Emission Characteristics of alternative fuelsBiodiesel:It is produced from transesterification of vegetable oil in which the fatty acid triglycerides are reacted with a suitable alcohol (Methyl, Ethyl, or others) in the presence of a catalyst(KOH, NaOH) under a controlled temperature(60-70 0C) for a given length of time.Combustion:0.04437 A+0.01675 B+0.08432 C+0.1766 D+0.0205 E+9.037 O2+33.98 N26.42 CO2+5.92 H2O+33.98 N2Chemical Composition

Emissions:

Reduced CO emission:A 100% sulphur dioxide reduction is reasonable taking into account that biodiesel, by its vegetal origin, does not contain sulphur. The CO emissions for biodiesel combustion in diesel engines are 40 to 50% lower than those for conventional diesel; this happens due to the presence of oxygen molecules in the biodiesel, mainly in the methyl or ethyl ester, helping to obtain complete combustion. Reduced Particulate Matter:High gas temperatures and high temperatures of the combustion chamber wall contributes to less smoke and particulates. PM emissions among biodiesels could be due to either their chemical composition or their physical properties.The oxygen content of biodiesel is favorable in reducing the PM emission.PM emission decreases with increasing degree of unsaturation. The reduction of smoke is due to the dilution of aromatics, which are soot precursors.PM emission depends on viscosity and surface tension. Fuels with low cetane value undergo prolonged premixed combustion phases that are responsible for less soot formation.

Reduced Hydrocarbons:Since biodiesel is an oxygenated fuel, it promotes combustion and results in the reduction of unburned hydrocarbon emissions. A decrease of unburned hydrocarbons due to complete combustion takes place, because the chains of carbon-hydrogen and oxygen in esters help the formation of CO2 and water unlike to what occurs with diesel fuel.Increased NOX EmissionThe possible reasons are: The shorter ignition delay, caused by biodiesels higher cetane number, because of advanced combustion timing which increases peak pressure and temperature and hence NOX emission. An increase in flame temperature in either premixed or diffusion burn, which is caused by reduction in the concentration of carbonaceous soot a highly effective heat radiator. The double bonds present in biodiesel may cause a higher adiabatic flame temperature, and hence a higher temperature at the flame front and hence increased NOX emission. Unsaturated compounds present in biodiesel may form higher levels of radicals during pyrolysis and combustion. Prompt NO is formed by reaction of radical HC species with nitrogen, ultimately leading to formation of NO.Ethanol:Ethanol refers both to ethyl alcohol and to a blend of ethyl alcohol and gasoline used as a motor vehicle fuel. In the U.S. most ethanol is used in blends of up to 10% ethanol and 90% gasoline (E10 or gasohol) to reduce carbon monoxide emissions and prevent air pollution. E10 is not considered an alternative fuel, and conventional gasoline engines can run on E10.Although motor vehicle gasoline engines can run on E10, only flexible fuel vehicles (FFVs) with specially modified engines can use the more corrosive E85. The main difference between FFVs and conventional gasoline vehicles are the materials used in the fuel management system and modifications to the engine calibration system.FFV engine parts are modified to resist corrosion, and a fuel system sensor in the engine analyzes the fuel mixture and adjusts the fuel injection and ignition accordingly.CO emission:The reduction in CO concentration using blended fuels is due to the fact that ethanol (C2H5OH) has less carbon than gasoline (C8H18). Another significant reason of this reduction is that the oxygen content in the blended fuels increases the oxygen-to-fuel ratio in the fuel-rich regions. The most significant parameter affecting CO concentration is the relative airfuel ratio (). Relative airfuel ratio () approaches 1 as the ethanol content of the blended fuel increases, and consequently combustion becomes complete.CO2 Emission:CO2concentration increases as the ethanol percentage increases. CO2emission depends on relative airfuel ratio and CO emission concentration. As a result of the lean burning associated with increasing ethanol percentages, the CO2emission increased because of the improved combustion.HC Emissions: Ethanol can significantly reduce HC emissions. The concentration of HC emission decreases with the increase of the relative airfuel ratio, the reason for the decrease of HC concentration is similar to that of CO concentration described above.

NOX Emission: As the percentage of ethanol in the blends increases NOX emission increases. When the combustion process is closer to stoichiometric, flame temperature increases, therefore, the NOxemission is increased, particularly by the increase of thermal NO.Carbonyl Emissions:Carbonyl emissions(acetaldehyde,formaldehayde,propionaldehyde) increases with increasing ethanol percentage in the ethanol gasoline blend. Carbonyls are formed primarily from the reaction of hydrocarbons with OH radicals. Combustion of ethanol tends to form carbonyl compounds due to its hydroxyl structure. In addition, the combustion of ethanol with two carbons in structure can easily form acetaldehyde which contains two carbons as well. The higher emission of carbonyls can be attributed to the addition of rich oxygen-containing ethanol.

LPG(Liquified Petroleum Gas):LPG is a quite niche alternative fuel that can be used in special spark ignition engines or as an auxiliary fuel in dual fuel compression ignition engines together with diesel oil. LPG is recovered directly from oil and gas fields (WLPGA) in which case no actual refining is needed and also formed as a by-product in crude oil processing either in distillation phase or after-treatment (cracking) processes. The use of LPG in transportation is concentrated in few countries (Korea, Turkey, Russia, and Poland) and it is mainly used in bi-fuel light duty vehicles. LPG forms easily a homogenous mixture with air. This combined with the relatively simple chemical structure of LPG, it burns cleanly and is well-suited for spark-ignition engines. For compression ignition (diesel) engines, LPG is not suitable as the sole fuel. In spark ignition engines, similar compression ratios are typically used with LPG as with gasoline, even though the octane number of LPG (112 for propane, 94 for butane) is higher than that of gasoline. This is due to the fact that the combustion temperature is higher when LPG is used and this lowers the knock limit especially at high engine loads. Exceptions to this are the engines in which LPG is injected in liquid form. In bi-fuel cars, the upper limit for compression ratio is restricted by gasoline. Efficiency of LPG engines is similar to gasoline engines. A higher thermal efficiency and, therefore, improved fuel economy can be obtained from internal combustion engines running on LPG as opposed to unleaded gasoline. This is because LPG has a higher octane number, typically 112 research octane number(RON) for pure propane, which prevents the occurrence of detonation at high engine compression ratio. In dual fuel engines under low loads, when the LPG concentration is lower, the ignition delay of the pilot fuel increases and some of the homogeneously dispersed LPG remains unburned, resulting in poor emission performance. Poor combustion of LPG under low loads because of a dilute LPGair mixture results in high CO and unburned HC emissions. However, at high loads, increased admission of LPG can result in uncontrolled reaction rates near the pilot fuel spray and lead to knock.

Hendriksen(2003),Verbeek 2008)Emission Characteristics:LPG CO emissions are significantly reduced .This may be due to the better mixing obtained by gaseous fuel dosification and due to the higher cylinder-to-cylinder uniformity achieved. In some cases LPG having a slightly greater tendency to produce CO may be due to its higher combustion chamber surface to volume ratio and thus a proportionally higher charge cooling and flame quenching effect.LPG combustion normally produces higher temperatures due its slightly superior heating value, its higher burning speed and its lack of charge cooling effect (obtained with gasoline by its evaporation).Therefore NOX emission is increased with increasing proportion of LPG in gasoline.(HC) emissions were reported as 40% lower, carbon monoxide (CO) as 60% lower and carbon dioxide (CO2) as substantially reduced, principally due to the high hydrogen/carbon ratio(propane,butane) of LPG when compared to gasoline. An increasing proportion of LPG in gasoline promotes faster burning velocity of mixture and hence reduce the combustion duration and subsequently the in-cylinder peak temperature increases.

References:[1]California Air Resources Board, National Biodiesel Board and A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions, United States Environmental Protection Agency, EPA420-P-02-001, October 2002