Overview
1.3 Fuel Modification Technique
Compression ignition engines are considered as prime movers in light, medium and heavy duty applications such as automobiles, power plants, marine and industrial sectors due to their reliable operation i.e., lower fuel consumption, and better power performance. The fossil fuels are very limited and may exhaust in coming few decades. In addition, the utilization of fossil fuels is increasing at faster rate due to the population growth and living advancements. Furthermore, the scientific community searches for alternative fuels those are renewable, safe and non-polluting.
The renewable fuels such as vegetable oils of methyl ester biodiesel oils are an alternative to petroleum based fuels. However, the problems like high viscosity and density, and poor volatility of the biodiesel oils put obstacle on the end users. Fuel modification technique is being used by various researchers to gain specific fuel properties so as to improve performance and reduce the exhaust emissions of diesel engine. The impact of blending biodiesel with the diesel and preheating biodiesel and blended with diesel are one of the current scopes of research with regards to the fuel modification techniques. Intensive research is underway to separately to utilize the methyl ester of biodiesel judiciously without affecting our ecological environment. So far, much attention has been paid to biodiesel blend with diesel for many years. The majority of blends found in different countries are BD5− BD20.whereas, on preheating only few studies are available.
Initially, it is required to investigate the fuel properties of diesel, JOME, COME and POME, including the cetane number, density, kinematic viscosity, flashpoint, calorific value, cetane number, etc., and compared with international standards of ASTM D6751, EN14214 and IS, and then, characterize the fuel properties various blend ratios of biodiesel. Later, experimentation was carried out to examine the effect of various blend ratios of fuel run in a diesel engine for performance and emission parameters analysis to recommend the best the ranges of blend ratios of biodiesel that strongly offered better performance and emission parameters.
Similarly, the effects of heating on the fuel properties of neat biodiesel and different blend ratios of biodiesel at different fuel inlet temperature was studied to utilized more percentage volume fraction of biodiesel as a fuel in a diesel engine with enhanced its engine performance and emission parameters.
1.3.1 Blending Technique
The method used to blend the fuel is the most important factor contributing to blend accuracy. The two major blending techniques used are splash blending and in-line (injection) blending. Currently, the most widely implemented technique is splash blending. This blending process involves adding biodiesel to a fuel storage tank that is partially filled with diesel fuel. The blending occurs as the magnetic stir rotates and the fuel splashes around in the tank. Unfortunately, in many cases, the truck does not drive far enough for the two fuels to blend uniformly. In addition, environmental factors such as temperature and humidity can affect the speed at which the fuels blend. A second, more accurate blending method is in-line blending. This type of blending occurs at a fuel rack, where dedicated blending equipment delivers a metered amount of fuel into a waiting fuel line.
With in-line blending, the correct ratio of biodiesel is metered with automated control valves into the diesel fuel before it is dispensed into a fuel injection pump. Since the resulting fuel is blended prior to entering the fuel tank, the mixing problem associated with splash blending is eliminated.
Although in-line blending offers a more accurate blending method than splash blending, any mechanical system is subject to wear and/or failures. The need to test the biodiesel blend ratio after final mixing is necessary regardless of the blending method. An accurate method to determine the biodiesel blend is just as important as an accurate blending method. In this study in-line blending technique was applied.
Blending of biodiesel with mineral diesel is a widely used methods for enhancing the biodiesel property (such as kinematic viscosity and density) and overcoming the fuel cold flow problems. Biodiesel from different feedstock's can blend with mineral diesel and used as a fuel for diesel engine under the ASTM blended fuel standard (Atabani et al., 2012). Diesel engines can be fuelled directly or blended with various diesel proportions with biodiesel on some modifications or no modification at all (Abu-Hamdeh and Alnefaie, 2015). Biodiesel and diesel being advantageous and disadvantageous in their respective terms, blending them would yield a fuel with intermediate properties which may improve the combustion and emission characteristics of neat biodiesel and thus its usage as a fuel in a diesel engine. The measurement and evaluation of blended fuel property is an important indicator for the maximum blending ratio of biodiesel from different sources (feedstock) that can meet the fuel specification requirements. Hence, the determination of right blend ratio is the most important criteria for utilizing biodiesel in a diesel engine. Previous investigations were conducted to study the effect of different volume fractions of biodiesel ratios
in a blended fuel for enhancing density and viscosity the fuel mixture operated in a diesel engine (Lahane and Subramanian, 2015). The studies indicated that, the maximum blend ratio of biodiesel currently limited to blends of 20 % or less as commercial fuels for many countries in existing diesel engine operate without engine modification (Smith et al., 2010). This blend is approved as a fuel for the existing diesel engines at low blending ratio up to 20% biodiesel (B20) according to the ASTM D7467 fuel standard specification. Accordingly, it is used as a commercial fuel in many countries. Conversely, at high blending ratios problems related to fuel properties are worsening (Ali et al., 2016). Furthermore, the engine performance and emission parameters results also showed worsen with increasing biodiesel ratio beyond B20 in the blend, when the key properties of the used fuel (biodiesel fuels properties) differ from those of mineral diesel fuel, which means different combustion characteristics.
1.3.2 Preheating Technique
As an alternative fuel for diesel engines, biodiesels are the principal renewable and carbon neutral sources. The causes of technical problems arising from the use of various biodiesel are the high surface tension, high density, and the high viscosity. Transesterification is the processes generally performed in order to reduce the viscosity of biodiesel but still it is higher to that of the diesel. To increase the fraction of biodiesel in blends, it is required to reduce the viscosity by preheating.
Fuel preheating technique offers the advantage of easy conversion of the normal diesel engine to work on heavy fuels. It needs no modifications in the engine. Engine with fuel preheating has indeed in principle superior characteristics to that of normal fuel operation (Nwafor, 2003). This technique is economical, feasible and real for improved fuel performance of biodiesel run as a fuel in a diesel engine, and engine overall efficacy without modification existing diesel engine or fuel injection system (Martin et al., 2017). The preheating of the biodiesel improves the injection characteristics by enhancing the fuel properties of biodiesel (surface tension, density, kinematic viscosity and poor flow characteristics). The preheating of biodiesel at different temperatures reduces the viscosity and surface tension which enhances better fuel injection and atomization, improves mixture formation, and it will influence the fuel-air mixing due to the changes of spray evaporation and consequently influence the combustion, performance and emissions of diesel engine. From the fuel properties, viscosity can affect fuel flow rate and cause poor fuel atomization during the combustion process (Khalid et al., 2017). The preheating of biodiesel results in complete combustion of the biodiesel or fuel that results in decreased in amount of carbon dioxide, carbon
monoxide and particulate exhaust emission is also complete combustion of biodiesel. The cleaner exhaust can be obtained while elevated temperature of the fuel increases NOx emissions. Thereby, minimize the troubles due to poor fuel droplet formation and atomization that results in lots of carbon deposit formation on the valves and injector choking (Mustaffa et al., 2014b). Moreover, increasing fuel temperature or heating also will ease the problem of injection process because it results in a decrease of the arithmetic diameter of the fuel droplets due to the effect of surface tension and viscosity changes with temperature (Augustine et al., 2012). Thus, it gives better spray formation and combustion process.
1.3.3 Preheating and Blending Techniques
Biodiesel is an alternative fuel similar characteristic to diesel fuel. It can be produced from vegetable oil, animal fat and waste cooking oil. The reduction of fossil fuel causes the increasing using the biodiesel fuel. However, use of biodiesel fuel can affect on engine performance and exhausts emission. Biodiesel are not efficiently in cold weather and it is the biodiesel major problem. It influences the fuel spray characteristics during the combustion process. The aim of this study was to determine the effects of biodiesel to temperature and is carried out using of room temperature, 40°C and 60°C. It is required to recommend the biodiesel blending ratio and biodiesel temperature that optimizes the engine performance and lower exhaust emissions. There are three types of biodiesel oil is used to carry out this study (crude palm oil, waste cooking oil and jatropa oil). While there are 3 blending ratios that have been made towards biodiesel (5%, 10% and 15%) except for crude palm oil biodiesel for which there is additional another type of blending ratios provided for carrying out this study (20%). The study about biodiesel is important for investigation as it gives some solution for reduction of fossil fuel. Performance of engine and emissions exhausted from biodiesel fuels is a measure of the results by using the biodiesel in diesel engines.
Research and development of biodiesel fuels and its blends are very important to study and investigate in reducing problem in diesel engine.
In this sense, research and focus on preheat biodiesel fuels on these three types of biodiesel sources i.e. POME and COME are very important to be performed in promising alternative to conventional diesel fuel in India and for further comprehensive improvements as well. Increased of load condition and preheated biodiesel blends temperature promotes more rapid engine performance but exhibit relatively small variations in emissions production (Amir et al., 2014).
From the fuel properties, the use of biodiesel or its blends effects on fuel droplet formation,
vaporization and air fuel mixing process due to its higher viscosity (Ma and Hanna, 1999). These effects cause important engine failures such as fuel filter clogging, piston ring sticking, injector choking and carbon formation deposits (Jazair et al., 2011). High viscosity fuel also leads to high smoke, HC and CO emissions. The high viscosity and the major chemically bound oxygen component in the biodiesel fuel play as a keyelement in combustion process especially during the fuel-air premixing. Chemical and physical properties of biodiesel were determined using standard ASTM and American Oil Chemists Society (AOCS). For example, jatropa oil kinematic viscosity is high at 35.98 cSt compare to the mineral diesel at 2.44 cSt. Fuels with high viscosity tend to form larger droplets on injection which can cause poor combustion. While biodiesel is cleaner than standard diesel fuel in many other ways, it’s still dirtier (more air polluting) than gasoline. Bio fuels in general “result in more atmospheric CO2 pollutants than burning an energy equivalent amount of oil when considering the entire production and consumption cycle. Therefore, the major reduction of CO2 emission should be achieved in road transportation (Shahid et al., 2014). Further studies on the effects of preheat biodiesel blends fuel from crude palm oil, jatropha oil and waste cooking oil on the performance and emissions characteristic was conducted. Preheat is one of the effective method to reduce the viscosity of biodiesel fuels and its blends and viscosity will gradually decrease as the temperature increase.
1.3.4 Intake Air Preheating
The intake temperature also plays a vital role in increasing of peak pressure and brake thermal efficiency. The heating of the intake air helps to decrease engine warm-up times, improving fuel economy and emissions. When intake temperature increases the ignition delay changes and causes a change in the occurrence of peak pressure. Hence, the optimum intake temperature that gives maximum performance is to be identified. An air preheater is positioned in communication with the inlet of the engine. Preheating ensures proper combustion of the fuel mixture and rise in inlet temperature. This higher inlet temperature leads to increase in efficiency, torque, horse power and evaporation of fuel. Air preheating decreases ignition delay and improves the combustion rate. The effect of preheated air on standard diesel fuel engine indicated a good result on emission control.
Higher inlet air temperature causes lower ignition delay, which is responsible for lower NOx formation. Uniform or better combustion is occurred due to pre-heating of inlet air, which also causes lower engine noise. Easy vaporization and better mixing of air and fuel occur due to warm up of inlet air, which causes lower CO emission. Lower temperature intake air leads to inadequate
final compression temperature, increase in emission delay, and longer time between the injection of the fuel to ignition, local over-enrichment, incomplete combustion and high pressure gradients due to abrupt mixture conversion in the cylinder. These factors lead to knocking of the engine, increase in emission of hydrocarbons in the exhaust leading to severe loading of the environment.