We present and analyze three-dimensional calculations of the spray, mixing and combustion in the UPS-292 stratified charge engine for three different operating conditions, corresponding to overall air-fuel ratios between 22.4 and 61.0. The numerical calculations are performed with KIVA, a multidimensional arbitrary-mesh, finite-difference hydrodynamics program for internal combustion engine applications. The calculations use a mesh of 10,000 computational cells. Each operating condition is calculated from intake valve closure at 118° BTDC to 90° ATDC and requires approximately three hours of CRAY-XMP computer time.Combustion occurs primarily in the wake of the spark plug, and to include the effects of the spark plug on the flow field, we use a novel internal obstacle treatment. The methodology, in which internal obstacles are represented by computational particles, promises to be applicable to the calculation of the flows around intake and exhaust valves.Comparisons are made of several computed and measured quantities. Pressure histories compare well at the highest load conditions. Computed emissions of NO agree well with experiment, but those of HC and CO agree poorly. Predicted wall heat losses are approximately two-thirds of the measured values for all test conditions. We discuss the reasons for agreement or disagreement and tell how the results change when some of the controlling physical parameters are varied.The calculations also yield much information that is not obtainable experimentally. Plots are given of the histories of several global quantities, including fuel vapor mass in premixed regions, chemical heat release, and wall heat loss. In addition, we analyze plots of velocities, temperature, and concentrations at various times, and describe a computer-generated movie that helps visualize the complex three-dimensional flow fields.