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 03/31/09
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 from 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 was believed that further improvements would be available at higher EGR levels. This theory was tested in this project by modifying a V6 engine such that one bank of cylinders was configured for dedicated EGR, thereby providing 50 percent EGR at all times. Two issues arise when attempting to operate an engine with 50 percent EGR. First, ignition of such a highly dilute mixture requires very high ignition energy, and it is unknown if a spark ignition system is capable of providing sufficient energy. Second, the boost system must produce approximately twice the intake manifold pressure of the base engine in order to provide both air and EGR to the engine, and it is unclear what boost system architecture is needed to achieve this target across the entire speed-load range of the engine.
Approach - One-dimensional cycle simulation was used to identify the appropriate layout of the air handling system, including supercharger and/or turbocharger size and arrangement. Experiments were conducted on a 3.5L V6 direct injected gasoline engine to assess whether it was possible to operate an engine with 50 percent EGR, and determine what fuel economy improvement was available.
Accomplishments - On the simulation side, one-dimensional cycle simulation results showed that a properly sized supercharger and turbocharger arrangement in series could produce the high intake manifold pressures needed for 50 percent dedicated EGR and to meet the baseline engine torque curve. Simulation results also confirmed that fuel economy at full load was improved relative to the baseline since fuel enrichment at high load could be eliminated and spark timing could be advanced.
On the experimental side, six iterations of spark ignition hardware, charge motion control, and compression ratio were evaluated in an attempt to achieve combustion at 50 percent EGR. Because of the inherent difficulty of igniting such a dilute mixture, none of the configurations involving a spark plug was able to provide enough energy to reach the 50 percent EGR target. As such, the spark ignition system was replaced with an SwRI-developed pilot ignition system, which provided much higher ignition energy than could be realized with a spark plug. This pilot ignition system could be described as a "chemical spark plug" in which a small quantity of highly reactive "ignition fluid" was injected into the combustion chamber to initiate combustion. With the addition of the pilot ignition system, the engine was able to operate with 50 percent EGR across a range of engine speeds and loads. Investigations were then performed to evaluate the effects of pilot timing, pilot quantity and air-fuel ratio on combustion parameters, emissions, and fuel economy. The two main highlights of the results were: 1) up to 14 percent fuel economy improvement was observed relative to the baseline, and 2) engine-out NOx emissions were low enough to meet the U.S. 2010 heavy duty emissions standard at all test conditions.