06-12-2012, 12:00 PM
Using LNG as a Fuel in Heavy-Duty Tractors
1LNG as a Fuel.pdf (Size: 2.64 MB / Downloads: 89)
Executive Summary
The U.S. Department of Energy (DOE) has designated the National Renewable Energy
Laboratory (NREL) as the “field manager” for implementing certain portions of the Alternative
Motor Fuels Act of 1988. Section 400BB of the Act makes a commitment to develop and
evaluate alternative fuels in heavy-duty vehicles.
By the early 1990s the passenger car, light-duty truck, and transit bus segments of the
automotive industry had all accumulated some experience with natural gas, but there was very
little experience with natural gas in heavy-duty (line-haul) automotive applications. In 1994,
NREL contracted with the Trucking Research Institute (TRI) to obtain a cooperative agreement
with Liquid Carbonic. The purpose of this agreement was to (1) purchase and operate liquid
natural gas- (LNG-) powered heavy-duty tractor-trailers with prototype Detroit Diesel
Corporation (DDC) Series 60 natural gas (S60G) engines in over-the-road commercial service
applications; and (2) collect and provide operational data to DDC to facilitate the on-road
prototype development of the engine and to NREL for the Alternative Fuels Data Center.
The vehicles operated from August 1994 through April of 1997 and led to a commercially
available, emissions-certified S60G in 1998. This report briefly documents the engine
development, the operational characteristics of LNG, and the lessons learned during the project.
Background
In early 1992, one of the largest transit bus operations in the country, the Metropolitan Transit
Authority of Harris County, Texas, (Houston Metro), decided to convert its entire fleet of transit
buses to natural gas. This was years ahead of a Texas law (Appendix A) that requires a phasedin
conversion to alternative fuels by metropolitan transit authorities. This law limited alternative
fuel choices to natural gas, propane, or electricity. With the selection of LNG in April 1992,
Houston Metro awarded Liquid Carbonic a 7-year fuel supply contract. To produce and
distribute the LNG required for the Houston Metro buses and the Greater Austin Bus Company,
which also opted to convert to LNG, Liquid Carbonic began to purchase large sources of natural
gas and to construct a new facility in Willis, Texas. During construction of the Willis plant,
Liquid Carbonic supplied LNG to its growing list of customers from a plant in Geismar,
Louisiana. The Willis plant was designed to produce approximately 100,000 gallons per day
when it was completed in 1994.
With the plant’s completion, Liquid Carbonic took the lead in U.S. LNG production. Because
the Willis facility was built to supply and sell LNG exclusively for transit bus fleets and heavyduty
trucks, Liquid Carbonic decided it should use LNG-powered trucks for the distribution
fleet. Four Freightliner LNG-fueled tractors, with DDC S60G engines, were leased from Ruan
Transportation Management Systems (RTMS). Three were operated in the distribution fleet and
the fourth was used for demonstration and show purposes.
Equipment
Engines
Many of the 12.7L, 6-cylinder, DDC S60G engine components are common with the Series 60
diesel (S60) version. The engine block, cylinder liners, pistons, and cooling and lubricating
systems are identical. They also share the features of a waste-gated turbo-charger, air-to-air
aftercooling, lean-burn combustion, and a DDC electronic engine control module (DDEC). The
S60G crankshaft, camshaft, connecting rods, and exhaust system are modified variations from
the S60.
The big differences between the S60G and the S60 are the compression ratio (c/r) and the fuel
delivery and ignition systems. The S60G has a 10:1 c/r, single point (prior to the throttle plate)
fuel injection, and spark ignition of the air/fuel mixture. The S60 has a 15:1 c/r, multipoint
(directly into the cylinder) fuel injection and ignites the air/fuel mixture by the heat of
compression.
Background
Although emissions were not measured in this project, natural gas can provide significant
reductions in regulated emissions, especially oxides of nitrogen (NOx) and particulate matter
(PM)1. This makes LNG particularly attractive for urban routes, where NOx and PM emissions
are a major concern and fuel can be dispensed from a central fuel storage system. Natural gas is
also one of the most abundant fuel sources in the United States, so its use in transportation is
strategically sound.
Tank Design
On-board fuel storage is considerably different between diesel and LNG. Standard diesel tanks
are single-wall aluminum containers and cost in the hundreds of dollars, depending on size.
Tanks for cryogenic LNG require double-wall construction from stainless steel with super
insulation and vacuum inter-tank space. Typical tank pressures are between 20 to 150 psig, but
the design must withstand more than two times that amount to compensate for heat gain when
not in use. LNG fuel storage systems are significantly more complicated to design and
manufacture and their cost is an order of magnitude greater than that of a diesel tank.
The weight of LNG is approximately 3.5 lb/gal, compared to diesel at 7.6lb/gal. Unfortunately,
the more complex LNG fuel tank is substantially heavier. In total, given the difference in tank
design and fuel density, LNG-powered tractors have suffered a weight penalty.
Engine Technology
Current dedicated natural gas engine technology, which includes the DDC S60G, uses a throttle,
a low compression ratio, and spark ignition. Inherent to this engine strategy is a thermal
efficiency penalty of 15% or more versus a diesel-cycle engine. By comparison, the diesel
engine does not use a throttle, has a high compression ratio and is ignited by heat of
compression. The National Renewable Energy Laboratory (NREL), with the support of the U.S.
Department of Energy (DOE), is sponsoring research that will potentially lead to natural gas
engines with efficiencies similar to diesel engines. However, these new engine designs may take
many years to perfect and commercialize.
LNG Weathering
One effect of LNG’s cryogenic properties is unique, and important to understand, especially
when LNG is to be used as a vehicle fuel. This effect, called “weathering” or “enrichment,” is a
phenomenon that arises from the fact that natural gas is a chemical mixture. LNG produced
from pipeline gas has varying percentages of methane and other hydrocarbons. The methane
content can vary from 92%−99%. The other hydrocarbons found in natural gas are ethane
(1%−6%), propane (1%−4%), butane (0%−2%), and other compounds. Each chemical element
or compound in the liquid vaporizes at its own unique boiling point. Consequently, over time,
concentrations of the heavier hydrocarbons (such as ethane, propane, and butane) increase.
Higher concentrations of these hydrocarbons will cause premature ignition and “knock.”
Because uncontrolled knock causes engine damage, LNG must be used before it becomes
weathered. Recognizing this potential difficulty, Liquid Carbonic set out to manufacture 99.4%
pure methane LNG. With this high percentage of methane, LNG weathering cannot create
harmful fuel mixtures because the potentially harmful constituents are largely absent.