unburned Hydrocarbons (UHC) Concentrations The variation of unburned hydrocarbon concentration under full load condition as a function of engine speeds for the tested systems is given in Fig. 10. As known, the rate of UHC formation depends on the quality of combustion process occurring inside the combustion chamber [24,25,27]. Overall trends show that UHC concentrations decreased with increasing engine speeds and DDF system either with or without CNG-air venturi mixjector produced less UHC concentrations compared to solely diesel fuel system. The trends obtained were similar with previous works done; however, their results show higher UHC concentrations in DDF sys140


hc (ppm)

120

100

80

60 800

1200

1600

2000

2400

2800

speed (rpm)

Fig. 10 Unburned hydrocarbons concentrations at different engine speeds under full load condition

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tem compared solely diesel fuel system throughout the tested speeds range [16,26]. The differences might be due to the different ratio in diesel and CNG fuels consumption used for the tested engine. DDF system with four injectors in CNG-air venturi mixjector results in lowest UHC concentrations and at rated power speed, the UHC concentrations value were slightly over 60 ppm. This lowest UHC concentrations value might be mainly due to the suitable air fuel ratio used and homogeneous CNG-air fuel mixture produced inside the combustion chamber by application of CNG-air venturi mixjector with four injectors. As a result, only small quantities of fuel escape the combustion process and thus improve the quality of combustion process.

3.3 Flow Behavior of Methane and Air in CNG-Air Venturi Mixjector Simulation works have been conducted to investigate the possible mechanism for an optimum numbers of injectors that resulted in better engine performance focused in flow behavior of methane and air in the CNG-air venturi mixjector connected to the intake manifold. For comparison purposes, only three different cases were considered which are CNG-air venturi mixjector with two, four and six injectors and it is depicted in Figs. 11 12 and 13 respectively. Based on the power output and exhaust gas concentrations, it can be conclude that the best design was CNG-air venturi mixjector with four injectors. It is followed by CNG-air venturi mixjector with six injectors and lastly CNG-air venturi mixjector with two injectors. The results from laboratory works are also in agreement with simulation works. Figure 11 indicates that poor quality of CNG and air mixture and it was shown by four contours at the exit of the intake manifold. The mixing distance for CNG-air venturi mixjector with two injectors is long and even at the exit of the intake manifold, the concentration ratio of mixture still has not reach equilibrium. The concentration ratio of mixture was found to be slightly improved with CNGair venturi mixjector with six injectors, and it is depicted in Fig. 13. Eventhough the flow was not homogeneous at the exit of the intake manifold, the contours were reduced to three compared to the CNG-air venturi mixjector with two injectors. Figure 12 shows homogeneous mixture produced by CNG-air venturi mixjector with four injectors prior to entry to the combustion chamber. Only short homogeneous mixing distance required and just one contour appeared at the exit of the intake manifold. This homogeneous mixture will result in high combustion efficiency and thus produced high power output and less exhaust gas concentrations. Similar results were also obtained by previous researcher in terms of power output and exhaust gas concentrations [15–17].

Author's personal copy Arab J Sci Eng Fig. 11 CNG-air concentrations for two injectors in venturi mixjector

Fig. 12 CNG-air concentrations for four injectors in venturi mixjector

Fig. 13 CNG-air concentrations for six injectors in venturi mixjector

4 Conclusions New design of CNG-air venturi mixjector was successfully developed with CNG-air venturi mixjector with four injectors exhibiting highest power output at the engine speed range

from 1,000 to 3,000 rpm. Moreover, the exhaust gas concentrations of NOx , CO, CO2 and UHC within the tested speed and load ranges also reduced significantly compared to the other system. From simulation point of view, homogeneous mixture of CNG and air was produced by CNG-air venturi

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Author's personal copy Arab J Sci Eng

mixjector with four injectors and thus lead to the high combustion efficiency. This new design can be applied to the existing conventional diesel engines and provide promising technique for controlling exhaust gas concentrations without power sacrificing.

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