26-10-2016, 04:13 PM
EFFECT OF INJECTION PARAMETERS ON PERFORMANCE, COMBUSTION AND EMISSION CHARACTERISTICS OF DIESEL ENGINE WITH ALTERNATE FUELS – A REVIEW
[attachment=73961]
ABSTRACT
According to the current progress engine performance improvement and exhaust emissions reduction are the two most important issues to develop a high efficient engine with low environmental impact. For a diesel engine, injection timing and injection pressure are one of the major parameters. In this review the type of injection on engine performance and emissions studied, and characterizing influence of injection timing on engine performance and exhaust emissions have been reviewed. In diesel fuel, advancement in injection timing results in lower carbon monoxide (CO) and hydrocarbon (HC) emission; though it rises nitrogen oxides (NOx) emission. Advance injection timing helps to increases brake thermal efficiency (BTE) and reduces brake specific fuel consumption (BSFC). And also injection pressures in diesel engine plays an important part for engine performance obtaining treatment of combustion. The present diesel engines the fuel direct injection, the pressures can be increased to 100 – 200 Mpa bar in fuel pump injection system. The experimental proof effects of fuel injection pressure on engine performance. The injection pressure was varied by changing the fuel injector spring system. The performance and emission characteristics were presented graphically and finalized that increase in injector opening pressure increases the brake thermal efficiency
Keywords: injection timing, injection pressure, bmep, bsfc
1. INTRODUCTION
One of the most efficient energy conversion devices is diesel engines. The fuel injection is the most vital component in the working of CI engines. The engine performance such as power output, economy etc is greatly dependent on the effectiveness of the fuel injection system.[1-5] The injection system has to perform the important duty of initiating and controlling the combustion process. However they have one serious drawback: the amount of exhaust emissions from the products of combustion like NOx and particulate matter (PM) is comparatively larger than that of gasoline engines.[7] The emissions formed are dependent upon the engine design, power output and load. The complete combustion of fuel leads to major reductions in the formation of the exhaust emissions. Complete combustion is a result of careful matching of air-fuel mixture and accuracy in the injection process.
2. REQUIREMENTS OF AN INJECTION SYSTEM
In a CI engine, for good efficiency and performance, the following requirements should have in the injection system:
• Accurate metering of the fuel injected per cycle. This is very critical due to the fact that very small quantities of fuel being handled.[10]
• The quantity of the fuel metered should vary to meet changing speed and load requirements of the engine.
• Timing the injection of the fuel correctly in the cycle so that maximum power is obtained ensuring fuel economy and clean burning[9]
• Proper control of rate of injection so that the desired heat release pattern is achieved during combustion.
• Proper atomization of fuel into very fine droplets
• Proper spray pattern to ensure rapid mixing of fuel and air
• Uniform distribution of fuel droplets in the combustion chamber
• To supply equal quantities of metered fuel to all cylinders in case of multi cylinder engines
2. PERFORMANCE AND EMISSION ANALYSIS.
For critical analysis the performance of an IC engine under all the conditions of load and speed is shown by a performance map.
In CI engine the bsfc increase at high loads owing to the increased fuel waste associated with high fuel air ratios. At lower load bsfc increases due to decrease in mechanical efficiency.[14] As the speed is reduced from the point of best economy along a line of constant bmep, the mechanical and indicated thermal efficiency appears to remain a constant down to the lowering operating speed. The reduction in fmep with speed is apparently balanced by a reduction is indicated thermal efficiency due to poor spray characteristics at very low speeds. An interesting feature of the performance curves is that they show the power at maximum economy is about half of the maximum power
FUEL INJECTION PARAMETERS
Fuel injection timing essentially controls the crank angle at which combustion starts[19]. While the state of air into which the fuel is injected as injection timing is varied, and thus ignition delay will vary, these effects are predictable[20]. The fuel injection rate, fuel nozzle design and fuel injection pressure all affect the characteristics of the diesel fuel spray and it’s mixing with air in the combustion chamber
The effect on the performance and emissions of varying injection showed, in a) a medium swirl DI diesel engine and b) an IDI engine. At fixed speed and constant fuel delivery per cycle, the DI engine shows an optimum bsfc and bmep at a specific start of injection for a given injection duration. IDI engine experiments are at fixed bmep, bsfc at full load and fueling rate at idle show a minimum at specific injection timings.
3.1.2.1 INJECTION TIMING
Injection timing plays a vital role in combustion process and pollutant formation. The injection timing affects the ignition delay because the air temperature and pressure change significantly close to TDC. As advancing the injection timing, the initial air temperature and pressure are lower so ignition delay will increase while retarding the injection timing i.e. closer to TDC when air temperature and pressure are slightly higher, results in shorter ignition delay [21-22].
However, retarding or advancing injection timing beyond certain limits which varies from engine to engine may result in poor combustion. Injection timing variations also have strong effect on NOx formation. Retarding injection timing may help to control NOx emissions with substantial penalty in fuel consumption, and also increasing unburned hydrocarbons, smoke and particulate emissions Therefore, finding optimum injection timing for best performance and lesser emissions are required. Injection timing which is more advanced than this optimum results in combustion starting too early before TC
Swirl chamber IDI engine
Injection timing variations have a strong effect on NOx emissions for DI engines; the effect is significant but less for IDI engines. Retarded injection is commonly used to help control NOx emissions. It gives substantial reductions, initially with only modest bsfc penalty[23]. For the DI engine, at high load, specific HC emissions are low and vary only modestly with injection timing. At lighter loads, HC emissions are higher and increase as injection becomes significantly retarded from optimum. This trend is especially known as idle. For IDI diesel engine HC emissions show the same trends but are much lower in magnitude than DI engine HC emissions.
Retarding timing generally increase smoke, though trends vary significantly between different types and designs of diesel engine. Mass articulate emissions increase as injection is retarded. The injection rate depends on the fuel injector nozzle area and injection higher heat release rates[24,25]. For a amount of fuel injected per cylinder per cycle, as the injection rate id increased the optimum injection timing moves closer to TC. The effects of injection rate and timing on bsfc in a naturally aspirated DI diesel engine are shown.
The higher heat release rates and shorter overall combustion process that result from the increased injection rate decreases the minimum bsfc at optimum injection timing: however, a limit to these benefits is eventually reached.
Increasing the injection rate increases NOx emissions and decrease smoke or particulate emissions. The controlling physical process is the rate of fuel-air mixing in the combustion chamber so, at constant fuel injected per cylinder per cycle, both increased injection pressure at fixed nozzle orifice area and reduced nozzle area at fixed injection duration produce these trends.[26]
Smoke is plotted versus NOx for a range of speed, loads, injection timing, injection pressure, and EGR rates. This indicates that for well optimized DI diesel engine, the smoke nitric oxide tradeoff is relatively independent of engine speed, injection rate, injection timing and amount of EGR [27-30] A given reduction in one of these pollutants through changing any one of these variables results in a given increase in the other pollutant. This tradeoff exists for essentially all types of diesel engine, though the magnitude depends on engine details.
3.1.2.2 INJECTION PRESSURE
In present diesel engines, fuel injection systems have designed to obtain higher injection pressure. So, it is aimed to decrease the exhaust emissions by increasing efficiency of diesel engines. When fuel injection pressure is low, fuel particle diameters will enlarge and ignition delay period during the combustion will increase. This situation leads to increase pressure. Engine performance will be decrease since combustion process goes to a bad condition. When injection pressure increased of fuel particle diameters will become small. Since formation of mixing of fuel to air becomes better during ignition period, engine performance will be increase [31] If injection pressure is too higher, ignition delay period becomes shorter. Possibilities of homogeneous mixing decrease and combustion efficiency falls down. The fuel injection system in a direct injection diesel engine is to achieve a high degree of atomization in order to enable sufficient evaporation in a very short time and to achieve sufficient spray penetration in order to utilize the full air charge [32-34]. The fuel injection system must be able to meter the desired amount of fuel, depending on engine speed and load, and to inject that fuel at the correct time and with the desired rate.
AIR SWIRL AND BOWL IN PISTON DESIGN
Increasing amount of air swirl within the cylinder are used in DI diesel engines, as engine size decreases and maximum engine speed increases, to achieve adequately fast fuel air mixing rates[35]. In the medium to small size engines, use of a bowl-in-piston combustion chamber results in the substantial swirl amplification at the end of the compression process
EFFECTS OF FUEL INJECTION PARAMETERS ON DIFFERENT FUEL
The effect of fuel injection parameters such as
i) Injection timing
ii) Injection pressure
on different fuel are as follow:
4.1 EFFECT OF INJECTION TIMING ON DIFFERENT FUEL
Based on the study about the alternate fuels, in diesel engine the biodiesel gives excess of NOx emissions at same engine conditions due to various factors. From biodiesel, one of the factors for increase in NOx emissions from biodiesel is because of its properties like high viscosity, isentropic bulk modulus and lower compressibility factor. By varying injection timing from its default values has been proved to improve performance and emissions [43]. Many authors studied about the effects of fuel injection timing on
With retarded injection timing using Pongamia methyl ester it was possible to achieve low amount of NOx, HC and CO emissions with very less effect in fuel consumption rate.[1] Similarly based on the current trends it observed with BTE and exhaust gas temperature. However in-cylinder gas pressure and ignition delay was reduced [45,46]
With the advancing in injection timing of diesel engine with Jatropha oil by 31, it results in increased BTE and reduced HC and smoke were observed when compared to original injection timing of diesel,[2]
With advancing in injection timing in diesel engine using Jatropha methyl ester, from default value it has been observed that results in considerable reduction in BSFC, CO, HC and smoke levels and an increase in BTE, maximum in-cylinder pressure, increase in heat transfer rate and NOx emission. However retarded injection timing caused effects in the other way, hence optimum balance between the performance and emission of Jatropha methyl ester is compared to diesel at optimum injection timing [3,47]
With three different injection timing were preferred such as 151, 201 and 251 bTDC, using the bio diesel-blended diesel fuel in CI engine. When it is compared to actual advancing and retarding injection timing, the original injection timing of 201 bTDC gave best results of BSFC, BSEC and BTE . Since advance injection timing gives more time for carbon oxidation, led to lesser smoke, HC and CO, whereas by retarding the injection timing NOx emissions were reduced because of lower combustion temperature in the cylinder [48,49]
When injection timing was advanced with Pongamia biodiesel-diesel blends in CI engine,and the best performance parameters were obtained at 211 Btdc[47-50] and from 181 to 301 bTDC, so that the level of CO, HC, and smoke emissions were reduced with an increase in NOx emissions.
With the advancing injection timing in CI engine using Algal oil methyl ester (AOME). it could improve BSFC and reduce HC, CO and smoke with increase in rate of heat transfer , BMEP and NOx, while during retarding it results in very less amount of increase in heat release rate, BMEP and NOx emissions with increase in BSFC, HC, CO and smoke. Finally they found it can be done with better combustion and performance with minimal emissions at optimum injection timing to be 340 CAD [6]
With using blend of Mahua methyl ester with diesel fuel when injection timing was advanced, it results that there was rapid decrease in BSFC and EGT and an increase in BTE to meet the performance in par [7]
With the advancing in injection timing in CI engine using dual fuel to reduce emissions and to improve the thermal efficiency at lower loads, thermal efficiency was improved with some increase in the NOx emissions and reduction in CO and HC. [51-53]
However at medium and high loads advancing injection timing led to early knocking. Therefore at medium and high loads, the advanced timing of pilot fuel in dual fueled engines was not useful [8-15]
4.2 EFFECT OF INJECTION PRESSURE ON DIFFERENT FUEL
According to general information, for better results in air fuel mixing it causes an earlier start of combustion because of improved atomization, when the injection pressure is increased. As a result of this, during combustion the cylinder charge gets compressed if the piston moves towards top dead center (TDC) results in relatively higher temperatures in which it facilitates increase in NOx formation and decrease the HC formation due to high premixed rate of heat transfer. Similarly engine power output gets improved, resulting in better BSFC due to lower ignition delay period at high injection pressure and on contrary lower injection pressure results in poor BSFC due to longer ignition delay because of deteriorated atomization and mixing process
Based on the results of many review papers studied about the effects of fuel injection pressure for performance and emission characteristics on diesel engine some are as follows:
By varying the lower injection pressures to 100, 150, 200, 250 bar they found that the effect at higher pressure of 250 bar, NOx was increased and smoke was decreased and also found from the engine there is considerable increase in torque and power output[9-12,54-57]
In IDI engine the effect of injection pressure on engine performance and exhaust emissions was studied. They varied the injection pressure from 101 to 255 bar in steps of 50 bar. As a result they found that 150 bar injection pressure results in increase of performance, after which the performance become progressively worse. [58]
This may due to the fact that for IDI engine, high fuel injection pressure may lead to increase of burnt fuel in the swirl chamber/prechamber, which results in reduction in output power. They also found that higher injection pressure reduces SO2, CO2 and O2 while lower injection pressure was preferred for reducing NOx and smoke emissions [10-14, 58-61]
Effect of fuel injection pressure on diesel engine fueled with biodiesel-diesel fuel blends. They examined the effect on performance and exhaust emissions at four different injection pressures 180, 200, 220, 240 bar. At higher percentage biodiesel-diesel blends BSFC decreased with increased injection pres-sure, smoke, HC and CO also increased but CO2, O2 and NOx decreased[11,62]
According to an author, the impact of injection pressure on performance and emission of biodiesel-diesel blends results in increased injection pressure for BSFC, BTE and BSEC compared to original and reduced injection pressures. It caused when fuel droplets are breakup to fine droplets which provided increased surface area and better mixing with air leading to better combustion. Increase in injection pressure for all fuel blends results in reduction of smoke, HC and CO decreased while NOx emissions were increased [63-70]
With methanol blended diesel fueled in CI diesel engine, the effect of injection pressure on the performance and emission characteristics is studied. They carried out by varying three different injection pressures 185, 205, 225 bar to study its effect on four different loads 10, 15 and 20 Nm at constant engine speed of 2200 rpm.[69-72]
The results showed that with increase in injection pressure, increase in NOx and CO2 with decrease in smoke, CO and HC. On contrary at original injection pressure of 200 bar the performance parameters like BSFC, BSEC and BTE were better and gets relatively worse on either increased or decreased injection pressure and also the peak in-cylinder pressure and rate of heat transfer increases with increase in injection pressure. This may due to the fact that the fuel injection timing is predicted and fixed for the injection pressure of 200 bar, it may lead to an earlier combustion before TDC due to high fuel injection pressure, resulting in a higher peak in-cylinder pressure. In another words, to get the increased fuel injection pressure the fuel injection timing can be retarded [13].
On diesel engine fueled with orange skin powder diesel solution the effect of injection pressure on heat release rate and emissions are studied. For the study they done for three different injection pressures as 210, 230 and 250 bar. For injection pressure of 230 bar, they obtained increase in peak cylinder pressure, BTE and peak heat release rate, and NOx also increased by 26%, whereas there is decrease in CO, HC and smoke by 39, 66 and 27% respectively for 30% orange skin powder solution compared with diesel fuel[14,15]
Based on the study of effect of injection pressure on high linolenic linseed oil methyl ester fueled diesel engine, the thermal efficiency and BSFC is improved at injection pressure of 240 bar, accompanied with decrease in CO, smoke and HC but slight increase in NOx emissions[75-77]
Effect of injection pressure on Jatropha methyl ester fuelled engine was studied, in that they chose three injection pressures 150, 200, 250 bar for their analysis. It was found that at injection pressure of 250 bar, BTE was improved by 8.89%, with decrease in HC and smoke compared to the base injection pressure
EMISSIONS AND THEIR CONTROL IN CI ENGINE
In CI engines the catalytic converters are being tried but are not efficient at reducing NOx due their overall lean operation. HC and CO can be adequately reduced, although there is greater difficulty because of the cooler exhaust gases of a CI engine. This is counter balanced by the fact that less HC and CO are generated in the lean burn of the CI engine. NOx is reduced in a CI engine by the use of EGR, which keeps the maximum temperature down. EGR and lower combustion temperatures, however contribute to an increase in solid soot [80]
Platinum and palladium are the two main catalyst materials used for converters on CI engines. They promote the removal of 30-80% of the gaseous HC and 40-90% of the CO in the exhaust. The catalysts have little effect on solid carbon soot but do remove 30-60% of the total particulate mass by oxidizing a large percent of the HC absorbed on the carbon particles.[82-84] Diesel fuel contains sulphur impurities, and this leads to poisoning of the catalyst materials. However, this problem is getting minimized as legal levels of sulphur in diesel fuels continue to be lowered.
5.1 REDUCING EMISSIONS BY CHEMICAL METHODS
In order to reduce NOx emission, some work has been done on large stationary engines using cyanuric acid. Cyanuric acid is a low cost solid material that sublimes in the exhaust flow in the exhaust flow.[81-83] The gas dissociates, producing iso-cyanide that reacts with NOx to form N2, H2O and CO2. Operating temperature is about 500ºC.[84-88] Up to 95% NOx reduction can be achieved with no loss of engine performance. At present, this system is not practical for automobile engines because of its size, weight and complexity.
5.1.1AMMONIA INJECTION SYSTEMS:
Some large marine engines and stationary engines reduce emissions with an injection system that sprays NH3 into the exhaust flow[85-87]. In the presence of a catalyst, the following reactions occur
4NH3 + 4NO +O2 → 4N2 + 6H2O
6NO2 + 8NH3 → 7N2 + 12H2O
Careful control must be adhered to, as NH3 itself is an undesirable emission.
6. CONCLUSION
From the study about the effect of injection parameters in performance, combustion and emission of diesel engine it is more concern about emission control in diesel engine by different fuel injection strategies. The strategies covered in this review are varying injection pressure, injection timing etc. Researchers have carried out many experimental works to study the effect of injection strategies on engine performance and emission formation. The table gives an insight on the effect of various strategies on performance and emissions achieved experimentally. Some of the prominent points showing the effect of above listed strategies on engine performance and emissions for diesel and other bio-fuel are listed below:
• Increasing injection pressure, in general, results in increased thermal efficiency and better fuel consumption and less CO, HC and smoke emissions, however with higher NOx.
• Advanced injection timing results in increased NOx while reduced fuel consumption, and emissions like CO, HC and smoke, although advancing beyond certain limit may result in high smoke and poor performance. Combined with high injection pressures may lead to reduced particle number concentration
• Similarly retarding injection timing results in reduced NOx, while increasing other emissions such as CO, HC and smoke and also deteriorates fuel consumption.
• In general, an optimized timing has to be found for any engine and fuel to strike a balance between performance and emissions
[attachment=73961]
ABSTRACT
According to the current progress engine performance improvement and exhaust emissions reduction are the two most important issues to develop a high efficient engine with low environmental impact. For a diesel engine, injection timing and injection pressure are one of the major parameters. In this review the type of injection on engine performance and emissions studied, and characterizing influence of injection timing on engine performance and exhaust emissions have been reviewed. In diesel fuel, advancement in injection timing results in lower carbon monoxide (CO) and hydrocarbon (HC) emission; though it rises nitrogen oxides (NOx) emission. Advance injection timing helps to increases brake thermal efficiency (BTE) and reduces brake specific fuel consumption (BSFC). And also injection pressures in diesel engine plays an important part for engine performance obtaining treatment of combustion. The present diesel engines the fuel direct injection, the pressures can be increased to 100 – 200 Mpa bar in fuel pump injection system. The experimental proof effects of fuel injection pressure on engine performance. The injection pressure was varied by changing the fuel injector spring system. The performance and emission characteristics were presented graphically and finalized that increase in injector opening pressure increases the brake thermal efficiency
Keywords: injection timing, injection pressure, bmep, bsfc
1. INTRODUCTION
One of the most efficient energy conversion devices is diesel engines. The fuel injection is the most vital component in the working of CI engines. The engine performance such as power output, economy etc is greatly dependent on the effectiveness of the fuel injection system.[1-5] The injection system has to perform the important duty of initiating and controlling the combustion process. However they have one serious drawback: the amount of exhaust emissions from the products of combustion like NOx and particulate matter (PM) is comparatively larger than that of gasoline engines.[7] The emissions formed are dependent upon the engine design, power output and load. The complete combustion of fuel leads to major reductions in the formation of the exhaust emissions. Complete combustion is a result of careful matching of air-fuel mixture and accuracy in the injection process.
2. REQUIREMENTS OF AN INJECTION SYSTEM
In a CI engine, for good efficiency and performance, the following requirements should have in the injection system:
• Accurate metering of the fuel injected per cycle. This is very critical due to the fact that very small quantities of fuel being handled.[10]
• The quantity of the fuel metered should vary to meet changing speed and load requirements of the engine.
• Timing the injection of the fuel correctly in the cycle so that maximum power is obtained ensuring fuel economy and clean burning[9]
• Proper control of rate of injection so that the desired heat release pattern is achieved during combustion.
• Proper atomization of fuel into very fine droplets
• Proper spray pattern to ensure rapid mixing of fuel and air
• Uniform distribution of fuel droplets in the combustion chamber
• To supply equal quantities of metered fuel to all cylinders in case of multi cylinder engines
2. PERFORMANCE AND EMISSION ANALYSIS.
For critical analysis the performance of an IC engine under all the conditions of load and speed is shown by a performance map.
In CI engine the bsfc increase at high loads owing to the increased fuel waste associated with high fuel air ratios. At lower load bsfc increases due to decrease in mechanical efficiency.[14] As the speed is reduced from the point of best economy along a line of constant bmep, the mechanical and indicated thermal efficiency appears to remain a constant down to the lowering operating speed. The reduction in fmep with speed is apparently balanced by a reduction is indicated thermal efficiency due to poor spray characteristics at very low speeds. An interesting feature of the performance curves is that they show the power at maximum economy is about half of the maximum power
FUEL INJECTION PARAMETERS
Fuel injection timing essentially controls the crank angle at which combustion starts[19]. While the state of air into which the fuel is injected as injection timing is varied, and thus ignition delay will vary, these effects are predictable[20]. The fuel injection rate, fuel nozzle design and fuel injection pressure all affect the characteristics of the diesel fuel spray and it’s mixing with air in the combustion chamber
The effect on the performance and emissions of varying injection showed, in a) a medium swirl DI diesel engine and b) an IDI engine. At fixed speed and constant fuel delivery per cycle, the DI engine shows an optimum bsfc and bmep at a specific start of injection for a given injection duration. IDI engine experiments are at fixed bmep, bsfc at full load and fueling rate at idle show a minimum at specific injection timings.
3.1.2.1 INJECTION TIMING
Injection timing plays a vital role in combustion process and pollutant formation. The injection timing affects the ignition delay because the air temperature and pressure change significantly close to TDC. As advancing the injection timing, the initial air temperature and pressure are lower so ignition delay will increase while retarding the injection timing i.e. closer to TDC when air temperature and pressure are slightly higher, results in shorter ignition delay [21-22].
However, retarding or advancing injection timing beyond certain limits which varies from engine to engine may result in poor combustion. Injection timing variations also have strong effect on NOx formation. Retarding injection timing may help to control NOx emissions with substantial penalty in fuel consumption, and also increasing unburned hydrocarbons, smoke and particulate emissions Therefore, finding optimum injection timing for best performance and lesser emissions are required. Injection timing which is more advanced than this optimum results in combustion starting too early before TC
Swirl chamber IDI engine
Injection timing variations have a strong effect on NOx emissions for DI engines; the effect is significant but less for IDI engines. Retarded injection is commonly used to help control NOx emissions. It gives substantial reductions, initially with only modest bsfc penalty[23]. For the DI engine, at high load, specific HC emissions are low and vary only modestly with injection timing. At lighter loads, HC emissions are higher and increase as injection becomes significantly retarded from optimum. This trend is especially known as idle. For IDI diesel engine HC emissions show the same trends but are much lower in magnitude than DI engine HC emissions.
Retarding timing generally increase smoke, though trends vary significantly between different types and designs of diesel engine. Mass articulate emissions increase as injection is retarded. The injection rate depends on the fuel injector nozzle area and injection higher heat release rates[24,25]. For a amount of fuel injected per cylinder per cycle, as the injection rate id increased the optimum injection timing moves closer to TC. The effects of injection rate and timing on bsfc in a naturally aspirated DI diesel engine are shown.
The higher heat release rates and shorter overall combustion process that result from the increased injection rate decreases the minimum bsfc at optimum injection timing: however, a limit to these benefits is eventually reached.
Increasing the injection rate increases NOx emissions and decrease smoke or particulate emissions. The controlling physical process is the rate of fuel-air mixing in the combustion chamber so, at constant fuel injected per cylinder per cycle, both increased injection pressure at fixed nozzle orifice area and reduced nozzle area at fixed injection duration produce these trends.[26]
Smoke is plotted versus NOx for a range of speed, loads, injection timing, injection pressure, and EGR rates. This indicates that for well optimized DI diesel engine, the smoke nitric oxide tradeoff is relatively independent of engine speed, injection rate, injection timing and amount of EGR [27-30] A given reduction in one of these pollutants through changing any one of these variables results in a given increase in the other pollutant. This tradeoff exists for essentially all types of diesel engine, though the magnitude depends on engine details.
3.1.2.2 INJECTION PRESSURE
In present diesel engines, fuel injection systems have designed to obtain higher injection pressure. So, it is aimed to decrease the exhaust emissions by increasing efficiency of diesel engines. When fuel injection pressure is low, fuel particle diameters will enlarge and ignition delay period during the combustion will increase. This situation leads to increase pressure. Engine performance will be decrease since combustion process goes to a bad condition. When injection pressure increased of fuel particle diameters will become small. Since formation of mixing of fuel to air becomes better during ignition period, engine performance will be increase [31] If injection pressure is too higher, ignition delay period becomes shorter. Possibilities of homogeneous mixing decrease and combustion efficiency falls down. The fuel injection system in a direct injection diesel engine is to achieve a high degree of atomization in order to enable sufficient evaporation in a very short time and to achieve sufficient spray penetration in order to utilize the full air charge [32-34]. The fuel injection system must be able to meter the desired amount of fuel, depending on engine speed and load, and to inject that fuel at the correct time and with the desired rate.
AIR SWIRL AND BOWL IN PISTON DESIGN
Increasing amount of air swirl within the cylinder are used in DI diesel engines, as engine size decreases and maximum engine speed increases, to achieve adequately fast fuel air mixing rates[35]. In the medium to small size engines, use of a bowl-in-piston combustion chamber results in the substantial swirl amplification at the end of the compression process
EFFECTS OF FUEL INJECTION PARAMETERS ON DIFFERENT FUEL
The effect of fuel injection parameters such as
i) Injection timing
ii) Injection pressure
on different fuel are as follow:
4.1 EFFECT OF INJECTION TIMING ON DIFFERENT FUEL
Based on the study about the alternate fuels, in diesel engine the biodiesel gives excess of NOx emissions at same engine conditions due to various factors. From biodiesel, one of the factors for increase in NOx emissions from biodiesel is because of its properties like high viscosity, isentropic bulk modulus and lower compressibility factor. By varying injection timing from its default values has been proved to improve performance and emissions [43]. Many authors studied about the effects of fuel injection timing on
With retarded injection timing using Pongamia methyl ester it was possible to achieve low amount of NOx, HC and CO emissions with very less effect in fuel consumption rate.[1] Similarly based on the current trends it observed with BTE and exhaust gas temperature. However in-cylinder gas pressure and ignition delay was reduced [45,46]
With the advancing in injection timing of diesel engine with Jatropha oil by 31, it results in increased BTE and reduced HC and smoke were observed when compared to original injection timing of diesel,[2]
With advancing in injection timing in diesel engine using Jatropha methyl ester, from default value it has been observed that results in considerable reduction in BSFC, CO, HC and smoke levels and an increase in BTE, maximum in-cylinder pressure, increase in heat transfer rate and NOx emission. However retarded injection timing caused effects in the other way, hence optimum balance between the performance and emission of Jatropha methyl ester is compared to diesel at optimum injection timing [3,47]
With three different injection timing were preferred such as 151, 201 and 251 bTDC, using the bio diesel-blended diesel fuel in CI engine. When it is compared to actual advancing and retarding injection timing, the original injection timing of 201 bTDC gave best results of BSFC, BSEC and BTE . Since advance injection timing gives more time for carbon oxidation, led to lesser smoke, HC and CO, whereas by retarding the injection timing NOx emissions were reduced because of lower combustion temperature in the cylinder [48,49]
When injection timing was advanced with Pongamia biodiesel-diesel blends in CI engine,and the best performance parameters were obtained at 211 Btdc[47-50] and from 181 to 301 bTDC, so that the level of CO, HC, and smoke emissions were reduced with an increase in NOx emissions.
With the advancing injection timing in CI engine using Algal oil methyl ester (AOME). it could improve BSFC and reduce HC, CO and smoke with increase in rate of heat transfer , BMEP and NOx, while during retarding it results in very less amount of increase in heat release rate, BMEP and NOx emissions with increase in BSFC, HC, CO and smoke. Finally they found it can be done with better combustion and performance with minimal emissions at optimum injection timing to be 340 CAD [6]
With using blend of Mahua methyl ester with diesel fuel when injection timing was advanced, it results that there was rapid decrease in BSFC and EGT and an increase in BTE to meet the performance in par [7]
With the advancing in injection timing in CI engine using dual fuel to reduce emissions and to improve the thermal efficiency at lower loads, thermal efficiency was improved with some increase in the NOx emissions and reduction in CO and HC. [51-53]
However at medium and high loads advancing injection timing led to early knocking. Therefore at medium and high loads, the advanced timing of pilot fuel in dual fueled engines was not useful [8-15]
4.2 EFFECT OF INJECTION PRESSURE ON DIFFERENT FUEL
According to general information, for better results in air fuel mixing it causes an earlier start of combustion because of improved atomization, when the injection pressure is increased. As a result of this, during combustion the cylinder charge gets compressed if the piston moves towards top dead center (TDC) results in relatively higher temperatures in which it facilitates increase in NOx formation and decrease the HC formation due to high premixed rate of heat transfer. Similarly engine power output gets improved, resulting in better BSFC due to lower ignition delay period at high injection pressure and on contrary lower injection pressure results in poor BSFC due to longer ignition delay because of deteriorated atomization and mixing process
Based on the results of many review papers studied about the effects of fuel injection pressure for performance and emission characteristics on diesel engine some are as follows:
By varying the lower injection pressures to 100, 150, 200, 250 bar they found that the effect at higher pressure of 250 bar, NOx was increased and smoke was decreased and also found from the engine there is considerable increase in torque and power output[9-12,54-57]
In IDI engine the effect of injection pressure on engine performance and exhaust emissions was studied. They varied the injection pressure from 101 to 255 bar in steps of 50 bar. As a result they found that 150 bar injection pressure results in increase of performance, after which the performance become progressively worse. [58]
This may due to the fact that for IDI engine, high fuel injection pressure may lead to increase of burnt fuel in the swirl chamber/prechamber, which results in reduction in output power. They also found that higher injection pressure reduces SO2, CO2 and O2 while lower injection pressure was preferred for reducing NOx and smoke emissions [10-14, 58-61]
Effect of fuel injection pressure on diesel engine fueled with biodiesel-diesel fuel blends. They examined the effect on performance and exhaust emissions at four different injection pressures 180, 200, 220, 240 bar. At higher percentage biodiesel-diesel blends BSFC decreased with increased injection pres-sure, smoke, HC and CO also increased but CO2, O2 and NOx decreased[11,62]
According to an author, the impact of injection pressure on performance and emission of biodiesel-diesel blends results in increased injection pressure for BSFC, BTE and BSEC compared to original and reduced injection pressures. It caused when fuel droplets are breakup to fine droplets which provided increased surface area and better mixing with air leading to better combustion. Increase in injection pressure for all fuel blends results in reduction of smoke, HC and CO decreased while NOx emissions were increased [63-70]
With methanol blended diesel fueled in CI diesel engine, the effect of injection pressure on the performance and emission characteristics is studied. They carried out by varying three different injection pressures 185, 205, 225 bar to study its effect on four different loads 10, 15 and 20 Nm at constant engine speed of 2200 rpm.[69-72]
The results showed that with increase in injection pressure, increase in NOx and CO2 with decrease in smoke, CO and HC. On contrary at original injection pressure of 200 bar the performance parameters like BSFC, BSEC and BTE were better and gets relatively worse on either increased or decreased injection pressure and also the peak in-cylinder pressure and rate of heat transfer increases with increase in injection pressure. This may due to the fact that the fuel injection timing is predicted and fixed for the injection pressure of 200 bar, it may lead to an earlier combustion before TDC due to high fuel injection pressure, resulting in a higher peak in-cylinder pressure. In another words, to get the increased fuel injection pressure the fuel injection timing can be retarded [13].
On diesel engine fueled with orange skin powder diesel solution the effect of injection pressure on heat release rate and emissions are studied. For the study they done for three different injection pressures as 210, 230 and 250 bar. For injection pressure of 230 bar, they obtained increase in peak cylinder pressure, BTE and peak heat release rate, and NOx also increased by 26%, whereas there is decrease in CO, HC and smoke by 39, 66 and 27% respectively for 30% orange skin powder solution compared with diesel fuel[14,15]
Based on the study of effect of injection pressure on high linolenic linseed oil methyl ester fueled diesel engine, the thermal efficiency and BSFC is improved at injection pressure of 240 bar, accompanied with decrease in CO, smoke and HC but slight increase in NOx emissions[75-77]
Effect of injection pressure on Jatropha methyl ester fuelled engine was studied, in that they chose three injection pressures 150, 200, 250 bar for their analysis. It was found that at injection pressure of 250 bar, BTE was improved by 8.89%, with decrease in HC and smoke compared to the base injection pressure
EMISSIONS AND THEIR CONTROL IN CI ENGINE
In CI engines the catalytic converters are being tried but are not efficient at reducing NOx due their overall lean operation. HC and CO can be adequately reduced, although there is greater difficulty because of the cooler exhaust gases of a CI engine. This is counter balanced by the fact that less HC and CO are generated in the lean burn of the CI engine. NOx is reduced in a CI engine by the use of EGR, which keeps the maximum temperature down. EGR and lower combustion temperatures, however contribute to an increase in solid soot [80]
Platinum and palladium are the two main catalyst materials used for converters on CI engines. They promote the removal of 30-80% of the gaseous HC and 40-90% of the CO in the exhaust. The catalysts have little effect on solid carbon soot but do remove 30-60% of the total particulate mass by oxidizing a large percent of the HC absorbed on the carbon particles.[82-84] Diesel fuel contains sulphur impurities, and this leads to poisoning of the catalyst materials. However, this problem is getting minimized as legal levels of sulphur in diesel fuels continue to be lowered.
5.1 REDUCING EMISSIONS BY CHEMICAL METHODS
In order to reduce NOx emission, some work has been done on large stationary engines using cyanuric acid. Cyanuric acid is a low cost solid material that sublimes in the exhaust flow in the exhaust flow.[81-83] The gas dissociates, producing iso-cyanide that reacts with NOx to form N2, H2O and CO2. Operating temperature is about 500ºC.[84-88] Up to 95% NOx reduction can be achieved with no loss of engine performance. At present, this system is not practical for automobile engines because of its size, weight and complexity.
5.1.1AMMONIA INJECTION SYSTEMS:
Some large marine engines and stationary engines reduce emissions with an injection system that sprays NH3 into the exhaust flow[85-87]. In the presence of a catalyst, the following reactions occur
4NH3 + 4NO +O2 → 4N2 + 6H2O
6NO2 + 8NH3 → 7N2 + 12H2O
Careful control must be adhered to, as NH3 itself is an undesirable emission.
6. CONCLUSION
From the study about the effect of injection parameters in performance, combustion and emission of diesel engine it is more concern about emission control in diesel engine by different fuel injection strategies. The strategies covered in this review are varying injection pressure, injection timing etc. Researchers have carried out many experimental works to study the effect of injection strategies on engine performance and emission formation. The table gives an insight on the effect of various strategies on performance and emissions achieved experimentally. Some of the prominent points showing the effect of above listed strategies on engine performance and emissions for diesel and other bio-fuel are listed below:
• Increasing injection pressure, in general, results in increased thermal efficiency and better fuel consumption and less CO, HC and smoke emissions, however with higher NOx.
• Advanced injection timing results in increased NOx while reduced fuel consumption, and emissions like CO, HC and smoke, although advancing beyond certain limit may result in high smoke and poor performance. Combined with high injection pressures may lead to reduced particle number concentration
• Similarly retarding injection timing results in reduced NOx, while increasing other emissions such as CO, HC and smoke and also deteriorates fuel consumption.
• In general, an optimized timing has to be found for any engine and fuel to strike a balance between performance and emissions