Chemical kinetic studies were employed to better understand how and at what rate air pollutants are formed in an external combustion process. With this background, an experimental combustion test rig employing a recirculating step, staged burner was built. Tests were conducted with various liquid and vaporized liquid fuel injectors using aviation turbine fuel (Jet A) and 2,2,5-trimethylhexane. A wide range of fuel-air ratios were examined by individually controlling primary and secondary air flow rates. Fuel flows were varied from a maximum corresponding to a heat release of 500,000 BTU/hr to 1/30 of this value. Two runs were also made using gaseous methane fuel. The range of test variables were: injector configurations - pressure atomizing, vaporizing, 2nd vaporizing premixed; fuel flows from 0.15 to 4.5 gallons per hour; air flows from 3.4 to 103 standard cubic feet per minute; fuel equivalence ratio - primary from 0.53 to 1.59, and overall from 0.40 to 0.84; number of test conditions - 140; and cumulative combustion time - 22 hours. The tests demonstrated that gaseous and particulate emissions less than those established as the 1980 Federal Research goals can be achieved simultaneously in a high heat release, low pressure drop, burner configuration. The emission data measured at steady state conditions is compared to current and future emission goals for automobiles. (Author)