ARTICLE pubs.acs.org/EF
Measurement of Enthalpies of Vaporization of Isooctane and Ethanol Blends and Their Effects on PM Emissions from a GDI Engine Longfei Chen* and Richard Stone Department of Engineering Science, University of Oxford, Oxford, U.K. ABSTRACT: Experimental measurements have been made of the enthalpies of mixing of isooctane and ethanol blends so as to calculate the enthalpies of vaporization of these mixtures. The enthalpy of vaporization is very important for the performance of spark ignition engines, especially those that use gasoline direct injection (GDI). High enthalpies of vaporization increase the charge cooling effect so that the volumetric efficiency is improved (thereby increasing the specific output) and a higher compression ratio can also be used, because there is a lower temperature at the start of compression. The higher compression ratio increases both the efficiency and the specific output. However, measurements reported here show that the increased enthalpy of vaporization has an adverse effect on the particulate matter (PM) emissions from a GDI engine. This is attributed to the air fuel mixture being less homogeneous.
1. INTRODUCTION The increased market share of ethanol/gasoline blends as transportation fuels and the increasingly stringent particle emissions legislation make it important to study the properties of such blends and their effects on PM (particulate matter) emissions. Previous research has focused on thermodynamic properties of the blends, such as Reid vapor pressure (RVP), distillation properties, and the temperature at a vapor/liquid ratio of 20 (T at V/L20).1,2 However, these parameters primarily reflect volatility properties that are mostly important in terms of the tendency for vapor lock to occur and evaporative emissions. As far as engineout PM emissions are concerned, the latent heat of vaporization might be of more relevance because it refers to the energy required to vaporize the entire liquid fuel, rather than the value for equilibrium evaporation measurements when there is no restriction on the heat input, as measured in these vapor pressure and distillation experiments. The enthalpies of vaporization of gasoline/ethanol blends have not been widely covered in the literature, but they are necessary as they serve as basic thermodynamic data for determining the dynamics of fuel vaporization, which, in turn, can affect PM emissions significantly. A number of researchers3,4 have derived the enthalpies of vaporization (hfg) of such blends from their vapor pressure data using the Clausius-Clapeyron equation. However, this method may only give an estimate value for the enthalpy of vaporization because the Clausius-Clapeyron equation is only valid for an isothermal phase change, rather than mixtures, such as ethanol/hydrocarbons blends, with a wide boiling point range.5 Instead of an estimated measure of the latent heat of evaporation (hfg), this paper uses a method to derive this enthalpy by subtracting the enthalpy of mixing from the mass weighted specific enthalpies of vaporization of the individual blend components. In this work, isooctane was used as a proxy for gasoline hydrocarbons and the enthalpy of vaporization has been measured for a wide range of isooctane/ethanol blends (E5, E10, E20, E35, E50, E70, and E85), where E## means ##% by volume ethanol in the mixture. Blends up to E10 are used interchangeably with r 2011 American Chemical Society
unleaded gasoline in Europe. Flex-fuel vehicles can be used with any ethanol gasoline blend up to E85, but it should be noted that, in cold weather, the ethanol content in E85 is reduced and might in reality be E70. Finally, the measured PM emissions data on both a number and a mass basis from a GDI engine have been measured and the implication of the enthalpy of vaporization of the ethanol blends on PM emissions will be discussed.
2. EXPERIMENTAL SECTION 2.1. Enthalpy of Vaporization Measurement. 2.1.1. Fuels. The base fuel used in this study was isooctane. The enthalpies of mixing for a range of isooctane/ethanol blends in different blending proportions (E5, E10, E20, E35, E50, E70, and E85 as ##% by volume) have been measured. A number of citations for the enthalpy of vaporization of ethanol have been found, and there is inconsistency in the values and units among them. Some reference books give discrete values for discrete temperatures, and others provide an empirical formula for a specified temperature range. Figure 1 gives a summary of the values for the enthalpy of vaporization of ethanol cited from a number of references. In this work, 922 kJ/kg was used for the enthalpy of vaporization of ethanol at 25 °C, which is cited in ref 9, the reference selected by NIST. Table 1 lists some properties of the two blend components. The improvement in the research octane number is not a linear function of the volume fraction. As the measured research octane number of a blend is higher than the linear prediction, this leads to the concept of a blending value for the octane number. Owen and Coley12 quote a blending value of 112-120 for the research octane number of ethanol; the value will depend on the composition of the base fuel and the proportion of ethanol. An additional issue in the use of ethanol blends for transport is due to the hygroscopic nature of ethanol. If too much moisture (for example, from the air) is absorbed, then it might lead to phase separation and a Received: November 23, 2010 Revised: January 5, 2011 Published: February 22, 2011 1254
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Figure 1. Summary of data for the enthalpy of vaporization of ethanol.6-11
Table 1. Properties of Isooctane and Ethanol ethanol density @ 15 °C (kg/L) research octane number
0.786 107
isooctane 0.690 100
boiling point at 1 atm (°C)
78.3
99.2
stoichiometric air fuel ratio lower heating value (MJ/kg, 25 °C)
9.00 26.9
15.13 44.3
heat of vap. (kJ/kg, 25 °C)
924.2
308
H/C
3
2.25
O/C
0.5
0
Figure 2. Typical temperature trace when mixing isooctane with ethanol. and the thermal conductivity] than the borosilicate glass; hence, the thermal capacity of a plastic beaker will be lower and produce less heat transfer into its wall than a glass beaker.14 The heat flux at the contact surface between the liquid mixture and the plastic beaker during the temperature drop period has been taken into account. A semi-infinite solid heat conduction model has been adopted. The semi-infinite √ assumption is fully justified as the depth of thermal penetration ( (Rt)) is only 1.1 mm for a 10 s duration experiment; although this is only slightly smaller than the minimum wall thickness of the beaker (1.5 mm), this is acceptable as the heat flow is very small compared to the change in the internal energy of the mixture. The analytic expression for the heat flux at the boundary surface is given as follows15 qs ðtÞ ¼
further increase in the enthalpy of vaporization. Johansen and Schramm (2009) studied the miscibility of ethanol-gasoline-water blends experimentally, and their results showed that the ethanol/ water azeotrope (4.4% water by mass) is miscible with Euro95 gasoline, for a gasoline concentration of at least 95% in the blend, at temperatures as low as -25 °C.13 Therefore, the phase separation should not be an issue for ethanol with less water content than 4.4% by mass. Owen and Coley12 point out that the miscibility of water with a gasoline/ethanol blend will depend on the aromatic content of the gasoline, and whether or not a cosolvent (such as higher alcohols) has been added. 2.1.2. Methodology. A total of seven fuel blends (E5, E10, E20, E35, E50, E70, and E85) were mixed on a volume basis. The ethanol was taken from a 2.5 L bottle of anhydrous ethanol that was at other times kept carefully sealed to avoid any issues of moisture absorption. A thermocouple with an exposed junction (to give a fast response rate) was used to measure the temperature profiles for the mixing process, and the amplified data were logged to a PC by using an NI USB-6008 data acquisition card at a 100 Hz sampling rate. The two fuel components were allowed to reach the same room temperature prior to mixing. A plastic beaker was used to contain the major liquid fuel (isooctane in the cases of E5 to E50 and ethanol in the cases of E70 and E85), and the corresponding minor fuel (ethanol in the cases of E5 to E50 and isooctane in the cases of E70 and E85) was injected into the beaker using a calibrated syringe with the needle kept below the free surface of the liquid in the beaker. This was followed by a vigorous stir to ensure complete mixing of the fuel components. The temperature traces were recorded throughout the whole process, and a typical temperature trace is illustrated in Figure 2. The reason for using a plastic beaker rather than a glass beaker is that polypropylene (the base material of the plastic) has a lower thermal contact coefficient [(ckF)1/2, where c and k are the specific heat capacity
kðT0 - Ti Þ pffiffiffiffiffiffiffiffi ðW=m2 Þ πRt
ð1Þ
where T0-Ti refers to the temperature drop when mixing and R refers to the thermal diffusivity, which is the thermal conductivity divided by the volumetric heat capacity, as expressed in eq 2. R¼
k ðm2 =sÞ Fc
ð2Þ
It is assumed that the boundary temperature is suddenly changed to its minimum point and is maintained at that temperature for the period of temperature drop, which is about 10 s in all cases. The heat flux can then be calculated by analytically integrating qs over the first 10 s. The heat flow into the liquid via the plastic beaker is Z10 Qbeaker ¼ A 3
qs ðtÞ dt ¼ 0
AkðT0 - Ti Þ pffiffi t ¼ 10 pffiffiffiffiffiffiffi ð2 t jt ¼ 0 Þ πR
ð3Þ
where A is the contact surface area between the liquid mixture and the beaker, which can be derived using the measured diameter of the beaker and the height of the fuel. The heat flow from the beaker walls was typically only about 10% of the enthalpy of mixing. To assess the heat flow from the top surface of the liquid mixture, two different total volumes were used, namely, 50 and 100 mL. As shown in Figure 3, there was no significant difference in the temperature profile for the two different volumes; therefore, it is fair to say that the heat flow from the free surface is negligible. Lastly, according to Hess’s law, the energy change for any chemical or physical process is independent of the pathway or number of steps required to complete the process provided that the final and initial reaction conditions are the same.16 As the enthalpy of mixing refers to the energy obtained from the atmosphere for the blends to reach its initial state, that is, atmospheric pressure at room temperature, the 1255
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Figure 3. Measured temperature drop on mixing for all the test fuels with two different total volumes (∼50 and 100 mL) on a volume and molar basis.
Figure 4. Corrected enthalpy of mixing for all the test fuels with two different total volumes (50 and 100 mL) on a volume and molar basis. enthalpy of vaporization for the ethanol/hydrocarbons blends (hfg)mix can then be calculated on a mass basis using eq 4 mi ðhfg Þi þ me ðhfg Þe ¼ ðmi þ me Þ 3 ðhfg Þmix þ ΔT½mi ðcp Þi þ me ðcp Þe þ Qbeaker
ð4Þ
where the subscripts (i and e) refer to isooctane and ethanol, respectively, and ΔT[mi(cp)i þ me(cp)e] refers to the enthalpy of mixing. 2.1.3. Results and Discussion. The measured temperature drop is presented in Figure 3 for each of the isooctane/ethanol blends with total volumes of both 50 and 100 mL. It can be seen from Figure 3 that there is no significant difference in temperature drop during mixing between the two total mixture volumes (50 and 100 mL). Given the fact that the heat flow via the plastic beaker is less than about 10% of the enthalpy of mixing, such small differences between the 50 and 100 mL volumes are negligible in determining the enthalpy of vaporization of the blends. In other words, the heat flow from the free surface of the liquid fuels can be ignored. The temperature drop rises rapidly for samples with small ethanol contents (
