Spark ignition gasoline engine efficiency is limited by a number of factors; these include the pumping losses that result from throttling for load control, spark ignition and the slow burn rates that result in poor combustion phasing and a compression ratio limited by detonation of fuel. A new combustion process has been developed based on the patented concept of injection-ignition known as Transonic Combustion or TSCi™; this combustion process is based on the direct injection of fuel into the cylinder as a supercritical fluid. Supercritical fuel achieves rapid mixing with the contents of the cylinder and after a short delay period spontaneous ignition occurs at multiple locations. Multiple ignition sites and rapid combustion combine to result in high rates of heat release and high cycle efficiency. The injection-ignition process is independent from the overall air/fuel ratio contained in the cylinder and thus allows the engine to operate un-throttled. Additionally, the stratified nature of the charge under part load conditions reduces heat loss to the surrounding surfaces, resulting in further efficiency improvements. The short combustion delay angles allow for the injection timing to be such that the ignition and combustion events take place after TDC. This late injection timing results in a fundamental advantage in that all work resulting from heat release produces positive work on the piston. Other advantages are the elimination of droplet burning and increased combustion stability that results from multiple ignition sources.Engine test results are presented over a range of speed, load and operating conditions to show fuel consumption, emission and combustion characteristics from initial injector and combustion system designs. The results are correlated with thermo-dynamic modeling and comparisons are made with contemporary engines.