Natural CNG. Although NG has been used as a

Natural gas as a transportation fuel is not a new
idea; however large finds of natural gas, and the technology to recover this
fuel at reasonable costs, have spurred increased national interest in CNG.
Although NG has been used as a fuel in IC engines for a number of years, much
development and optimization are still possible, both for the case of dedicated
and  bi-fuel engines. World-wide emphasis
on CO2 emissions reduction and fuel economy improvements suggests that it may
be worthwhile to make a small investment to understand what can be achieved
with a CNG (dedicated or bi-fuel) engine in passenger car applications as well
as in stationary engines. Not all automobile engines are suitable for use
with CNG as a fuel. CNG provides less upper cylinder lubrication than petrol or
diesel, so CNG-fueled engines are more prone to valve wear if they are not
suitably modified. To benefit from the usage of CNG in IC engines, it is essential
to know its combustion under the suitable conditions and to study the influence
of several parameters on it. Methane, the primary component of natural gas, is
composed of one carbon atom and four hydrogen atoms. This H/C ratio of 4:1 is
advantageous to an engine’s emissions as compared to gasoline which has an H/C
ratio of about 1.85 4-methane. The reason being that during combustion, heat
energy and oxygen mix with the methane to break the molecular bonds and
re-combine them. This ideally turns carbon into CO2 and hydrogen into H2O.
Thus, if there is less carbon and more hydrogen in the reactants, there should be
less CO2 and H2O. This is indeed the case for CNG as compared to gasoline as
natural gas observes a CO2 emissions reduction of about 20% 5-methane. This
is the reason that natural gas is typically regarded as a ‘greener’ fuel than
its petroleum based brethren. Being a gas at normal temperature and pressure CNG
mixes readily with air in any proportion. The ability of an engine to pump air termed
as the volumetric efficiency (VE), which if reduced has an impact on the
maximum power output. Liquid fuel (Gasoline) when atomized, generally consumes
very small space in the intake system and thus do not affect volumetric
efficiency significantly. Gaseous fuels require about 4 to 15% of intake passage
volume. Space occupied by the fuel reduces the amount of air entering the
engine; hence the power output of the engine is reduced. Due to vaporization CNG
of in manifold as they mix with air in the engine’s intake to enter the
cylinders, it absorbs energy and cools the fuel/air mixture.  Cooler the mixture, higher is the engine VE.
Most gaseous fuels, on the other hand, are already in a vapor form and do not
provide any cooling of air/fuel mixture. This loss of cooling constitutes an
additional power loss of gas-fueled engine as compared to liquid fueled engine.
Theoretically, loss in power output for CNG is around 4-10%.  Octane rating of a fuel indicates how lately
the fuel will ignite and how well the fuel will resist pre-ignition before the
spark plug fires. Higher compression ratios can be used due to the higher
auto-ignition temperature and octane rating that avoids the mixture to
pre-ignite and makes the engine more knock-resistant. CNG has high octane
rating great than 110 that allows CR to be higher than gasoline engines. The
higher the CR of an engine, the more efficient and powerful is the engine
output. Utilization of CNG at its efficient operation involves different
combustion techniques which is dealt in the following section.