In a paper in the journal Progress in Energy and Combustion Science, Gautam Kalghatgi, newly retired from Saudi Aramco and a visiting professor at Imperial College London and Oxford; Howard Levinsky, Senior Principal Specialist in Combustion Processes at DNV GL Oil and Gas and Professor of Combustion Technology at the University of Groningen; and Med Colke, retired as a Senior Fellow of United Technologies Research Center and currently technical coordinator for the National Jet Fuels Combustion Program, outline their view of the future of transportation fuels.
Since internal combustion engines will remain the primary mode of choice for mobility in the foreseeable future, both alternative (renewable) fuels and high-performance combustion concepts/ engines using fuels adapted for that purpose will be preferred for reducing the combustion impact on climate change. Multiple step-change reductions in the environmental/climate footprint of combustion-based transportation systems, while maintaining the fitness for purpose and convenience for the end user, are achievable.
The need for scientific and engineering advancements, as well as the opportunities to achieve them, are great. The potential societal impact of these advancements may seem incremental individually; however, collectively they will have a significant impact on the challenge of mitigating climate change with limited impact on mobility and its economic importance. It also must be recognized that the needs, costs, and ability to adopt changes readily are different for different countries: the economic and societal concerns can be different in countries with mature and ageing transportation modes than in those undergoing slower or rapid economic growth, which is strongly reflected in their needs for mobility.
The societal and technical challenges are great, and the physical and engineering aspects are complex. The intertwining of fuel and engine properties, microscopic in nature but macroscopic in impact, demands that engine manufacturers, fuel producers and research institutions work together to meet the challenges of the future. Government involvement, to enable financially the sound assessment of the future fuel options, investments, and to understand policy implications, is essential.—Kalghatgi et al.
The authors identified a number of challenges for new fuels for a low-carbon transportation sector:
Fuels tailored to engines with cycle and system advances that increase overall conversion of chemical energy to useful work. Step-change improvement in the internal combustion engine is limited by the properties of existing fuels and may require fuels with high (super)knock resistance, altered compression-ignition behavior, increased heat sink capability, and equal or improved pollutant emissions. As an example, the gasoline compression ignition (GCI) engine could use low-octane gasolines that are ignited by compression to achieve diesel-like efficiencies.
Impact on future/advanced cycle engine designs may also be critical as preferred fuel properties may be altered. As future cycles are developed, new constraints on fuel formulation may be required to satisfy cycle needs, and safe operation. Such constraints may include tighter limits on ignition characteristics, such as octane (RON/MON) or (derived) cetane number.
Renewable fuels that are chemically converted to equivalent petroleum products matched to the combustion engine that optimizes performance. Matching fuels and engines is essential to guaranteeing fitness for purpose for the end user and maximizing fuel efficiency. Co-optimization of engines and fuels may be required. (This is the basis of the DOE Co-Optima work.)
Fuels that optimize the complex interactions of physical and chemi- cal properties. The generation of power in a cylinder or combustor is an interaction of physical/chemical processes, such as the evaporation of multicomponent fuels in a spray, turbulence in the charge, (auto)ignition of the fuel-air mixture, combustion rates, piston motion and pollutant formation, which are each dependent on the specific operating conditions. All of these processes are time-dependent and interwoven by physical-chemical couplings, including the specific fuel properties.
There are preferred physical and chemical properties of a fuel for each set of operating and environmental conditions; hybrid systems that include an engine and fuel optimized to operate at one or two conditions may have an advantage. A fuel tailored to these operating conditions and ambient environments might be preferred for optimum energy conversion to useful energy. These targets challenge the design and optimization process: the engine itself is macroscopic, but the processes determining its behavior are microscopic and usually dependent on the fuel properties, as well as the specific engine design.
Jet aircraft fuels with more stringent requirements and specific characteristics to maximize cycle efficiency. Changes in design or fuel composition must satisfy the necessity of keeping the aircraft in the air. For example, advanced cycle gas turbine engines may need an upper limit to cetane number (CN) to avoid pre-ignition in lean-direct injection designs, and perhaps a lower limit to avoid premature blow-out events. A constraint on CN did not exist with petroleum fuels since other jet fuel constraints resulted in a practical narrow limit for CN with such fuels.
With respect to aromatics, there is no lower limit for petroleum fuels, yet blends with synthetic fuels must have aromatic concentrations between 8 and 25%, to ensure that seal swelling occurs. If the lower limit of aromatics could be reduced and H/C ratio increased, lower (non-volatile) particulate matter emissions (and smoke), and increased heat release (per unit fuel volume) should result. In addition, increased thermal stability of fuels will increase the ability to recover more energy in the fuel, increase the cycle efficiency, as well as decrease operational maintenance costs, all of which will enhance the large-scale introduction of new fuels and increased cycle efficiency.
Kalghatgi, G., Levinsky, H., & Colket, M. (2018) “Future transportation fuels.” Progress in Energy and Combustion Science, 69, 103–105. doi: 10.1016/j.pecs.2018.06.003