Alternate
fuels for Indian automobiles
Dr
Anupam Dewan discusses the advantages and disadvantages of alternative
fuels that can be used to power automobiles
Automobiles
employ internal combustion engines, where thermal energy obtained from
the combustion of a fuel is finally converted into mechanical form of
energy. A typical automobile fuel consists of several hydrocarbons which
when combined with oxygen release appreciable quantity of thermal energy.
With the possibility of increasing gap between demand and supply of
crude oil and with fast depleting fossil fuel reserves, it has now become
essential to find out alternative fuels for internal combustion engines.
The
search for alternative fuels is also influenced by pollution and environmental
concerns and this has forced mankind to switch over to renewable and
environmental friendly fuels. The important pollutants discharged by
the automobile engines include carbon mono-oxide, nitrogen oxides, sulphur
oxides, lead and suspended particulate matter.
Normally
fuels are rated for their antiknock qualities, and two parameters, called
Octane number and Cetane number, for petrol and diesel, respectively,
are used for this purpose. Diesel knock is caused by too rapid combustion
and a diesel with a high value of Cetane number is desirable. Petrol
knock is caused by the self-ignition of the un-burnt mixture and the
resulting collision of flame fronts. Octane number of petrol is a measure
of how much it can be compressed before it ignites spontaneously.
Synthetic
gasoline / diesel
Synthetic
and reformulated hydrocarbons are produced from coal, natural gas and
petroleum gas, biomass (organic waste and crops), tar and residual fuel
oil from crude oil refining. These hydrocarbons are of similar characteristics
to conventional gasoline and diesel. However, the final yield, processing
requirement and cost depend largely on feedstock. Synthetic hydrocarbons
are not renewable fuel resources, although, they can extend considerably
the life span of the existing hydrocarbon reserves.
Reformulated
gasoline and diesel were introduced mainly for their emission/evaporative
qualities and sulphur content. They are produced by adding/blending
the conventional gasoline and diesel with oxygenates such as alcohols
(methanol and ethanol). Reformulated fuels are well suited for urban
and densely populated areas as they provide a significant environment
benefit.
Liquefied
petroleum gas
Liquefied
Petroleum Gas (LPG) consists mainly of butane (C4H10) and propane (C3H8)
in addition to traces of ethane (C2H6) and pentane (C5H12). It is a
by-product of the extraction and refining of crude oil and natural gas.
The composition of LPG varies across countries due to difference in
quantity of crude oil and methods of refining.
LPG
is useful for a number of reasons. For starters, it has higher octane
rating than gasoline. Second, it gives improved thermal efficiency as
engines can operate at higher compression ratios compared to engines
using gasoline. It can be easily mixed with air in engines in any proportion
allowing leaner combustion than that of gasoline. It has lower carbon
contents than both gasoline and diesel. Hence it produces less carbon
dioxide. If used in a spark ignition engine, it produces no particulate
matter, very little carbon monoxide and moderate nitrogen oxide and
hydrocarbon emission.
The
problem with LPG is its limited supply. Perhaps it can be used in vehicles
operating in densely populated and pollution sensitive areas.
Natural
gas
Natural
gas, which consists mainly of methane, has a high octane number (125-130)
and is used in vehicles either as a compressed natural gas (CNG) or
liquefied natural gas (LNG), which has a higher energy density than
CNG. It can be used with petrol in a bi-fuelled vehicle, with some engine
modification, or in a single fuelled natural gas engine.
There
are several advantages to the use of natural gas in automobiles. First,
methane is the simplest hydrocarbon, hence it is much easier to mix
uniformly with air than petrol or diesel. Therefore, combustion in engines
that are driven by natural gas is more complete, leading to lower carbon-monoxide
and hydrocarbon emissions. Second, it has high Octane number (125-130)
and therefore possesses good anti-knock characteristics. Therefore it
can be used with a high compression ratio, which results in significantly
high thermal efficiency
....CONTD
