Engine-fuel relationships of the Ford PROCO stratified charge engine have been examined. The test program was conducted in three phases to assess the interrelationships between exhaust emissions, fuel economy, octane requirement, and fuel properties in an experimental, research, single cylinder, stratified charge PROCO (programmed combustion) engine.In Phase I, tests were conducted at a steady-state speed-load condition to determine the effect of engine calibration parameters on emissions and fuel economy after an initial evaluation of engine operation with three different ignition system configurations. A dual ignition system produced reliable, misfire-free operation with the dilute mixtures and high EGR rates tested.In Phase II, five fuels with significantly different volatility properties and composition were tested to determine their effect on emissions and fuel economy of the PROCO engine. At the same steady-state speed-load condition examined in Phase I, satisfactory operation was obtained at both 0 and 50% EGR with air/fuel ratios as lean as 32:1. None of the fuels tested produced differences in emissions or fuel consumption which could be of practical use.In Phase III, the Primary Reference Fuel (PRF) and Full Boiling Range Unleaded Fuel (FBRU) octane requirements were determined for the single-cylinder PROCO engine at a series of 13 speed-load points which represents both the CVS-H cycle and certain other engine operating conditions. Subsequently, at six speed-load points where the PRF octane requirement was above 80, the effects of ignition timing, fuel-injection timing, intake mixture temperature, air/fuel ratio, and exhaust gas recirculation rates on FBRU octane requirement, exhaust emissions, fuel consumption, and power output were examined. The data show that the best selection of adjustable calibration parameters for controlling octane requirements depends on the mode of engine operation and the emission, fuel economy and performance constraints associated with the mode of operation.