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 perfect balance.

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 (NO_x) and particulate matter, or soot. NO_x 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 NO_x, 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 refuse haulers.

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 NO_x 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 NO_x 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.

 Lessons 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 NO_x 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 NO_x 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 air.

The main benefit of lean burn is that the dilution of the fuel-air charge lowers combustion temperatures. Because NO_x formation is a strong function of the temperature of the combustion products, a reduction in temperature yields a dramatic reduction in the amount of NO_x 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 NO_x 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 tested.

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 engines.

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 NO_x, 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 NO_x 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 NO_x, 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 NO_x 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 NO_x emissions so that the overall NO_x emissions are nearly zero.

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 NO_x 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 significantly.

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.

Acknowledgements

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 & Company.

Published in the Fall/Winter 2000 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Martinez.

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