The US Department of Energy’s (DOE’s) Co-Optima initiative—a broad, joint effort to co-optimize the development of efficient engines and low greenhouse-gas fuels for on-road vehicles with the goal of reducing petroleum consumption by 30% by 2030 beyond what is already targeted (earlier post)—has published a year-in-review report for FY 2016—the initiative’s first 12 months.
Co-Optima’s premise is that current fuels constrain engine design—and thus engine efficiency. The researchers suggest that there are engine architectures that can provide higher thermodynamic efficiencies than available from modern internal combustion engines; however, new fuels are required to maximize efficiency and operability across a wide speed/load range. The report details the technical progress in a selection of projects across the initiative’s two main thrusts: spark ignition (SI) and advanced compression ignition (ACI).
Thrust I has concentrated on improving near-term SI engine efficiency and assessing desirable properties for bio-derived or bio-based components (blendstocks) to be blended with petroleum-derived gasoline. The research teams have identified more than 40 promising blendstock candidates. The teams are also evaluating the impact of discrete fuel properties on advanced SI engine performance.
The Thrust II ACI combustion strategies focused on achieving higher efficiency with lower levels of harmful emissions than can be achieved today. The researchers evaluated fuel property impacts on kinetically controlled and low-temperature combustion (LTC) strategies and began to improve the understanding of how co-optimized fuel properties and engine strategies can contribute to longer-term solutions.
A great deal of foundational work early in the project’s first 12 months has laid the basis for the specific projects:
Tiered screening approach narrows the field of potential fuel candidates;
Assessment of fuel-enabled efficiency gains embodied in “merit function”
Analysis assesses the benefits of proposed fuels and engines;
Database developed for bio-derived fuel blendstocks and their gasoline blends;
Model predicts octane numbers for blended fuels with bio-derived components;
Machine learning tool classifies fuel properties in blendstock evaluation;
Co-optimizer software solves merit function for fuel blendstocks;
New tool automatically generates physically accurate surrogate fuel mixtures;
Parallel-cycle simulations yield fuel velocity statistics in bulk;
Microliter fuel ignition tester detects octane number dependence;
Spray chamber simulates engine temperatures and pressures.
Specific Thrust I accomplishments in the first year include:
Co-Optima researchers performed the first systematic assessment of the suitability of oxygenate functional groups in fuel blends for SI engine, and identified more than 40 bio-derived materials as potential blendstocks.
Co-Optima analysts developed 21 metrics in the categories of process economics, technology readiness, sustainability, and market barriers against which to evaluate a subset of 20 promising Thrust I candidate blendstocks with a RON exceeding 98. Five candidates were identified for more rigorous analyses in the project’s second year.
Co-Optima researchers examined the compatibility of 32 fuel candidate blendstocks with six elastomers, using a theoretical chemical method called the Hansen solubility analysis. For many candidates, the solubility potential (or potential swelling) increases with higher blendstock content and peaks at low- to mid-range concentrations. For many elastomers, the solubility is within an acceptable range, but experimental testing will be undertaken in FY17 to confirm their compatibility.
At a compression ratio of 12:1, use of 98 RON fuels (including E20) yielded significant fuel efficiency improvements relative to 92 RON fuel, offering the possibility of fuel economy parity with E10 fuels for some driving cycles. The benefit of octane increases with the aggressiveness of the driving cycle due to the higher fraction of time spent in knock-limited combustion phasing conditions. Higher RONs reduce the need to retard combustion phasing under these conditions, improving fuel efficiency. Engine downsizing and down-speeding are well suited to take advantage of higher-octane fuels because these engines encounter the knock limit more frequently.
Higher fuel laminar flame speed expands dilute or lean operation. Five fuel blends (all based on pure components) were designed to test the effect of laminar flame speed (LFS) on the lean and exhaust-gas recirculation (EGR) dilution limits of SI combustion with various constant heats of vaporization (HoV). While having a low HoV is important for stable combustion, the higher LFS is also important because it allows for leaner mixtures or increased EGR dilution even while the HoV is high. Compared to typical engine design parameters (spark energy, tumble ratio, and direct injection (DI), fuel LFS had similar impacts on lean and EGR-dilute SI combustion limits.
Operation under boosted conditions is essential to Co-Optima’s Thrust I effort to maintain acceptable power density for downsized and downsped SI engines. However, investigations revealed that with sufficient boost, some fuels exhibit heat release prior to SI. This behavior may have far-reaching implications on understanding the boundary conditions for engine knock and other abnormal combustion phenomena.
Co-Optima is successfully applying chemical–kinetic modeling to provide insight into the regions of reactivity and how that relates to different fuels. Experimental temperature and pressure traces combined with kinetic simulations of autoignition chemistry are demonstrating how boosted operating conditions interact with regions of enhanced reactivity at highly boosted conditions.
Research with a single-cylinder, gasoline DISI engine at a fixed retarded combustion phasing and intake air temperatures higher than 60 °C found that fuels with higher HoV values enabled operation at significantly higher load. This important Co-Optima research decouples the physical HoV effect from the RON and octane sensitivity effects, and for the first time, demonstrates engine operating conditions where HoV effects on knock resistance can be measured.
Two new studies were performed to further investigate the relationship between HoV and octane sensitivity. One study found similar combustion phasing under knock-limited spark advance (KLSA) conditions for three similar fuels with differing HoVs, while a second study found that KLSA is well-correlated with the RON. The study results indicate that HoV anti-knock effects can be viewed as a contributor to octane sensitivity.
Co-Optima has determined that Thrust I fuel blends will likely influence particulate matter (PM) oxidation behavior in the absence of an aftertreatment solution, which could have regulatory implications.
Co-Optima researchers characterizing a set of 15 gasoline blends made the unanticipated discovery that ethanol suppresses aromatic evaporation and pushes it to higher temperatures. Incomplete fuel vaporization and mixing produces higher PM emissions—especially for aromatic compounds with a high sooting tendency.
A new high-speed infrared fuel-vapor-imaging technique demonstrated that tumble flows are generated in conjunction with the swirl and create asymmetric fuel distributions, leading to excessively rich gas volumes that promote soot formation. This research may lead to clean and efficient stratified-charge SI engine operation and guide the formulation of new high-octane gasoline fuels.
Co-Optima Thrust II research efforts are exploring the factors important for enabling ACI technologies, along with the supporting fuel technologies necessary. The researchers are identifying fuel properties that critically impact kinetically controlled combustion and LTC. In addition, they are mapping the interplay of molecular structure, fuel properties, engine design, and operating strategies needed to enable high-efficiency ACI engine operation across a broad range of applications. Among the FY 2016 accomplishments:
The Co-Optima project employed constant-volume combustion-vessel experiments to measure soot reductions enabled by a variety of ducted fuel injection (DFI) configurations studied over a range of ambient temperatures, densities, and oxygen mole fractions. Soot was typically reduced by half to more than two orders of magnitude, depending on ambient conditions and duct configuration. DFI could be a key Co-Optima technology because it is an effective, simple, mechanical approach that is not highly sensitive to the use of dilution for control of NOx or to fuels having significant variability in their properties. This discovery has particularly important implications to compression ignition engine technologies, which are efficient but have sooting challenges.
Co-Optima researchers developed a new perspective on ACI combustion strategies by classifying them along a spectrum that features increasing degrees of air and fuel stratification at the start of combustion. This new perspective unites how all of these previously thought “individual” combustion modes relate to one another along this spectrum.
Co-Optima engineers tested two fuels with different anti-knock properties—one similar to E10 and the other similar to a naphtha distillate—and found that the boost level had the most significant impact on autoignition, while stratification was just as important as the fuel choice. In other words, engine designs that promote stratification can yield reliable autoignition.
he Co-Optima project is employing infrared imaging to improve the understand- ing of in-cylinder mixing and fuel-property effects on ignition and combustion processes in reactivity-controlled compression ignition (RCCI) combustion.
FY 2017. By the 18-month mark, the Co-Optima teams plans to assess the validity of the central fuels hypothesis:
If target values for critical fuel properties that maximize efficiency and reduce harmful emissions for a given engine architecture are identified, then fuels that have properties with those values (regardless of chemical composition) will provide comparable performance.
Co-Optima anticipates selecting at least three bio-based blendstock candidates as potential Thrust I blend components this year.
Thrust II R&D on ACI will proceed, with analyses planned to identify how fuel properties and ACI operating strategies can be co-optimized to achieve high efficiency and low-emissions. Eight university teams will be working with Co-Optima researchers to carry out fundamental technical studies.