Fossil Based and Renewable Fuels

Transcription

1 Fossil Based and Renewable Fuels Requirements for fuel The most important requirements, for fuel for IC engines have to meet, are: 1) Plentiful availability, high processing and dressing facility, low costs and minimum environmental impact; 2) High heating values per unit mass and unit volume and easy transport and storage; 3) rapid start and development of combustion processes at any ambient temperature; 4) complete and speedy combustion, without formation of harmful pollutant; 5) Absence of carbon and corrosive deposits on surfaces of combustion chamber; So the availability of liquid fuels derived from crude oil distillation and their relatively easy processing, in order to meet previous listed requirements, made them up to now be the main (over 90%) energy source for IC engines. However, starting from last decades, it became clear that natural resources such as coal, petroleum and natural gas took millions of year to form naturally and cannot be replaced as fast as they are being consumed. Therefore alternative fuels, also known as renewable (i.e. fuels that can be naturally reproduced on a time scale which can sustain their consumption rate) have been extensively tested on engines. Fossil Fuel for Engines Over 90% of fuel used in IC engines is derived by fraction distillation from the raw petroleum coming from oil wells. It consists of several hydrocarbon types of different molecular weights, including a small fraction of organic compounds containing: sulphur, nitrogen, etc... The real composition of crude oil widely differs according to its source. On the basis of its residue after distillation, which may be mainly paraffin, asphalt or a mixture of the two, crudes are called paraffin, naphthenic or mixed based, with a large variety of composition for each group. Refining of crude oil usually starts with distillation at atmospheric pressure, during which the distillate is separated into various fraction according to their volatility. Distillate and residue may then be subjected to chemical and thermal treatments at various pressures and temperatures, in order to reduce main hydrocarbon molecular weights (cracking) or to obtain the reverse process (polymerization). The structure of the resulting products depends both on the details of the undergone process and on the composition of the treated fraction and of the initial crude oil. Fuels Derived from Petroleum The products, resulting from the petroleum refinement, are classified by their use and also according to their density and volatility in the following groups: 1) Gaseous products: gaseous hydrocarbons are usually associated with liquid petroleum, either dissolved in the liquid or standing above it in the earth. They are the first products that are separated during distillation process, in the form of natural gas or liquefied petroleum gas (LPG) 2) Gasoline: is the lighter petroleum-derived liquid, whose density range from 0,73 to 0,76 kg/dm3, obtained by distillation within 25 and 200 C, that is used primarily as a fuel in spark ignition engines. 3) kerosene: is the next fraction heavier than gasolinewith density lying between 0,77 and 0,83 kg/dm3, obtained by distillation in the range of 170 to 260 C and primarly intended for use as fuel in gas turbines

2 and jet engines. 4) Diesel oils: are petroleum fraction that lie between kerosene and lubricating oils and cover a wide range of density (0,815 0,855 kg/dm3) and of distillation temperature ( C). On the basis of their characteristics they are divided into seven grades, closely controlled to make them suitable for use in various types of Diesel engines. 5) Fuel oils: cover a range of density and distillation temperature similar to that of Diesel oils, but since they are designed for use in continuous burners, their composition does not require such accurate control as for Diesel oils. Composition of each of these fuel varies widely depending on the nature of the original crude oil and on the processes used in refining. In any case the fuel consists of a mixture of hydrocarbon compounds having different molecular weight and chemical structure and classified on the basis of the number and of the position of carbon atoms in the molecules. Molecular Structure From this point of view the main classes of petroleum fuel constituents are: 1) Alkanes (or paraffin CnH2n+2) single bonded open chain saturated hydrocarbons; 2) Alkenes (or olefins CnH2n) open chain unsaturated hydrocarbons with a double bond; 3) Alkines (or acetylenes CnH2n-2) open chain unsaturated hydrocarbons with one carbon-carbon triple bond; 4) Cycloalkanes (or naphthenes CnH2n) single bond ring unsaturated hydrocarbons; 5) Aromatics (CnH2n-6) circular structure (the basis is the benzene ring: C6H6) and double bonds hydrocarbons; 6) oxygenated hydrocarbons (including: alcohols, ethers, ketones and aldehydes). Gaseous Fossil Fuels Natural gas (NG) is a naturally occurring hydrocarbon gas mixture consisting primarily of methane (typically 80-95% mass), with small amounts of light hydrocarbons (ethane, propane, butane and pentane), nitrogen, carbon dioxide, etc..., but its composition depends on its origin. The major source is the extraction from geological deposits known as natural gas or oil fields. Really NG and oil are often found together in the subsoil, since both are produced by the same geological process: an anaerobic decay of organic matter deep under the Earth s surface. Moreover NG can be as well extracted from coal beds, where it is naturally adsorbed into the solid matrix of the coal and finally it can also be generated by the anaerobic digestion or fermentation of organic materials, in the absence of oxygen. In the last case it is called biogas, since it is no longer a fossil based fuel, but it is derived from renewable energy sources. Because of its low volumetric energy density, natural gas is not easy to store or transport, but it is generally distributed by pipelines in its natural gas form by a close network in Europe, from Near East and Northern Africa to Europe, in Northern America, etc... Or it can be transported as liquefied natural gas (LNG) or compressed (CNG).

3 Effects on Engine Performances, Cylinder Filling and Emissions 1) First of all, to compare the effects of fuel of different lower calorific values on engine performances, it must be considered that the engine mean effective pressure is: proportional to the filling coefficient λv (i.e. to the mass of air inducted in the cylinder at the beginning of each working cycle) and to the ratio Hi/αs (i.e. to the lower calorific value per unit mass of air in the stoichiometric mixture). About this last parameter for all the gaseous fuels commonly used in IC engines (in spite of quite different value of Hi and αs separately) the ratio (Hi/αs) takes approximately a similar value (around 3 MJ/kg). This result means that all the gaseous fuels are potentially able to give similar powers, after a proper adjustment of engine parameters. Effect of gaseous fuel on cylinder filling process (the blue balls represent the air, while the red ones represent the fuel): a gasoline indirect injection, b gaseous fuel indirect injection, c liquefied gas indirect injection, d fuel direct injection, e gaseous fuel indirect injection in a supercharged engine. 2) Moreover the way used to add gaseous fuels to the new air, which is inducted in the cylinder at the beginning of each working cycle, influences the cylinder filling process (and so also the engine delivered power). This effect is schematically shown in figure, where first of all is represented the case of a liquid fuel indirectly injected in the intake manifold. In this case the fuel is supposed to partially evaporate later in the cylinder, so initially occupying a very small volume (represented by a little red ball) If a gaseous fuel, instead, is mixed with air during the intake stroke(by means of a carburettor or a multipoint indirect injection system) the cylinder filling with air decreases, since gases have a larger volume and hence the displace more air than a liquid fuel, occupying a substantial portion of the cylinder volume. However, if the gaseous fuel is injected in the intake manifold in its liquid state, the subsequent evaporation process is going to cool the inducted charge, so increasing the density (and thus the mass) of air introduced in the cylinder. This mass reduction of intake air is completely avoided if (gaseous or liquid) fuels are directly injected into the combustion chamber, when all the valves are closed, so that the air induction process is no longer affected. In this case, figure d represents the gaseous fuel outside the cylinder volume during the intake stroke, since it will be injected later in the cylinder. Finally, supercharging the engine, the amount of inducted air can be controlled through the valve of the supercharging pressure, while the fuel can be injected outside or inside the cylinder. 3) Better gaseous fuel characteristic in comparison with liquid fossil fuels (such as: larger octane number, higher laminar combustion velocity... and so on) clearly can be fully exploited (increasing, for example, the compression ratio) only if the engine is designed for exclusively running with the gaseous fuel, losing the advantage of a bi-fuel or multi-fuel feeding.

4 4) Gaseous fuels are a cleaner alternative to liquid fossil fuels, since their combustion doesn t produce particulates nor dangerous unburned hydrocarbon of high molecular weight. Moreover they usually have larger energy/carbon ratio in comparison with liquid fossil fuel, and thus they generate less CO2 per unit of energy. However, they may behave themselves as greenhouse gases, when released into the atmosphere. Use of CNG in SI and Diesel Engines In particular the previous consideration can help understand why compressed natural gas (CNG and less commonly LNG) is used in traditional gasoline SI engine cars, which have been converted into bi-fuel vehicles (gasoline/cng). Really any existing gasoline vehicle can be converted, by installing: gas tank in the trunk (where CNG is stored at a pressure of MPa), relative hydraulic plant, injection system and the electronics. However better technical results are obtained in factory-built bi-fuel vehicles, both in terms of engine parameter optimization and plant solutions. with high gasoline prices an increasing number of vehicles are being manufactured to operate with CNG, since the main advantage obtained from the use of this fuel is a reduction of engine running costs. For the same reason NG is frequently burned in Diesel engines, powering stationary plants for combined generation of heat and power. In any case, the main reason for employing NG in IC engines is due to its availability on the market at a lower cost than liquid fuels. Liquefied Petroleum Gas (LPG) Liquefied petroleum gas (LPG) is a mixture of hydrocarbon gases including blends that are primarily propane (C3H8), primarily butane (C4H10) and, most commonly, mixtures including propane and butane, depending on the season (in winter more propane, in summer more butane). Propylene, butylenes and various other hydrocarbons are usually also present in small concentrations. LPG is prepared by refining crude oil or drying wet natural gas, so it is entirely derived from fossil fuel sources. As its boiling point is below room temperature, LPG evaporates quickly at normal temperatures and pressures, so it is stored in pressurized vessels. LPG is the third (after gasoline and Diesel oil) most popular automotive fuel in the world, covering about 3% of the total market share, for similar reasons to those seen for the use of CNG. However it must be consider that LPG has a slightly lower octane number than CNG (RON-LPG = versus RON-CNG = , both higher than gasoline), but lower are limitations due to fuel storage in the vehicle as liquid with volumetric energy density about 80%, versus 25% of CNG, of that of gasoline, at relative low pressure (0,2 2 MPa) and greater fuelling station availability. In gas vehicles powered by a SI engine LPG is injected in the intake manifold either as a gas or a liquid. Fuel Composition During combustion processes fuel constituents are oxidised by air oxygen. The development of oxidation reactions is strongly affected by the actual air/fuel ratio α of reacting mixture, defined as: α = m air/m fuel The optimum value of this ratio primarily depends on fuel composition. Now, for hydrocarbon fuels, including in their molecules: carbon C, hydrogen H and oxygen O atoms, their elemental composition can be defined giving either:

5 1) the number (m, n, r) of atoms in fuel molecules: Cn Hm Or 2) the mass percentages of the elemental constituents: C[kgc/kgf]; H[kgh/kgf]; O[kgo/kgf] It is possible to move from first expression to the second by computing the fuel relative molecular mass μf by the values of constituent molecular masses: μc = 12,011; μh = 1,008; μo = 16: μf = 12,011*n + 1,008*m + 16*r and the mass percentages of elemental constituents: C = 12,011 n/μf; H = 1,008 m/μf; O = 16 r/μf. Two typical examples: Stoichiometric Air/Fuel Ratio Now the overall complete combustion equation of a type fuel, taking into account that (n + m/4 r/2) O2 molecules are necessary to oxidize carbon to CO2 and hydrogen contents to H2O and that is standard atmospheric air the value of the ratio N2 moles/o2 moles is equal to 3,773, can be written in the following terms: This expression only relates the elemental composition of reactant and product species, but it does not indicate the real process of combustion, which actually involves a large number (several hundreds) of chemical reactions. However this relation allows to express, as a function of the fuel composition, the stoichiometric air/fuel ratio αs, i.e. the mixture where there is the correct oxygen amount necessary to completely oxidize the fuel constituents. Considering the value of oxygen and nitrogen molecular masses (μo2 = 32 and μn2 = 28,16), the αs analytical expression can be written as: αs can also be related to mass percentages of elemental constituents of the fuel: Actual Air/Fuel Ratio Since the actual α achieved in engine may be larger, equal or smaller then αs, the real mixture composition can be express either (more commonly) by means of the equivalence ratio Φ, which defines the richness in fuel of the mixture (Φ < 1: fuel-lean; Φ = 1: stoichiometric; Φ > 1: fuel-rich mixture) or by means of the Φ reciprocal: i.e. the air excess coefficient λ, which defines the richness in air of the mixture (λ < 1: fuel-rich; λ = 1: stoichiometric; λ > 1: fuel-lean mixture). Today the actual air/fuel ratio used in given engine operation condition can be determined by means of these relation: 1) predicting with reliable simulation models the mass flow rates of air m a and fuel m f employed by the engine; 2) or measuring in a test room, where the engine is steadily running the actual values of m a and mf entering the cylinders. then, on the basis of fuel composition the value of αs can be calculated, and hence also Φ and λ can be

6 easily obtained. Fuel Heating Value An important property to assess the fuel quality is its heating value or heat of combustion. It is defined as the energy released as heat when a specified amount of oxygen undergoes standard conditions. Heating values are commonly determined by burning a fuel sample in a bomb or constant-pressure calorimeter. A difference definitely large occurs between the higher heating value (Hh) and the lower heating value (Hl). The first one is determined by bringing all the products of combustion back to the original pre-combustion temperature. Such measurements usually use initial standard temperature of 25 C for reactants. The lower heating value (Hl), instead, is determined by considering that the water component of a combustion process is in vapour state at the end of combustion. Therefore, during its measurement the calorimetric vessel and its contents are not cooled till the initial 25 C (as in the case of Hh determination). In conclusion this procedure assumes that the latent heat of vaporization in the reaction products is not recovered, since the burned gases are exhausted at high temperature. This is the case of most technical applications, included IC engines, so Hl is the heating value in practice most commonly used and reported in tables. Adiabatic Flame Temperature Adiabatic flame temperatures at constant volume (Tad,v) or at constant pressure (Tad,p) and equilibrium pressure (pe,v) reached in adiabatic constant volume combustion of isooctane-air mixture as a function of Φ. Knock Rating of Fuels Really knock is a complex phenomenon strongly influenced by engine design and almost every operating variable, but certainly the occurring of it primarily depends on the fuel anti-knock property. In particular hydrocarbons show very different abilities in resisting to knock, depending on their molecular size and structure. Moreover it has been clearly proven that even the relative knocking tendency of any two fuels significantly depends on the method used to compare them. Thus it was recognized that a comparative scale of the fuel knocking resistance must be based on the use of standardized: test engine, operation conditions, measurement of knock intensity and comparison with reference fuels. All these variables were fixed by the Cooperative Fuel Research committee, which defined first of all the characteristics and the operating condition of the variable-compression mono-cylinder engine to be used for tests. The two standard reference fuels used in test are isooctane, a fuel that has a lower knocking tendency than the common engine fuels and normal heptane with a higher knocking tendency than the average engine fuel. The tested fuel is supplied to the engine, running under standard conditions, and its compression ratio is raised to obtain a standard knock intensity. Octane Number Then, holding the compression ratio and other engine conditions constant, different mixtures of the two

7 reference fuels are tested until two mixtures are found, one of which gives slightly more knock and the other slightly less than the fuel being rate. The volumetric percentage of isooctane in the matching mixture is called the octane number of the tested fuel. The test conditions were set by CFR committee, trying to represent as far as possible the most critical service conditions (full load, low rpm, high inlet temperature...). In particular two main octane rating methods were standardized and the two sets of operating conditions are called the research method and the motor method. These conditions are more severe for the motor method, because of its higher speed, greater inlet mixture temperature and more advanced spark timings. So research octane number (RON) for a given fuel is usually larger than correspondent motor octane number (MON). The difference between the two octane numbers is designed as the fuel sensitivity (S): fuel sensitivity(s) = RON MON, typical values for Italian unleaded gasoline: RON=95, MON=85-86,S=9-10. Anti-Knock Index (AKI) In fuel filling stations of most countries, including Australia and Europe, the octane rating shown on the pump is the RON, but in Canada, USA and Brazil, is used the average of the RON and MON, called the Anti- Knock Index (AKI, and often written on pumps as (R+M)/2): Anti-Knock Index (AKI)=(RON+MON)/2. Because of the 8 to 1o point difference between RON and MON, the octane rating shown in Canada, USA and Brazil is from 4 to 5 point lower than the rating shown elsewhere in the world for the same fuel. In any case it should be clear that higher octane numbers offer a better protection against knocking combustion, but do not improve engine power output or fuel economy. Road Octane Number (Road ON) Moreover it must be considered that Research and Motor test are performed in a mono-cylinder engine with a flat roof combustion chamber, spark plug on side wall, carburetted, running at low and constant speed, while modern SI multi-cylinder engines have pent roof combustion chambers, spark plug in centre, fuel injection and operate with a great variety of speeds and loads. Therefore several methods of rating a fuel in actual vehicles, running on the road, have been developed, keeping however the same principle of laboratory tests, i.e. the comparison with the knocking resistance of primary reference fuels. Typically these road test are based initial full throttle accelerations, made from low speed and using primary reference fuels. For each of them the ignition timing is adjusted until trace knock is detected at some stage of the acceleration. Using several reference fuels a settling graph is obtained, where the measured Road Octane Numbers are reported versus basic ignition timings. Then the fuel sample is tested in the same full throttle acceleration test. The ignition timing setting, able to produce the same trace knock, is measured, by means of which from the settling graph the fuel Road ON is derived. The original tests were done in cars running on the road, but as technology developed they were moved to chassis dynamometers in controlled environments. ON Requirement or Octane Index On the other side car manufacturers are more interested in defining the fuel qualities necessary to satisfy a correct operation of engine-vehicle sets, i.e. their Octane Number Requirement (ONR) or Octane Index (OI). The actual octane requirement of a vehicle is determined by using series of standard blends of isooctane and normal heptanes and commercial gasoline of known RON and MON, to test the vehicle under a wide range of speeds and loads. Usually the conditions that require maximum ON fuels are full-throttle

8 accelerations from low starting speeds, using the highest gear available. The test is carried on using decreasing ON fuels until trace knock is detected, at some critical engine speed. Reporting the observed results on a graph as tested mixture ON versus the engine speeds, at which a trace knock was observed, the curve of vehicle octane requirement versus engine speed is obtained. Its maximum ON is of most interest, as that defines the ON of the recommended fuel for that specific vehicle fleet, since it allows engines to work on their whole operating fields without any knocking risk. Octane Number Control Finally it must be considered that in order to control the octane rating of commercial gasoline the following actions are made: 1) Refining processes have been set up to obtain a proper mixture of hydrocarbon with a good knock resistance. 2) basic gasoline is usually extended with oxygenates (such as alcohols and ethers) with high anti-knock quality; 3) small quantities of additives, able to improve the knock resistance of gasoline components, are added to final mixture. Then the use of organic compounds containing oxygen (oxygenates) as gasoline extenders is continuously increasing, mainly for the following reasons: 1) oxygenates (those of major interest are: methanol, ethanol, tertiary butyl alcohol and methyl tertiary butyl ether) have all good anti-knock properties, so their blending (typical value <20%) with basic gasoline improves ON. 2) Since oxygenates can also be produced from non-petroleum sources (natural gas, coal...) or renewable sources (biogas, biomass, waste materials...), their use may offer economic or strategic advantages. Ignition Quality Rating Therefore good ignition quality of a fuel means a short delay when it is used in an engine at a given: speed, compression ratio, inlet air and coolant temperature, turbulence motion... So, to integrate the effects of all these variables and to avoid entering into the detail of the complex physical and chemical processes of auto-ignition of a Diesel fuel it has been standardize method of rating Diesel fuels in respects to their ignition quality, that is based on engine test comparisons with primarily reference fuel mixtures, as in case of octane rating. Hence the ignition quality of a Diesel fuel is measured by its cetane number, defined by means of blends of two primary reference fuels: n-cetane (C16H34: n-hexa-decane), a straight chain paraffin with high ignition quality, is used to represent the top of the scale with a cetane number of 100, while the iso-cetane (C16H34: 2, 2, 4, 4, 6, 8, 8 heptamethylnonane (HMN)) of poor ignition quality defines the bottom of the scale with a cetane number of 15. Then the percentage of cetane in a blend of this two fuels, giving the same delay as the fuel under test, defines its cetane number according to the following expression: CN = n-cetane [%]+0,15 HMN [%]. Determination of CN The test is performed on a special compression-ignition mono-cylinder engine whose characteristic and operating condition have been defined by the cooperative fuel research (CFR) committee. As in the case of

9 the spark-ignition CFR, the compression-ignition CFR has a variable compression ratio and special loading, control and measurement equipments. The tested fuel is supplied to the engine, running under standard conditions, and its compression ratio r is raised to obtain fuel ignition at TDC. Then, recorded the r value required by the tested fuel, different mixtures of the two reference mixture are found, one of which requires a slightly higher r and the other a slightly lower r than the fuel being rated, the cetane number CN of the tested fuel is derived by interpolation of CN values of the two matching reference mixtures, weighted by means of the three measured r values. Diesel Index In particular it is quite common the use of Diesel Index (whose procedure is fixed) to predict the ignition quality of Diesel fuels as functions of their percentage of normal-paraffinic hydrocarbons. The last ones are derived from the values of two simple and easily measured properties: 1) the Aniline Point, i.e. the lowest temperature (expressed as Fahrenheit degrees F) at which equal volumes of fuel and aniline become just miscible. This point measures the content of normal-paraffinic hydrocarbons in the fuel, since they become miscible at higher temperatures than hydrocarbons of other families. 2) the fuel density expressed in API (American Petroleum Institute) degrees, a non-dimensional quantity computed by means of the following relation: API density=141,5/rho 131,5, where rho is the fuel density relative to the water density. For a given boiling temperature range, the normal-paraffinic hydrocarbons have a lower relative density and hence a higher API density, which can be assumed as index of good ignition quality. Therefore the Diesel Index can be obtained by: Diesel Index = API density*aniline Point[ F]/100. Cetane Number Control High NC values (NC>55) are required for high speed Diesel engines, since their available times for the total combustion process are relatively short. In low speed Diesel engines, instead, longer times available for combustion allow the use of worse ignition quality (lower NC). Comparing the meaning of NC and ON it is possible to recognize that they measure two opposite properties of fuels: the first one considers the fuel ignition aptitude, the second its resistance to oxidation processes. Therefore the chemical structure desired in petroleum fuels for compression-ignition engines in opposed to that wanted for spark-ignition engines. As seen in the case of octane numbers, also cetane numbers of commercial Diesel fuels can be controlled through the following actions: 1) refining processes have been set up to control the molecular structure of fuel components, so that to obtain a proper mixture of hydrocarbon with a good ignition quality. Therefore the cetane numbers of commercial Diesel fuels are no longer mainly influenced by the characteristics of the base crude, but they are now subjected to a higher and higher degree of control in the refining process (Diesel fuel typical composition: 70% paraffin, 5% olefin, 25% aromatic) 2) Basic Diesel fuels may be extended with sulphur-free and aromatic-free synthetic liquid fuels (such as liquid fuels derived from natural gas, biomass or coal), and/or biodiesel, a vegetable oil-based or animal fat based fuel consisting of long-chain alkyl esters. Their blending (typical value <20%) with basic Diesel oil can help in meeting its high CN demand and in controlling pollutant formation. Moreover, since these extenders are usually produced from non-petroleum sources (natural gas, coal...) or renewable sources (animal or vegetal fats, biomass, waste materials...), their use may offer economic or strategic advantages

10 (employ of different or renewable energy sources). 3) small quantities of additives, are added to final mixture to improve the fuel quality (to increase fuel lubricity (after removal of sulphur compounds) to prevent excessive engine wear, detergents to clean the fuel injectors and minimize carbon deposits, water dispersants...), but principally to improve its ignition quality. Among the most effective substances in reducing the ignition delay period (ignition accelerators) are: amyl nitrate, ethyl nitrate (typically 1% in volume of these ignition accelerators could increase the mixture cetane number of about 10 units) Liquid Fuel Volatility Volatility is the tendency of a liquid to vaporize, propery directly related to the liquid s vapour pressure, i.e. at a given temperature, to the pressure at which its gaseous phase is in equilibrium with the liquid phase. Fuels used in both spark ignition or compression ignition engines (gasoline and Diesel fuels) consists of mixtures of various chemical compounds. Therefore their volatility depends on their fractional composition, which can be determined by heating a fuel sample in a special device and successively removing the fractions that boil within a specific temperature range. A diagram showing the volume percentages of the evaporated fuel versus the correspondent temperatures is known as fractional distillation curve. The distillation temperatures, where 10%, 50% and 90% by volume of the fuel is evaporated (briefly called T10, T50 and T90), are often used to characterize the fuel volatility. Another common measure of fuel volatility (used for gasoline) is the Reid Vapour Pressure (RVP). It is defined as the absolute vapour pressure exerted by gasoline vapours at 100 F (37,8 C) in a standardized bombe having a 4:1 ratio of air to liquid fuel. Gasoline volatility The value of RVP is essentially determined by the vapour pressure of light fuel fraction. Thus RVP and T10 are important values in relation with a correct operation of gasoline powered (especially when engines were carburetted) vehicles. Really high values of volatility are desirable for cold winter starting and operation, while lower levels are required in avoiding vapour lock during hot summers. Vapour lock is a problem that occurs when the liquid fuel vaporizes while still in the fuel delivery circuit, causing loss of feed pressure to the injectors and so resulting in engine power losses. The fuel can vaporize due to being heated by the engine, by the local climate or due to its low boiling temperature. However modern vehicles with fuel injection use electric pumps located in the fuel tank, so that the entire fuel delivery system is under positive pressure and its value is regulated by returning unused fuel to the tank. Therefore, vapour lock is almost never a problem in modern vehicles with fuel injection. More generally the entire distillation curve of a given gasoline, represented by its point T10, T50 and T90, influences cold-start and warm-up vehicle driveability, while its end point (the highest value of boiling temperature) affects reactivity of HC emissions, combustion chamber deposits and dilution of crankcase oil. Diesel Fuel Volatility In Diesel engines volatility influences the spray evolution in combustion chamber, affecting droplet vaporization and mixing of fuel vapour and air. So, if T10 point of the diesel fuel is too high, poor starting may result and an excessive temperature between T10 an T50 points may produce a weak driveability during warm-up time. In high speed diesel engines at T50 point above 300 C might cause smoke formation, emission of unburned

11 hydrocarbons of irritating odour, contamination of lubricating oil, promotion of carbon deposits. Moreover a decrease of T90 distillation temperature reduces the emissions of PM and unburned hydrocarbons of high molecular weight, but also the yield of diesel fuel extracted from a barrel of crude oil. Finally, the boiling range of a Diesel fuel affects also its Cloud Point, i.e. the temperature below which dissolved waxes are no longer completely soluble, but precipitate as a solid phase, giving the fluid a cloudy appearance. The presence of solidified waxes thickens the liquid and clogs filters and injectors. The use of electric heaters in tanks and around delivery lines, of mixtures with lighter fuels and of treatments with proper additives can allow diesel engines to continue to operate in cold weather conditions. Renewable Fuels Fuels presented in the previous section have a fossil origin. They were formed in a very long period of millions of years by an anaerobic decay of organic material in the Earth s depths. Fossil based fuels are energy resources that are consumed much faster than nature can reproduce them, and so called nonrenewable. Renewable fuels instead are produced from natural resources, which can be restored with a renewal rate exceeding their consumption, through naturally recurring processes. Examples include: biofuels (ethanol, methanol, biodiesel and biogas), derived from biomasses and hydrogen when produced by means of biofuels or renewable energies (such as wind and solar). In the following section motor properties and application of biofuels will be presented. Biofuels for Engines A biofuel is a type of fuel whose energy is derived from biological reduction of atmospheric inorganic carbon (CO2) to organic compounds by living organisms (e.g. photosynthesis process). Biofuels for engines include liquid fuel derived from biochemical, thermal or chemical conversion of biological material from recently (in contrast with prehistoric of fossil) living organisms, as well as biogases. Converted biomasses include: a large variety of vegetal species (sugarcane, corn, bamboo, oil palm...), remains (municipal waste, plant material, organic/slaughtering waste...), animal effluents (sewages, manure...) During the last decades biofuels have gained increased technical and scientific attention, mainly because of their: 1) sustainability, allowing meeting the present needs, without compromising those of future generations, since biofuels can be derived from renewable resources, respecting natural ecosystem and environments. This result is particularly true with the second generation of biofuels, produced from waste, manufacturing remains, non-food crops, lignocelluloses biomass and algae. In fact first generation biofuels (like those derived from: corn, sugar cane or edible vegetal species), present the risk of diverting farmland or crops for biofuels production in detriment of the food supply, while second generation biofuels can potentially combine farming both for food and for fuel generation. 2) contribution to the diversification of energy sources, so that farmers of each country could farm domestically biomasses for biofuel production, reducing their dependence on unstable foreign sources of oil. This issue is particularly true for Europe, whose regulation fixed the ambitious target of replacing at least 10% of all transport fossil fuel with biofuel by the year Accordingly technologies for biofuel production in current years are facing a quite rapid development in several European countries. 3) Emission of lower amounts of greenhouse gases when burned, since they only emit back to the environment the CO2 that their source biomass absorbed out of the atmosphere during its life cycle. For example considering the case of the ethanol, which is most commonly produced through fermentation of sugar, during its formation process one mole of glucose (C6H12O6) is decomposed into two of ethanol

12 (C2H5OH) and two of CO2, while the combustion of two moles of ethanol gives: so that the global cycle (fuel production + its combustion) is represented by the sum of the two previous reaction: which is the opposite of the photosynthesis reaction: Therefore biofuels aim to be carbon neutral, since the CO2 released during the fuel combustion is balanced by the carbon previously absorbed by biomass used to produce them. Subsequently carbon neutral fuels lead to no net increase in human contributions to the atmospheric CO2 levels, so reducing the human impact to the global warming. Global Warming Potential (GWP) The carbon neutrality concept may be extended to include the other greenhouse gases, whose contribution is measured in terms of their CO2 equivalence. This comparison is done by means of the index called Global-Warming Potential (GWP), which is a relative measure of how much heat a given greenhouse gas traps in the atmosphere, comparing the amount of heat trapped by a definite mass of the considered gas to the amount of heat trapped by the same mass of CO2. The GWP index depends both on the action of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is calculated over a specific time interval (commonly 20, 100 or 500 years) and it is expressed as a factor of the CO2 effect (whose GWP is standardize as value 1). Assessment of Biofuel Effects Now, to fairly assess the full range of environmental effects produced by the biofuel use, it is necessary to calculate the total amount of carbon dioxide and other greenhouse gases emitted during all their life cycle, from putting seed in the ground to their final use in cars and trucks. So calculating correctly how much greenhouse gas is produced in burning biofuels is a complex and difficult process, which depends very much on: the used biomasses, the method by which the fuel is produced and the other assumptions made in the calculation. Therefore many different results have been obtained, but is commonly accepted that the second generation biofuels offer potentially greater savings of greenhouse gas emission (of about 50-60% in comparison with petroleum fuels) than those obtained by first-generation biofuels (which offer saving of only 5-20%). Bio-Alcohols Aliphatic alcohols (and particularly the first two: methanol and ethanol) are attractive fuels for i.c. engines, because of their technical characteristics. They can be produced either from fossil or from biological materials, but the second way is the most interesting. When obtained from biomasses and/or biological processes, they are known as bio-alcohols. At present, methanol is commonly produced using natural gas or coal as raw materials, so obtaining non-petroleum based fuel. However bio-methanol may be produced by gasification of nearly any biomass (included animal waste), to get synthesis gas in a gasifier, followed by a conventional methanol synthesis process. Bio-ethanol is mostly made by fermentation of carbohydrates coming from sugar or starch crops (sugarcane or corn). However cellulosic biomasses, derived from nonfood sources (trees and grasses), are also being developed as feedstock for ethanol production. Properties of Methanol and Ethanol

13 The power output expected from different liquid fuels is proportional to the filling coefficient λv and to the ratio Hi/αst. Gasoline, methanol and ethanol have similar values of ratio Hi/αst (about 3 MJ/kg_air) and so if λv is kept constant, this three fuels are able to give the same power output. Gasoline Mixtures with Ethanol Because of all these positive properties alcohols are commonly used as gasoline additives or extenders, in order to increase the mixture octane number, but they may also feed specially designed engines as pure fuels. In particular ethanol can be added to gasoline up to 10% without problems in common engines, while flex-fuel engine, that have fuel system and components designed for long life using ethanol, can operate with any fuel blend between gasoline and ethanol alone. This is possible thanks to a special software for fuel composition identification, which allow the engine management unit to adapt injection and ignition timing. With flex-fuel engine the compression ratio con be raised (because to higher fuel RON), with a resulting increase of engine efficiency. Generation of Biodiesel Biodiesel is the most common biofuel in Europe, where it is made using several feedstock based on: animals fats, derived from by-products of meat processing and cooking, recycled oil or virgin vegetable oils (soy, sunflowers, palm oil...). The common process to obtain biodiesel from vegetable includes two stages: at first vegetable oil are derived and then biodiesel and glycerine are generated by mean of transesterification reactions. More generally the most common process used to synthesize biodiesel is a base-catalyzed transesterification process, by which lipids (of animal fats or vegetable oils) react with alcohols (typically methanol or ethanol) to produce mostly fatty acid methyl (or ethyl) esters. Besides this usual process, there are several methods for carrying out lipid transesterification reactions. Biodiesel vs Petro-Diesel Biodiesel and petroleum diesel fuel are not chemically similar: biodiesel is composed of fatty acids esters, whereas petro-diesel is a mixture of aliphatic and aromatic hydrocarbons that contain about 10-15% carbon atoms. However, in spite of its different chemical composition, biodiesel, unlike straight vegetable oil, has combustion properties quite similar to those of petro-diesel and can replace it in most current uses. Biodiesel has many important technical advantages over petro-diesel, such as higher cetane number, no sulphur content, reduced pollutant emissions, low toxicity... Because of its good lubricating properties and high cetane numbers, biodiesel is often used as an additive to increase the ignition and lubrication quality of today s ultra low sulphur petro-diesel, since the sulphur compound provide much of lubricity. Furthermore better fuel lubrication reduces pump and injection wear. Moreover, biodiesel has virtually no sulphur content and it is oxygenated fuel, meaning it contains a reduced amount of carbon and higher hydrogen and oxygen content than petro-diesel. Thus improves the combustion process and reduces emission levels of particulates carbon monoxide and unburned hydrocarbons. Usually NOx emission increase, but in modern diesel engines equipped with EGR the presence of biodiesel seems to show no significant difference in NOx emissions. Despite its many advantages as a renewable alternative fuel, biodiesel presents a number of drawbacks, which include high

14 feedstock cost, inferior oxidative stability, and inferior low-temperature operability. Moreover from the point of view of material compatibility it was found that in the case of plastics the high density polyethylene is compatible with biodiesel, but polyvinyl chloride is slowly degraded and polystyrene is dissolved. Several types of synthetic rubbers are unaffected. Because of its properties mixtures of biodiesel and petro-diesel are commonly distributed for use in the fuel market. Blends up to 10% of biodiesel can be used in actual Diesel engines with no, or only minor modifications, while higher mixtures and pure biodiesel require some engine modifications to avoid maintenance and performance problems. Green-Diesel Green-diesel has properties and chemical composition very similar to petro-diesel, but is made from recently living biomass (variety of vegetable oils and animal fat). This type of fuel is composed of long-chain hydrocarbons, so it can be mixed with petro-diesel in any proportion for use as diesel engine fuel. The main method to produce green-diesel is hydroprocessing, that means to make vegetable oils or animal fats react with hydrogen under elevated temperature and pressure in order to change the chemical composition of the initial feedstock. This process allows to convert liquid feedstock into distillate fuels (there is also propane as co-product). This technology is diffused and very used in petroleum industry to crack (to convert) large organic molecules into smaller ones. A second method to produce green-diesel involves partially combusting a biomass source to produce carbon monoxide and hydrogen and then utilizing a particular reaction to produce complex hydrocarbons. This process is called biomass-to-liquid process (BTL process). Frequently green-diesel technology is referred to as second generation of renewable diesel fuels, which presents over the first generation the following advantages: 1) superior cold weather properties 2) large heating value 3) greater cetane number 4) propane by-product is preferable than glycerine 5) lower capital cost and operating cost of the production processes Biogas Biogas is a gas produced by the anaerobic digestion or fermentation of biodegradable materials (municipal waste, plant material, crops...) in the absence of oxygen. It is practically generated as landfill or digested gas, as result from chemical reactions and microorganism actions upon the feedstock as putrescible materials begins to break down. The biogas composition varies depending upon the origin of anaerobic digestion process (landfill, digesters, of farm wastes...), but typically it includes primarily methane and carbon dioxide, together with small amounts of nitrogen, oxygen, water vapour, hydrogen sulphide and other contaminants. This raw biogas is not high quality enough to be used as fuel in thermal machinery, principally because of its content of dangerous contaminants. In particular sulphur compounds (H2S), formed during the decomposition of organic waste, when condense out of the fuel gas are highly corrosive and damage metal engine components. When halogenated hydrocarbons are burned, chlorine and fluorine are released, which react with water, forming HCl and HF, both very corrosive to internal engine components. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or

15 various other elements present in the combustion gases. Deposits are then formed containing mostly silica (sio3) or silicates and wear metals from the engine. These very hard deposits accumulate specially in the combustion chamber, from where they cannot be easily detached. Application of Biogas Therefore the raw biogas must be upgraded and purified by means of processes whereby contaminants are absorbed or removed, leaving more methane per unit volume of gas. Among several methods of biogas upgrading, the prevalent is water washing where high pressure gas flows into a column, where CO2 and other elements are removed by cascading water running counter flow to the gas. Because of its positive characteristics, when upgraded and purified, biogas can be used in all the applications seen for NG. In particular biogas not only increase engine performance as a gaseous fuel, but it is also a renewable energy source, produced from regionally available raw material and recycled waste. Hydrogen On Earth hydrogen is a very abundant chemical element (present in most of substances), but it is not available in its free state. Therefore it must be produced from its most common composites such as: natural gas, biomass, alcohols or water. In all cases it takes energy to obtain pure hydrogen from its compounds and for this reason, hydrogen must be considered an energy carrier or storage medium rather than energy source. So the resulting impacts of its use depend on the energy source used to produce it. In particular, if hydrogen is generated by steam reforming of methane or natural gas, its production and distribution cycle releases into the atmosphere approximately the same amount of carbon dioxide that gasoline, used in a traditional vehicle, would produce. However these emissions can be significantly reduced, if hydrogen is obtained by water electrolysis, where the required electrical energy is provided by low-carbon electricity plants. Positive Properties of Hydrogen Hydrogen has been tested as fuel for i.c. engines, to prove that it could replace petroleum fuel in transportation vehicles. Hydrogen show some favourable characteristics: 1) wide range of flammability and low ignition energy in comparison with gasoline/natural gas. As a result, hydrogen can be burned in i.c. engines over a wide range of fuel/air mixtures, so hydrogen engines can operate with lean mixtures, obtaining great fuel economy and quite complete oxidation reactions. In addition the final combustion temperature is low, so reducing the amount of nitrogen oxides emitted. Be careful that lean operation can significantly reduce the power output, due to a reduction in the volumetric heating value of the air/fuel mixture. On the other side, the low ignition energy means that hot gases or spot on the cylinder can easily act as sources of ignition, creating problems of pre-ignition and backfire in the intake manifold. 2) high flame speed at stoichiometric ratio, which is nearly five time larger than that of gasoline. This means that hydrogen engines can closely approach the ideal thermodynamic engine cycle, with resulting higher efficiency. At leaner mixtures the flame velocity decreases, but it is still sufficient to allow the engine to be controlled in a qualitative manner, where air is not varied with the load, but engine power results proportional to the mass of injected fuel. So removing the pumping losses during the fluid exchange

16 process, the engine efficiency at partial loads is improved. 3) relatively high auto-ignition temperature and octane number that allows the use in hydrogen engines of compression ratios larger than in hydrocarbons engines, therefore obtaining greater engine thermal efficiency. On the other hand these properties also mean that hydrogen is a fuel difficult to be ignited in a compression ignition engine, showing that its use better fits spark ignition than diesel engines. Negative Properties of Hydrogen On the other side main unfavourable characteristics are: 1) very low volumetric energy density of gaseous hydrogen, which is due the its very low density, in spite of its quite large lower heating value per unit mass. The result is that storage of enough energy, to give vehicle an adequate driving range is a difficult technical task. 2) the very low density of gaseous H2 also results in a high volume fraction occupied by gaseous fuel in a stoichiometric mixture with air. The power output of a volumetric engine is proportional to the filling coefficient λv and to the ratio Hi/αst. Since hydrogen ratio Hi/αst is larger than that of gasoline, the final variation of power output mainly depends on the air mass trapped in the cylinder per cycle. Storage of Hydrogen There are possible different approaches: 1) gaseous storage, compressing gaseous hydrogen to increase its volumetric energy density and to permit the use of smaller but not lighter container tanks. Using pressure up to MPa, volumetric energy densities of compressed hydrogen can reach about 12-15% of those of hydrocarbon fuels, but losing about 2% of the fuel energy content to power the compressor. 2) liquid storage, which requires the liquefaction of hydrogen that boils at -253 C. Hence, its liquefaction imposes a large energy loss in a complex process of several steps of compression and cooling it down to that temperature (about 40% of the fuel energy content). The final results is that liquid liquid hydrogen has a volumetric energy density that is about 27% of that of gasoline. The tank must also be well insulated to maintain heat transfer from the ambient to the hydrogen to a very low value and to minimize liquid evaporation. 3) Chemical storage, based on the use of compounds that reversibly release hydrogen upon heating. Several metal hybrids with vary degrees of efficiency, can be used as a storage medium for hydrogen. Some are liquid at ambient conditions, other are solids which could be turned into pellets. These materials have good energy density by volume, although their energy density by mass is often worse than that seen for compressed storage. Moreover they often require quite high temperatures (around C) to release their hydrogen content. 4) physical storage at molecular level in appropriate adsorbent materials, such as carbon nanotubes or synthetic porous structures. Like liquid storage, this approach uses cold liquid hydrogen at a temperature little above -253 C, in order to reach high energy density by unit mass of the storage system. However, the main different is that, when the hydrogen would warm-up due to heat transfer with the environment, the tank is allowed to go to pressure much higher.