Patent Publication Number: US-2017355915-A1

Title: Spark ignition fuel mixture and method of making the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent document claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 62/349,664, filed on Jun. 14, 2016, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a spark ignition fuel mixture and a method of making and handling the fuel mixture. In particular, the present invention relates to a spark ignition fuel mixture comprising ethanol, diethyl ether, and water as a gasoline-alternative fuel in spark ignition engines. 
     BACKGROUND 
     The presence of water hi a vehicle&#39;s fuel system presents several problems. Among these problems are freezing in the fuel tank of the vehicle, ignition delay, phase separation, and low cold-start ability. For instance, the longer evaporation time of water relative to conventional hydrocarbon fuels increases ignition delay, which is defined as the time delay between the start of fuel injection and the combustion in an engine. Cold-start is an attempt to start a vehicle&#39;s engine when it is cold, relative to its normal operating temperature, often due to cold weather. In short, the presence of water in a fuel was known to adversely affect the functionality of a spark ignition engine. 
     Ethanol is used as a gasoline fuel blend in a spark ignition engine due to its good anti-knock performance, which increases the compression ratio of the engine and, consequently, the efficiency factor thereof. However, an ethanol fuel blend has significant problems in cold-start and cold operation due to low Reid vapor pressure (RVP) (measured, by definition, at 100° F.), high boiling point, and high latent heat of vaporization. For instance, a high content of ethanol adversely affects cold-start and warming up in low-temperature conditions because more heat is needed to vaporize ethanol than gasoline. Such problems limit the ethanol content allowed in a gasoline-ethanol mixture. 
     Diethyl ether, even though it can be produced directly from ethanol, is quite different from ethanol in terms of combustion characteristics and fuel properties. Ethanol has an octane rating of around 113, a long ignition delay of about 100 ms, and a low RVP of about 2 psi. Ethanol is hydrophilic and completely miscible with water in all proportions. By contrast, diethyl ether has a much higher cetane number of over 200, a short ignition delay of less than 2 ms, and a relatively high RVP of 16 psi. Diethyl ether has limited solubility in water (6.05 g/100 ml at 25° C.). Because of its high volatility and relatively low flash point, diethyl ether was used as an ignition enhancer added to a diesel fuel in a compression ignition engine and was known to improve cold-start of a fuel containing ethanol. 
     A widely-held belief is that ethanol used in fuel blends with a hydrophobic fuel such as gasoline must be dehydrated, resulting in anhydrous ethanol, to avoid phase separation. As well known in gasoline-ethanol blends, phase separation with water can cause damages to fuel storing or delivering equipment, and the remaining fuel may be out of specification. The necessity of anhydrous ethanol requires an additional dehydration step mainly due to the fact that ethanol is hygroscopic, absorbing water even from air. With respect to diethyl ether, the difficulties and expense in producing dry diethyl ether further arise because diethyl ether presents many handling and storage problems with safety issues. For example, diethyl ether has a high vapor pressure and presents a high risk of explosion when contacted with high temperature sources in air. Thus, more efficient, practical, and economic approaches are needed for utilizing fuel blends comprising ethanol and/or diethyl ether. 
     U.S. Publication No. 2014/0275636 studied the possibility of making a fuel from a mixture of diethyl ether and ethanol. However, the advantageous effects of water on the fuel properties of a diethyl ether-ethanol mixture are largely unknown. For this reason, along with the known adverse effects of water on a fuel, no meaningful attempts have been made to utilize water as a component in a fuel for a spark ignition engine. 
     BRIEF SUMMARY 
     The present invention relates to a fuel mixture comprising diethyl ether, ethanol, and water that may be used as a spark ignition engine fuel, satisfying the requirements for a contemporary automobile gasoline fuel. 
     One embodiment of the present invention provides a spark ignition fuel comprising: a) diethyl ether with a content from 33.3 to 50 vol % of the mixture; b) ethanol with a content of at least 27 vol % of the mixture; and c) water with a content of at least 6 vol % of the mixture and not exceeding the ethanol content; wherein the mixture remains in a form of homogeneous liquid at −40° C. 
     The spark ignition fuel mixture is further characterized by: wherein the diethyl ether content is smaller than or equal to 68 minus 0.52 times the ethanol content in vol % of the mixture; and wherein the mixture has a Reid vapor pressure (RVP) of less than 5 psi; wherein the diethyl ether content is larger than or equal to 47.3 minus 0.23 times the ethanol content in vol % of the mixture; wherein the mixture enables a cold-start at −30° C.; and further comprising lubricants, antioxidants, and/or denaturants. 
     One embodiment of the present invention provides a method of making a spark ignition fuel mixture, comprising: forming a mixture of diethyl ether, ethanol, and water in pre-determined relative amounts such that the mixture forms a homogeneous liquid. 
     The method of making a spark ignition fuel mixture is further characterized by: wherein the forming comprises dehydrating an initial volume of a hydrous ethanol reactant using a catalyst to form an intermediate product mixture consisting of diethyl ether and water; wherein the forming further comprises mixing the intermediate product mixture and an additional volume of the initial hydrous ethanol reactant such that the pre-determined relative amounts are obtained; and wherein the dehydrating dehydrates ethanol in the initial volume of the hydrous ethanol reactant such that the-predetermined relative amounts are obtained. 
     One embodiment of the present invention provides a method of handling a spark ignition fuel mixture, comprising: providing the spark ignition fuel mixture, wherein the spark ignition fuel mixture remains in a form of homogeneous liquid and no step of removing a portion of the water is performed before the fuel mixture is used as a fuel. 
     The method of handling a spark ignition fuel mixture is further characterized by: wherein the providing step comprises providing hydrous ethanol and is devoid of removing water from the anhydrous ethanol. 
     The fuel mixtures of the present invention enable operating a spark ignition engine with a non-petroleum based fuel comprising a diethyl ether/ethanol/water mixture without other ignition enhancers, without emulsifiers, and without making major adjustments to the engine as compared to operation with conventional gasoline fuels. The ignition delay time of the fuel mixture of the present invention is similar to that of gasoline fuels having a Research Octane Number (RON) between 85 and 100, and therefore ignition enhancers are no longer necessary to avoid in-cylinder temperature increase due to excessively advanced spark timing commonly encountered with ethanol fuel blends. Additional components such as lubricants, antioxidants, and denaturants may be present in the fuel mixture. 
     Additionally, through excellent charge cooling due to a higher heat of vaporization of water and ethanol, thermodynamic efficiency in a spark ignition engine may be in good company with conventional diesel engines. With the fuel mixtures of the present invention, it is possible to operate a turbo-charged or super-charged spark ignition engine without an intercooler or a heat exchanger. The emissions of unburned hydrocarbon, nitrogen oxides, and carbon dioxide are reduced significantly as well. Therefore, elaborate exhaust gas cleaning or three-way catalytic converter as applied in gasoline fueled spark ignition engines can be significantly reduced or completely removed. 
     The present invention also relates to a method of making the fuel mixtures comprising diethyl ether, ethanol, and water. One embodiment relates to mixing diethyl ether, ethanol, and water under mixing conditions sufficient to form a homogeneous mixture. One advantage of the present invention is that no additional step of removing water from ethanol is needed. Diethyl ether may be made by any known method. The present invention needs neither the step of removing water produced in a process of forming diethyl ether from ethanol, nor the step of enhancing diethyl ether concentration by other means. 
     One particular advantage of using diethyl ether-hydrous ethanol mixtures is that they can be produced from less purified ethanol as a starting material. This process is made under dehydrating conditions using a catalyst with high selectivity to diethyl ether. The fuel mixture made by this process may be used as a fuel without the further step of removing water or enhancing the diethyl ether yield. As a result, the production cost associated with purifying ethanol and diethyl ether is reduced, and therefore the consequent low cost per unit energy delivered makes the fuel mixture of the present invention competitive with conventional fuels. 
     The spark ignition fuel mixtures of the present invention can be transported pre-mixed, stored, and used without substantial risks of water condensation/freezing or explosion of the fuel. The present invention eliminates the significant drawbacks encountered in utilizing diethyl ether-ethanol blends. At the same time, the disclosed fuel mixture offers the advantages of high octane rating, proper evaporation properties, good ignition quality to cold start, and a complete miscibility without phase separation. All these properties together make the fuel mixture disclosed herein economically viable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  displays measured ignition delay times with respect to temperature for the fuel mixtures disclosed in the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a spark ignition fuel mixture comprising ethanol, diethyl ether, and water. 
     In the present invention, water in the fuel mixture advantageously contributes to the fuel properties even though it is not a combustible component. 
     A high content of diethyl ether in a fuel can cause preignition in a spark ignition engine because it has a very short ignition delay and low autoignition temperature of about 180° C. The present invention found that, to avoid undesired preignition during operating a spark ignition engine, the diethyl ether content should be less than 50 vol % of the mixture. 
     The present invention resolves problems relating to two important characteristics of hydrous fuel blends: the freezing point and the maximum allowable content of diethyl ether prior to the onset of phase separation. For general use in the North America region, a fuel is required to remain as a liquid at temperatures as low as −40° C. To remain in a liquid phase at −40° C. for ethanol-water mixtures, the water content was found to require less than about 40 vol % according to the freezing point data of hydrous ethanol. Hydrous ethanol is defined in the present invention as ethanol having some water content in the mixture. The maximum water content in a diethyl ether-ethanol-water mixture is calculated using Blagden&#39;s freezing-point depression (ΔT F ), which depends only on the solute concentration with the cryoscopic constant (K F ), as follows: 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       T 
                       F 
                     
                   
                   = 
                   
                     
                       
                         T 
                         
                           F 
                           , 
                           solvent 
                         
                       
                       - 
                       
                         T 
                         
                           F 
                           , 
                           solution 
                         
                       
                     
                     = 
                     
                       
                         K 
                         F 
                       
                       × 
                       
                         
                           moles 
                            
                           
                               
                           
                            
                           of 
                            
                           
                               
                           
                            
                           solute 
                         
                         
                           kilograms 
                            
                           
                               
                           
                            
                           of 
                            
                           
                               
                           
                            
                           solvent 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     wherein T F, solvent  and T F, solution  is the freezing point of the solvent and the solution, respectively. For a diethyl ether-ethanol-water mixture, the solvent is water, and the solute is ethanol and/or diethyl ether. The cryoscopic constant is dependent on the properties of the solvent, not the solute. For water, K F =1.853 K·kg/mol. Accordingly, a fuel mixture condition to avoid freezing is found as follows: W≦26.2+0.21×E, wherein W and E is the content in vol % of water and ethanol, respectively. Additional conditions found in the present invention are as follows: the water content does not exceed the ethanol content in vol % of the fuel mixture, and the diethyl ether content is larger than or equal to 33.3 vol % of the fuel mixture. 
     A high content of water in the liquid phase of a diethyl ether-ethanol-water mixture may result in phase separation due to immiscibility of water and diethyl ether. However, the present invention found that the phase separation problem of the mixture can be avoided by controlling the amount of diethyl ether in the mixture. Liquid-liquid equilibrium (LLE) data for ternary mixtures consisting of water, ethanol, and diethyl ether and experimentation were used to examine the phase of the mixture. As a result, to obtain a homogeneous liquid in the diethyl ether-ethanol-water mixture without phase separation, the following condition should be met: D≦127−3×E, wherein D and E is the content in vol % of diethyl ether and ethanol, respectively. 
     A more preferred condition is as follows: the ethanol content is at least 27 vol % of the mixture; the water content does not exceed the ethanol content in vol % of the mixture; and the water content is at least 6 vol % of the mixture, preferably at least 15 vol % of the mixture. 
     One of the criteria in fuel properties for a spark ignition engine is volatility. Volatility, customarily measured in terms of Reid Vapor Pressure (RVP) (measured, by definition, at 100° F.), is closely linked to emission of volatile organic components (VOCs) and cold-start ability. The recommended RVP for E85 fuel is 5 psi. Raoult&#39;s law states that total vapor pressure of a liquid mixture is equal to the mole-fraction-weighted sum of the vapor pressures of each component in the mixture. Given that the RVP value of diethyl ether, ethanol, and water is 16, 2, and 1 psi, respectively, a condition for the RVP of the mixture to be less than 5 psi is expressed in Eq. (2): 
         x   D   ≦x   E /15+80/3  (2)
 
     where x D  and x E  are the concentrations in mole percent of diethyl ether and ethanol, respectively. Accordingly, to avoid the emission problem relating to VOC, the present invention found a desired condition: D≦68−0.52×E, wherein D and E are the content in vol % of diethyl ether and ethanol, respectively. 
     Regarding cold-start considerations, the industry standard for cold-starting an automobile is that a spark ignition fuel should have sufficient vapor pressure to start a vehicle at a temperature down to −30° C. The following method of the present invention is used to determine the ethanol conversion in dehydration of hydrous ethanol to provide sufficient diethyl ether yield (and thus sufficient vapor pressure) to cold-start a vehicle at the cold-start temperature, −30° C. 
     The pressures of ethanol (P E ) and diethyl ether (P D ) in the vapor phase, and the mole concentrations of ethanol (x E ) and diethyl ether (x D ) in the liquid phase, at a cold-start temperature of −30° C. were obtained from ASPEN PLUS, which provides results for a liquid-vapor system of diethyl ether-ethanol-water ternary mixture at static equilibrium. To ignite a vapor mixture consisting of air, ethanol, diethyl ether, and water, the combined vapor pressures of the flammable components (ethanol and diethyl ether) must exceed the lower flammability limit, which is expressed as 0.23P E +0.52P D =1.0. For example, for anhydrous ethanol (i.e., 100 mol % ethanol) as an input, the ASPEN PLUS calculation yields that ethanol conversion of 15.4% to form diethyl ether (i.e., x E =84.6 mol % and x D =7.7 mol %) would provide sufficient vapor pressure to cold-start at −30° C. 
     However, when a fuel is injected to a combustion chamber in a modern vehicle, the actual conditions may be different from those of an ideal static system. To produce a more accurate condition applicable to a real vehicle, the present invention considered four factors. First, when a fuel mixture is injected into the cold cylinder of an engine, part of the fuel is vaporized and part of remains as droplets suspended in air. These vapors and droplets of the fuel injected are assumed as a quasi-static system which is equivalent to the static liquid-vapor system dealt with in ASPEN PLUS. Second, an excess amount of fuel mixture is injected during the cold-start period in a real engine operation in order to ensure ignition and to avoid any possibility of misfire. In this invention, the injection amount of fuel mixture is assumed to be twice as much a fuel per unit stoichiometric amount of air (i.e., fuel-rich mode, relative air-fuel ratio=2). Third, the latent heat of vaporization is ignored. Fourth, the contribution of water to the vapor phase is neglected since water is nonflammable and its vapor pressure is negligibly small at the cold-start temperature. 
     At a given cold-start temperature, an engine is only able to start when a fuel mixture with the appropriate composition is injected. The injected fuel contains N E  moles of ethanol and N D  moles of diethyl ether. The stoichiometric air-to-fuel ratios are 14.3 for pure ethanol and 28.6 for pure diethyl ether. When twice as much a stoichiometric amount of fuel (relative air-fuel ratio=2) is injected and completely combusted with 1 mol of air, then N E  and N D  are related as follows: 
       14.3 N   E +28.6 N   D =2  (3)
 
     For several hydrous ethanol fuels with different water contents, the vapor pressures (P E  and P D ) and the mole fractions (x E  and x D ) are obtained from ASPEN PLUS at −30° C. The moles of vaporized ethanol (n E ′) and diethyl ether (n D ′) per 1 mole of air are given as follows: 
     
       
         
           
             
               
                 
                   
                     n 
                     E 
                     ′ 
                   
                   = 
                   
                     
                       P 
                       E 
                     
                     
                       101.3 
                       - 
                       
                         ( 
                         
                           
                             P 
                             E 
                           
                           + 
                           
                             P 
                             D 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     n 
                     D 
                     ′ 
                   
                   = 
                   
                     
                       P 
                       D 
                     
                     
                       101.3 
                       - 
                       
                         ( 
                         
                           
                             P 
                             E 
                           
                           + 
                           
                             P 
                             D 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Then, the moles of ethanol (n E ) and diethyl ether (n D ) in the liquid phase after vaporization are expressed as follows: 
         n   E   =N   E   −n′   E   (6)
 
         n   D   =N   D   −n′   D   (7)
 
     Defining x=n E /(n E +n D ) and using Eqs. (6) and (7), N D  and N E  can be written in terms of n E ′, n D ′, and x. 
     
       
         
           
             
               
                 
                   
                     N 
                     D 
                   
                   = 
                   
                     
                       
                         n 
                         D 
                         ′ 
                       
                       + 
                       
                         x 
                          
                         
                           [ 
                           
                             
                               2 
                               14.3 
                             
                             - 
                             
                               n 
                               D 
                               ′ 
                             
                             - 
                             
                               n 
                               E 
                               ′ 
                             
                           
                           ] 
                         
                       
                     
                     
                       1 
                       + 
                       x 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     N 
                     E 
                   
                   = 
                   
                     
                       2 
                       14.3 
                     
                     - 
                     
                       2 
                        
                       
                         N 
                         D 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Here, x can be obtained from x=x E /(x E +x D ), if the small change in water concentration in the liquid phase during vaporization is neglected. Therefore, N D  and N E  can be calculated using Eqs. (8) and (9) with P E , P D , x E  and x D . Also, the ethanol conversion A (%) required to cold-start a vehicle can be calculated by using N D  and N E  as follows: 
     
       
         
           
             
               
                 
                   
                     A 
                      
                     
                       ( 
                       % 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         2 
                          
                         
                           N 
                           D 
                         
                       
                       
                         
                           2 
                            
                           
                             N 
                             D 
                           
                         
                         + 
                         
                           N 
                           E 
                         
                       
                     
                     × 
                     100 
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     In view of the foregoing, it was found that the ethanol conversion in excess of 38.1% for anhydrous ethanol (x E,0 =100 mol %), and the ethanol conversion to form diethyl ether in excess of about 60.8% for hydrous ethanol with 50 mol % ethanol (x E,0 =50 mol %), should be used to cold-start at −30° C. The corresponding concentrations of the mixture are (x D , x E )=(19.06, 61.89) for the anhydrous ethanol and (x D , x E )=(15.21, 19.59) for the 50 mol % hydrous ethanol, respectively. By repeating the calculation with a different water content (or ethanol content) in hydrous ethanol, the required ethanol conversion as function of ethanol content in hydrous ethanol and the corresponding diethyl ether content as a function of ethanol in mol % are determined. The relations in mol % obtained according to the above procedure are converted to the relations in vol %. Finally, the cold-start requirement is expressed as follows: D≧47.3−0.23E, wherein D and E are the contents in vol % of diethyl ether and ethanol, respectively. 
     The present invention also relates to a method of making a homogeneous fuel mixture of diethyl ether, ethanol, and water. To avoid phase separation due to the immiscibility of water and diethyl ether, mixing may be conducted by mixing ethanol and water with the subsequent addition of diethyl ether, or by mixing ethanol and diethyl ether with the subsequent addition of water. The content of each component must be pre-determined to form a workable fuel mixture. 
     In another embodiment, a diethyl ether-ethanol-water mixture may be produced by dehydrating a hydrous ethanol reactant using a catalyst with high selectivity to diethyl ether. In this method, even less purified ethanol may be used as a starting material. The water content in a hydrous ethanol reactant and the required ethanol conversion % of the dehydration process may be pre-determined according to the composition of the fuel mixture to be obtained. This method is advantageous in that the produced fuel mixture can be used without further treatment. The pre-determination step is set forth in detail in the following paragraphs. 
     During dehydration of a hydrous ethanol reactant to form diethyl ether, if a selectivity to diethyl ether is to be assumed 100%, the mole concentrations of diethyl ether (x D ) and ethanol (x E ) in the mixture can be expressed, depending on the initial concentration of ethanol x E,0  (mol %) in the hydrous ethanol reactant, and the ethanol conversion A (%), as follows: 
     
       
         
           
             
               
                 
                   
                     x 
                     D 
                   
                   = 
                   
                     
                       
                         x 
                         
                           E 
                           , 
                           0 
                         
                       
                       2 
                     
                      
                     
                       A 
                       100 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
             
               
                 
                   
                     x 
                     E 
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         - 
                         
                           A 
                           100 
                         
                       
                       ) 
                     
                      
                     
                       x 
                       
                         E 
                         , 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     To directly determine the initial ethanol content and the ethanol conversion, Equations (11) and (12) are rearranged as follows: 
     
       
         
           
             
               
                 
                   
                     x 
                     
                       E 
                       , 
                       0 
                     
                   
                   = 
                   
                     
                       x 
                       E 
                     
                     + 
                     
                       2 
                        
                       
                         x 
                         D 
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
             
               
                 
                   A 
                   = 
                   
                     
                       
                         2 
                          
                         
                           x 
                           D 
                         
                       
                       
                         ( 
                         
                           
                             2 
                              
                             
                               x 
                               D 
                             
                           
                           + 
                           
                             x 
                             E 
                           
                         
                         ) 
                       
                     
                     × 
                     100 
                      
                     
                       ( 
                       % 
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     This means that, necessary values for x E,0  and A may be pre-determined using Equations (13) and (14). A desired fuel mixture having x D  and x E  may be obtained by dehydrating a hydrous ethanol reactant having x E,0  until the conversion % reaches A. 
     In another embodiment, a desired fuel mixture may be obtained by dehydrating ethanol in entirety (A=100%) in an initial volume of a hydrous ethanol reactant to form an intermediate product and adding an additional volume of the hydrous ethanol reactant thereto. A preferred intermediate product is a binary mixture of diethyl ether and water. The required mixing ratio (by mole or weight) of the final product is equivalent to the ethanol conversion, A (%), which can be calculated as Eq. (14). In this method, neither removal of the water produced in the conversion of ethanol nor separation processes to enhance the diethyl ether yield is required. 
     Example 1 
     A fuel mixture of 40 vol % diethyl ether, 40 vol % ethanol, and 20 vol % water was made in Example 1. 
     The diethyl ether content of 40 vol % is present between 33.3 and 50 vol %, the ethanol content of 40 vol % is larger than 27 vol %, and the water content of 20 vol % does not exceed the ethanol content of 40 vol %. The diethyl ether content of 40 vol % is smaller than 47.2 vol % (=68−0.52*40), but larger than 38.1 vol % (=47.3−0.23*40). 
     Example 2 
     This example shows that the required ethanol content in the initial hydrous ethanol and the required conversion percentage thereof can be determined from a desired composition of the final fuel mixture. 
     First, the amounts of diethyl ether, ethanol and water in vol % in the final fuel mixture are converted to mol %, as follows: 
     
       
         
           
             
               
                 
                   
                     C 
                     
                       
                         mo 
                          
                         
                             
                         
                          
                         l 
                       
                       , 
                       i 
                     
                   
                   = 
                   
                     
                       
                         
                           C 
                           
                             vol 
                             , 
                             i 
                           
                         
                          
                         
                           
                             ρ 
                             i 
                           
                           
                             M 
                             i 
                           
                         
                       
                       
                         
                           ∑ 
                           i 
                         
                          
                         
                           
                             C 
                             
                               vol 
                               , 
                               i 
                             
                           
                            
                           
                             
                               ρ 
                               i 
                             
                             
                               M 
                               i 
                             
                           
                         
                       
                     
                     × 
                     100 
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     where the index i indicates the three components, diethyl ether, ethanol and water; C mol  and C vol  are the concentrations in mol % and vol %, respectively. M is the molar mass of the components, which is 74 for diethyl ether, 46 for ethanol, and 18 for water, respectively, and p is the density of the components, which is 0.71 g/cm 3  for diethyl ether, 0.79 for ethanol and 1.0 for water. Thus, 40 vol % diethyl ether in the fuel mixture corresponds to 17.6 mol % as follows: 
     
       
         
           
             
               
                 100 
                 * 
                 40 
                 * 
                 
                   0.71 
                   74 
                 
               
               
                 
                   40 
                   * 
                   
                     0.71 
                     74 
                   
                 
                 + 
                 
                   40 
                   * 
                   
                     0.79 
                     46 
                   
                 
                 + 
                 
                   20 
                   * 
                   
                     1.0 
                     18 
                   
                 
               
             
             = 
             
               17.6 
                
               
                   
               
                
               mol 
                
               
                   
               
                
               % 
             
           
         
       
     
     Likewise, the molar concentrations of ethanol and water are 31.5 mol % and 50.9 mol %, respectively. Using Eqs. (13) and (14), the ethanol content, x E,0 , in the initial hydrous ethanol and the ethanol conversion A are determined, respectively, as follows: 
     
       
         
           
             
               x 
               
                 E 
                 , 
                 0 
               
             
             = 
             
               
                 31.5 
                 + 
                 
                   2 
                   * 
                   17.6 
                 
               
               = 
               
                 66.66 
                  
                 
                     
                 
                  
                 mol 
                  
                 
                     
                 
                  
                 % 
               
             
           
         
       
       
         
           
             A 
             = 
             
               
                 
                   
                     2 
                     * 
                     17.6 
                   
                   
                     
                       2 
                       * 
                       17.6 
                     
                     + 
                     31.5 
                   
                 
                 × 
                 100 
               
               = 
               
                 52.77 
                  
                 % 
               
             
           
         
       
     
     The initial concentrations in mol % are then converted to the vol % as follows: 
     
       
         
           
             
               
                 
                   
                     C 
                     
                       vol 
                       , 
                       i 
                     
                     0 
                   
                   = 
                   
                     
                       
                         
                           C 
                           
                             
                               mo 
                                
                               
                                   
                               
                                
                               l 
                             
                             , 
                             i 
                           
                           0 
                         
                          
                         
                           
                             M 
                             i 
                           
                           
                             ρ 
                             i 
                           
                         
                       
                       
                         
                           ∑ 
                           i 
                         
                          
                         
                           
                             C 
                             
                               
                                 mo 
                                  
                                 
                                     
                                 
                                  
                                 l 
                               
                               , 
                               i 
                             
                             0 
                           
                            
                           
                             
                               M 
                               i 
                             
                             
                               ρ 
                               i 
                             
                           
                         
                       
                     
                     × 
                     100 
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     The ethanol content in the initial hydrous ethanol then becomes E 0 =86.61 vol % as follows: 
     
       
         
           
             
               
                 100 
                 * 
                 66.66 
                 * 
                 
                   46 
                   / 
                   0.79 
                 
               
               
                 
                   66.66 
                   * 
                   
                     46 
                     0.79 
                   
                 
                 + 
                 
                   33.34 
                   * 
                   18 
                 
               
             
             = 
             
               86.61 
                
               
                   
               
                
               vol 
                
               
                   
               
                
               % 
             
           
         
       
     
     In summary, the fuel mixture consisting of 40 vol % diethyl ether, 40 vol % ethanol, and 20 vol % water was produced by dehydrating hydrous ethanol containing 86.61 vol % of ethanol at the ethanol conversion of 52.77%. 
     Example 3 
     This example shows that a fuel mixture can be made by mixing an intermediate product, resulted from dehydrating ethanol in an initial volume of a hydrous ethanol reactant, and an additional volume of the hydrous ethanol reactant. In Example 2, it is shown that the volume concentration of (D, E, W)=(40, 40, 20) is converted to the mole concentration of (x D , x E , x W )=(17.6, 31.5, 50.9), and the corresponding initial mole concentrations and the ethanol conversion are (x D , x E , x W ) 0 =(0, 66.66, 33.34) and A=52.77%, respectively. 
     After a complete conversion of the ethanol (i.e., A=100%) in the initial volume of the hydrous ethanol reactant, according to Eq. (11), the mole concentration of diethyl ether is equal to a half of the ethanol content of the initial hydrous ethanol, i.e., x D,f =x E,0 /2. Eq. (12) gives x E,f =0, meaning there is no ethanol in the intermediate product. Therefore, the mole concentrations of the intermediate product is given as (x D , x E , x W ) f =(33.33, 0, 66.67). 
     Therefore, the fuel mixture comprising 40 vol % diethyl ether, 40 vol % ethanol, and 20 vol % water can be produced as follows: 
     (1) provide an initial volume of a hydrous ethanol reactant containing 86.61 vol % of ethanol; 
     (2) produce an intermediate product consisting of diethyl ether and water by completely dehydrating the ethanol in the initial volume of the hydrous ethanol reactant; and 
     (3) mix together 52.77 wt % (or mol %) of the intermediate product and 47.23 wt % (or mol %) of an additional amount of the hydrous ethanol reactant. 
     Example 4 
     A number of fuel mixtures obtained by the methods disclosed above were tested to measure various characteristics such as operation in general and ignition delay time in an ignition quality tester engine. Tests were performed at initial charge pressure of 21 bar, and the initial air temperature of the chamber was regulated within the range between 400 to 580° C. Neither lubricant nor emulsifier was added to the fuels. 
     In  FIG. 1 , a fuel mixture comprising diethyl ether, ethanol, and water blended in predetermined volumetric mixture ratios is compared to iso-octane having a RON  100  in terms of ignition delay time with respect to temperature. The fuel mixture of the present invention labeled as “DEW 221” represents a fuel mixture of 40 vol % diethyl ether, 40 vol % ethanol, and 20 vol % water (i.e., volumetric content ratio is D:E:W=2:2:1).  FIG. 1  shows that the ignition delay characteristics of DEW 221 are close to those of iso-octane. Such finding confirms that the fuel mixture of the present invention may be used for spark ignition engines. It was also observed that decreasing water content results in decreasing ignition delay. 
     Those skilled in the art may recognize that the present invention can be embodied in other specific forms without departing from the spirit and essential characteristics of the present invention. Therefore, it should be understood that the embodiments described above are illustrative and not restrictive to limit the scope. The scope of the present invention is represented by the claims to be described below rather than the detailed description, and it is to be interpreted that the meaning and scope of the claims and all the changes or modified forms derived from the equivalents thereof come within the scope of the present invention. 
     Furthermore, the advantages described above are not necessarily the only advantages, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment.