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ELECTRONIC FUEL INJECTION SYSTEM
1. INTRODUCTION
In developed and developing countries considerable emphasis is being laid on the minimization of pollutants from internal combustion engines. A two-stroke cycle engine produces a considerable amount of pollutants when gasoline is used as a fuel due to short-circuiting. These pollutants, which include unburnt hydrocarbons and carbon monoxide, which are harmful to beings. There is a strong need to develop a kind of new technology which could minimize pollution from these engines.
Direct fuel injection has been demonstrated to significantly reduce unburned hydrocarbon emissions by timing the injection of fuel in such way as to prevent the escape of unburned fuel from the exhaust port during the scavenging process.
The increased use of petroleum fuels by automobiles has not only caused fuel scarcities, price hikes, higher import bills, and economic imbalance but also causes health hazards due to its toxic emissions. Conventional fuels used in automobiles emit toxic pollutants, which cause asthma, chronic cough, skin degradation, breathlessness, eye and throat problems, and even cancer.
In recent years, environmental improvement (CO2, NOx and Ozone reduction) and energy issues have become more and more important in worldwide concerns. Natural gas is a good alternative fuel to improve these problems because of its abundant availability and clean burning characteristics.
1.1. The objectives of present study are:
To compare the performance of a carbureted and injected engine at constant speed. Direct injection system was developed which eliminates short circuiting losses completely and injection timing was optimized for the best engine performance and lower emissions.
In a lean burn engine, air fuel ratio is extremely critical. Operation near the lean mixture limit is necessary to obtain the lowest possible emission and the best fuel economy. However, near the lean limit, a slight error in air-fuel ratio can drive the engine to misfire. This condition causes drastic increase in hydrocarbon emission; engine roughness and poor throttle response.
A reliable electronic gaseous fuel injection system was designed and built in order to control the engine and also for the evaluation of control strategies. The electronic control unit is used to estimate the pulse width of the signal that would actuate the fuel injector and the start of fuel injection. The experiments were carried out on the engine using state-of-art instrumentation.
2. COMPRESSED NATURAL GAS
The search of alternative fuels is becoming a major concerned worldwide. This is due to few obvious reasons; an increased of oil price, a declining trend of oil production globally, health-issues due to pollution and an alarming global climate change.
One of the most affected industries due this current situation is an automotive sector. Automotive sector is one of the highest contributions to pollution and global warming. Thus, the demand for alternative fuel is always a burning issue.
Natural gas engine has been acknowledged for having an almost zero emission but lacked of power and torque as reported by many researchers worldwide. However, to keep the output power and torque of natural gas engine comparable to gasoline or diesel counterparts, a high boost of pressure should be used.
In terms of exhaust emission reduction, high activity of catalyst for methane oxidation and lean NOx system or three-way catalyst with precise air-fuel ratio control strategies should be developed to meet future stringent emission standards.
Malaysia Energy Centre (PTM) has estimated that in year 2030, there major industries that contributes to 2 CO emission; (i) electricity 49%, (ii) transportation 28% and (ii) industry 20%. The percentage indicates the automotive industry will be the second biggest contributor towards global emission.
A study conducted by World Energy Council, (WEC) on the energy demand and the availability of energy has shown that the trend of natural gas consumption estimated to increase in contrast to oil consumption during the period of 2005 to 2025. The ratio of natural gas reservation to production worldwide estimated to be 62 years, compared to oil ratio, 38 years as depicted in Figure 1 .
Natural gas is regarded as one of the most promising alternative fuels and probably the cleanest fuels. Natural gas vehicles (NGVs) typically offergreenhouse gas reductions as much as 30%. With climate change now firmly on the agenda at all levels of society, NGVs offer an economic and environment advantage unmatched by any other fuel. It also has been noted that is has high-octane value, a good coldstarting characteristics that cause less wear on engine and the vehicle can be fueled at home,.
The objective of this study is to examine the potential of CNGDI with two shapes of piston crown (i) homogenous (ii) stratified. The outcome of this initial experiment will be used as a benchmark of multi-cylinder engine of CNGDI.
3. DIRECT INJECTION PROCESS
Spark ignition engine, uses more volatile gasoline fuel, has compression ratios between 8 and 11 while the compression-ignition requires typically 18 to 21 to accomplish its combustion process. The compressed ignition engine uses heating air during compression stroke to vaporize a liquid fuel and heat the vapor to its self-ignition temperature. Such heating processes take time.
A practical mechanism that can be used to speed the process up is to raise the heat transfer rate between the hot air and the cold fuel droplets by increasing the relative velocity between these two fluids. This presents two options (1) either have fuel droplets that move rapidly through the air, or (2) have slower-moving fuel whirled by faster-moving air. The first option gives rise to the direct-injection and the second to the indirect type.
One significant characteristic of natural gas is the flammability limit as tabulated in Table 1. Natural gas has high flammability limit compares to gasoline and diesel, thus requires high energy of spark ignition for combustion.
5. FUEL INJECTION TECHNIQUES
The performance characteristics of an engine and the concentration level of the exhaust emissions depend, to a large extent, on the combustion pattern. It directly depends on fuel system, which provides an appropriate mixture of fuel and air to the engine at the appropriate point in the cycle. The fuel air mixture must be in right proportion as per the condition of the speed and load on the engine.
The overall engine behavior depends upon the fuel induction mechanism. Introduction of a CNG kit to the existing gasoline engine hardware does not involve any substantial modifications except inducting the mixture into the intake manifold.
However, in spite of the excellent characteristics and various advantages of CNG as a fuel in vehicles, it has certain problems, when used in vehicles, like backfiring during suction, knocking at higher compression ratio with advanced spark timing; these problems are due to inappropriate technology used for the formation of the mixture.
In consideration of the inherent constraints in the design of carburetor, the engine manufacturers and automobile industries now are switching over to fuel injection system. The mode of fuel injection from an injector plays a critical role in determining the performance characteristics of an engine.
The lean burn for the engine operation can be easily achieved with this technique. Keeping in view the requirement of the CNG fuel, an electronic direct CNG injection system was designed and developed in the present experimental work.
6. NEW DIRECT CNG INJECTION SYSTEM
The short-circuiting losses of the two-stroke engine can be eliminated by directly injecting the fuel into the cylinder after the closure of the exhaust port. This requires the development of an electronically controlled direct fuel injection system fitted with suitable modification to the engine.
The Figure 1 shows the cylinder wall injection, with an injection nozzle installed in the cylinder wall. The injection nozzle was tilted by 400 from the horizontal and injects the fuel upward, different from the method of injecting the fuel at a right angle to the cylinder axis as employed by Vieillendent [5], Blair [6], etc. The spray would be concentrated on the upper position of the combustion chamber near the spark plug.
The location of the nozzle on the cylinder was determined from the pressure crank angle diagram corresponding to an in-cylinder pressure of 2 bar attained after the closure of the exhaust port. Corresponding to this crank angle a hole is drilled in the cylinder bore at an inclination of 400 from horizontal. A water-cooled adaptor was designed for cooling the injector to prevent excess heating of the injector.
The Figure 2 shows the schematic diagram of experimental setup and the engine instrumentation. The fuel system consists of high-pressure storage cylinders with a filling pressure of about 22 MPa, a regulator to reduce the line pressure to 200 kPa and a high-pressure line for connecting the cylinder to the regulator.
An injection system controller was used to control the pulse width of the injector. The engine was connected to brake dynamometer for loading purposes. Fuel consumption is measured using weighing machine and rotameter. Air consumption is measured by an air flow meter.
A pressure transducer in conjunction with a charge amplifier was used to measure the cylinder pressure. The transducer was mounted in the cylinder head. Signals of crankshaft angle were derived from a shaft encoder rigidly attached to the engine crankshaft. The top dead center (TDC) signal of the encoder was checked with the engine TDC, under dynamic conditions.
The encoder provides the necessary signals for the data acquisition system to collect cylinder pressure at every degree during engine cycle. The exhaust emissions of HC and CO are measured with an exhaust gas analyzer. The performance testing of the engine is carried out at constant speed.
8. RESULTS
8.1 Performance of Direct CNG Injection
The performance of the direct injection system was tested at 3500 rpm. The injection timing was initially set such that the start of injection takes place immediately after the closure of exhaust port (2500 ATDC). The crank position sensor sends a signal corresponding to the closure of the exhaust port and then the injection is immediately started. The injection timing was optimized for the best engine performance and emissions.
Any further advancement of the injection timing above the optimum injection angle results in poor performance and higher emissions as short-circuiting losses predominate. The performance of direct injection system with the optimized injection advance angle is compared with carbureted engine at a compression ratio of 12:1.
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