Previous studies of gasoline direct-injection compression-ignition (GDCI) showed good potential for very high efficiency, low NOx, and low PM over the full speed-load range. Low-temperature combustion was achieved using multiple-late injection (MLI), intake boost, and cooled EGR. Advanced injection and valvetrain were key enablers. In the current study, a new piston was developed and matched with the injection system. Single-cylinder engine tests were conducted with the objective to reduce injection pressure, intake boost, and swirl levels. Results showed that ISFC could be further improved while maintaining low levels of NOx, PM, and combustion noise. Efficiency loss analysis indicated a very efficient thermodynamic process with greatly reduced heat losses. Injection parameters could be used to control combustion phasing with good combustion stability.Engine simulations were performed to develop a practical boost system for GDCI. Sufficient intake boosting is difficult for low-temperature-combustion engines because the exhaust enthalpy to drive a turbocharger is very low. Several boost architectures with various boost devices were studied. A two-stage system with a supercharger, a turbocharger, and two charge coolers was selected and optimized using simulation-based calibration techniques. Results showed that attractive BSFC levels over the operating range with good full-load BMEP could be obtained. Subsequent vehicle simulations showed greatly reduced fuel consumption relative to a GDI baseline on both urban and highway drive cycles. Engine down-sizing, down-speeding, and up-loading with good transmission matching were key factors to achieve the most efficient operation.A multi-cylinder GDCI engine was designed and built. Preliminary parametric tests were conducted on an engine dynamometer and compared to single-cylinder tests at similar operating conditions. Overall, results for the two engines were comparable for ISFC with combustion noise, NOx, and PM emissions below targets. More work is needed to characterize the engine over the operating map, conduct cold starts and transient tests, and optimize the powertrain calibration for minimum fuel consumption at target emissions levels.