Researchers at Oak Ridge National Laboratory have used on-board catalytic fuel reforming to increase the brake efficiency of a multicylinder, stoichiometric spark-ignited (SI) engine by increasing EGR dilution tolerance.
In an open-access paper published in the ACS journal Energy & Fuels, they report that under an engine operating condition of 2000 rpm and brake mean effective pressure (4 bar), catalytic EGR reforming made it possible to sustain stable combustion with a volumetric equivalent of 45%–55% EGR. Under this same operating condition with stoichiometric engine exhaust (and no reforming), they could only sustain stable combustion with EGR under 25%.
The researchers suggest that these results indicate that multicylinder gasoline engine efficiency can be increased substantially with catalytic reforming combined with and higher EGR operation, resulting in a decrease of more than 8% in fuel consumption, compared to baseline operation.
Typically, you incur a fuel penalty when reforming fuel. We’ve created a systematic approach that addresses that issue and can be used with conventional fuels and conventional emissions controls.—Jim Szybist, ORNL
External cooled exhaust gas recirculation (EGR) provides known thermodynamic benefits toward improving the efficiency in SI engines while maintaining compatibility with conventional three-way catalysts for emissions control. These benefits include: reduced pumping work at partial-load conditions; decreased heat transfer due to lower cylinder temperature; and increased ratio of specific heats.
External EGR also decreases the propensity of knocking for a given fuel, which can be used as the basis for additional increases in efficiency through more advanced combustion phasing or higher compression ratio. EGR also reduces NOx emissions over a broad range of speed and load conditions.
However, the ORNL team notes, the amount of EGR dilution that can be used is limited due to cycle-to-cycle combustion instability, thereby limiting its potential efficiency benefits.
The root cause of the cyclic instability with EGR has been linked to a decrease in flame speed, which elongates the initial flame kernel development process, making it more susceptible to stochastic turbulence variation. While several technologies unrelated to fuel reforming are being developed to extend the EGR dilution limit, including high-energy long-duration ignition systems and incorporation of different higher turbulence combustion chamber flows, the focus of this work is to extend the EGR dilution limit by using the reformed products of the fuel to increase the EGR dilution tolerance.
… Although there are many approaches to generating reformate on board, they can generally be classified into two broad categories. The first category is where fuel is reformed in one or more cylinders in an engine using noncatalytic processes. This category includes the Dedicated EGR (D-EGR) strategy developed by Southwest Research Institute, which is the most developed strategy in this category. [Earlier post.] D-EGR uses fuel-rich combustion in one cylinder and recirculates its exhaust to the intake system, generating brake thermal efficiency as high as 42.5% and demonstrating a vehicle-level fuel consumption decrease of more than 10%. This category also includes injecting fuel during the negative valve overlap period for a homogeneous charge compression ignition engine to manipulate the fuel–air mixture autoignition propensity.
The second category of fuel reforming, and the category of the present work, is where a catalyst is used to reform the fuel outside of the engine cylinders.—Chang et al.
The ORNL team used a 2.0 L GM Ecotec LNF SI engine equipped with the production side-mounted direct injection fueling system. The combustion chamber geometry and camshaft profiles were unchanged from the stock configuration. They used two EGR configurations for the engine: conventional and a catalytic EGR-loop reforming strategy. All data were collected at an engine speed of 2000 rpm and a brake mean effective pressure (BMEP) load of 4 bar.
|Schematic of the engine and test cell configuration for (a) the conventional EGR configuration and (b) the catalytic exhaust gas recirculation (EGR)–loop reforming strategy. Source: Chang et al. Click to enlarge.|
In the catalytic reforming strategy a single isolated cylinder feeds the reforming catalyst prior to passing through the EGR cooler and entering the same intake plenum and static mixer arrangement used in the conventional EGR case, thereby providing the entirety of the reformate-EGR mixture to the other three cylinders.
The isolated cylinder (referred to as cylinder 4 in the study), does not have the ability to breathe the EGR–reformate mixture as the other cylinders do. However, because all of the cylinder 4 exhaust is used to feed the reforming catalyst and is recirculated to the other three cylinders, it is not necessary to maintain stoichiometric conditions in cylinder 4 to have a stoichiometric exhaust. Rather, to provide the catalyst with the proper feed for good performance, cylinder 4 is intentionally operated under nonstoichiometric conditions.
Based on their earlier work, they operated cylinder 4 so as to provide the catalyst with a fuel-rich mixture that also contained oxygen.
Among their other findings noted above, they also found that the catalytic reforming strategy could produce intake manifold H2 concentrations as high as 5% at the 2000 rpm and 4 bar BMEP condition.
While they successfully demonstrated progress toward increasing the efficiency of stoichiometric SI engines using a catalytic EGR-loop reforming strategy, the team also noted that the development of this combustion strategy is at a very early stage. As a result, they said, there are a number of additional undeveloped benefits that could produce further efficiency improvements, and there are also a number of unresolved challenges and barriers to its use in vehicle applications.
Undeveloped benefits include:
As the results of the current study were achieved without and engine or catalyst optimization, significant optimization should be possible, such as increasing the compression ratio.
The geometry of the engine could be optimized. The study engine uses a square combustion chamber geometry (stroke = bore), whereas newer engines that are highly tolerant of EGR dilution are under-square (stroke > bore). An under-square geometry increases the mean piston speed and, by extension, the in-cylinder turbulence, which scales with mean piston speed.
Higher turbulence shortens the duration of the early flame kernel development by transitioning to turbulent combustion sooner, thereby increasing combustion stability, and shortens combustion duration, which increases efficiency, they noted.
No attempt was made to optimize the fuel injection timing or targeting, or more broadly, optimize the overall catalyst fueling strategy.
Transient performance represents a difficult control challenge even for conventional EGR. The EGR-loop reforming system provides further transient complications because achieving a steady-state temperature in the catalyst takes several minutes—significantly longer than engine transient operation.
Because hybrid electric vehicles may not require the engine performance to be as transient-capable as conventional powertrain vehicles, hybridization may be synergistic with this technology, the team observed.
The study focused on a single speed/load operating condition; the team does no know if there are limitations of the reforming process itself that prevent it from being applicable over the entire operating strategy. From idle to full power, the mass flow rate of a typical engine varies by more than a factor of 50. This will cause a significant variation in space velocity and enthalpy flux, as well as catalyst temperature. Further, the speed/load demands of a modern vehicle likely require operating under boosted intake manifold pressure—further complicating the engine system.
The long-term durability of the reforming catalyst, as well as the reforming response to a variety of fuels that a vehicle will encounter has not been investigated.
Yan Chang, James P. Szybist, Josh A. Pihl, and D. William Brookshear (2018) “Catalytic Exhaust Gas Recirculation-Loop Reforming for High Efficiency in a Stoichiometric Spark-Ignited Engine through Thermochemical Recuperation and Dilution Limit Extension, Part 2: Engine Performance” Energy & Fuels 32 (2), 2257-2266 doi: 10.1021/acs.energyfuels.7b02565