Environmental Engineering Reference
In-Depth Information
Although air quality has improved over the last 20-30 years, there are still
significant pollution problems throughout industrialized and emerging countries,
especially in urban areas and densely populated regions. At this regard, the research
in the field of new engine and fuel technologies has been mainly forced by legis-
lators, rather than by marketplace. Governmental legislators are mandating
increasingly stringent standards for pollutant emissions from vehicles' exhaust,
with a general warning also about fuel economy issues and lowering of greenhouse
gas emissions. In particular, reduced emissions from road transport are seen as an
important factor to improve air quality, particularly because the share of Diesel
vehicles in the overall sales of light duty vehicles is increasing. In the European
Union the Euro 5 and Euro 6 emission standards for cars have been fixed by the
introduction of Regulation (EC) N. 715/2007 [
45
] and it's implementing regulation
(EC) N. 692/2008 [
46
]. These regulations revise the current emission limits for
motor vehicles (the Euro 4 standards, in force since 1, January 2005), and their
technical requirements take effect in two stages, with Euro 5 emission limits
coming into force since 1, September 2009, and Euro 6 emission limits since 1,
September 2014. The main effect of Euro 5 is to reduce the emissions of particulate
matter from Diesel cars from 25- 5 mg/km. This will make the introduction of
particle filters for Diesel car mandatory. Euro 6 limits mainly reduce the emissions
of nitrogen oxide from Diesel cars further, from 180 mg/km to 80 mg/km. As the
introduction of new technologies implies additional costs and determines an
increase in consumer prices of new vehicles, the new emission limit values of Euro
5 and Euro 6 have been calculated in order to ensure a benefit in air quality
standards without compromising affordability of cars for the consumers.
The necessity to meet the always more severe emission limits imposed by
regulators, together with concerns about the extent of crude oil reserves and
excessive dependence on oil-producing countries, have favored in the past decades
a strong interest towards the possibility of using alternative fuels to propel road
vehicles.
Natural gas is a fossil fuel very abundant in nature, but, at variance of crude oil,
whose reserves are located prevalently in the Middle East (about 65%), it is evenly
distributed around the world, with the largest known reserves in the areas of
ex-Soviet Union (35%) and Middle East (40%), followed by Asia, Africa, America
and Western Europe. It is mainly constituted by methane (85-99%, in dependence
of its source), with other compounds present in much lower concentrations: hea-
vier hydrocarbons (ethane, propane, butane, and their isomers), inert gases
(nitrogen and carbon dioxide), sulfur compounds (hydrogen sulfide, carbonyl
sulfide, mercaptans), traces of noble gas and water. This composition is respon-
sible for the main advantage of natural gas with respect to gasoline and gas oil
from an environmental point of view, in fact natural gas vehicle have much lower
non-methane hydrocarbon and particulate matter emissions, but higher methane
emissions, which imply less toxicity of the engine exhaust gas [
47
]. However, also
natural gas vehicles need the adoption of three-way catalytic converters (with
enhanced reactivity with respect to those designed for gasoline vehicles, due to the
higher chemical stability of methane) and modern electronic fuel control systems
