ALCOHOL AS AN ALTERNATIVE FUEL IN I.C ENGINES
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17-02-2011, 02:59 PM
ALCOHOL AS AN ALTERNATIVE FUEL IN I.C ENGINES.docx (Size: 75.01 KB / Downloads: 339)
In this century, it is believed that crude oil and petroleum products will become very scarce and costly. Day-to-day, fuel economy of engines is getting improved and will continue to improve. However, enormous increase in number of vehicles has started dictating the demand for fuel. With increased use and depletion of fossil fuels, alternative fuel technology will become more common in the coming decades. Because of the high cost of petroleum products, emission problems some developing countries are trying to use alternate fuels for their vehicles.
1. Extensive research and development is difficult to justify until the fuels are accepted as viable for large numbers of engines.
2. Most alternate fuels are very costly at present since the quantity used is very less.
3. There is lack of distribution points (service stations) where fuel is available to the public.
Liquid fuels are preferred for IC engines because they are easy to store and have reasonably good calorific value. The main alternative is the alcohol
Alcohols are attractive alternate fuels because they can be obtained from both natural and manufactured sources. Methanol and ethanol are two kinds of alcohols that seem most promising.
1. It is a high octane fuel with anti-knock index numbers of over 100.Engines using high octane fuel can run more efficiently by using higher compression ratios. Alcohols have higher flame speed.
2. It produces less overall emissions compared to gasoline.
3. When alcohols are burned, it forms more moles of exhaust gases, which gives higher pressure and more power in the expansion stroke.
4. It has high latent heat of vaporization which results in a cooler intake process. This raises the volumetric efficiency of the engine and reduces the required work input in the compression stroke.
5. Alcohols have low sulphur content in the fuel.
6. Reduced petroleum imports and transportation.
1. Alcohols have low energy content or in other words the calorific value of the fuel is almost half. This means that almost twice as much as gasoline must be burned to give the same energy input to the engine. With equal thermal efficiency and similar engine output usage, twice as much fuel would have to be purchased, and he distance which could be driven with a given fuel tank volume would be cut in half. Automobiles as well as distribution stations would require twice as much storage capacity, twice the number of storage facilities, twice the volume of storage at the service stations, twice as many tank trucks and pipelines, etc. Even with the low energy content of the fuel, engine power for a given displacement would be about the same. This is because of the lower air-fuel ratio needed by alcohol. Alcohol contains oxygen and thus requires less air for stoichiometric combustion. More fuel can be burned with the same amount of air.
2. Combustion of alcohols produces more aldehydes in the exhaust. If as much alcohol fuel was consumed as gasoline. Aldehyde emissions would be a serious problem.
3. Alcohol is much more corrosive than gasoline on copper, brass, aluminum, rubber, and many plastics. This puts some restrictions on the design and manufacturing of engines to be used with this fuel. Fuel lines and tanks, gaskets, and even metal engine parts can deteriorate with long-term alcohol use (resulting in cracked fuel lines, the need for special fuel tank, etc). Methanol is very corrosive on metals.
4. It has poor cold weather starting characteristics due to low vapor pressure and evaporation. Alcohol-fuelled engines generally have difficulty in starting at temperatures below 10 C. Often a small amount of gasoline is added to alcohol fuel, which greatly improves cold-weather starting. However, the need to do this greatly reduces the attractiveness of alcohol.
5. Alcohols have poor ignition characteristics n general.
6. Alcohols have an almost invisible flame, which is considered dangerous when handling fuel. A small amount of gasoline removes this danger.
7. There is the danger of storage tank flammability, due to low vapor pressure. Air can leak into storage tanks and create combustible mixtures.
8. There will be less NOx emissions because of low flame temperatures. However, the resulting lower exhaust temperatures take longer time to heat the catalytic converter to efficient operating temperatures.
9. Many people find the strong odor of alcohol very offensive. Headaches and drizzles have been experienced when refueling an automobile.
10. There is a possibility of vapor lock in fuel delivery systems.
Of all the fuels being considered as an alternate to gasoline, methanol is one of the most promising and has experienced major research and development. Pure methanol and mixtures of methanol and gasoline in various percentages have been extensively tested in engines and vehicles for a number of years. The most common mixtures are M85 (85% methanol and 15% gasoline). The data of these tests which include performance and emission level levels are compared with pure gasoline (M0) and pure methanol (M100). Some smart flexible fuel (or variable fuel) engines are capable of using any random mixture combination of methanol and gasoline ranging from methanol to pure gasoline. Two fuel tanks are used and various flow rates of the two fuels can be pumped to the engine, passing through a mixing chamber. Using information from sensors in the intake and exhaust, the electronic monitoring systems (EMS) adjust to the proper air-fuel ratio, ignition ratio, ignition timing, injection timing, and valve timing (where possible) for the fuel mixture being used.
Methanol can be obtained from many sources, both fossil and renewable. These include coal, petroleum, natural gas, biomass, wood, landfills, and even the ocean. However, any source that requires extensive manufacturing or processing raises the price of the fuel.
Emissions from an engine using M10 fuel are about the same as those using gasoline. The advantage (and disadvantage) of using this fuel is mainly 10% decrease in HC and CO exhaust emissions. However, there is an increase in NOx and a large (500%) increase in formaldehyde emissions.
Methanol is used some dual-fuel CI engines. Methanol by itself is not a good CI engine fuel because of its high octane number, but if a small amount of diesel oil is used for ignition, it can be used with good results. This is very attractive for developing countries, because methanol can often be obtained from much cheaper source than diesel oil. Methanol fuel has received less attention than ethanol fuel as an alternative to petroleum based fuels.
Use in racing
Methanol fuel is also used extensively in drag racing, primarily in the Top Alcohol category.
Formula One racing continues to use gasoline as its fuel, but in pre war grand prix racing methanol was often used in the fuel.
Use as internal combustion engine fuel
Both methanol and ethanol burn at lower temperatures than gasoline, and both are less volatile, making engine starting in cold weather more difficult. Using methanol as a fuel in spark ignition engines can offer an increased thermal efficiency and increased power output (as compared to gasoline) due to its high octane rating (114) and high heat of vaporisation. However, its low energy content of 19.7 MJ/kg and stoichiometric air fuel ratio of 6.42:1 mean that fuel consumption (on volume or mass basis) will be higher than hydrocarbon fuels. The extra water produced also makes the charge rather wet (similar to hydrogen/oxygen combustion engines)and combined with the formation of acidic products during combustion, the wearing of valves, valve seats and cylinder might be higher than with hydrocarbon burning. Certain additives may be added to motor oil in order to neutralize these acids.
Methanol, just like ethanol, contains soluble and insoluble contaminants. These soluble contaminants, halide ions such as chloride ions, have a large effect on the corrosivity of alcohol fuels. Halide ions increase corrosion in two ways; they chemically attack passivating oxide films on several metals causing pitting corrosion, and they increase the conductivity of the fuel. Increased electrical conductivity promotes electric, galvanic, and ordinary corrosion in the fuel system. Soluble contaminants, such as aluminium hydroxide, itself a product of corrosion by halide ions, clog the fuel system over time.
Methanol is hygroscopic, meaning it will absorb water vapor directly from the atmosphere. Because absorbed water dilutes the fuel value of the methanol (although, it suppresses engine knock), and may cause phase separation of methanol-gasoline blends, containers of methanol fuels must be kept tightly sealed.
Methanol is poisonous; ingestion of only 10 ml can cause blindness and 60-100 ml can be fatal, and it doesn't have to be swallowed to be dangerous since the liquid can be absorbed through the skin, and the vapors through the lungs. US maximum allowed exposure in air (40 h/week) is 1900 mg/m³ for ethanol, 900 mg/m³ for gasoline, and 1260 mg/m³ for methanol. However, it is less volatile than gasoline, and therefore decreases evaporative emissions. Use of methanol, like ethanol, significantly reduces the emissions of certain hydrocarbon-related toxins such as benzene and 1, 3 butadiene. But as gasoline and ethanol are already quite toxic, safety protocol is the same.
Since methanol vapour is heavier than air, it will linger close to the ground or in a pit unless there is good ventilation, and if the concentration of methanol is above 6.7% in air it can be lit by a spark, and will explode above 54 F / 12 C. Once ablaze, the flames give out very little light making it very hard to see the fire or even estimate its size, especially in bright daylight. If you are unlucky enough to be exposed to the poisonous substance through your respiratory system, its pungent odor should give you some warning of its presence. However, it is difficult to smell methanol in the air at less than 2,000 ppm (0.2%), and it can be dangerous at lower concentrations than that.
Joined: Oct 2012
22-11-2012, 11:20 AM
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