This paper summarizes a review of dual-fuel natural gas/diesel engine technology carried out for the Gas Research Institute.(1)* In the past, dual-fuel natural gas/diesel engines have been relegated to a few small niche markets, but our review has shown that dual-fuel engine technology has significant potential. Potential advantages of dual-fuel engines include diesel-like efficiency and brake mean effective pressure (BMEP) with much lower emissions of oxides of nitrogen (NOx) and particulate matter. New technologies offer solutions to the problems of poor efficiency and emissions at light load. Dual-fuel engines can be designed to operate interchangeably on natural gas with a diesel pilot, or on 100% diesel fuel. Many existing diesels can be converted to dual-fuel operation. Preliminary economic analyses show that such conversions could be justified from the fuel cost savings alone in applications such as railroad locomotives, marine vessels, mine trucks, and diesel power generation systems.A dual-fuel engine is an internal combustion engine in which the primary fuel (usually natural gas) is mixed more or less homogeneously with the air in the cylinder, as in a spark-ignition engine. Unlike a spark-ignition engine, however, the air/fuel mixture is ignited by injecting a small amount of diesel fuel (the “pilot”) as the piston approaches the top of the compression stroke. This diesel pilot fuel rapidly undergoes preflame reactions and ignites due to the heat of compression, just as it would in a diesel engine. The combustion of the diesel pilot then ignites the air-fuel mixture in the rest of the cylinder.Because the air and the primary fuel are premixed in the cylinder, dual-fuel engines have many features in common with spark-ignition, Otto-cycle engines. Because they rely on compression-ignition of the diesel pilot, however, they also share some characteristics with diesels, as well as some unique advantages and drawbacks of their own.Among the advantages of dual-fuel engines is that - in most cases - they can be designed to operate interchangeably on natural gas with a diesel pilot, or on 100% diesel fuel. This makes them especially valuable in circumstances where use of natural gas is desired for environmental or economic reasons, but where gas supply may not be fully reliable. For example, a dual-fuel truck could operate on compressed natural gas where that fuel was available - such as urban areas suffering from severe air pollution. If the truck had to travel beyond range of its compressed natural gas supply, however, it could still fall back on 100% diesel fuel. Similarly, a generator set could operate most of the time on relatively inexpensive pipeline gas, but switch instantly to 100% diesel if gas supply were interrupted. Other potential applications in which this capability would be important include diesel-electric locomotives, marine vessels, farm equipment, construction and industrial equipment, and engines using biogas, sewage gas, or other variable gas supplies.Another advantage of dual-fuel engines is the ease with which most existing diesels can be converted to dual-fuel operation. In contrast to the difficulty involved in converting a diesel engine to spark-ignition, many diesel engines can be converted to dual-fuel operation without even removing the cylinder heads. Given the large number of diesel-powered vehicles, equipment, and machinery in use, such dual-fuel conversions could make possible the widespread substitution of natural gas for diesel fuel, with a large savings in capital cost and time compared to that required to convert to spark-ignition engines.Dual-fuel engine performance and emissions vary depending on operating conditions and the sophistication of the control system. Dual fuel engines perform best under moderate to high load, and can often equal or better the fuel-efficiency of a pure diesel under these conditions. Operating with a lean air-fuel ratio, they can also achieve much lower emissions (especially of NOx and particulate matter (PM)) than a pure diesel. Existing dual-fuel conversions suffer from major increases in carbon monoxide (CO) and hydrocarbon (HC) emissions and loss of fuel efficiency at light loads. This is because they operate unthrottled, so that the air-fuel mixture becomes leaner as the load is reduced. As the mixture becomes leaner, combustion eventually degrades, leaving large amounts of partial reaction products in the exhaust. Since large amounts of light-load operation are characteristic of many diesel engine applications - especially vehicles - the high emissions and poor efficiency of dual-fuel engines in this condition are a major handicap. This is perhaps the major reason that virtually all new heavy-duty natural gas truck engines under development are spark-ignition rather than dual-fuel designs. But recent technological developments in large dual-fuel engines, combined with a new generation of electronic fuel metering and control systems, could make it possible to overcome the problems of light-load emissions and fuel-efficiency in the dual-fuel engine, while maintaining and even enhancing the advantages of the dual-fuel approach.This report documents the conclusions of a research project carried out for the Engine Technology Group of the Gas Research Institute (GRI). The purpose of this study was to define and assess present dual-fuel technology, to identify potentially promising applications of this technology, and to identify appropriate areas of focus for future GRI-funded research and development on dual-fuel engines. This paper reviews the present state of dual-fuel engine technology, and characterizes current and potential future performance and emission levels, as well as the basic combustion phenomena responsible for performance and emissions. It presents data on emissions and performance of a number of commercial dual-fuel engines. In addition, it discusses the potential for retrofitting dual-fuel technology to existing diesel engines. Finally, it presents recommendations for future research and development.