Page 216 - Biofuels Refining and Performance

Ethanol and Methanol as Fuels in Internal Combustion Engines 199

the same specific gravity. All those properties are of great advantage if
used in SI engines. Some important advantages of alcohol-fueled engines
compared with gasoline engines are listed below:

1. The alcohols (both) have higher heat of vaporization. As the liquid
fuel evaporates into the air stream being charged to the engine, a
higher heat of vaporization cools the air, allowing more mass to be
drawn into the cylinder. This increases the power produced from the
given engine size. High latent heat of vaporization leads to higher
volumetric efficiency and provides good internal cooling.
2. The high octane number of alcohols compared to petrol means higher
compression ratios can be used, which results in higher engine effi-
ciency and higher power from the engine.
3. Ethanol burns faster than petrol, allowing more uniform and
efficient torque development. Both alcohols have wider flamma-
bility limits, which results into a rich air–fuel (A:F) ratio being
used when needed to maximize power by injecting more fuel per
cycle.
4. Alcohols also have lower exhaust emissions than gasoline engines
except for aldehydes. Both alcohols have lower carbon–hydrogen
ratio than petrol and diesel, and produce less CO . For the same
2
power output, CO 2 produced by an ethanol-fired engine is about
80% of the petrol engine. Because of high heat of vaporization, the
fuels burn at lower flame temperatures than petrol, forming less
NO . The CO percentage in both cases (alcohol and petrol) remains
x
more or less the same.
5. Contamination of water in alcohols is less dangerous than petrol or
diesel because alcohols are less toxic to humans and have a recog-
nizable taste.
6. The alcohols can also be blended with gasoline to form the so-called
gasohol (80% petrol and 20% alcohol), which is widely used in the
United States.

7. Ethyl alcohol as a fuel offers great safety due to its low degree of
volatility and higher flash point (17 C).
8. The heating value of alcohol is 60% of that of petrol (60% only), and
it shows equally good thermal efficiency and lower fuel consumption,
because the air required for petrol and alcohol is in the ratio of 15:9
by weight, which is the same as their calorific value, i.e., the same
heat is developed per cylinder charge in petrol and alcohol engines.
The power per unit volume of cylinder for petrol, ethanol, and
methanol are closely similar.