The Study of Dedicated EGR Cylinders to Improve the Fuel Economy and Transient Performance of a Spark-Ignited Gasoline Engine, 03-R9798Printer Friendly Version
Inclusive Dates: 04/01/08 Current
Background - SwRI has demonstrated in the HEDGE® program that a spark-ignited (SI) gasoline engine can be operated with high levels of exhaust gas recirculation (EGR) to achieve fuel economy approaching that of a diesel engine. A key technology associated with this success is the air handling system, which must provide the EGR in a manner that does not adversely affect engine pumping, turbocharger performance, or transient response. SwRI has proposed a new EGR system that separates the exhaust runner of one (or more) cylinder from the main exhaust manifold and reroutes it directly into the intake manifold. With this configuration, all of the burned gases from the separated exhaust runner are considered EGR, and hence the cylinder is called a dedicated EGR cylinder. In the case of a four-cylinder engine with one dedicated EGR cylinder, the engine is essentially operated at a fixed EGR rate of 25 percent. This fixed EGR rate greatly simplifies transient control of EGR, as the fixed EGR rate is maintained as air flow rates are increased or decreased during the transient. More importantly, the air-fuel ratio of the EGR cylinder is controlled separately of the other cylinders, and is made to operate fuel-rich such that a small amount of hydrogen is generated. This small quantity of hydrogen significantly improves the EGR tolerance and burn rate of the engine, both of which contribute to fuel economy improvement. The EGR cylinder can also be made to operate fuel-lean if high oxygen content EGR is needed. While preliminary tests with 25 percent dedicated EGR showed significant improvements in fuel economy, it is believed that further improvements are possible at higher EGR levels. This theory is being tested in this IR project by modifying a V6 engine such that one bank of cylinders is configured for dedicated EGR, thereby providing 50 percent EGR at all times.
Approach - One-dimensional cycle simulation is being used to identify the appropriate layout of the air handling system, including supercharger and/or turbocharger size and arrangement. Experiments are being conducted on a 3.5 L V6 direct injected gasoline engine to assess whether it is possible to operate an engine with 50 percent EGR, and determine what fuel economy improvement is available.
Accomplishments - On the simulation side, a one-dimensional cycle simulation model was created to determine an appropriate air handling system. Results showed that series boosting will be needed to produce the high intake manifold pressures needed for 50 percent dedicated EGR. A supercharger and turbocharger combination in series provided the necessary flows to meet the baseline engine torque curve. Simulation results confirmed that fuel economy at full load was improved relative to the baseline. On the experimental side, a state-of-the-art turbocharged and direct injected V6 engine donated by a U.S. automaker was instrumented and installed in the test cell. Steady-state measurements of performance and emissions were made at several speeds and loads to establish a baseline. The engine plumbing was then reconfigured for 50 percent dedicated EGR. An advanced ignition system developed at SwRI was added to help overcome the known ignitability challenges associated with high EGR. High tumble inserts are being added to the intake ports to increase charge motion and counteract the slow burn tendency of high EGR. A water-gas shift catalyst is being added to increase the hydrogen content of the dedicated EGR gas to improve ignitability. Upon completion of these modifications, testing with 50 percent dedicated EGR will begin.