Individual-Cycle Measurements of Exhaust-Hydrocarbon Mass from a Direct-Injection Two-Stroke Engine

Unburned hydrocarbon (HC) emissions and processes leading thereto are quantified in a single-cylinder version of an experimental V6 direct-injection (DI) two-stroke engine. Fast-response HC sampling at the exhaust port of the engine is integrated with simultaneous acquisition of individual-cycle cylinder-pressure data and with high-speed imaging of the fuel spray and spectrally resolved combustion luminosity. For every engine cycle, both the total HC mass and the fractions thereof that leave the cylinder during the cylinder-blowdown, main-scavenging, and port-closing phases are determined using a pressure-based calculation of the individual-cycle exhaust mass flow rate. At light load, HCs exhausted during the main-scavenging phase (when the transfer ports are open) account for 60-70% of the total HC mass and are strongly correlated with the amount of unburned fuel in each cycle. In this DI two-stroke engine, incomplete combustion of the stratified charge (due primarily to overmixing of fuel during and after injection, slow mixing-limited combustion of rich zones, misfires, and fuel trapped in fuel-injector crevices) appears to be a much more important HC source than the piston top-ring-land crevice, which is the dominant HC-emission source in premixed-charge four-stroke engines.