Fuel Characteristics of Ethanol
As discussed in the earlier section, ethanol was first introduced on a larger scale in the 1970s as a blend in fuel to serve as a replacement for methyl tertiary butyl ether (MTBE). Ethanol has a octane number of 108.6, comparable to 110 of MTBE [7]. Mixing about 10% of ethanol to gasoline can boost the octane number of the fuel [8]. The octane number is a measure of the gasoline quality for prevention of early ignition, which leads to cylinder knocking. Fuels with higher octane numbers are preferred in spark-ignition internal combustion engines (ICEs). An oxygenate fuel such as bioethanol provides a reasonable antiknock value [8]. In addition to a higher octane number, ethanol has broader flammability limits, higher flame speeds and higher heats of vaporization. These properties allow for a higher compression ratio and shorter burn time, which lead to theoretical efficiency advantages over gasoline in an internal combustion engine. A comparison of important physical properties and fuel characteristics of ethanol, methanol and gasoline is shown in Table 2.1.
Property |
Methanol |
Ethanol |
Gasoline |
CH3OH |
C2H5OH |
C4 — C12 |
|
Molecular weight (g/mol) |
32 |
46 |
~ 114 |
Specific gravity |
0.789 (298 K) |
0.788 (298 K) |
0.739 (288.5 K) |
Vapor density rel. to |
1.10 |
1.59 |
3.0-4.0 |
air |
|||
Liquid density (g cm-3 at 298K) |
0.79 |
0.79 |
0.74 |
Boiling point (K) |
338 |
351 |
300-518 |
Melting point (K) |
175 |
129 |
— |
Heat of evaporation (Btu/lb) |
472 |
410 |
135 |
Heating value (kBTU gal-1) |
|||
Lower |
58 |
74 |
111 |
Upper |
65 |
85 |
122 |
Table 2.1 Physical properties and some fuel characteristics of ethanol, |
methanol, and gasoline [9]. |
Property |
Methanol |
Ethanol |
Gasoline |
CH3OH |
C2H5OH |
C4 — C12 |
|
Tank Design/pressure |
15 |
15 |
15 |
(psig) |
|||
Viscosity (cp) |
0.54 |
1.20 |
0.56 |
Flash Point (K) |
284 |
287 |
228 |
Flammability/ Explosion limit (%) Lower (LFL) |
6.7 |
3.3 |
1.3 |
(%) Upper (UFL) |
36 |
19 |
7.6 |
Auto ignition |
733 |
636 |
523-733 |
temperature (K) |
|||
Solubility in water |
100% |
100% |
Negligible |
Miscible |
Miscible |
(~ 0.01%) |
|
Azeotrope with water |
None |
95% Ethanol |
Immiscible |
Peak flame |
2143 |
2193 |
2303 |
temperature (K) |
|||
Minimum ignition |
0.14 |
0.23 |
— |
energy in air (mJ) |
|||
Research Octane num- |
108.7 |
108.6 |
91-93 |
ber (RON) |