Powering the Way to Better Fuel Economy
The SwRI D-EGR development team (L to R): Principal Engineer Jess Gingrich, Principal Designer Douglas McKee, Assistant Director Dr. Terry Alger, Principal Engineer Mark Jones, Manager Christopher Chadwell and Research Engineer Raphael Gukelberger. Not shown are Research Engineer Jacob Zuehl and Research Technologist Roger Huron. All are within the Engine, Emissions and Vehicle Research Division.
SwRI’s high-efficiency D-EGR engine contains custom-designed airpath parts.
A schematic drawing of a D-EGR layout shows the path of air flow through the engine. Recirculated exhaust is shown in yellow.
A fuel consumption comparison shows the relative performance parameters of a D-EGR engine and a modern diesel. (BSFC = brake specific fuel consumption, a measure of how much fuel is required to achieve a given power level.)
This CAD model of the D-EGR engine shows the unique parts required to accomplish D-EGR operation.
An SwRI technician installs the D-EGR engine in a demonstration vehicle.
As engine manufacturers race to bridge the gap between current and future emissions and fuel economy regulations, a team of engineers at Southwest Research Institute (SwRI) has developed a new engine design that may quicken the pace.
The SwRI-developed Dedicated Exhaust Gas Recirculation (D-EGR®) gasoline engine is on average 10 percent more efficient than the next-best gasoline engine with certain operating conditions exceeding a 30-percent improvement. The engine, conceived under the High Efficiency Dilute Gasoline Engine (HEDGE®) research consortium managed by the Institute, is based on cooling and then re-circulating burned exhaust gases into the engine to improve the engine’s thermal efficiency (see “Clean and Cool,” Summer 2010 Technology Today). Dedicated EGR® takes the technology a step further by using a subset of an engine’s cylinders to re-circulate exhaust gases to the others. Tests at SwRI have shown that the engine has fuel economy comparable to a diesel engine of similar displacement, but at Dedicated EGR offers superior efficiency, emissions less than two-thirds the cost and with lower smog-forming and particulate emissions.
The cost-effective, high-efficiency, ultralow emissions D-EGR engine combines the efficiency improvements of recirculated exhaust gas with the combustion benefits of reformed fuel. The fuel reformation process occurs inside a power cylinder that is operated with excess fuel. Rich combustion leads to the formation of large amounts of hydrogen (H2) and carbon monoxide (CO), which are then recirculated to the engine. Since the fuel reformation occurs in a power-producing cylinder and all the combustion products are recirculated, the normal losses associated with fuel reformation in an external device are avoided. The synergy between cooled, recirculated exhaust gas and reformed fuel is at the crux of D-EGR technology.
The recirculated exhaust gas helps partially overcome engine limitations that have historically reduced spark-ignition engine efficiency, such as thermal losses, pumping work losses and engine knock. At low EGR levels, increasing the EGR rate leads to increased efficiency. Eventually, however, the engine reaches a limit where efficiency no longer improves with additional EGR. The efficiency limiting mechanisms are associated with combustion efficiency and flame speed. Recirculated exhaust gas at high levels slows combustion reaction rates, reducing knock but also reducing the flame speed. Slow flame speeds lead to unstable combustion, particularly at low power conditions. In a similar manner, the in-cylinder temperature reductions with cooled EGR that reduce heat transfer losses and improve efficiency also lead to a reduction in post-flame hydrocarbon oxidation rates and increased flame quench, which increases the emissions of unburned hydrocarbons and reduces combustion efficiency.
Addressing these deficiencies in cooled EGR is where the fuel reformation aspect of D-EGR is important. The reformate, primarily consisting of H2 and CO, is recirculated with the rest of the exhaust gas. The combustion properties of the reformate mitigate some of the drawbacks of cooled EGR alone. Reformate improves the engine’s tolerance of recirculated exhaust gas, primarily by increasing burn rates, leading to improved stability at high EGR levels. The reformate also has a very low minimum ignition energy, which means that it has a reduced quench distance and improved fuel oxidation. This, in turn, leads to reduced emissions of unburned hydrocarbons and improved combustion efficiency. The combination of the two factors allows an engine with 25 percent dilution to operate with nearly the same combustion efficiency and identical stability as the baseline, non-dilute engine, but with a significant efficiency improvement.
Because the air-fuel ratio differs among cylinders, D-EGR relies on a sophisticated control system to maintain optimum efficiency across the engine’s combustion spectrum and to ensure that all cylinders produce the same power.
After exploring cooled EGR technology in the earliest phases of the HEDGE consortia, the latest phase has involved constructing a prototype D-EGR system in a modified four-cylinder engine to prove the performance gains indicated in earlier computer-generated analyses of D-EGR technology.
Other benefits of reformate
In-cylinder fuel reforming is used to convert a fuel-rich mixture of air and gasoline into a gas stream containing high levels of reformate in the form of hydrogen and carbon monoxide along with burned gas containing carbon dioxide, water vapor and nitrogen. Combusting reformed fuel (or, in chemical terms, converting complex hydrocarbons to a blend of hydrogen and carbon monoxide molecules) improves the efficiency and emissions of internal-combustion engines, especially in highly dilute spark-ignited engines. As mentioned above, reformate improves the combustion process by increasing flame speeds and enabling engine operation at higher dilution levels, creating more stable and complete combustion.
In addition to improving on the limiting factors associated with cooled EGR, the nature of reformate means that the engine charge is improved in several fundamental ways. First, both H2 and CO have very high octane ratings — both have research octane numbers greater than 100 — and their presence in the fuel mixture increases the octane rating of the charge significantly. In a D-EGR engine, the knock response of the engine using regulargrade (87 AKI) gasoline is the same as for the baseline engine using super-premium (greater than 93 AKI) gasoline, which enables operation at very high (>11:1) compression ratios with high efficiency, even at very high specific power levels.
In addition, since reformate is primarily diatomic molecules, it has a high ratio of specific heats. Because the ultimate efficiency potential of the engine increases as the ratio of specific heats of the charge increases, the presence of reformate improves the efficiency potential of the engine by increasing the charge’s ratio of specific heats. The charge’s ratio of specific heats is a function of both composition and temperature. By combining the composition- changing impact of reformate with the cooler combustion temperatures from cooled EGR, the D-EGR engine operates at a much higher ratio of specific heats over the cycle, increasing the efficiency potential over either reformate or cooled EGR alone.
By improving the tolerance for dilution, D-EGR engines can run higher levels of recirculated exhaust gas and further reduce losses typically associated with throttled stoichiometric engines, which account for virtually all modern gasoline engines. Dedicating a cylinder to reformate production also ensures that while the dedicated cylinder runs with excess fuel, the rest of the engine can operate at stoichiometric air-fuel ratios (meaning the combination of air and fuel that results in complete combustion) and use the current, high-efficiency and lowcost aftertreatment available for automotive applications. The result is an engine design that has demonstrated very high engine efficiency with very low associated cost, delivering real-world engine efficiencies greater than 42 percent along with ultra-low emissions.
Reforming low-quality fuels
Historically, engines have been designed to operate within fairly narrow fuel quality parameters, any deviation from which might significantly limit engine efficiency. For example, some modern, high-performance engines require premium gasoline. However, the SwRI team discovered that D-EGR engines can operate on very lowquality fuels, whether liquid or gaseous, by reforming a portion of the fuel internally and thus improving its effective quality. By modulating the amount of in-cylinder fuel reformation, the D-EGR power plant in effect provides “octane on demand.” With marketplace fuel costs generally directly proportional to quality, the lower the quality of fuel the engine can burn, the greater in-use cost benefit D-EGR can provide.
Because nearly all the hazardous emissions of a modern engine are released before the catalyst becomes active, the combination of EGR and reformate, plus the ability of reformate to accelerate catalyst light-off, creates the potential for ultra-low emissions. Recent tests at SwRI indicate that cooled EGR reduces the emissions of particulates from gasoline engines. When D-EGR is employed, the particulate emissions are reduced even further from the cooled EGR baseline, indicating that it may be possible to meet future particulate emissions standards without a costly particulate filter.
Torque is not necessarily compromised with D-EGR, because a turbocharger can be scaled and added to the engine to meet vehicle requirements. However, adding a turbocharger and other D-EGR components does increase costs, as does the requirement for higher cylinder pressures due to high compression ratios and improved combustion phasing. On the other hand, D-EGR may enable some potential cost-saving changes to engine architectures. Potentially, D-EGR eliminates the need for costly technologies such as a gasoline direct injection (GDI) fuel system because the EGR system, and the reformate that a Dedicated EGR cylinder produces, both suppress knock to a far greater degree than the GDI system does. D-EGR also reduces the requirement for cam phasers to improve efficiency because it achieves part-load efficiency through dilution and in-cylinder fuel reformation.
D-EGR hardware and control
The D-EGR engine uses conventional low-cost and durable automotive components in a unique way to yield an elegant solution for high-efficiency engines. This technology uses cooled EGR and reformed fuel without requiring the hardware complexity and cost typically associated with such systems. The control system, integral to the product’s high efficiency, is the final piece of this technology. The control system actively determines the amount of fuel that must be reformed to achieve best efficiency. In a transient environment, it can adapt the fueling rate independently for each cylinder. In addition to fueling, the controller adjusts ignition timing based on the reformate level to ensure optimal combustion phasing in each cylinder on a cycle-by-cycle basis. Finally, the control algorithms monitor the actual dilution in the system and continually adjust the airflow through the dedicated cylinder to maintain misfire-free operation in a transient environment. A sophisticated electronic control system enables the benefits of the technology to be realized in real-world conditions on a variety of platforms.
While a Dedicated EGR engine’s primary intended application is as either the sole powerplant for a vehicle or as part of a hybrid system, there are several other potential applications. In non-automotive applications, the engine can be used to generate electricity, pump fluids or power industrial machinery. In such applications, the D-EGR engine can produce more mechanical power per fuel input than other engines in its class, with potentially higher torque and power than a small, off-road certified diesel. This best-in-class efficiency is coupled with the capability to operate with super-ultra-low emissions (SULEV) using a conventional three-way catalyst aftertreatment system. As modern, emissions-compliant diesel engines have significantly increased in cost, the sparkignition D-EGR engine for off-road or stationary applications also provides a significant cost reduction.
The engine can also be configured to produce high-quality heat for co-generation or combined heat and power (CHP) applications for large natural gas engines. Because the recirculated exhaust gas is cooled prior to introduction to the induction system, the heat extracted from this circuit can be coupled with exhaust heat to produce steam or preheat a subsequent power generating system. Conventional CHP systems that use lean-burn natural gas engines require emission control technologies that are more costly and less efficient than a three-way catalyst system. Also, lean-burn emissions control systems have a limited thermal operating range, which limits the thermal energy available for co-generation. D-EGR is less restricted by aftertreatment systems and can therefore increase the total energy recovered by a co-generation system.
Future development and applications
A major commercial automotive manufacturer has announced a D-EGR engine using HEDGE-developed technology for production in 2018 and several predevelopment efforts with other companies are in progress. The HEDGE-III consortium will also continue work on the D-EGR concept, investigating additional ways of improving efficiency in the system. In addition, an internally funded research project is under way at SwRI to build a demonstration vehicle for D-EGR technology based on a 2012-model sedan platform.
Questions about this article? Contact Alger at (210) 522-5505 or email@example.com.