For years we heard about gadgets and gizmos improving fuel efficiency. Some were snake oil, others worked to varying degrees. This story grabbed my attention because of its simplicity and credibility.
Gasoline-powered automobiles could achieve an 8 percent or greater fuel efficiency gain through a new combustion strategy developed at Oak Ridge National Laboratory. Scientists have demonstrated a new method for reforming fuel over a catalyst, a process that chemically converts fuel into a hydrogen-rich blend. This blend allows more work to be extracted from the engine cylinders, increasing efficiency and saving fuel. “Typically, you incur a fuel penalty when reforming fuel,” said ORNL’s Jim Szybist. “We’ve created a systematic approach that addresses that issue and can be used with conventional fuels and conventional emissions controls.” The team published the method in Energy & Fuels and is working at ORNL’s National Transportation Research Center to demonstrate similar fuel savings at a wider range of engine operation. [Contact: Kim Askey, (865) 576-2841
Efficiency improvements in spark-ignited (SI) engines are desired as a way to meet increasingly challenging fuel economy and CO2
External cooled exhaust gas recirculation (EGR) provides known thermodynamic benefits while maintaining compatibility with conventional three-way catalysts for emissions control.(2)
summarized the thermodynamic benefits of EGR, which include reduced pumping work at partial-load conditions, decreased heat transfer due to lower cylinder temperature, and increased ratio of specific heats. External EGR is also a proven way to decrease the knocking propensity 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.(4)
Lastly, EGR reduces NOx emissions over a broad range of speed and load conditions.(5)
The amount of EGR dilution that can be used is limited due to cycle-to-cycle combustion instability, thereby limiting the potential efficiency benefit of EGR.(6-8)
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.(9-11)
While several technologies unrelated to fuel reforming are being developed to extend the EGR dilution limit, including high-energy long-duration ignition systems(12)
and incorporation of different higher turbulence combustion chamber flows,(13)
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.
The addition of the major products of fuel reforming, H2
in particular, increases the combustion rate and can increase engine efficiency because of the shorter combustion duration.(14-20)
Alger et al.(21)
reported that H2
can also be used to extend the EGR dilution limit, with the addition of 1 vol % H2
extending the EGR limit from 25% to more than 50% for gasoline and from 20% to 28% for compressed natural gas. Fennell et al.(22)
showed that simulated H2
-rich reformate could extend EGR dilution in an SI engine from 21% to 27% at the same combustion stability. Ivanič et al.(23)
added reformate at levels of 15% and 30% gasoline energy equivalent in a single cylinder engine and found that, under partial-load conditions, lean dilution can improve engine efficiency by as much as 12% while EGR dilution delivers 8% improvement.
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.(24)
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%(25)
and demonstrating a vehicle-level fuel consumption decrease of more than 10%.(26)
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.(27-32)
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(19, 22, 33-40)
Catalytic reforming processes often use the sensible enthalpy in the exhaust to promote reforming reactions over the catalyst to improve the energetics of reforming.(18, 19, 33-36, 31-43)
The range of possible combustion and reforming reactions that can occur has been presented by Ahmed and Krumpelt(44)
and by Jamal and Wyszynski,(18)
and is presented below for completeness using iso-octane as the starting fuel. Equation 1
shows the reaction stoichiometry for the complete combustion of iso-octane, while eq 2
defines the equivalence ratio (Φ). For the purposes of this manuscript, Φ refers to the O2
/fuel ratio of complete combustion in eq 1
, relative to the actual O2
/fuel ratio. Equations 3
show the primary types of reactions that can occur in a reformer. When O2
is present such that the molar O/C ratio is unity, partial oxidation (POx) occurs where the parent hydrocarbon is converted to CO and H2
, resulting in an exothermic process, as shown in eq 3
. For an environment devoid of O2
but where H2
O is present, the resultant steam reforming reaction, shown in eq 4
, is highly endothermic and results in much higher concentrations of H2
. Similarly, eq 5
shows a dry reforming reaction where CO2
is consumed to form H2
and CO in a process that is substantially more endothermic than steam reforming, but where the concentrations of H2
are lower. Ideally, the endothermic steam reforming reactions are driven by the sensible enthalpy in the exhaust, allowing for waste heat recovery through thermochemical recuperation (TCR).(45)
In this work, we studied reforming environments that contain an insufficient amount of O2
to convert all of the fuel to CO and H2
through eq 3
and thus consist of a balance of POx, steam, and dry reforming processes.
The work by Leung et al.(38)
showed that, for ethanol, the steam reforming reaction (eq 4
) is very active at engine exhaust temperatures, converting almost 100% of the fuel energy at 600 °C. In contrast, dry reforming (eq 5
) produces <40% fuel conversion at the same temperature. This indicates that dry reforming is not the favored reaction pathway, likely due to the larger energy input required, especially at lower initial temperatures. Regardless of whether reforming occurs via the steam or dry reforming pathway, however, the product concentrations can equilibrate via the water–gas shift (WGS) reaction, shown in eq 6
It can be challenging to operate the engine and catalyst systems together, such that good performance of the catalytic reformer and engine are achieved simultaneously. Hwang et al.(36)
integrated a reforming catalyst containing Rh and Pt into the exhaust manifold of a diesel engine. While they were able to produce high concentrations of H2
(>10%), the reforming system resulted in an overall engine efficiency decrease, because the reformer catalyst equivalence ratio was too low (Φcatalyst
as low as 1.5), and the reforming process was too dependent on the exothermic reactions in eq 3
. This is consistent with much of the diesel reforming work where high concentrations of H2
can be produced, but because of the dependency on POx reforming, there is a large fuel energy penalty to produce the H2
In contrast, Ashida et al.(33)
used a steam reforming catalyst (4 wt % Rh/Al2
, La additive) in the EGR loop of an SI engine without any additional O2
approaching ∞). They initially found high levels of fuel conversion and H2
production, but the steam reforming catalyst began deactivating immediately, with 35 ppm of S fuel resulting in a 90% deactivation within 5 h.
In the companion paper (Part 1, “Catalyst Performance”), we reported on the reformer catalyst performance over a range of Φcatalyst
conditions in an effort to find a balance between the POx activity required for catalyst durability and the endothermic steam reforming reactions required to minimize energy losses or to achieve TCR during reforming.(48)
In this paper, we characterize the multicylinder engine performance while maintaining stoichiometric exhaust emissions with the EGR-loop reforming strategy, focusing on whole-engine performance, including dilution tolerance, combustion performance, and brake thermal efficiency.
In this investigation we demonstrated significant progress toward increasing the efficiency of stoichiometric SI engines using a catalytic EGR-loop reforming strategy. The development of this combustion strategy is at a very early stage, and as a result, 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.
The efficiency increase observed here was achieved without any optimization of the engine geometry or of the catalyst, and there is reason to believe that significant optimization is possible. EGR has been shown to be an effective knock suppressant(4)
that could allow a substantial increase in the engine compression ratio, which could increase engine efficiency. While increasing the compression ratio benefits efficiency by extracting more work from the combustion products, it also reduces the temperature in the exhaust, thereby exacerbating a process that is already enthalpy-limited. The tradeoff between these two factors has yet to be explored.
This 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.(52)
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.
Finally, no attempt was made to optimize the fuel injection timing or targeting, or more broadly, optimize the overall catalyst fueling strategy. This engine used a side-mounted fuel injector, making fuel spray impingement on the cylinder wall during the post-injection probable. A centrally mounted injector and specifically designed fuel injector targeting for these conditions would likely provide better control. Whether the post-injection fueling strategy is preferable to direct fueling to the reforming catalyst in the exhaust system has not been studied and may provide improved performance and/or improved control.
Barriers to Implementation
In this engine, as with any combustion strategy using high levels of EGR, the transient performance of the engine poses a difficult control challenge. This is because the reforming and EGR loop represents a substantial volume, making it difficult to rapidly change the intake manifold pressure required when varying load to follow transients in SI engines. The EGR-loop reforming system provides further transient complications because Figures 9
–11 showed that achieving a steady-state temperature in the catalyst took several minutes, which is significantly longer than engine transient operation. Intake system optimization to minimize the volume combined with calibration optimization can provide improvements to the transient response, but also has risks. A smaller volume may lead to poor mixing of the reformate both spatially and temporally. Hybrid electric vehicles may not require the engine performance to be as transient-capable as conventional powertrain vehicles, thus hybridization may be synergistic with this technology.
In addition, the current investigation focused on a single speed/load operating condition. It is not yet known whether 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. Additionally, meeting the speed/load demands of a modern vehicle likely requires operating under boosted intake manifold pressure, which further complicates the engine system.
Finally, 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. This investigation used iso-octane, which is a paraffinic fuel that contains no sulfur. Marketplace gasoline will have some level of sulfur and aromatics, and the real-world H/C ratio of the fuel will change. Whether the EGR-loop reforming strategy can perform consistently has not yet been determined.