Page 224 - Biofuels Refining and Performance

Ethanol and Methanol as Fuels in Internal Combustion Engines 207

(e) wax content, and (f) hydrocarbon composition. Methanol solubility
in diesel increases as the aromatic content goes up.

7.7.2 Alcohol–diesel fuel emulsions
Here, an emulsifier extends the water tolerance of alcohol–diesel blends.
In general, equal volumes of alcohols and emulsifiers are required for
suitable emulsions. No component changes, but injection volume and
timing are adjusted for diesel fuel with alcohol then solutions, i.e., up
to 35% diesel substitution is possible. Addition of ignition improvers, e.g.,
cyclohexanol nitrate, up to 1% helps increase the alcohol percentage up
to 35% while maintaining a cetane rating at permissible levels. Cost of
emulsifiers and poor low-temperature physical properties of emulsions
limit the use of this technique. Stable emulsion requires the use of costly
surfactants. Using higher-order alcohols improves the stability of blends
at temperatures as low as 20 C.

7.7.3 Spark ignition
This technique replaces 100% diesel. The injection system can be retained
as is or replaced by carburetion or port-type fuel injection. A spark plug
is introduced in the combustion chamber, and the associated ignition
system is added. High compression ratio and positive ignition result in
smooth combustion, thereby improving thermal efficiency.
This approach is quite attractive as it uses the high latent heat of the
vaporization of alcohols and their octane rating to good advantage.
Power output is reduced due to lower heat content of alcohols. Changes
in engine operability are not noticeable with alcohol-fired SI engines, rel-
ative to the same engines using diesel fuel due to their similar torque.
The engines are as efficient as their diesel-fueled counterparts. In fact,
huge torque is available at engine speeds below 1400 rpm, which increases
engine flexibility and response in use. Converting an existing diesel
fleet to an SI technique involves engine modification. Space at the appro-
priate place must be available for spark plugs in the cylinder head.
Lubricants need to be added to alcohols to increase lubricity and pre-
vent wear. Small amounts of cetane improvers may be added, but they
are not required. It is not easy to switch between fuels after conversion
to the SI technique.

7.7.4 Ignition improvers
Neat alcohols are used in diesel engines by increasing the cetane number
sufficiently using ignition improvers. This technique saves the expense
and complexity of engine component changes but adds the cost of igni-
tion improvers. The cost of 10–20% ignition improvers is quite prohibitive.