![]() ![]() ![]() Intake valve timing influences the providing of internal exhaust gas recircula- tion (EGR), which reduces the emissions of nitro- oxide (NOx) and fuel consumption with the longer valve overlap period at part load. In conventional CVVT engines, valve overlap used to be eliminated to improve the startability and stable idling quality at the start phase. However, some amounts of overlap promote fuel atomization by the blow-back gases which are at high temperature. In this study, combustion phenomena according to various intake valve timings and spark timings were evaluated to achieve low cold-start emissions and rapid catalyst light-off performance during the cold start phase including the idle transient operation of the gasoline CVVT engine. Also, time-resolved THC and NOx were measured at the exhaust port with high resolution gas analyzers to investigate the characteristics of their formation and reduction mechanisms. An in-line 4, 1,998cc gasoline engine with 16 valves was used in the experiment. The engine has a CVVT system which controls the intake valve timing continuously. The CVVT system is a ‘cam-phasing type’ with a fixed valve duration therefore, when the intake valve opening (IVO) timing is advanced or retarded, the closing timing is simultaneously changed. Table 1 shows the detailed specifications of the test engine. To determine behaviors of the engine and the exhaust gases during a fast idle condition after a cold start, the temperature of the cooling water was main- tained at 25 ̊ C with 1400 rpm, idle and λ =1.0. Intake valve timings were changed from BTDC 10 ̊ (with 20 ̊ valve overlap) to ATDC 10 ̊ (without valve overlap) by 10 degrees and spark timings from BTDC 20 ̊ to ATDC 5 ̊. Cylinder pressure traces were measured at the 4th cylinder with spark-plug type pressure sensor and the signals from the sensor were resolved by the combustion analyzer. ![]()
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