Evolution of an Engine
Natural gas engines show great
promise for meeting the strictest emissions regulations. But with technical
hurdles yet to tackle and cost and refueling concerns still an issue, what does
their future hold?
by John T. Kubesh
John T. Kubesh is a principal engineer in the Engine Research Department of the Engine and Vehicle Research Division. Since joining the Institute in 1991, he has developed lean-burn, spark-ignited, natural gas-fueled engines that use advanced technologies to reduce emissions. Kubesh also has directed considerable effort to developing accurate fuel metering systems, viable engine diagnostic methods for knock and misfire detection, and new combustion systems for natural gas-fueled engines.
Designing a clean and efficient
engine is no easy task, particularly when adjustments to one variable often
create undesired outcomes elsewhere. The key to a successful design is finding
the optimum balance between high efficiency and low emissions. The most abundant
and least expensive energy source, natural gas, is proving to be ideal for achieving a nearly
Engineers in the Engine and Vehicle
Research Division at Southwest Research Institute (SwRI) have recognized this
fact and have worked to exploit the advantages of natural gas as an engine fuel
while also developing technologies to overcome some of its shortcomings. In
particular, a significant amount of work has focused on the development of
heavy-duty engines, such as those used in over-the-road trucks, off-highway
equipment, locomotives, and power generation.
Nearly all heavy-duty engines
operate on diesel fuel, which is readily available and relatively inexpensive.
However, the characteristics of the diesel combustion cycle pose problems for
current efforts aimed at cleaning up the nation's air. Diesel fuel is injected
directly into the combustion chamber at the time of combustion, and the high
temperatures and rich combustion zones associated with the burning of a liquid
fuel jet cause the formation of large amounts of oxides of nitrogen (NOx) and
particulate matter, or soot. NOx is an ingredient for the production of ground
level ozone (smog), and diesel particulates are believed to be carcinogenic.
The use of natural gas as a
heavy-duty engine fuel potentially would eliminate these problems. The biggest
drawbacks to natural gas use at this time are problems associated with
compression, storage, and the delivery of a gaseous fuel in short, the lack
of a nationwide refueling infrastructure. For now, this alternative fuel is
particularly well-suited to fleet trucks or buses that can return to a home base
every evening for refueling. Despite these drawbacks, ever-more stringent
regulations for NOx, carbon monoxide, nonmethane hydrocarbon, and particulate
emissions could soon bring natural gas engines to the forefront.
Vehicle emissions have dramatically
decreased since the Environmental Protection Agency began regulating in the
1960s, but further reductions may only be possible using alternative fuels and
technologies. SwRI engineers plan to improve natural gas engine technologies
that surpass the proposed 2007 emissions levels the toughest set by the EPA.
The evolution begins
When most people hear the name John
Deere, an image of green farm tractors comes to mind. At SwRI, the name has an
entirely different meaning: natural gas engines. For the past seven years, the
Institute has led a continuous program of research and development aimed at
producing natural gas engines for John Deere. These low-emission, high-efficiency engines have been
designed for use in medium- and heavy-duty trucks, buses, tractor-trailers, and
John Deere entered the world of
on-highway natural gas engines in 1994, when the company contracted with SwRI to
develop a natural gas-fueled, 8.1-liter (L) engine. It was designed to produce
250 horsepower (hp) at 2,200 revolutions per minute (rpm) and 800 pound-feet
(lb-ft) of torque at 1,350 rpm. In addition to meeting power and efficiency
goals, the engine also significantly reduced emissions.
Testing over the U.S. Environmental
Protection Agency Federal Test Procedure (FTP) transient cycle, an industry test
that simulates the average cycle of traffic in and around city roads and
expressways, showed that the new engine could achieve the California Air
Resources Board (CARB) ultra-low emissions vehicle (ULEV) standards some of
the toughest in the nation. Once in production, the engine quickly gained a
reputation for reliability and fuel economy. One measure of the engine's
success is that the power density and efficiency remain competitive even today,
six years after development. The engine was so successful that similar
technology was later applied to the John Deere 6.8L engine for a 225-hp version.
Following these programs, SwRI
embarked on a project funded by the National Renewable Energy Laboratory (NREL),
in partnership with John Deere and other manufacturers, to develop an ultra-low
emission, alternative-fueled engine for a prototype school bus that also
included advanced safety features. SwRI engineers aggressively pursued the
project goals of reducing NOx emissions to 1.0 gram per brake horsepower-hour
(g/bhp-hr) as measured over the FTP transient cycle. Capitalizing on the low
emissions nature of the production 8.1L engine, SwRI engineers used it as the
basis for the improved bus powerplant. The final engine design, which did not
require catalytic exhaust aftertreatment, achieved an NOx level of 1.0 g/bhp-hr,
and carbon monoxide, nonmethane hydrocarbon, and particulate matter emissions
below ULEV standards.
SwRI's latest heavy-duty engine
designed for John Deere, the 8.1L 280-hp natural gas engine, successfully met
CARB Optional Low NOx standards by integrating a redesigned combustion chamber,
an efficient turbocharger, a higher flow exhaust manifold, and an updated electronic
controller. This design and several others are now in production.
learned during the NREL project regarding calibration of the engine control
system for transient operation were applied to further improve the emissions
characteristics of the existing John Deere engines. These modifications allowed
both the production 250-hp 8.1L and 225-hp 6.8L gas engines to be certified to
the CARB Optional Low NOx standards.
A subsequent program, the High
Efficiency, Advanced Technology (HEAT) project*, put forth two new objectives:
to continue the investigation into advanced engine technology begun during the
NREL project and to guide the most promising of these technologies to production
status. The project resulted in an engine with a power rating of 280 hp and a
peak torque of 900 lb-ft. The design also incorporated many advanced features,
such as a redesigned combustion chamber, a more efficient turbocharger, a higher
flow exhaust manifold, and an updated electronic controller equipped with
humidity compensation and knock detection. The low emissions characteristics of
its predecessors were retained, with emissions levels certified to the CARB
Optional Low NOx standards.
The medium- and heavy-duty engines
and engine technologies described have since reached production status and are
now in service on American roads and highways.
The technology behind the engines
Each of these engines uses the same
basic combustion technology a spark-ignited, open chamber, lean-burn
combustion system. Spark-ignited, open combustion chambers are typical of the
light-duty automobile and truck gasoline engines driven by the average consumer
every day. The "lean-burn" technique dilutes the fuel-air mixture in the
combustion chamber with excess air so that the ratio of the mass of fuel to the
mass of air (fuel-air ratio) is lower than that required for chemically correct,
or stoichiometric, combustion. Fuel flow to the engine is metered so that excess
air is always present in the combustion chamber. Because the fuel flow is less
than that of a stoichiometric situation, the power output of the engine is
reduced accordingly. To increase the fuel flow to the engine and raise the power
output of the engine, a turbocharger supplies an increased amount of dilution
The main benefit of lean burn is
that the dilution of the fuel-air charge lowers combustion temperatures. Because
NOx formation is a strong function of the temperature of the combustion
products, a reduction in temperature yields a dramatic reduction in the amount
of NOx formed.
An added benefit is that the
reduced cycle temperatures provide a cooler engine overall, particularly around
the cylinder head, exhaust ports, and exhaust manifold. Lower mean material
temperatures help prolong component life.
Lean-burn technology does have its
limits, however. The "lean limit," the level of dilution at which combustion
can no longer be sustained, dictates the amount of NOx reduction possible.
Adopting a lean-burn strategy for heavy-duty gas engines also posed other
technical hurdles. For example, to maintain the proper fuel-air ratio, systems
for accurately estimating the air flow into the engine and precisely metering
fuel flows were required. Lean-burn engines are also susceptible to combustion
problems such as misfire and knock, the onset of which can be caused by factors
such as changes in fuel-gas composition or inlet air humidity.
Most of these technical hurdles
were solved by applying advanced electronic controls. In particular, a concerted
effort devoted to improving natural gas engines was a major part of the program
sponsored by NREL. The SwRI-developed Rapid Prototyping Engine Control System
(RPECS), a tool that rapidly evaluates and adjusts complex control and
diagnostic algorithms needed to test and modify engines under development,
proved to be invaluable in tackling these problems.
One of the improvements was to
include an air-flow observer, which is essentially a real-time model of air flow
inside the engine used for accurately estimating the air flow. The accuracy of
the fuel-air ratio control algorithm was significantly improved in this way. New
hardware and control algorithms for fuel metering, knock detection, misfire
detection, gas composition sensing, and turbocharger boost control were also
Arguably, the most significant
discovery from this program was the recognition of the effects of humidity on
engine performance and emissions. In particular, higher humidity increased the
engine's propensity to misfire, and it simultaneously caused the electronic
control module to "think" the engine
was running too rich. This triggered a shift to a leaner mode, which increased
misfire. By quantifying these effects, practical methods for humidity
compensation and improved engine controls were addressed. In line with SwRI's
long-held policy of assigning patents to the sponsoring organization, John Deere
received the patent for these improvements, which have largely eliminated the
deleterious effects of humidity changes on natural gas engine performance.
The 280-hp engine now in production
embodies several of these new control methods, including humidity compensation
and knock detection. Future John Deere engines will integrate more advanced
control features as they are developed.
Since the first 8.1L 250-hp natural
gas engine (left) designed for John Deere in 1995, SwRI engineers have
incorporated advanced technologies to significantly
improve emissions. Although this engine is still competitive, the latest 8.1L 280-hp
engine has improved the emissions rating of medium- and heavy-duty natural gas
Natural gas engines of the future
The development of future engines
undoubtedly will be driven by increasingly stringent emissions standards.
It is unlikely that current
lean-burn technology can achieve the proposed 2007 standards without a
significant efficiency or driveability penalty. Another strategy will be
necessary to address these extremely low emissions limits. Three-way catalysts,
such as those used on light-duty automobiles, efficiently reduce NOx, but the
overall exhaust composition must be stoichiometric. This prevents the use of air
as a diluent, such that the engine will need another method, exhaust gas
recirculation (EGR), to dilute the fuel-air charge. The use of EGR is similar in
principle to lean burn, with recycled exhaust gas taking the place of excess air
to dilute the intake fuel charge. The advantage of using EGR is that a three-way
catalyst can then be used to reduce the NOx to nearly zero because the overall
exhaust mixture is stoichiometric.
Although this engine configuration
isn't common in heavy-duty gas engines, it is similar to that of a modern
light-duty automobile. Light-duty vehicles use stoichiometric fueling, EGR, and
three-way catalysts to produce the lowest emissions levels achieved thus far.
Natural gas engines create a new
set of technical hurdles with the addition of EGR technology. While dilution
with EGR can reduce NOx, there is a limit to the amount of dilution
analogous to the lean-burn engine at the lean limit. The amount of EGR dilution
is usually less than can be achieved with air so the NOx emissions produced by
the engine are higher, but it enables a three-way catalyst to be used. Because
the overall exhaust mixture is stoichiometric, the catalyst can significantly
reduce the remaining NOx emissions so that the overall NOx emissions are nearly
Development is still necessary to
maximize the amount of dilution. Another technical hurdle is thermal management.
Because the amount of effective EGR dilution is limited, engine temperatures
rise and heat transfer to critical components increases. Accurate metering of
EGR under transient situations will be required, and the development of suitable
EGR metering valves and coolers will be important. Diesel engine manufacturers,
currently interested in using EGR to reduce NOx in diesel engines, might be able
to provide some guidance by way of technology transfer.
NOx emissions vary greatly with
different engine configurations. This NOx comparison of a stoichiometric
engine, a stoichiometric engine with a 15 percent EGR rate, and a stoichiometric
engine with a 15 percent EGR rate and a three-way catalyst suggest that new
engine designs and aftertreatment technologies can reduce emissions
What comes next?
Modern natural gas engines such as
the John Deere 8.1L engine have proved to be clean, economical, and reliable
powerplants for medium- and heavy-duty trucks. Keys to this success have been
the incremental improvements in lean-burn combustion systems and the associated
technology required for control of these systems. By applying gas engine
expertise from SwRI and elsewhere to the challenges of proposed emissions
regulations, it is reasonable to expect that the technical issues might
eventually be overcome.
Other practical issues affect the
broad adoption of natural gas engines. The initial cost for a natural gas engine
is higher than that for a comparable diesel, mostly because fewer are made and
production costs are accordingly higher. However, as emissions limits drop, the
cost of reaching these levels with the diesel combustion cycle will grow
increasingly high. To attain the proposed levels, diesel engines will require
more expensive fuel injection systems and exhaust aftertreatment devices. The
cost-effectiveness and practicality of these new systems have yet to be proven.
Natural gas engines are more likely to achieve the proposed emissions levels
without resorting to these measures, making them much more
competitive. Some natural gas proponents even expect the cost of these engines
to fall below that of diesels. In any case, natural gas engines remain one of
the best choices for medium- to heavy-duty vehicles, especially in regions
striving to comply with clean air regulations.
Up until now, the expense and
scarcity of refueling stations and higher initial costs have undermined the
market for natural gas fueled vehicles. But as emissions limits are pushed
lower, the natural gas engine will gain an opportunity to displace diesels in
the heavy-duty workplace.
*The HEAT Project subcontract was
part of a larger engine development program funded by GRI, the Department of
Energy, and the South Coast Air Quality District. SwRI performed all research
and development for this program.
The author would like to acknowledge the
contributions from the staff of the SwRI Engine Research Department,
particularly Staff Technicians Joe Osborne and Mickey Milward Sr.,
Research Assistant Dennis Kneifel, Staff Engineer Lee Dodge, and Daniel
Podnar (now with Electronic MicroSystems). Thanks also to the Deere Power
Systems Group for their permission to publish this article and for their
continued support of SwRI natural gas engine development programs.
John Deere is an official trademark of Deere
Published in the Fall/Winter 2000 issue of
Technology Today®, published by Southwest Research Institute. For more
information, contact Maria
Fall/Winter 2000 Technology Today