DESIGN AND DEVELOPMENT OF ALTERNATIVE ...

DESIGN AND DEVELOPMENT OF ALTERNATIVE FUEL IN DIRECT INJECTION HYDROGEN FUELED ENGINE TO ENGINE PERFORMANCE

N. Shamimi Shahirol Faculty of Mechanical Engineering, Universiti Malaysia Pahang 26600 Pekan, Pahang, Malaysia E-mail: [email protected] Phone: +60136447209 ABSTRACT Regarding for environmental concern issues, improvement and research need to be done concerning about performance and emissions from our vehicle. High quality of fuel is vital for cleaner environmental air quality. Homogeneous charge compression ignition (HCCI) combustion has been attracting considerable attention due to higher efficiency with lower nitrogen oxide (NOX) and particulate matter (PM) emissions. The combustion process in dual fuel systems in HCCI engines is very complex due to the interactions of mixing and chemistry inside the combustion chamber during the late stages of the compression stroke and the ignition process. The ignition process is very critical with significant heat release from chemical reactions occurs and the timing is difficult to predict. In this paper, comparison of performance between a single cylinder research compression ignitions (CI) engine fuelled with a compressed natural gas (CNG) – diesel system and a conventional CI engine fuelled by conventional diesel. The present study focuses on the effect of air-fuel ratio on the performance of four cylinder hydrogen fuelled direct injection internal combustion engine. Results from this paper shows about the emissions in exhaust gas components (NOX, CO and CO2) from the usage of alternative fuel diesel compared to conventional diesel engine. The average power consumption by the dual-fuel engine was higher than the diesel over the power curve. Airfuel ratio (AFR) was varied from rich limit to a lean limit. The rotational speed of the engine was varied from 1250 to 4500 rpm. The acquired results shown that the air fuel ratio are greatly influence on the brake mean effective pressure (BMEP), brake efficiency (BE), brake specific fuel consumption (BSFC) as well as the maximum cylinder temperature. It can be seen that the decreases of BMEP, BE and maximum cylinder temperature with increases of air fuel ratio and speed, however, increases the brake specific fuel consumption. For rich mixtures (low AFR), BMEP decreases almost linearly, then BMEP falls with a non-linear behaviour. Keywords: Compressed natural gas; gas- mixer; gas emission, Hydrogen fuelled engine, air fuel ratio, engine performance, direct injection, engine speed INTRODUCTION Human community’s health becomes a major concern in invention of any new product. Proper development also needed to improve the human factor issues that may existed. Nowadays in vehicle industry, vital problems regarding the performance and emissions from diesel ignition intake become more popular among researches. Previous research has made a statement that long-term exposure of particulate matter in emissions can lead to higher percentage of lung disease [1,2,3,4,5]. Possible effective steps and procedure that has been research is the usage of alternative clean air diesel fuel. One such fuel which has shown great promise in reducing emissions in general and fine particulates specifically is compressed natural gas (CNG) [6,7,8,9]. Properties such as very high hydrogen to carbon atom ratio, 4:1 compared to 1.63:1 for diesel[10], a very high octane rating (OR) with a research octane number (RON) in

the vicinity of 130 depending on composition and wider ignition limits allowing a leaner burn, are some of the properties that make it an attractive clean substitute for diesel fuel. It has been demonstrated that CNG can significantly reduce combustion emissions, which include particulates, but ultrafine particulates and NOx have proven to be more elusive and have been the subject of a great deal of research effort in recent times. Some earlier researchers designed and studied the gas mixer to mix the fuel [11,12,13]. One very practical method of converting a diesel engine to accept CNG without major modifications is to fit a CNG–diesel dual-fuel system to the engine intake and, if necessary, reduce the compression ratio to suit the octane rating of the CNG. With this arrangement, the air/fuel mixer can be used to admit a mixture of air and CNG into the combustion chamber prior to compression, while a small amount of diesel fuel is injected to ignite the air/CNG homogeneous charge and maintain combustion within the chamber. The purpose of the Venturi mixer is to admit the CNG into the air stream in a manner that achieves efficient mixing [14], and to regulate the air/fuel ratio with the assistance of a butterfly valve in the CNG supply line or a pressure regulator upstream of the mixer [15]. Hydrogen induction techniques play a very dominant and sensitive role in determining the performance characteristics of the hydrogen fuelled internal combustion engine (H2ICE) [16]. Hydrogen fuel delivery system can be broken down into three main types including the carburetted injection, port fuel injection and direct injection (DI) [17]. In direct injection, the intake valve is closed when the fuel is injected into the combustion cylinder during the compression stroke [17]. Like PFI, direct injection has long been viewed as one of the most attractive choices for supplying hydrogen fuel to combustion chamber [18]. This view is based on: its prevention for abnormal combustion: pre-ignition, backfire and knock; and the high volumetric efficiency, (since hydrogen is injected after intake valve closing). The improved volumetric efficiency and the higher heat of combustion of hydrogen compared to gasoline, provides the potential for power density to be approximately 115% that of the identical engine operated on gasoline [18]. However, it is worthy to emphasize that while direct injection solves the problem of preignition in the intake manifold, it does not necessarily prevent pre-ignition within the combustion chamber [17]. Metal hydrides can only provide low pressure hydrogen, compressed hydrogen could be used but this limits the effective tank contents as the tank can only be emptied down to the fuel injection pressure. Compressing gaseous hydrogen on board would mean an extra compressor and a substantial energy demand [19]. THEORY AND PHYSICAL LAW The inlet velocity is dependent on the engine speed. The air inlet cross-sectional area and the total CNG cross-sectional area are constant, and the ratio was around 17:1. In order to investigate the effect of engine speed on the air/fuel ratio and mixing efficiency, further simulations were carried out using the 8hole Venturi mixer. As the engine speed increases, velocity increases at the throat, increasing the depression. This increased depression will “suck” more fuel through the Venturi, automatically compensating for increased air flow rate through the Venturi. The distribution of CNG concentration through the mixer for engine speeds of 250 rpm, 500 rpm, 750 rpm and 1000 rpm was simulated. The simulated results showed that as the engine speed increases, the amount of CNG inducted into the mixer increases. Further analysis of CNG concentration at the exit of the mixer revealed that the air-fuel ratio decreases and hence the equivalence ratio increases with engine speed. The model showed that the mixing efficiency improved with engine speed due to an increase in flow turbulence. The brake mean effective pressure (BMEP) can be defined as the ratio of the brake work per cycle Wb to the cylinder volume displaced per cycle Vd, and expressed as in Eq. (1):

BMEP 

Wb Vb

This equation can be extended for the present four stroke engine to:

(1)

BMEP 

2P NVb

(2)

Where Pb is the brake power, and N is the rotational speed. Brake efficiency ( b ) can be defined as the ratio of the brake power Pb to the engine fuel energy as in Eq. (3):

b 

Pb m f ( LHV )

(3)

Where m f is the fuel mass flow rate; and LHV is the lower heating value of hydrogen. The brake specific fuel consumption (BSFC) represents the fuel flow rate m f per unit brake power output Pb and can be expressed as in Eq. (4) [20]:

BSFC 

mf

(4)

Pb

The volumetric efficiency (  v ) of the engine defines the mass of air supplied through the intake valve during the intake period, ma , by comparison with a reference mass, which is that mass required to perfectly fill the swept volume under the prevailing atmospheric conditions, and can be expressed as in Eq. (5) [21]:

v 

ma aiVd

(5)

Where  ai is the inlet air density. EXPERIMENTAL SETUP The second part of this project included modifying an existing four-stroke single-cylinder diesel engine to be operated using a dual-fuel system. The most practical way of converting a diesel engine to accept CNG is by installing a CNG–air mixer at the air inlet before the combustion chamber. The engine specification is presented in Table 1. Table 1. Engine Specification [22] Parameters Model Bore and length Compression ratio Displacement Maximum power Injection timing

Description Y 170 vertical 4 st diesel engine 70 mm * 55 mm 16:1 211 cc 3.5 kW 14o btdc

Figure 2: Design of the Venturi mixer used for engine tests [23]. In this arrangement, the CNG will be admitted into the combustion chamber along with the air intake charge, while the diesel is used as a pilot in maintaining the flame inside the combustion chamber. Some minor modifications were made to the engine while the compression ratio remained unchanged. The design of the Venturi mixer used in this study was based on the CFD study as mentioned above. An eddycurrent dynamometer was used to apply variable torque to test the ability of the engine to produce power. An exhaust gas analyser was employed to study the concentration of exhaust gases components. From the results of the study it was found that the dual-fuel system produced higher power than the diesel system, at engine speeds up to 3000 rpm. Maximum power of 4.0 kW was achieved at 2700 rpm and dropped off as expected due to the increase in engine friction power. This trend was observed for both dual-fuel and diesel, as shown in Figure 2. At engine speeds beyond 3000 rpm, the power output from the diesel proved superior to the dual-fuel engine due to increased resistance to intake flow through the Venturi restriction and to the onset of knock, resulting from the rapid in-cylinder pressure rise due to combustion. RESULT AND DISCUSSIONS A lean mixture is one in which the amount of fuel is less than stoichiometric mixture. This leads to fairly easy to get an engine start. Furthermore, the combustion reaction will be more complete. Additionally, the final combustion temperature is lower reducing the amount of pollutants. Figure 4 shows the effect of airfuel ratio on the brake mean effective pressure. The air-fuel ratio AFR was varied from rich limit (AFR =27.464:1 based on mass where the equivalence ratio ( = 1.2) to a very lean limit (AFR =171.65 where  = 0.2) and engine speed varied from 2500 rpm to 4500 rpm. BMEP is a good parameter for comparing engines with regard to design due to its independent on the engine size and speed. Air Fuel Ratio It can be seen that BMEP decreases with increases of AFR and speed. This decrease happens with two different behaviours. For rich mixtures (low AFR), BMEF decreases almost linearly, then BMEP falls with a non-linear behaviour. Higher linear range can be recognized for higher speeds. For 4500 rpm, the linear range is continuing until AFR of 42.9125 (=0.8). The non-linear region becomes more predominant at lower speeds and the linear region cannot be specified there. The total drop of BMEP within the studied range of AFR was 8.08 bar for 4500 rpm compared with 10.91 bar for 2500 rpm. At

lean operating conditions (AFR = 171.65,  =0.2 the engine gives maximum power (BMEP = 1.635bar) at lower speed 2500 rpm) compared with the power (BMEP = 0.24 bar) at speed 4500 rpm. Due to dissociation at high temperatures following combustion, molecular oxygen is present in the burned gases under stoichiometric conditions. Thus some additional fuel can be added and partially burned. This increases the temperature and the number of moles of the burned gases in the cylinder. These effects increases the pressure were given increase power and mean effective pressure. Oxides of Nitrogen Figure 3 compares the relationship between NOx emission and engine speed for diesel and dual-fuel during maximum operating conditions. It is clear that the dual-fuel establishes an overall superiority over the diesel, and the NOx concentration in the exhaust gases for dual-fuel, on average, is 54% lower than diesel. However, during the no-load operating condition this trend is generally inverted. For the no-load operating conditions, the NOx emission was higher with the dual-fuel operation during high-speed than with diesel operation. This is due to the higher air-fuel ratio and higher combustion temperatures [24]. As the speed increases, the diesel fuel engine is running at a lower air to fuel ratio compared to dual-fuel, which means the reduction of oxygen intake [25]. Therefore, to reduce the NOx emissions for dual-fuel the CNG injection should be increased. Another method is to reduce the airflow, making the mixture richer than that under normal dual-fuel operation. This can be done by throttling the intake air at light loads using the more sophisticated control of an electric control unit (ECU). Another solution is to mix CNG with only part of the incoming airflow. Nevertheless, in the application of power generation the no-load condition is only a very small proportion of its operation. At maximum engine torque, @ 2000 rpm, dualfuel has a clear-cut superiority (less NOx), at all applied torques, over the diesel.

Figure 3. Variations of NOx with engine speed when set at maximum load operating conditions [26]. Carbon Monoxide For the maximum operating condition, the CO for dual-fuel is 59% lower than for diesel, as shown in Figure 4. Generally, the dual-fuel runs better than diesel at maximum-load operating conditions. This shows that the dual-fuel can achieve better combustion at higher loads compared to diesel. The CO emission from dual-fuel under light operating conditions tended to be higher than that of diesel. At a constant engine speed of 2000 rpm, the dual-fuel starts to represent an improvement only at torques higher than 16 Nm. CO is usually attributed to a high equivalence ratio and/or poor mixing. With the straight diesel, this would indicate that as the load increases, the combustion approaches the smoke line as additional fuel is injected. With the dual-fuel engine, this rapid rise in CO does not occur, indicating a

more efficient combustion with the addition of the premixed CNG/air charge, and lowering the requirement for additional diesel fuel [27].

Figure 4. Variations of CO with engine under maximum load operating conditions [28]. Carbon Dioxide CO2 concentration in the exhaust gases is mainly a function of the chemical composition of the fuel and its hydrogen to carbon ratio, and the efficient conversion of CO to CO2 via the chemical kinetic reaction path. The hydrogen to carbon ratio for diesel fuel is 1.8:1 compared to 4:1 for methane (the main component of CNG). When the theoretical chemical combustion equations at stoichiometric are calculated for the combustion of CH4 and CnH1.8n, then it works out that for every kilogram of diesel burned, 3.18 kg of CO2 is produced and for every kilogram of CH4 burned, 2.74 kg of CO2 is produced. This represents a theoretical reduction of 13.84% (mass basis) for a 100% combustion efficiency at stoichiometric. Test results for maximum operating conditions in Figure 5 show that the dual-fuel system gave consistently lower CO2 emission at maximum operating conditions throughout the entire speed range, with greater divergence at higher speeds. At the maximum operating condition, the dual-fuel gave an average of 31% less CO2 emission than diesel, as shown in Figure 5. On the other hand, the no-load operating conditions resulted in 5% reduction in CO2 emission when using the dual-fuel compared to the diesel. At the constant engine speed of 2000 rpm, dual-fuel gave lower CO2 emission at all the tested torques with an average reduction of 18% compared to diesel. The results obtained in most cases returned better than theoretical reductions. This could be due to low conversion from CO to CO2, but the above results of CO would negate this. Another reason for the extra low reduction in CO2 emissions could be inefficient combustion due to a high equivalence ratio of the straight diesel system, resulting in higher CO and CO2 emissions. The foregoing results would tend to support this position.

Figure 5. CO2 emissions as a function of engine speed for dual-fuel and diesel at maximum-load operating conditions operating conditions [29].

CONCLUSION The present study considered the performance characteristics of a four cylinders hydrogen fuelled internal combustion engine with hydrogen being injected directly in the cylinder. The following conclusions are drawn: 1. At very lean conditions with low engine speeds, acceptable BMEP can be reached, while it is unacceptable for higher speeds. Lean operation leads to small values of BMEP compared with rich conditions. 2. Maximum brake thermal efficiency can be reached at mixture composition in the range of ( =0.9 to 0.8) and it decreases dramatically at leaner conditions. 3. The desired minimum BSFC occurs within a mixture composition range of ( = 0.7 to 0.9). The operation with very lean condition ( 4500) consumes unacceptable amounts of fuel. A CNG–air Venturi mixer for a CNG–diesel dual-fuel stationary engine has been designed using 3D CFD analysis. Venturi mixers with four and eight holes have been simulated. It was found that the 8-hole Venturi mixer gives better mixing performance than the 4-hole mixer. This study shows that the conversion of an existing small single-cylinder stationary diesel engine to a dual-fuel system can be done with minimum modification. Tests showed that the emission of CO2 consistent with theory was less for dual-fuel than for diesel at all operating conditions. Maximum-load operating conditions yielded the best results, with 31%, compared to only 5% at no-load. At maximum load, both NOx and CO emissions were less for dual-fuel compared to diesel by 54% and 59% respectively. However, at no-load operating conditions tests showed that diesel has an advantage over the dual-fuel. It has been convincingly demonstrated that a power generator CI engine fitted with a CNG dual-fuel system will have a positive impact on its environment due to a sizeable reduction in exhaust emissions with little or no loss of performance. The Venturi mixer system was proven to be a simple but effective method of converting a diesel engine generation set to dual- fuel. The effect of the amount of hydrogen addition on the fuel consumption and emission of DI has been studied. Overall, the benefits of hydrogen are significant since these results show that the diesel fuel with hydrogen additive can be further accomplished as there is plenty of room for continuous improvement on performances and emissions. ACKNOWLEDGMENT The authors would like to express their deep gratitude to Universiti Malaysia Pahang (UMP) for provided the laboratory facilities and knowledge support. REFERENCES 1

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