9.4 Why are CNG and LPG considered "cleaner" fuels.
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
9.4 Why are CNG and LPG considered "cleaner" fuels.
CNG ( Compressed Natural Gas ) is usually around 70-90% methane with 10-20%
ethane, 2-8% propanes, and decreasing quantities of the higher HCs up to
butane. The fuel has a high octane and usually only trace quantities of
unsaturates. The emissions from CNG have lower concentrations of the
hydrocarbons responsible for photochemical smog, reduced CO, SOx, and NOx,
and the lean misfire limit is extended [117]. There are no technical
disadvantages, providing the installation is performed correctly. The major
disadvantage of compressed gas is the reduced range. Vehicles may have
between one to three cylinders ( 25 MPa, 90-120 litre capacity), and they
usually represent about 50% of the gasoline range. As natural gas pipelines
do not go everywhere, most conversions are dual-fuel with gasoline. The
ignition timing and stoichiometry are significantly different, but good
conversions will provide about 85% of the gasoline power over the full
operating range, with easy switching between the two fuels [118]. Concerns
about the safety of CNG have proved to be unfounded [119,120].
CNG has been extensively used in Italy and New Zealand ( NZ had 130,000
dual-fuelled vehicles with 380 refuelling stations in 1987 ). The conversion
costs are usually around US$1000, so the economics are very dependent on the
natural gas price. The typical 15% power loss means that driveability of
retrofitted CNG-fuelled vehicles is easily impaired, consequently it is not
recommended for vehicles of less than 1.5l engine capacity, or retrofitted
onto engine/vehicle combinations that have marginal driveability on gasoline.
The low price of crude oil, along with installation and ongoing CNG
tank-testing costs, have reduced the number of CNG vehicles in NZ. The US
CNG fleet continues to increase in size ( 60,000 in 1994 ).
LPG ( Liquified Petroleum Gas ) is predominantly propane with iso-butane
and n-butane. It has one major advantage over CNG, the tanks do not have
to be high pressure, and the fuel is stored as a liquid. The fuel offers
most of the environmental benefits of CNG, including high octane.
Approximately 20-25% more fuel is required, unless the engine is optimised
( CR 12:1 ) for LPG, in which case there is no decrease in power or increase
in fuel consumption [27,118]. There have been several studies that have
compared the relative advantages of CNG and LPG, and often LPG has been
found to be a more suitable transportation fuel [118,120].
methane propane iso-octane
RON 120 112 100
MON 120 97 100
Heat of Vaporisation (MJ/kg) 0.5094 0.4253 0.2712
Net Heating Value (MJ/kg) 50.0 46.2 44.2
Vapour Pressure @ 38C ( kPa ) - - 11.8
Flame Temperature ( C ) 1950 1925 1980
Stoich. Flame Speed. ( m/s ) 0.45 0.45 0.31
Minimum Ignition Energy ( mJ ) 0.30 0.26 -
Lower Flammable Limit ( vol% ) 5.0 2.1 0.95
Upper Flammable Limit ( vol% ) 15.0 9.5 6.0
Autoignition Temperature ( C ) 540 - 630 450 415
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