25-01-2013, 12:39 PM
BIODIESEL IN DIESEL ENGINES:A CRITICAL ASSESSMENT
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ABSTRACT
Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl (methyl, propyl or ethyl) esters. Biodiesel is typically made by chemically reacting lipids (e.g., vegetable oil, animal fat) with an alcohol. Biodiesel production is a modern and technological area for researchers due to constant increase in the prices of petroleum diesel and environmental advantages. The use of fatty acids methyl esters either in mixtures with fossil diesel or in pure state in diesel engines is correlated to a series of technical problems which are to be taken into account in order to not spoil the important environmental and social advantages that this kind of bio-combustibles reports. Throughout this work, the most critical aspects of biodiesel use as well as their immediate consequences are introduced. In general, this work tries to be a guide, which allows identifying the most sensible areas within the engine, as well as the suitable means and solutions for the correct use of biodiesel in compression ignition engines (diesel engines).
INTRODUCTION
Biodiesel is a clean-burning diesel fuel additive produced from soybean and other vegetable oils instead of petroleum. Biodiesel is produced from vegetable oils by converting the triglyceride oils to methyl (or ethyl) esters with a process known as trans-esterification. Biodiesel is used in compression ignition (diesel) engines to enhance engine combustion performance, improve engine lubrication, and reduce air and water pollution caused by the exhaust. Biodiesel blends operate in diesel engines, from light to heavy-duty, just like petroleum diesel fuel. No engine conversions are required at all, unless an engine has old fuel lines. It is a renewable domestically produced liquid fuel that can help reduce the countries dependence on foreign oil imports.
HISTORICAL BACKGROUND
Trans-esterification of vegetable oil was conducted as early as 1853 by scientists E. Duffy and J. Patrick, many years before the first diesel engine became functional. Rudolf Diesel's prime model, a single 10 ft (3 m) iron cylinder with a flywheel at its base, ran on its own power for the first time in Augsburg, Germany, on 10 August 1893 running on nothing but peanut oil. In remembrance of this event, 10 August has been declared "International Biodiesel Day".
It is often reported that Diesel designed his engine to run on peanut oil, but this is not the case. Diesel stated in his published papers, "at the Paris Exhibition in 1900 (Exposition Universelle) there was shown by the Otto Company a small Diesel engine, which, at the request of the French government ran on arachide (earth-nut or pea-nut) oil, and worked so smoothly that only a few people were aware of it. The engine was constructed for using mineral oil, and was then worked on vegetable oil without any alterations being made. The French Government at that time thought of testing the applicability to power production of the Arachide, or earth-nut, which grows in considerable quantities in their African colonies, and can easily be cultivated there." Diesel himself later conducted related tests and appeared supportive of the idea.
MANUFACTURING PROCESS
A lot of research work has been carried out to use vegetable oil both in its neat form and modified form. Studies have shown that the usage of vegetable oils in neat form is possible but not preferable. The high viscosity of vegetable oils and the low volatility affect the atomization and spray pattern of fuel, leading to incomplete combustion and severe carbon deposits, injector choking and piston ring sticking.
The methods used to reduce the viscosity are
* Blending with diesel
* Emulsification
* Pyrolysis
* Transesterification Among these, the trans-esterification is the commonly used commercial process to produce clean and environmental friendly fuel. However, this adds extra cost of processing because of the trans-esterification reaction involving chemical and process heat inputs. Trans-esterification involves reaction of the triglycerides of Jatropha oil with methyl alcohol in the presence of a catalyst Sodium Hydroxide (NaOH) to produce glycerol and fatty acid ester.
The Biodiesel Standard (ASTM D 6751)
All engines are designed and manufactured for a fuel that has certain characteristics. In the US, the industry organization that defines the consensus on fuels is the American Society for Testing and Materials (ASTM). In the case of diesel fuel (and biodiesel), the responsibility for setting standards lies within ASTM Committee D02 on Petroleum Products and Lubricants. In order to assure that the standards are rigorous and robust, ASTM committee D02 is comprised of fuel producers, engine equipment manufacturers, and third party interests (users, government agencies, consultants). An ASTM standard is not easily achieved. Some standards can take over 10 years to gain agreement and be issued by ASTM. This rigorous, time-consuming process is why ASTM standards are recognized and adopted by others worldwide.
USE IN DIESEL ENGINES
Additives are abundantly manufactured and mixed with IC engine fuels to meet the proper performance of fuel in engine. Additives act like catalyst so that they aid combustion, control emission, control fuel quality during distribution and storage and reduce refiners operating cost. The performance of diesel engine using diesel additive and methyl-ester of Jatropha oil as the fuel was evaluated for its performance and exhaust emissions. The fuel properties of biodiesel such as kinematic viscosity, calorific value, flash point, carbon residue and specific gravity were found. Results indicated that B25 have closer performance to diesel and B100 had lower brake thermal efficiency mainly due to its high viscosity compared to diesel. The brake thermal efficiency for biodiesel and its blends was found to be slightly higher than that of diesel fuel at tested load conditions and there was no difference between the biodiesel and its blended fuels efficiencies. Its diesel blends showed reasonable efficiencies, lower smoke, CO2 and CO. Multi-DM-32 additives with methyl ester of Jatropha offer fuel conservation as well as reduce pollution.