Page 234 - Biofuels Refining and Performance

Ethanol and Methanol as Fuels in Internal Combustion Engines 217

as a replaceable fuel. To avoid this, blends (15–25%) of both alcohols are
preferred over pure ethanol or methanol. The properties of blends and
their effect lie in between pure alcohol and petrol. As we know, methanol
blends have lower stoichiometric air requirements compared to petrol.
Therefore, if we use a methanol–petrol blend without any modification
in the carburetor, we get more air for combustion, which will reduce the
emission of CO and HC as well as NO as the engine works cooler with
x
the blend compared with a petrol engine.
Oxidizing catalytic devices can control aldehyde emissions.
Platinum–rhodium and platinum–palladium catalysts are considered
the most effective in tackling aldehyde emissions of methanol fueling.
Concerning the alcohol fuels, the following conclusions can be drawn:
1. Alcohol is potentially a better fuel than gasoline for SI engines.
2. Its use improves the thermal efficiency as a higher compression ratio
(12:16) can be used.
3. It can avoid knocking even at a higher compression ratio because of
the high octane number.
4. It provides better fuel economy and less exhaust emissions.
5. High latent heat of alcohol reduces the working temperature of the
engine.
6. It gives more power, specially when used as a blend.
7. Easy availability of raw materials.
8. Cost of production is low because of the price hike in crude petroleum.
In agricultural countries like India, we can get ethyl alcohol easily from
vegetables, agricultural material, and sugarcane waste at a much lower
cost compared with the cost of petrol today. Therefore, replacing petrol
with alcohol in a SI engine has a good future.

7.9 Comparison of Ethanol and Methanol
Most of the properties are similar, with differences of only 5–10%.
Ethanol is superior to methanol as it has a wider ignition limit (3.5–17)
than methanol (2.15–12.8). Its calorific value (CV) (26,880 kJ/kg) is con-
siderably higher than methanol (19,740 kJ/kg).
Ethanol is a much more superior fuel for diesel engines as its cetane
number is 8 compared to the cetane number of 3 for methanol. There are
wide resources for manufacturing ethanol compared with methanol.
Therefore, ethanol is widely used as SI engine fuel in many countries.
Methanol is superior to ethanol in one respect: Its vaporization rate is much
higher than ethanol. Therefore, mixing with air rapidly forms a uniformly