into heat. These linings normally consist of a porous skin  supported by a honeycomb backing and provide a separation  between the face sheet and the engine duct. For optimum  suppression, the acoustic properties of the skin and the liner  are carefully matched.  Engineers are introducing new combustion technology that  has dramatically reduced emissions from gas turbine engines.  Lowering exhaust emissions from gas turbine, especially  oxides of nitrogen (NO$_{X}$), continue to require improvement.  Most of the research has centered around the combustion  section of the engine. New technology with unique combustor  design has greatly reduced emissions. One manufacturer  has a design called the Twin Annular, Pre-mixing Swirler  (TAPS) combustor. Most advanced designs rely on a method  of pre-mixing the fuel/air before it enters the combustion  burner area. In the TAPS design, air from the high-pressure  compressor is directed into the combustor through two high- energy swirlers adjacent to the fuel nozzles. This swirl creates  a more thorough and leaner mix of fuel and air, which burns  at lower temperatures than in previous gas turbine engine  designs. Most of the NO$_{X}$ is formed by the reaction of oxygen  and nitrogen at high temperatures. The NO$_{X}$ levels are higher  if the burning fuel/air mixture stays at high temperatures for a  longer time. Newly designed combustors also produce lower  levels of carbon monoxide and unburned hydrocarbons. The  increases in gas turbine engine component efficiencies have  resulted in fewer emissions from gas turbine engines. Turbine Engine Emissions 3-33