PSA concept D-EGR engine. Click to enlarge.

The engine was run naturally aspirated over a large portion of the performance map at an ultra-high (14:1) compression ratio. The results showed that the concept results in large gains in fuel efficiency over a baseline, low compression ratio engine and significantly reduces the exhaust emissions from the other 3 cylinders.

SwRI’s rationale in pursuing D-EGR as part of a larger program in gasoline engine efficiency is that technologies leading to knock reduction—i.e., to decrease the propensity of gasoline to auto-ignite under high pressure and temperature—will lead to technologies that can increase compression ratio, and support high-efficiency downsizing and downspeeding.

While the organization is working on a number of supporting technologies, the backbone of the concept is some form of cooled EGR for efficiency improvement.

Despite the benefits of EGR—e.g., improved cycle efficiencies through knock reduction, charge properties and pumping work; cooler combustion leading to lower emissions—there are challenges. These include misfire and stability; control; boosting; and design and materials.

• Dilution reduces laminar burning velocities, decreasing volumetric heat release and slowing reaction rates. In an engine, slower burn rates can lead to unstable operation and full misfire.

• High EGR rates (and low pre-turbine temperatures) can lead to low turbine efficiency.

• Transient control of EGR is vital.

• The design of the EGR system design will be key to control and packaging.

SwRI has been developing D-EGR as part of the current HEDGE II program; the lab currently has a 2.0-liter engine running as a full D-EGR engine from idle to full load, delivering more than 42% brake thermal efficiency with ultra-low exhaust emissions, says Dr. Terrence Alger II, Assistant Director, Engine and Vehicle R&D, Engine, Emissions, & Vehicle Research, SwRI.

At the US Department of Energy (DOE) DEER meeting in 2012, Alger noted that the test engine delivered lower BSFC (brake specific fuel consumption) than a Tier II Bin 2 diesel, with GDI performance and ultra-low emissions and no PM.

The D-EGR concept also combines the high efficiency potential of in-cylinder gasoline reformation. Under D-EGR, excess fuel is sent to the cylinder dedicated to producing the recirculated gases, the hydrogen (H₂) content of which then increases. The reformate along with the EGR is distributed to the other cylinders. The resulting H₂ levels increases EGR tolerance; increases knock resistance (i.e., higher compression ratios); reduces emissions; and improves fuel consumption.

(Researchers at Oak Ridge National Laboratory are exploring different aspects of in-cylinder reforming that could be applied with D-EGR, including reforming with a six-stroke cycle to better understand the chemistry associated with D-EGR.)

SwRI will apply the D-EGR concept to new platforms in the upcoming HEDGE III (High-Efficiency Dilute Gasoline Engine) program, which is targeting LEV III standards. In the HEDGE III program, which will run over the next four years, SwRI will seek to apply D-EGR on both light- and medium-duty engines.

Resources

• Alger, T.F. and B.W. Mangold (2009) Dedicated EGR: A New Concept in High Efficiency Engines. Presented at the Society of Automotive Engineers Congress, SAE Paper 2009-01-0694