Effects of Intake and Exhaust Continuously Variable Valve Duration (CVVD) on Fuel Consumption and Emission Characteristics in Engine and Vehicle Tests

An experimental study was conducted on a multi-cylinder engine equipped with both intake and exhaust continuously variable valve duration (CVVD). Thanks to CVVD, valve closing and opening timings are decoupled, therefore, four valve timings (opening and closing timings of intake as well as exhaust sides) can be optimized under entire engine conditions in terms of minimum brake specific fuel consumption and particle number (PN) emission. Consequently, λ = 1 operation under all engine conditions was achieved without compromise of both fuel economy and performance. Furthermore, to reduce raw emissions and shorten light-off time of catalyst under catalyst heating condition, valve and injection strategies were tested. As a result, it was found that certain level of negative valve overlap duration was beneficial to secure combustion stability, and early exhaust valve opening timing increased exhaust gas temperature for catalyst heating. Therefore, both shorted light-off time and raw engine out emissions were achieved with optimized valve and injection timings. To verify whether the positive effects of CVVD of both sides in the engine test can be realized in the vehicle test, initial calibrations of engine and vehicle were conducted with some data analysis techniques. After the calibration, FTP, HWFET, and US-06 driving cycle tests were conducted, so that, both of fuel economy and emission characteristics were improved compared to the base vehicle with an engine equipped with only intake CVVD. These results were mainly due to shortened catalyst light-off time and optimal valve timings which were shown in engine test results.