Two-Step Variable Valve Actuation for Fuel Economy, Emissions, and Performance

TLDR

Variable‑Valve Actuation improves engine efficiency, emissions, and performance by varying valve lift and timing, and Two‑Step VVA systems—using two discrete lift profiles and optionally variable cam phasing—offer a simple, low‑cost, and easily packaged solution attractive to manufacturers. The objective was to optimize Two‑Step system design and operation for maximum system benefits. The authors used engine‑modeling tools to analyze thermodynamic and fluid‑mechanical processes, selecting an Early‑Intake‑Valve‑Closing strategy for warmed‑up operation and a Late‑Intake‑Valve‑Opening strategy for cold start, and developed an automated procedure to evaluate a wide range of valve‑train parameters, comparing the Two‑Step system to fully flexible VVA, Three‑Step VVA, and dual‑independent cam‑phasing configurations. Modeling results indicate substantial improvements in fuel economy, NOx emissions, and performance, with reduced cold‑start HC emissions also expected.

Abstract

Variable-Valve Actuation (VVA) provides improvements in engine efficiency, emissions, and performance by changing the valve lift and timing as a function of engine operating conditions. Two-Step VVA systems utilize two discrete valve-lift profiles and may be combined with continuously variable cam phasing. Two-Step VVA systems are relatively simple, low cost and easy to package on new and existing engines, and therefore, are attractive to engine manufacturers. The objective of this work was to optimize Two-Step system design and operation for maximum system benefits. An Early-Intake-Valve-Closing (EIVC) strategy was selected for warmed-up operating conditions, and a Late-lntake-Valve-Opening (LIVO) strategy was selected for the cold start. Engine modeling tools were used to fundamentally understand the thermodynamic and fluid mechanical processes involved. A procedure was developed to rapidly and automatically process engine simulations for a wide range of engine operating conditions and valve train parameters including valve lift, duration, and timing. Modeling results indicate that substantial improvements in fuel economy, NOx emissions, and performance can be achieved. Reduced cold-start HC emissions are also expected. A comparison to fully flexible VVA, Three-Step VVA, and dual-independent cam phasing (only) is presented.