Environmental Engineering Reference
In-Depth Information
in order to meet the severe regulations on pollutant emissions. Furthermore, since
natural gas contains only 75% carbon by mass versus 86-88% for gasoline and
gasoil, less CO
2
per unit of energy released is present in the exhaust gas (com-
parable to CO
2
emissions from a Diesel vehicle, which operates with lean air-fuel
mixture and higher efficiency). Regarding the motor characteristics, the methane
has a very high resistance to autoignition, which means high potential for use in a
high compression engine, and wider flammability range than other hydrocarbons,
allowing the engine to operate lean (but the need of using three-way catalyst
imposes a working in stoichiometric conditions). On the other hand, the volume of
the gas reduces the air breathing capacity of the engine, with diminution of engine
performance, which could be partially recovered by increasing the compression
ratio. However, as the majority of natural gas vehicles are bi-fuels aftermarket-
converted gasoline vehicles, then designed for specific antiknocking characteristics
typical of gasoline, the entire potential of methane cannot be exploited on these
vehicles. The low energy density of natural gas also causes storage problems on
board of vehicles, due to the pressurized cylinders necessary to store the gaseous
fuel, and consequent driving range limitations, especially for passenger cars.
Although a large amount of both light- and heavy-duty vehicles propelled with
methane are running in many countries, the main obstacles to widespread diffusion
of
natural
gas
vehicles
are
still
the
absence
of
transportation
and
storage
infrastructure.
Liquid petroleum gas (LPG) is separated from natural gas by extraction of
heavier liquid hydrocarbons, removed before gas distribution, or derived by
processing of crude oil in refineries. It is constituted mainly by propane, and at
lower extension by butane, isobutane, propene, and butenes, in dependence of the
production method. In addition to these components, other species may be present
in trace amounts, such as sulfur compounds, water and tars. The major producers
and exporters of LPG are the Middle East, Canada, North Africa and the Far East,
while the main consumers are USA, Japan and Western Europe (about 10% in the
transportation sector). Presently, LPG supply exceeds demand in most petroleum-
refining countries, so its price is low compared to other hydrocarbons. It has good
fuel properties (higher octane rating and wider flammability limits than gasoline),
and as a fuel for spark ignition engines it presents many of the advantages as natural
gas, with the useful additional characteristic of being easier to store on board of a
vehicle, thanks to its liquid form at room temperature and low pressure (8-10 bar).
Also, emissions characteristics are similar to those of natural gas vehicles, with
differences regarding hydrocarbon composition at the exhaust, which is depending
on fuel composition (propane/butane mixture instead of methane, with lower H/C
ratio). The transformation of vehicles to operate with LPG is made either by the
user by retrofitting after vehicle purchasing, or by the manufacturer before sailing.
The LPG transformation kit interfaces with the standard vehicle electronic control
unit, so that all emission control systems are fully employed. LPG is considered
both as an alternative fuel and as an intrinsically clean fuel by many governmental
agencies, with consequent tax incentives or exemption from no thoroughfare in
some historical centers, which have promoted its diffusion.
