Fuel–egr Fuel Mixtures Supercritical Fuel

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y W W W . S T L E . O R G wide range of approaches have been undertaken to determine how to reduce emissions from automotive engines. in a previous TLT article, the development of the homogeneous charge compression ignition (hCCi) engine was discussed. When used in place of a conventional gasoline engine, an hCCi engine can produce a 15% to 20% improvement in fuel economy and a significant reduction in No x emissions. The hCCi process relies on spontaneous ignition of the gasoline, air mixture similar to the auto-ignition process in a diesel engine. as we all know, three specific states of matter (solid, liquid and gas) exist, and most substances can be found in one of these states at any given combination of temperature and pressure. When the temperature and pressure is increased above a value known as the critical point, the substance reaches a fourth state known as supercritical. a supercritical fluid exhibits properties somewhere between a liquid and a gas. it is a homogeneous fluid that is maintained in a single phase. dr. George anitescu, research scientist in the department of biomedical and chemical engineering at syracuse university, in syracuse, N.Y., says, “a supercritical fluid cannot be turned into a liquid solely by compression/expansion (e.g., air in the engine cylinders).” Two supercritical fluids also mix easily with one another. They exhibit molecular diffusivities and no surface tension, which facilitates the mixing process. supercritical fluids are used in applications such as decaffeination and power generation. anitescu says, “The diesel engine displays greater efficiency than the gasoline engine. The air-fuel mixture in the latter, although homogeneous, needs a spark to ignite due to low reactivity. in contrast, the mixing of air and fuel in a diesel engine is not uniform. diesel fuel, which is composed of heavier hydrocarbons, is more viscous and less volatile than gasoline. The resulting incomplete combustion usually leads to the generation of undesirable levels of particulate matter, while hightemperatures at the flame front generate No x .” These deficiencies in the diesel engine can be overcome if the diesel fuel is moved into a supercritical state. anitescu says, “enabling diesel fuel to become supercritical will allow for mixing with air in a homogeneous phase upon injection, in a similar fashion to a gasoline engine.” an approach has now been devised to use diesel fuel in a supercritical state.