Researchers at Oak Ridge National Laboratory are pursuing investigations into the use of a non-catalytic in-cylinder reforming process—i.e., the conversion of liquid hydrocarbon fuel to a hydrogen- and CO-rich syngas—potentially for controlling combustion phasing in homogeneous charge compression ignition (HCCI) and other forms of advanced combustion.
When fuel is injected during negative valve overlap (NVO) in O2-deficient conditions, a portion of the fuel is reformed to products containing H2 and CO. In a paper presented at the recent SAE 2014 World Congress, the ORNL team and colleague from Sandia National Laboratories reported on the chemistry of an NVO in-cylinder reforming process as experimentally determined from a single-cylinder engine. The Oak Ridge team plans to pursue the in-cylinder reforming technique in a multi-cylinder configuration in which one of the engine cylinders would act as the reformer, “essentially breathing in reverse compared to the other cylinders (breathing in from the exhaust manifold and exhausting into the intake system).”
In concept, the use of one cylinder to generate reformate for consumption in the other cylinders is similar to the approach being take by Southwest Research Institute (SwRI) and its Dedicated-EGR (D-EGR) project. SwRI has implemented its in-cylinder reforming technology in a multi-cylinder engine, and installed it in a demonstrator. (Earlier post.)
In the SwRI D-EGR system, one cylinder is converted to operate under fuel-rich conditions to produce reformate with significant concentrations of H2 and CO. The reformate is exhausted to the intake of the remaining cylinders and consumed in spark-ignited (SI) combustion.
The SwRI D-EGR demonstrator—a converted 2012 Buick Regal with a 2.0-liter gasoline direct injection engine—shows improved engine efficiency and fuel consumption og 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.
The distinction in the ORNL concept is how the reformate is generated rather than how it is used. The SwRI approach uses fuel-rich combustion during partial oxidation to produce power. The conceptual ORNL approach requires input work from the crankshaft to the reforming cylinder rather than extracted from it. The applicability of this approach resides in the present study results which show that O2 deficient operation provides the possibility of chemistry that is more favorable to the overall system energy balance.
Specifically, the present approach may be a pathway toward a chemical reforming proces that is not thermodynamically expensive, and may even enable thermodynamic recuperation. It should be noted that this in-cylinder reforming process is being pursued at ORNL in a multi-cylinder configuration and results will be disclosed in future publications.—Szybist et al.
|A comparison of the conceptualized ORNL approach to in-cylinder reforming (left) and the SwRI D-EGR approach (right). Source: Szybist 2013. Click to enlarge.|
In the ORNL work reported at the World Congress, the team compared experimental results from two very different engine cycles and facilities. ORNL developed a unique 6-stroke engine cycle that continually exhausts gases following NVO recompression, allowing real-time chemical analysis. (The 6-stroke cycle is only for research purposes, not commercialization.)
The base engine was a highly modified GM 2.0L Ecotec SI engine with a stock direct injection system and an aftermarket port fuel injection (PFI) system. Three cylinders are disabled to allow single-cylinder operation; a custom piston increased compression ratio to 11.85, up from the stock 9.2.
The Oak Ridge team examined results form a range of operating conditions, including multiple fuels, varying exces O2, charge temperature, and NVO fuel injection timing.
To confirm the ORNL experimental results, colleagues at Sandia performed collaborative experiments. Sandia used a dump valve to capture the exhaust from a single NVO event for analysis.
The teams found that the results from the two experiments are in excellent trend-wise agreement and indicate that the reforming process under low-O2 conditions produces substantial concentrations of H2, CO, methane, and other short-chain hydrocarbon species.
Major findings of the study included:
Work has to be put into the in-cylinder non-catalytic reforming process. This is mainly attributable to heat transfer during NVO, but the reforming process under sufficiently low-O2 conditions also requires work input.
The concentration of reformate species exhibits a stronger dependency on fuel injection timing than on the available O2 or the exhaust temperature, indicating the fuel reforming is a kinetically-limited process.
NVO reforming does not require a large energy input from the engine. The majority of fuel energy can be recovered as reformate for advanced fuel injection timing conditions, indicating that this process may be thermodynamically inexpensive.
Application of the in-cylinder reforming technique investigated here will continue to be pursued by ORNL in a multi-cylinder configuration. In follow-on work, the reforming cylinder will breathe in from the exhaust manifold and exhaust into the intake. In that configuration, the advantageous fuel properties of reformate (i.e., improved anti-knock properties combined with improved dilution tolerance) will be utilized to determine if a sufficiently large efficiency benefit can be realized in the combustion cylinder to overcome the added friction and thermodynamic penalties associated with the NVO reforming process.—Szybist et al.
Szybist, J., Steeper, R., Splitter, D., Kalaskar, V. et al. (2014) “Negative Valve Overlap Reforming Chemistry in Low-Oxygen Environments,” SAE Int. J. Engines 7(1) doi: 10.4271/2014-01-118
James P. Szybist, Derek A. Splitter, Vickey Kalaskar, Josh A. Pihl, and C. Stuart Daw (2013) “An Investigation Of Non‐Catalytic In‐Cylinder Fuel Reforming,” SAE 2013 High Efficiency Internal Combustion Engine Symposium