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تعداد صفحات این فایل: ۲۴ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Cylinder air-charge estimation for advanced intake valve operation in variable cam timing engines~~en~~

Abstract

High efficiency of three-way automotive catalysts is achieved by regulating cylinder air–fuel ratio in a narrow band around the stoichiometry. Due to a delay present in the exhaust gas oxygen sensor signal, performance of the air–fuel ratio regulation depends on accuracy of the cylinder air-charge estimate. Variable cam timing (VCT) introduces a challenge to the air-charge estimator due to its effect on engine pumping and, in some cases, due to unmeasured back-flow of the exhaust gas into the intake manifold. The objective of this paper is to illuminate some of these issues and suggest methods to improve accuracy of air-charge estimation in VCT engines.

۱ Introduction

Variable valve timing (VVT) systems are used in spark ignition automotive engines to improve fuel economy, reduce emissions, and increase peak torque and power [1–۶]. Here we shall consider only the variable cam phasing (timing) systems, as opposed to other VVT systems such as cam profile switching [7], variable intake/exhaust duration [8], variable valve lift [9,10], and camless (electro-magnetic valve drive) engine systems [11]. In conventional (non-VCT) engines, the relative phase between the camshafts and the crankshaft is fixed at a value that represents a compromise between optimal phases at different operating conditions. In a VCT system, a mechanism varies the relative phase as a function of engine operating conditions [12]. Depending on the camshafts being actuated (exhaust, intake, or both), there are four variable cam timing system types: intake-only, exhaust-only, dual-equal, and dual-independent [2]. In each of these cases the VCT system alters engine pumping and affects cylinder air charge. A significant reduction in tailpipe emissions of regulated exhaust gases (NOx; HC, and CO), has been achieved with exhaust after-treatment devices such as three way catalysts. High efficiency of a three-way catalyst in removing regulated gases requires that the internal combustion engine be operated at the stoichiometric air–fuel (AF) ratio(approximately equal to14.6). Consequently, a high priority task of the engine control system is to maintain the air–fuel ratio at stoichiometry. A key component of this control system is the feedback of the measured oxygen content (and, hence, air–fuel ratio) in the exhaust gas by a HEGO or a UEGO sensor. Because of the inherent delay in the sensor measurement (induction to exhaust delay plus the transport delay from the exhaust port to the sensor that may be as large as 0:5 s), regulation of air–fuel ratio in transients mostly relies on a feedforward component provided by the aircharge estimator.

The mass of air inducted into the cylinders during the intake stroke (air-charge) is estimated from the signal of a sensor located upstream from the cylinder ports as illustrated in Fig. 1. This is either a mass air flow (MAF) sensor (hot wire anemometer) located upstream from the throttle body or an air pressure sensor located in the intake manifold (MAP). The air-charge estimate divided by the stoichiometric air–fuel ratio largely determines the amount of fuel to be injected. Feedback from the HEGO sensor is needed to maintain accurate AF regulation in steady state. A consistent AF ratio regulation error as small as one percent may result in manifold increase in tail-pipe emissions. To compensate for the time it takes to inject fuel, a predicted value of air charge is used for fueling [13]. In addition, an AF regulation system may include compensation for fuelpuddling dynamics in the intake port, and parameter adaptation to reduce the effect of slowly varying ambient conditions, aging, and sensor drift on AF ratio regulation accuracy [14].

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