Recent interest in cleaner air, stringent emission standards, clean burning fuels, energy security and renewable energies has led to increased interest in bio fuels-ethanol, methanol and bio diesel in many countries including India. Bio fuel is an efficient, environment friendly, 100 percent natural energy alternative to petroleum fuels.
In view of the potential of being produced from several agricultural sources and because of their low emission characteristics, bio fuels have a potential as auto fuels and in recent years are receiving a great deal of attention as a substitute to petroleum fuels. Ethanol and bio diesel are the two major bio fuels which are being looked upon as the potential fuels for vehicles.
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Bio fuels can play an important role in improving urban air quality and conserving conventional auto fuels especially in oil importing agricultural countries like India. Ethanol is being considered a feasible replacement for gasoline in cars, bio-diesel in diesel cars and heavy duty vehicles and methanol is being considered suitable for use in fuel cell vehicles.
Bio fuels have oxygen radicals in their molecules, higher octane/cetane values and have lower carbon content for equivalent energy compared to conventional fuels. This results in clean burning and lower emissions. In the absence of sulphur and aromatics, sulphur oxide and air toxins like benzene, 1-3 butadiene are not observed in the emissions of bio fuels.
Mandating the use of unleaded gasoline, resulted in increased benzene and aromatics content in gasoline in order to maintain the octane number of the fuel. Benzene is now considered a human carcinogen. Oxygenates like ethanol and methanol are being increasingly used to increase the octane number of gasoline. In many countries including India, ethanol is being blended to gasoline to improve the octane number and to improve the combustion characteristics.
Ethanol has been an auto fuel for almost as long as we have been driving vehicles. When Henry Ford designed the Ford Model- T, he believed that ethanol made from renewable biological materials, would be the major source of energy in vehicles. However interest in Ethanol as an auto fuel increased significantly after its success in Brazil. The idea of use of bio-fuels is not new and bio diesel is known for more than a century.
Rudolf Diesel, the father of the diesel engine, proposed the possibility of replacing the petroleum fuel by peanut oil about 100 years ago, when he presented his first engine in Paris. During the following decades, bio fuels were used only in emergency situations and in experiments.
Ethanol is being increasingly looked upon as a potential fuel for powering vehicles. The benefits of using ethanol include its renewable nature and lower emissions. Ethanol has attracted considerable attention as a motor fuel due to the success of the Brazilian pro-alcohol programme initiated in 1970s. In 1995, about 11 million motor vehicles in Brazil used ethanol or ethanol blends.
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Bio diesel (mono alkyl esters) is a cleaner – burning diesel fuel made from natural, renewable sources such as vegetable oils. Bio diesel is produced by reacting vegetable oils with methanol or ethanol to produce a lower viscosity fuel that is similar in characteristics to diesel, and can be used neat or blended with petroleum diesel. Bio diesel, apart from being produced from renewable sources, has high cetane number, low viscosity and very low sulphur content.
Bio diesel is today used extensively in European Union, U.S.A, Brazil, Argentina, Malaysia and many other countries. In most of these countries including Malaysia, bio diesel is produced from edible oils — palm oil, rape seed oil, corn, sunflower, pea nut etc. A programme to convert palm oil into bio diesel for use in taxis has been successful in Malaysia. Malaysia has recently built a bio diesel production unit with an annual capacity of 500,000 tons based on palm oil as feed stock.
Engines running on bio diesel or blended with petroleum diesel tends to have lower SO2, black smoke, polycyclic aromatic hydro carbons (PAHS) and CO but higher NOx emissions. Higher NOx emissions are due to higher cetane number of bio diesel which causes shorter ignition delay and higher cylinder pressures and temperatures.
Methanol is an oxygenated fuel. Today, most of the world’s methanol is produced using natural gas as feed stock. However the ability to produce methanol economically from non-petroleum feed stock such as coal or bio-mass is of interest in conserving petroleum. With an octane number of 112 and excellent combustion properties, methanol is a good alternative fuel in gasoline engines. However, methanol is being considered more for its use in the fuel cells rather than in internal combustion engines.
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Bio fuels (ethanol and bio diesel) and methanol when used in blends with conventional petroleum fuels (gasoline and diesel), enhances the combustion due to oxygen radicals and the increased octane/cetane value, resulting in a more efficient burn and reduced emissions. Recent developments world over have made the ethanol- gasoline blend (upto 22% ethanol) an interesting new alternative to conventional gasoline fuel.
Hydrogen is being considered a very promising alternate fuel for the future because it can be obtained from water using non- conventional energy sources, and thus has tremendous potential to become cleanest source of energy. Hydrogen powered vehicles can be driven by an internal combustion engine (similar to gasoline engine) or a fuel cell powered electric motor. The technical feasibility of using hydrogen has already been demonstrated in a range of vehicles, from cars to buses and trucks.
Di-Methyl Ether (DME) is similar to LPG. It is stored as a liquid under low pressure (5-10 bar at normal temperature). It has a very high cetane number and hence more suited for use in diesel engines. The main advantage of DME compared to diesel is the reduction of particulate emissions, visible smoke, HC and CO emissions to extremely low levels. DME vehicle technology is still in its infancy and most of the vehicle manufacturers are carrying out R & D to develop suitable vehicles.
1. Ethanol:
Ethanol (C2H5OH) is an alcohol. It is ethane (C2H6) with a hydrogen molecule replaced by a hydroxyl (OH) radical. Ethanol or ethyl alcohol is a chemical that can be produced through the process of fermentation and distillation of molasses, a by-product of sugar industry. Other possible raw materials are beet root, corn, cassava, rice straw and potato.
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In Brazil, the ethanol industry (the world’s largest) is based almost entirely on sugarcane, whereas more than 80 percent of the U.S ethanol production (the world’s second largest) is based on corn as the raw material. Ethanol can also be produced from other sources like damaged food grains and bio-mass.
As an auto fuel, ethanol has been used as a dedicated fuel in hydrous form or in combination with gasoline (the resulting fuel called “gasohol”). Ethanol when used in blends with gasoline, enhances the combustion of gasoline due to the oxygen present in the fuel, resulting in a more efficient burn and reduced emissions.
Recent developments world-over have made the ethanol-gasoline blends an interesting new alternative for conventional gasoline vehicles. While ethanol is completely miscible in gasoline, the presence of even a small amount of water can result in phase separation. Because of ethanol’s water affinity, it is usually added to gasoline in the market region rather than at the refinery.
Characteristics and Performance of Ethanol as an Auto Fuel:
Energy density or heat energy content of ethanol is about 65 percent of the heat energy content of gasoline for the same volume. The lower energy density implies that at equal engine efficiency (kms per BTU), a pure ethanol fueled vehicle would travel two-thirds as far as a gasoline fueled vehicle using the same size fuel tank. About 1.25 litres of pure ethanol gives the same mileage as one litre of gasoline – a significant fuel economy penalty.
Ethanol has an octane number of about 90. Higher octane values and other combustion characteristics of ethanol permit it to have higher thermal efficiency in an internal combustion engine. Hence upto 20 percent blend of ethanol, higher thermal efficiency compensates for the lower energy content, and hence there are no noticeable mileage penalties in substituting gasohol for gasoline.
Ethanol is less volatile than gasoline when used in neat form. The Reid vapour pressure for ethanol is 2.3 against gasoline’s 8 – 15. Hence ethanol is insufficiently volatile for cold starts in gasoline engines even at moderate temperatures. Because of low volatility, the most important issues for neat ethanol fuel are the cold – start problems and mis-firing during warm up.
However, when ethanol is dissolved in gasoline, the alcohol molecules become separated, and the molecular cohesion is weakened. The alcohol behaves more as a gas in such a mixture and results in a larger than anticipated increase in vapour pressure for the gasoline mixture than would be expected based on the alcohol’s and gasoline’s pure liquid vapour pressures.
Compared to the base gasoline, gasohol has a higher octane value. Under low speed and acceleration conditions, road performance of gasohol is generally similar or better than gasoline with the same octane number. The high speed, high load performance of gasohol, however, tends to be inferior to that of an equivalent octane quality gasoline.
Emissions from Ethanol and Gasohol:
Ethanol does not emit SO2 and has negligible PM emissions. Ethanol fueled vehicles generate 20 to 30 percent less CO and roughly 15 per cent less NOx compared to gasoline-fueled vehicles. Emissions of benzene, 1-3 butadiene and particulate matter are also substantially lower for ethanol powered vehicles. However ethanol produces increased aldehyde emissions in the exhaust of vehicles.
The toxic emissions of acetaldehyde and formaldehyde are significantly high with ethanol or gasohol compared to gasoline fuel. Formaldehyde and acetaldehyde emissions with ethanol or gasohol (with 85% ethanol 15% gasoline) are more than two times higher than gasoline-fueled vehicles. However these emissions can be controlled with the use of a catalytic converter.
The low vapour pressure of ethanol could cause cold – start problems in colder climates and result in higher cold – start exhaust emissions. Some of these limitations of pure ethanol can be addressed by blending ethanol with gasoline. The gasoline component vaporises more readily than ethanol, and makes cold – starting possible. But ethanol blended with gasoline reduces some of the benefits of using pure ethanol such as no benzene, lower CO and other emissions.
Studies indicate that typical emissions of CO and HC decrease with increase in ethanol content, while NOx emissions increase as ethanol content in the blend is increased.
Safety:
Ethanol is less volatile than gasoline, but is considered to be more explosive. Vapours that form above a pool of ethanol are potentially explosive. Repeated over exposure to ethanol will cause redness and irritation of the skin. Excessive ingestion of ethanol is dangerous and will require medical care. As an intoxicating beverage, ethanol presents a special supervisory challenge.
Diesel-Ethanol Blends-Problems and Prospects:
Studies have indicated about 22% less particulate emissions in 10% ethanol diesel blends compared to diesel fuels. HC, CO and NOx emissions also shows significant reduction, especially under high load conditions. The addition of ethanol to diesel fuel affects several of the fuel properties, especially the cetane number, flash point and lubricity apart from the problems of miscibility.
Petroleum — based diesel fuels are complex mixtures of hydrocarbon compounds while ethanol is relatively a simple and a single compound fuel. The compositional differences of petroleum based diesel fuel and ethanol results in miscibility problems. In order to avoid the separation problem in diesel and ethanol mix, a suitable coupler compound is required to be added to make a stable emulsion of ethanol in diesel.
The major problems in ethanol diesel blends are the cetane number and flash point. Cetane number of ethanol is only 8 against about 45-50 of diesel fuels and a flash point of around 15°C. Hence addition of ethanol to diesel fuel will degrade the cetane number of the blend. A diesel fuel’s 46 cetane number is likely to be reduced to 38 when 10% ethanol is added. Additives are necessary to boost cetane value in diesel-ethanol blends.
In diesel engines, diesel fuel is used as lubricant in the fuel injection pump and injectors. The addition of ethanol to diesel fuel normally results in reduced lubricity. Additives are necessary to improve the lubricity of the blend.
The dosage of additive package to take care of the above problems depends on the properties of diesel fuel used and the conditions under which the blend is used. Typical additive package can range from 1% to 3% of the volume of the fuel.
The recent studies and trials indicate that ethanol and diesel blends can be successfully used through a proper additive package. Caterpillar is successfully using a 10% ethanol blend in diesel in Brazil. Research and development work and trials on the use of ethanol in diesel fuel has been taken up in India recently.
Ethanol is produced primarily by fermentation of starch from corn, sugar or food grains. Hence production of ethanol for auto fuel competes directly with food production in many countries. In spite of the availability of large and inexpensive bio-mass resources, ethanol production as auto fuel has required massive government subsidies in Brazil and U.S.A for its viability.
The high cost of producing ethanol (compared to hydro carbon fuels) has required large direct and indirect subsidies amounting to U.S $1.9 billion per annum in Brazil. This amounts to U.S $0.15 per litre of ethanol in Brazil.
In the United States, subsidies ensure that sufficient ethanol is produced from biomass (mostly by corn fermentation). Ethanol production in the U.S costs about U.S $ 0.33 a litre including the subsidy. Since a litre of ethanol contains two thirds the energy of a litre of gasoline, the whole sale price of ethanol at the plant gate, on an energy equivalent basis, ranges from U.S $0.39 to U.S $ 0.61 a litre.
The State of California’s Energy Commission has concluded that ethanol production from conventional feed stocks (such as corn) would not be economical in California. In addition, the department of Agriculture in U.S argued that, since much of the benefits of ethanol subsidies goes to large producers and retailers instead of farmers, subsidized ethanol production is an inefficient way of raising farm incomes. The same argument holds good for India.
Although air quality improvements can be achieved through the use of ethanol as an alternative fuel, the Brazilian experience indicates that there is a delicate balance among sugarcane farmers, alcohol distillers, petroleum refineries, vehicle manufacturers and customers. If the oil prices rise sharply, the countries that would potentially have an interest in embarking on ethanol programme will be those with food surpluses and energy deficits like Brazil.
2. Methanol:
Widely promoted in the United States as a “clean fuel”, methanol has many desirable combustion and emission characteristics. Methanol (CH3OH) is an alcohol with only one carbon atom. Methanol is an oxygenated compound in which a hydrogen molecule of methane (CH4) is replaced by a hydroxyl (OH) radical.
Methanol is a liquid boiling at 148°F. Hence it has all the advantages of conventional fuels (gasoline and diesel) in storage, transportation and refining and at the same time, it produces significantly low pollutants, being an oxygenate. Methanol is dispensed from fuel pumps in a manner similar to gasoline.
Methanol is being considered more for its use in the fuel cells rather than in internal combustion engines. However, with an octane number of 112 and excellent lean combustion properties, methanol is a good fuel in spark-ignition gasoline engines.
Today, most of the world’s methanol is produced by a process using natural gas as feed stock. However, the ability to produce methanol economically from non-petroleum feed stocks such as coal or bio-mass is of interest for reducing petroleum imports.
Engine Technology and Performance:
Overall, neat methanol (100 percent methanol) in spark ignition engines can provide improved power output and greater thermal efficiency over gasoline. Difficulty in cold starts and warm up misfiring are methanol’s major performance problems in spark- ignition engines. In order to address these problems, methanol is blended with gasoline. M 85 is a blend with 85 percent methanol and 15 percent gasoline. Addition of gasoline apart from solving cold start problems, improves flame visibility and increases energy density over neat methanol.
Apart from anti knock advantages over gasoline, methanol has much higher heat of vaporisation. The higher heat of vaporization provides more power. This property makes methanol a superior racing fuel.
Methanol has a high octane value, and a low cetane value (below 15 versus 50-55 for diesel). In addition, methanol fails to provide adequate lubrication to the high pressure pumps used to inject fuel into the compressed air in the combustion chamber of a diesel engine.
These characteristics make neat methanol alone unsuitable for use in diesel engines, but various measures are being used to modify either the engine or the fuel. For example, diesel engines have been modified for use with methanol by adding an ignition source, such as a glow plug. Fuel additives are used to compensate the cetane number and lubricity.
A number of heavy duty methanol engines have been developed and are in operation in Europe and U.S. The most promising approach is to inject methanol in liquid form, as in diesel engines. Detroit Diesel Corporation in U.S.A has manufactured heavy duty transit bus engines using methanol in 1991, and a number of buses using methanol are now in operation. Since 1983, the Golden Gate Transit Company has been operating methanol-fueled buses in San Francisco.
Particulate emissions from these buses are reported to be lower than from comparable diesel fueled buses. The technical problems encountered with these buses have included fuel pump break downs and frequent glow plug failures.
Various transit authorities in the Los Angeles area have demonstrated the use of methanol on heavy duty urban buses. In addition, several diesel engines manufacturers (Cater pillar, Cummins, Ford, Detroit diesel and Navistar) have developed methanol-fueled diesel engines.
Methanol fueled vehicles have higher maintenance costs than gasoline fueled vehicles because methanol and the formic acid it forms on combustion are corrosive chemicals and can dissolve materials such as solders, aluminium and rubber.
Studies indicate that life of methanol fueled engines is likely to be considerably shorter than gasoline fueled engines. Although the excessive engine wear problems have been substantially solved by fuel and lubricant additives, methanol-fueled vehicles require frequent engine oil and filter changes.
Methanol as oxygenated fuel has 50% oxygen content by weight. As methanol contains no sulphur or complex organic compounds, it promises air quality benefits over gasoline- reduced toxic emissions (especially benzene and other polycyclic aromatic hydrocarbons), lower ozone-forming emissions and no SO2 or lead emissions.
HC emissions from methanol engines are mostly unburned methanol and formaldehyde. Emissions of formaldehyde however can be significantly higher, more than five times the amount emitted by comparable gasoline-fueled vehicles. Formaldehyde is toxic and probably carcinogenic.
Since methanol shows less photo chemical reactivity than most hydro carbons, it was long thought that methanol vehicles could help reduce urban ozone problems. More recent studies, however, have shown that the high reactivity of the formaldehyde offsets the low reactivity of methanol, so that net ozone benefits are small. Studies performed on catalytic converter-equipped methanol fueled cars indicate that NOx emissions are atleast twice as high as their gasoline counterparts.
As compared to diesel fuel, methanol fueled heavy duty vehicles emit substantially less PM and NOx. Methanol-fueled buses however require evaporative emission controls that are not needed with diesel-fueled buses as well as oxidation catalysts for controlling CO, methanol and formaldehyde emissions.
Major disadvantage of methanol compared to conventional fuels is associated with its safety characteristics. Methanol burns with a nearly invisible flame during day light making detection and control of fires difficult. This has generated concern over effects on fire fighters and passers-by in the event of a fire. In addition, at ambient temperatures, methanol vapours in the fuel tank are in the explosive range.
Methanol is a toxic chemical. Its adverse health effects include skin irritation, visual disturbances (including blindness), loss of muscular coordination, dizziness, nausea, abdominal cramps, delirium, coma, convulsions, respiratory failure, cardiac arrest and death. Its lack of taste or odour and its alcoholic properties indicate that poisoning may be a much greater problem than with gasoline.
Methanol can be produced from natural gas, crude oil, coal, bio mass etc. Currently, natural gas is the most cost-effective feed stock for methanol. A major drawback of methanol as a transport fuel is its relatively high and volatile price relative to conventional transportation fuels.
The price of methanol in the world market increased from US $ 0.06 in early 1980s to $ 0.17 a litre in late 1980s and to $ 0.47 a litre in 1994. Methanol should cost U.S $ 0.10 a litre to compare with the spot price of gasoline of $ 0.18 to $ 0.20 a litre on an equivalent energy efficiency basis. There is little prospect for methanol to become price competitive with conventional transportation fuels unless world crude oil prices increase substantially.
3. Hydrogen (H2):
Hydrogen is receiving worldwide attention as a “clean fuel” and efficient energy source for automobiles. Hydrogen can replace or supplement petroleum products used in road transport. Hydrogen operated engine is a practically achievable system today. It is possible to operate a spark ignition engine with total hydrogen.
Hydrogen’s potential for reducing exhaust emissions stems from the absence of carbon atoms in its molecular structure. Because of the absence of carbon, the only pollutant produced in the course of hydrogen combustion is NOx (the engine oil may- contribute small amounts of HC, CO and PM). Hydrogen combustion produces no emissions of CO2 or SO2.
Hydrogen gas is being explored for use in both internal combustion engines and fuel-cell electric vehicles. It is a gas at normal temperatures and pressures, which presents greater transportation and storage hurdles. Storage systems being developed include compressed hydrogen, liquid hydrogen and a storage material (for example, metal hydrides). On board storage as compressed gas involves a weight penalty, cryogenic storage as liquid and storage in the form of metallic hydrides pose a series of R & D challenges and costs.
Engine and Vehicle Technology:
Hydrogen, like the other alternative fuels, is best suited for spark-ignition engines, although there is hydrogen —fueled compression ignition (diesel) engines. A spark-ignition (gasoline) engine can be converted to hydrogen operation by either installing a gas carburetor or a gas injection system. Injection of hydrogen into the inlet manifold is preferred over carburetor system.
Timed manifold injection can be achieved using cam-actuated injection system or hydraulically operated injection system or electronically controlled solenoid actuated injection system. Low pressure direct cylinder injection systems are under research and development. Direct cylinder injection of hydrogen into the combustion chamber does have the benefits of late injection and lower NOx emissions. NOx emissions in hydrogen engines can be substantially reduced by adopting exhaust gas recirculation.
Hydrogen has a high flame speed, wide flammability limits and a high detonation temperature with lean burning, which gives an improved engine efficiency. Hydrogen is a gas at —253°C (or — 423°F). Therefore there are no problems in starting a hydrogen engine even in the coldest winter temperatures. Hydrogen has the highest energy content per unit mass of all fuels. Hydrogen contains 2.75 times more energy compared to gasoline of similar mass. Hydrogen engine is about 50% more efficient than a gasoline engine.
Research and development work on using hydrogen as a vehicle fuel has been in progress in several developed countries. Many countries in Europe are working on hydrogen vehicle technologies. Hydrogen filling stations to fuel buses and cars have been set up at Hamburg and Munich in Germany. A German car manufacturer has developed liquid hydrogen fueled cars.
Government of India is launching several R&D programmes on hydrogen energy for stationary and portable applications and for vehicular applications. R&D projects were taken up for the production of hydrogen using solar energy and water. Government of India is funding a major R&D project at Benarus Hindu University, Varanasi, for performance improvements of hydrogen and metal hydride-based vehicles.
Hydrogen engines do not produce health affecting emissions such as HC, CO, PM, SO2. Acid rain and the greenhouse effect due to CO2 are totally absent in hydrogen engines.
Hydrogen is the most plentiful element in the universe, and all the stars and many planets essentially consist of hydrogen. However, on earth, hydrogen is scarce and therefore, must be produced. Hydrogen gas can be produced from fossil fuels, bio mass and water.
Hydrogen production technologies can be classified as fossil fuel based and renewable resource based. Hydrogen gas is being produced on a commercial scale by steam reformation and thermal cracking of natural gas, naphtha etc., and from coal gasification. It is also produced from electrolysis of water.
Today, the most economical resource of hydrogen production is from reforming natural gas. The potential future source includes electrolysis of water using large scale, cheap photovoltaic devices, solar-electric systems or cheap, abundant, and environmentally benign hydroelectric, nuclear electric plants.
Electrolysis of water uses electric energy to split water molecules into hydrogen and oxygen. Hydrogen could also be produced from renewable bio mass. Bio mass gasification requires less primary energy and lower capital costs. Hydrogen could be produced on a large scale through gasification of coal, but this would result in large emissions of CO2, as well as localised environmental damage due to ground water contamination etc.
Hydrogen from water and solar energy can be produced in several ways. The cost of production of hydrogen from the electrolysis of water using photo voltaic devices needs to be reduced substantially, before it becomes economically viable.
Hydrogen Storage, Transportation and Delivery:
As in the case of natural gas, on – board storage of hydrogen is a key component to success. The three types of storage systems being pursued are compressed hydrogen, liquid hydrogen, and the chemical bonding between hydrogen and a storage material, for example metal hydrides.
Storage as a gas under high pressure is the most common current form of hydrogen storage requiring the same hard ware as for natural gas storage. Storage in gas cylinders under 200 bar pressure is feasible for stationary engines, but it appears as impracticable for vehicular applications as the weight of the gas cylinders required is almost 100 times greater than the weight of hydrogen fuel to be stored on board.
Compressed hydrogen occupies roughly fourteen times the space of an equivalent amount of gasoline. Thus compressed gas storage would add significant bulk and weight to a vehicle-about three times the bulk and weight of an equivalent volume of compressed natural gas. Vehicle range and refueling frequency would be significant constraints.
Hydrogen can also be stored on-board a vehicle as a liquid, but this requires that it be cooled to below its boiling point of — 423° F. The technology of large scale liquid hydrogen storage has been well developed for space programmes where the material is used as a rocket fuel.
The energy content per unit mass of liquid is about 2.75 larger than that of hydrocarbon fuels, so that hydrogen in liquid form is a more suitable fuel for rockets, air crafts and road transport applications. Even as a liquid, hydrogen would occupy roughly four times the volume of an equivalent amount of gasoline. The energy required to liquefy hydrogen (about 11 kwhs per kg of liquid hydrogen) could easily exceed the energy value of the fuel itself, resulting in an extremely inefficient system for vehicle propulsion.
The third option for hydrogen storage is in the form of metal hydrides. This system is considered the leading technology for storing hydrogen for vehicular use. Research is under way in several countries, and some vehicle manufacturers in Germany and Japan have already developed prototype vehicles.
In India, the use of hydrogen in two wheelers (motor cycles) through the metal hydride route has been demonstrated at Banaras Hindu University. Hydrides of vanadium, iron-titanium hydride and magnesium – nickel hydride are under various stages of trials. A recent study has indicated that the operating costs of liquid hydrogen system to be less than metal hydride systems.
Hydrogen contains no hydrocarbons, and hence has the potential to be the cleanest burning vehicle fuel. With the virtual elimination of CO, HC and PM exhaust emissions, only nitrogen oxide emissions are present in the vehicle exhausts. NOx emissions from hydrogen vehicles are similar to those from gasoline vehicles.
They can be lowered further by reducing the oxygen concentration in the combustion chamber, by reducing the combustion chamber temperature or by reducing combustion time at high temperatures. These steps are generally accomplished by exhaust gas recirculation, water injection into cylinder, retarding spark timing or by using an exhaust catalyst.
Due to the high cost of production, storage, transportation and delivery, it is unlikely that hydrogen will be a cost-effective fuel in the internal combustion engines in the near future. New production and storage technologies and facilities will be required before hydrogen fuel becomes economically feasible.
4. Dimethyl Ether (DME):
Dimethyl ether (DME) is an optimum substitute for diesel fuel. It is produced from natural gas and, although being a liquefied gas like LPG, represents the latest innovation in the field of the “gas to Liquid” fuels technology. If converted to DME, natural gas can be used in an economical and energy efficient way in diesel vehicles.
The most frequently used alternative fuels such as CNG, LPG, methanol and ethanol are typically otto-cycle fuels and thus primarily used in gasoline engined passenger cars. They have higher octane number and lower cetane number. Hence they are not very useful in diesel engines. Distribution, storage and compression problems of natural gas are increasing the cost of natural gas for vehicle application.
Physical measures to increase the energy density of natural gas (liquefaction by extreme cooling or high gas compression) are inefficient solutions. This is the motivation to search for chemical conversion of natural gas to liquid fuels. DME which is manufactured by oxidation of natural gas is a solution for better utilisation of natural gas in vehicles, especially diesel vehicles.
Typical alternative fuels for diesel vehicles operating on compression-ignition cycle are the synthetic petroleum diesel, bio diesel and di-methyl ether (DME). DME is an excellent alternative to conventional diesel fuel because it has optimum combustion related properties and economical to produce and distribute.
Di methyl ether (DME) is a very “new” alternative fuel. The development of new technologies for DME fuel systems, engines and vehicles has been pursued by several companies since the early 1990s. In the meanwhile, the potential of this new fuel has been clearly demonstrated.
Due to a new technology published in 1995, production costs of DME are relatively low and approximately comes down to the costs of conventional diesel fuel (on an equivalent energy basis) if a high production volume is assumed. DME appears to be an ideal diesel fuel substitute when high engine efficiency, ultra-low emissions, low combustion noise (similar to gasoline engines) and elimination of cold start problems are necessary and desirable.
For vehicular use, DME will most likely be introduced in urban areas where filling stations will be available first. Diesel powered vans and city buses are the most suitable vehicle categories to be converted to DME in the near future. The introduction of DME as fuel promotes not only environmental technologies in the fields of fuel production but also other future propulsion technologies like fuel cells.
Di methyl Ether (C2 H6 O) is a liquid under low pressure (5- 10 bar) at normal temperature like LPG and has a very high cetane value (about 55 compared to about 45 of diesel). High cetane value of DME makes it more suitable for use in diesel engines.
Due to low boiling point, DME vapourises instantly during injection, which allows relatively low fuel injection pressures (less than 300 atmospheres). It is an oxygenated fuel with about 35% oxygen content by weight. DME has high energy density compared to CNG and hence for equivalent storage volumes, the driving range of a DME vehicle is almost three times that of a CNG vehicle.
DME is closest to natural gas in its exhaust emissions profile and has a very high advantage over diesel in terms of emissions. The main advantage of DME compared to diesel is the reduction of particulate emissions to extremely low levels. Being an oxygenated fuel, no visible smoke is observed with DME even under transient operation. HC and CO emissions are reduced to ultra-low values with the help of an oxidation catalyst, which will work very efficiently since there is no sulphur in DME.
Because of the absence of carbon-carbon bonds, emissions of polycyclic aromatic hydrocarbons (PAHS), benzene, xylene and toluene will be totally absent. In converting a typical Euro-2 diesel engine to DME, the NOx emissions are approximately halved (from 7 grams per kwh to 3-4 grams per kwh). Halving of NOx emissions are achieved without additional technologies like exhaust gas recirculation. Consequently there is potential to fulfill more stringent emission regulations than Euro-4.
DME is no “greenhouse gas” nor does it contribute to the formation of other greenhouse gases. If DME escapes to the atmosphere, it has a short life and decomposes in the layer close to the earth. Higher hydrogen content in DME leads to lower CO2 production and hence lower greenhouse effect.
DME is produced from natural gas, crude oil, heavy residue, coal and bio mass. Since it is a liquid at low pressures, handling of DME is similar to the handling of LPG. Hence DME shows advantages compared to natural gas as it is transported as a liquid.
This has considerable consequences on the vehicle range. DME will be produced in India by a joint venture company set up by Indian oil Corporation, Gas Authority of India and Amoco U.S.A. The supply and distribution system is being set — up in the country in the near future.
DME engine requires new technologies in order to realize its full potential. To make use of DME in diesel engines, the fuel storage, injection and combustion systems need to be developed or modified suitably. AVL has developed a fuel injection system concept for DME, specially aimed at bus and truck engines and is developing the necessary combustion systems.
With respect to engine performance, the high thermal efficiency (which corresponds to a DI diesel engine) is emphasized. Engine efficiencies have to be considered in conjunction with emissions. Due to the oxygen in the fuel, the particulate emissions are extremely low and NOx emissions come down below Euro-4 limits by using electronically controlled common rail fuel injection system. In comparing DME engine technology with a typical Euro-3 diesel engine, DME looks advantageous, primarily due to cleaner combustion.
In comparing vehicle technologies for CNG and DME, the advantage of DME lie in practical aspects and mainly refer to the fact that DME is stored as a liquid at moderate pressure on board of vehicles. This (and the higher thermal efficiency) has considerable consequences on the vehicle driving range. Furthermore, DME refueling is simple and fast compared to CNG.
DME is a very new alternative fuel and the development of DME vehicle technology has started only in recent years.