SwRI D-EGR gasoline demo vehicle delivers diesel efficiency at lower cost; potential for LEV III/Tier 3 emissions
Southwest Research Institute (SwRI) announced preliminary test results of its Dedicated-Exhaust Gas Recirculation (D-EGR) demonstration vehicle (earlier post) at the SAE 2014 World Congress in Detroit. D-EGR is a SwRI-developed gasoline engine concept that uses in-cylinder fuel reforming to produce CO and H2 (with Research Octane Numbers of 106 and 130, respectively) and high levels of recirculated gas (EGR) to achieve very high levels of thermal efficiency.
The D-EGR demonstrator is a converted 2012 Buick Regal with a 2.0-liter gasoline direct injection engine. In their paper, the SwRI researchers reported that the conversion to D-EGR improved engine efficiency and fuel consumption at least 10% across the performance map, with some operating conditions seeing substantially higher improvements. The D-EGR engine offers efficiency similar to diesel engine (~40% BTE) but at half the cost; it also demonstrates the potential for meeting the very stringent LEV III/Tier 3 emissions.
When considering the high-efficiency alternatives out there today, we feel that our Dedicated-EGR technology is not only a high-efficiency, low-emissions alternative, but it’s the most cost-effective market-ready solution currently available.—Dr. Terry Alger, assistant director in SwRI’s Engine, Emissions and Vehicle Research Division
D-EGR brake specific fuel consumption (BSFC) at 2000 rpm 2 bar BMEP dropped 14.3% from 385 g/kWh to 330 g/kWh in the D-EGR engine. The lowest BSFC for the engine was 212 g/kWh, a reduction of 10.2% from 236 g/kWh in the series engine. Two-stage boosting allowed the engine to meet its torque targets of at least 17 bar BMEP from 1500-5500 rpm while maintaining good transient response and low engine-out emissions.
D-EGR technology was initially conceived and developed in SwRI’s HEDGE (High-Efficiency Dilute Gasoline Engine) consortium; SwRI used internal research funds to demonstrate production feasibility.
The 2.0-liter engine was modified so that exhaust from one dedicated cylinder is run with a rich mixture of fuel and air to reform hydrocarbon fuel into carbon monoxide and hydrogen. The reformulated exhaust gas is then cooled and looped into an SwRI-developed patented mixer where the EGR and reformate are mixed with fresh air before going into the engine intake. Engineers designed several new parts for the advanced combustion concept—the cooled EGR loop, the EGR mixer and high-energy ignition system—as well as engine-control software that enables in-cylinder fuel reformation.
By running one cylinder rich, the excess fuel is reformed into hydrogen and carbon monoxide. The in-cylinder reformation slightly reduces the carbon dioxide and water vapor while producing large volumes of carbon monoxide, which is a good fuel, and hydrogen, which is an outstanding fuel. That provides an octane boost and a flammability boost, and extends the EGR limit of the engine.—Chris Chadwell, manager of SwRI’s Spark Ignition Engine R&D section
One of the features of the D-EGR concept is that the EGR rate is not proportionately controlled; i.e., the engine will run continuously with a nominal 25% EGR rate—thereby making control less complex.
D-EGR provides additional knock tolerance via the reformate in addition to that already provided by EGR In-cylinder reformation is achieved by running the dedicated cylinder rich of stoichiometric; a typical operating target is to run an equivalence ratio (φ) beween 1.3 and 1.4. The SwRI team found that when the D-EGR cylinder was run in that range, the knock resistance was similar to an increase in RON of the fuel of approximately three points.
|Power and torque curves for the stock LEA engine and the converted D-EGR engine. Chadwell et al. Click to enlarge.|
The engine conversion. The base 2.0L GDI engine (LEA) produces 233 N·m of torque at 4900 rpm and 136 kw at 6700 rpm. The design target for the D-EGR engine was 270 N·from 1500-5500 rpm with a maximum power of 155 kW at 5500 rpm.
The D-EGR design targets exceeded the stock LEA engine to allow down-speeding of the engine when the vehicle could operate with a lower overall gear ratio for additional fuel economy benefits. Successful downspeeding requires elevated torque at lower engine speeds with good transient response and load acceptance, the SwRI researchers noted.
The air handling system consisted of boosting, EGR and EGR mixing subsystems. Initial simulation showed that turbocharging alone would not meet the torque requirements of the converted engine; hence, the D-EGR engine uses both a supercharger and a turbocharger.
The supercharger has a clutch that can be disabled under conditions when the turbocharger can take over, or during light load when no boosting is required. The supercharger is placed after the turbo compressor in the flow path such that the turbocharger provides boosted air to the inlet of the supercharger.
|Instantaneous representation of D-EGR mixing. The color contours represent the concentration of burned gasses; red is 100% burned gas while blue is 0% burned gas. Chadwell et al. Click to enlarge.|
Typical EGR mixers only operate in the spatial realm, the SwRI team noted; i.e., fresh air flow and EGR flow are relatively steady. Because D-EGR is produced in one cylinder and then spread evenly across the other cylinders, temporal mixing must also occur. SwRI designed a mixer with many small orifices along the axial length of a concentric tube containing the main air flow path.
Aside from the increased compression ratio, the combustion system of the converted D-EGR engine was largely unchanged. The engine uses custom pistons, and also adds a port fuel injector to the D-EGR cylinder.
Among the results and conclusion reported were:
It was possible to match a supercharger and turbocharger boosting system to the D-EGR engine to maintain a constant 17 bar BMEP from 1500 to 5500 rpm.
The new mixer designed for spatial and temporal mixing delivered a good balance of EGR rate across all cylinders of the engine.
Running rich in the D-EGR cylinder produced reformate that improved the knock tolerance of the engine. Based on measurements of hydrogen in the D-EGR exhaust port, the engine was running with approximately 1% hydrogen by volume. 1% hydrogen was sufficient to stabilize combustion with 25% EGR at low engine loads.
25% D-EGR combustion allowed the engine BSFC to be reduced by at least 10%. The reduction was lowest at low speeds and high loads where the elevated compression ratio required spark retard and the supercharger was used. The maximum reduction was at high speeds and high loads where the elimination of enrichment improved the BSFC by over 30%.
D-EGR combustion was feasible at loads from 2 bar to 17 bar BMEP at all engine speeds. Control of the engine during transient operation was successfully demonstrated.
|D-EGR demonstrator at SAE 2014 World Congress. Click to enlarge.|
Chadwell, C., Alger, T., Zuehl, J., and Gukelberger, R. (2014) “A Demonstration of Dedicated EGR on a 2.0 L GDI Engine,” SAE Int. J. Engines doi: 10.4271/2014-01-11907(1):2014