Compressed natural gas
Blue diamond symbol used on CNG-powered
vehicles in North America
Compressed Natural Gas (CNG) is a fossil
fuel substitute for gasoline (petrol), diesel, or
propane fuel. Although its combustion does
produce greenhouse gases, it is a more envir-
onmentally clean alternative to those fuels,
and it is much safer than other fuels in the
event of a spill (natural gas is lighter than
air, and disperses quickly when released).
CNG is made by compressing natural gas
(which
is mainly composed of methane
[CH4]), to less than 1% of its volume at stand-
ard atmospheric pressure. It is stored and
distributed in hard containers, at a normal
pressure of 200–220 bar (2900–3200 psi),
usually in cylindrical or spherical shapes.
CNG is used in traditional gasoline intern-
al combustion engine cars that have been
converted into bi-fuel vehicles (gasoline/
CNG). Natural gas vehicles are increasingly
used in Europe and South America due to
rising gasoline prices.
In response to high fuel prices and envir-
onmental concerns, CNG is starting to be
used also in tuk-tuks and pickup trucks,
transit and school buses, and trains.
CNG’s volumetric energy density is estim-
ated to be 42% of LNG’s (because it is not li-
quefied), and 25% of diesel’s.[1]
Technology
CNG can be used in Otto-cycle (gasoline) and
modified Diesel cycle engines. Lean-burn
A CNG powered high-floor Neoplan AN440A,
operated by ABQ RIDE in Albuquerque, New
Mexico.
Otto-cycle
engines
can achieve higher
thermal efficiencies when compared with
stoichiometric Otto-cycle engines at the ex-
pense of higher NOx and hydrocarbon
emissions.
Electronically-controlled
stoi-
chiometric engines offer the lowest emissions
across the board and the highest possible
power output, especially when combined with
exhaust gas recirculation, turbocharging and
intercooling, and three-way catalytic convert-
ers, but suffer in terms of heat rejection and
fuel consumption. A suitably designed natural
gas engine may have a higher output com-
pared with a petrol engine because