| |
contd...
- Alcohols are a preferred alternative for SI engines only due to their high octane number. A higher engine compression ratio could be used to obtain a higher engine thermal efficiency.
- Cetane number of methanol and ethanol are close to 5 and 8, respectively.
- Direct injection alcohol engine prototypes operating similar to diesel/CI engines have also been developed using either a positive source of ignition or high dosage of ignition quality improvers (3 to 7% by volume).
- Alcohols are not easily miscible in the diesel fuels. To prepare alcohol-diesel blends high amounts of emulsifiers or solublizers are required.
- Heating value of ethanol is approximately 60 percent and that of methanol is only 45 % of gasoline.
- The stoichiometric air-fuel ratio due to presence of oxygen in the molecule is much lower than the gasoline.
- The volumetric energy content of stoichiometric mixture (gaseous state) of alcohols and gasoline however, are not very different. Thus, engine specific power output that may be obtained with alcohols and gasoline is nearly the same.
- The latent heat of vaporization of methanol and ethanol is nearly 4 and 2.7 times, respectively compared to gasoline. Cold starting performance with neat alcohol is therefore, poor compared to gasoline.
- Flames of neat alcohols in air are not easily visible to the naked eye. Hence, 15% gasoline is mixed to alcohol for making the flame visible in case of an accidental fire.
A large number of investigations have been carried out with alcohols as engine fuel. Emissions with low level alcohol blends as well as with neat alcohols have been studied. CO and NOx emissions show no clear trend although lower flame temperatures with alcohol may result in lower NOx emissions. With neat alcohol operation, as the emissions of most of the volatile organic compounds in the exhaust would consist of alcohols or aldehydes only, the photo-chemical reactivity and the ozone forming potential is lower with alcohol fuel operation.
The main advantages and disadvantages of alcohols with respect to conventional gasoline and diesel fuels are summarized in Table 8.14.
| Table 8.14 |
| Advantages and Disadvantages of Alcohol Motor Fuels Compared to Gasoline and Diesel |
Performance Parameter |
Compared to gasoline and
diesel fuels |
Advantages/Disadvantages |
Flame temperature |
Lower |
Potentially lower NOx emissions and lower heat losses |
Vapour pressure |
Lower |
Poor cold starting and warm up performance, higher
unburned fuel emissions during starting/warm up phase |
PM emissions
|
Lower |
Due to clean burning characteristics PM emissions are even lower than the gasoline engines |
Air Toxic Emissions |
Lower |
Lower benzene and 1,3 butadiene emissions |
CO and NOx Emissions
|
Similar |
No definite trend is observed, So, no advantage over
petroleum fuels have been noted |
Aldehyde emissions |
Higher |
Formaldehyde and acetaldehyde emissions are higher |
Nature of sources
|
Renewable esp. of ethanol |
Sources more widespread around the world, hence
better energy security. Lower net CO2 emissions. |
|
