Unburned Hydrocarbon Emission Mechanisms in Small Engines

Unburned Hydrocarbon Emission Mechanisms in Small Engines

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The effect of the liquid fuel in the intake manifold, the ring pack crevices and the oil film on the unburned hydrocarbon (HC) emissions of a spark-ignited, carbureted, air-cooled V-twin engine was studied. Tests were performed for a range of engine load, two engine speeds, various air-fuel ratio and with a fixed ignition timing. To isolate liquid fuel effects due to the poor atomization and vaporization of the fuel when using a carburetor, a specially conditioned homogeneous, pre-vaporized mixture system (HMS) was developed. The results from carburetor and HMS are compared. To verify the existence of liquid fuel in the manifold, and to obtain an estimate of its mass, a carburetor-mounted liquid fuel injection (CMLFI) system was also implemented. Stop-injection tests performed with the CMFLI system show that 60-80 cycles worth of liquid fuel is held in the intake manifold depending on operating condition. The results of the comparison show that the liquid fuel in the intake manifold does not have a statistically significant influence on the averaged HC emissions. In addition, the cycle-resolved HC emissions for both systems follow the same trends and are comparable in magnitude. Heat release analysis showed little difference between fuel mixture delivery system. These results suggest that under steady state operation the HC emissions for this engine are not sensitive to the presence of liquid fuel in the intake manifold. The ring pack contribution to the engine-out HC emissions was estimated using a simplified ring pack gas flow model; the model was tested against the experimentally measured blowby. The tests were performed using the homogeneous fuel mixture system. The integrated mass of HC leaving the crevices from the end of combustion (the crank angle that the cumulative burn fraction reached 90%) to exhaust valve closing was taken to represent the potential contribution of the ring pack to the overall HC emissions; post-oxidation in the cylinder will consume so3.2.2.1 CARBURETOR FUEL SYSTEM The stock carburetor is calibrated by the manufacturer to provide a fixed amount of fuel according to the engine speed and load. This air-fuel ratio may vary between operating conditions. However, in thisanbsp;...

Title:Unburned Hydrocarbon Emission Mechanisms in Small Engines
Author:Victor M. Salazar
Publisher:ProQuest - 2008


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