The purpose of this paper is to describe a novel liquid fuel conditioning process that is incorporated within a 4-stroke internal combustion engine. The process takes place inside a small external vaporization chamber linked to the engine cylinder. A vaporization chamber transfer port is located adjacent to the cylinder bottom dead center. After an injected primary liquid fuel has evaporated and superheated inside the vaporization chamber it is transferred into the cylinder near the end of the intake stroke to form a homogenous mixture with a fresh air charge. Combustion is triggered by compression-ignition of a pilot fuel spray. Near the end of the expansion stroke, hot combustion products enter the vaporization chamber via the vaporization chamber transfer port. Thereafter, those products are entrapped inside the vaporization chamber during about 320° of crankshaft rotation since the vaporization chamber transfer port is sealed by the piston skirt for part of the cycle.Unlike spark ignition, compression ignition or homogeneous charge compression ignition engines, here the liquid fuel is injected into the vaporization chamber during the expansion stroke. Fuel droplets absorb heat from the hot entrapped combustion products and vaporization chamber walls, where they evaporate and reach a superheated gaseous state. Calculations predict that the residence time available inside a typical vaporization chamber of an engine running at 6,000 RPM is sufficient to evaporate and superheat gasoline fuel droplets of 180 micron SMD.It is anticipated that this novel concept could substantially reduce the untreated emission levels of nitrogen oxides, carbon monoxide, particulate matter and unburned hydrocarbons when compared to spark ignition, compression ignition or homogeneous charge compression ignition engines. This projection implies that less costly and simpler aftertreatment devices will suffice to comply with emission standard regulations. An improvement in engine fuel economy is expected because: (1) relatively high design compression ratio, (2) un-throttled operation and (3) faster heat release rate than that corresponding to either spark ignition or compression ignition engines.The combustion process prevents detonation and diesel knocking therefore the fuel does not need to be rated for octane or cetane number. These features allow this engine to efficiently employ gasoline or diesel fuels without additives or blends. Additionally, the system is expected to effectively utilize low-cost petroleum-derived fuel, biodiesel, bio-alcohol, vegetable oil, and in special applications coal-water-slurry fuels.