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Engine out (feedgas) emissions control during cold start operations has been a major technical challenge since mandated LEV/ULEV/SULEV/PZEV regulation compliance. Cold start emissions contribute to more than 85% of total emissions in a FTP test. Unburned Hydrocarbons are mostly generated during cold starts due to a rich Air/fuel ratio strategy. Cold intake and cylinder wall surfaces do not provide a quick vaporization bed for the rich fuel, therefore un-vaporized and unburned fuel result in excessive tailpipe emissions. Utilizing fuel vapor during cold starts reduces the Hydrocarbon (HC) emissions level and minimizes the transient fuel effect process. A vapor management system must function to control the Air-to-fuel ratio of the intake charge during “cold-starts”, idle, and drive-away, or until catalyst lights off to a desired level.

The recent availability of fast response (< 1.0 seconds) exhaust gas temperature sensors for exhaust gas temperature measurement enables a new method for closed loop feedback engine control in small engines and other non-stoichiometric applications (V-8 marine fuel injected engines for example). Conventional closed loop stoichiometric A/F control with traditional switching type oxygen sensors is often not applicable because these engines rarely if ever operate at stoichiometric A/F ratios. Richer A/F ratio control is necessary for maximum power and combustion temperature cooling. Wide range UEGO A/F sensors are an alternate solution but the cost of these sensors may be too high. Additionally, the very short exhaust systems found on many small engines can allow ambient air to back flow into the exhaust due to pressure pulsations and corrupt the signal from an oxygen concentration type sensor.

Engine performance under transients is greatly affected by the fuel behavior in the induction systems. To better understand the fuel behavior, a computer model has been developed to study the one-dimensional coupled heat and mass transfer processes occurring during the transient evaporation of liquid fuel from a heated surface into stagnant air. The energy and mass diffusion equations are solved simultaneously to yield the transient temperatures and species concentrations using a modified finite difference technique. The numerical technique is capable of solving the coupled equations while simultaneously tracking the movement of the evaporation interface. Evaporation results are presented for various initial film thicknesses representing typical puddle thicknesses for multi-point fuel injection systems using heptane, octane, and nonane pure hydrocarbon fuels.