Effect of exhaust valve timing on gasoline engine performance and hydrocarbon emissions

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Effect of exhaust valve timing on gasoline engine performance and hydrocarbon emissions

Bohac, Stanislav V. / Assanis, Dennis N.

Despite remarkable progress made over the past 30 years, automobiles continue to be a major source of hydrocarbon emissions. The objective of this study is to evaluate whether variable exhaust valve opening (EVO) and exhaust valve closing (EVC) can be used to reduce hydrocarbon emissions. An automotive gasoline engine was tested with different EVO and EVC timings under steady-state and start-up conditions. The work described herein shows that advancing EVO by 60 deg and advancing EVC by 40 deg reduces start-up hydrocarbon emissions by 27%. Engine operation is not adversely affected and the associated fuel consumption penalty is negligible if the strategy is only used until the close-coupled catalyst reaches light-off. Early EVO increases exhaust gas temperature and decreases catalyst light-off time, while early EVC causes a backflow of hot hydrocarbon-rich exhaust gas from the cylinder to the intake port, which enhances fuel vaporization and air/fuel mixing, and then returns this gas to the cylinder for reburning in the next cycle. Steady-state hydrocarbon emissions are also reduced, but higher fuel consumption makes early EVO / early EVC unattractive for steady-state use. If early EVO is used with standard EVC, hydrocarbon emissions are increased because although the hotter exhaust gas reduces catalyst light-off, the abrupt drop in cylinder temperature reduces in-cylinder post-flame oxidation to such a degree that overall hydrocarbon emissions are increased.