2-step variable valve actuation using early-intake valve closing is a strategy for high fuel economy on spark-ignited gasoline engines. Two discrete valve-lift profiles are used with continuously variable cam phasing. 2-step VVA systems are attractive because of their low cost/benefit, relative simplicity, and ease-of-packaging on new and existing engines.
A 2-step VVA system was designed and integrated on a 4-valve-per-cylinder 4.2L line-6 engine. Simulation tools were used to develop valve lift profiles for high fuel economy and low NOx emissions. The intake lift profiles had equal lift for both valves and were designed for high airflow & residual capacity in order to minimize valvetrain switching during the EPA drive cycle. It was determined that an enhanced combustion system was needed to maximize fuel economy benefit with the selected valve lift profiles. A flow-efficient chamber mask was developed to increase in-cylinder tumble motion and combustion rates.
A 2-step valvetrain mechanism was developed that features hydraulically-actuated switchable rocker arms and hydraulic lash adjusters (Type II valvetrain). The rocker arm is a dual-roller, single-slider design for compact packaging and low friction. The engine management system was modified for control and calibration of 2-step VVA, and to realize the full fuel economy potential of the system.
Dynamometer tests on a multicylinder engine indicated a 6.9 percent fuel economy benefit relative to the production engine with exhaust cam phasing alone on the EPA city cycle. Combustion enhancement significantly contributed to the overall fuel economy benefit. Vehicle tests showed less fuel economy improvement than steady state dynamometer tests due, in part, to cam phaser control limitations. For warmed-up Phase 3 EPA tests (cycles 19-23), a 5.5% improvement was measured with 46% reduction in NOx. For the whole EPA city test including cold start, a 4.8% improvement was measured. Further improvements in vehicle fuel economy are expected with refinement in transient control and calibration.