![]() The schematic diagram of the experimental apparatus is shown in Fig. 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. 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 ̊. 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. ![]() Table 1 shows the detailed specifications of the test engine. 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. The engine has a CVVT system which controls the intake valve timing continuously. An in-line 4, 1,998cc gasoline engine with 16 valves was used in the experiment. ![]() 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. 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. However, some amounts of overlap promote fuel atomization by the blow-back gases which are at high temperature. In conventional CVVT engines, valve overlap used to be eliminated to improve the startability and stable idling quality at the start phase. 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. With the CVVT system of the intake cam phaser, maximum torque and power are improved through the optimiza- tion of valve timing according to the overall engine operation condition. Also, a continuously variable valve timing (CVVT) system has been widely adopted since it can enhance the engine performance, reduce exhaust emissions and fuel consumption, simultaneously. To improve the combustion stability, research on advanced engine concepts such as variable valve timing and lift electronic control (VTEC) or swirl control valve (SCV) for high-turbulence in- cylinder flow engines has been conducted to achieve a large amount of spark timing retard. When spark timing is aggressively retarded, engine torque is decreased and combustion stability (such as cyclic indicated mean effective pressure (IMEP) variation) deteriorates. ![]() As spark timing at cold start is retarded from minimum spark advance for best torque (MBT), exhaust gas temperature increases, and hence it remarkably reduces the THC emissions at phase 1 of the FTP-75 mode. In addition to aftertreatment technologies, spark timing retard is considered as a very effective technique for reducing THC emissions. Technologies for reducing cold start emissions are exhaust aftertreatment systems, such as thin wall catalysts, electrically heated catalysts (EHC), flow optimized exhaust manifolds, and stainless steel exhaust manifolds, which have proven to be quite effective to meet the future emission regulations. Therefore, various factors of engine combustion, aftertreatment and engine control have been investigated for cold start emission reduction. More than 95% of total hydrocarbon (THC) emissions in Federal Test Procedure (FTP)-75 mode test were exhausted within the first few cycles before catalyst activation in super ultra low emission vehicle (SULEV). the last several years, the automobile indus- try has focused on the development of environmen- tally friendly vehicles to meet the reinforced emission legislation.
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