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Reactor dynamics in ICEs

The internal combustion engine (ICE) remains to be an indispensable part of the energy sector. The pursuit of higher fuel economy and reduced emission continuously fuels the development of advanced ICEs and modern combustion technologies. In the meantime, research efforts are needed to help the ICE industry diversify its application space in light of the inevitable electrification of the transportation sector. For example, retrofitting ICEs as chemical reactors could potentially enable small-scale chemical production and help reduce logistics costs.

Stratified reacting flow

Stratified combustion

In gasoline direct-injection engines, the in-cylinder fuel-air mixture is typically not well-mixed at the time of ignition. Flame propagation dynamics in a compositionally-stratified mixture can be significantly different from that in a homogeneous mixture. Understanding and quantifying these differences remains to be a fundamental challenge to modeling engine combustion and improving engine performance. A series of numerical and experimental studies was performed to unravel the basic physics of the problem to be discussed below.

One of the primary conclusions of the work was that regardless of the fuel (from hydrogen to n-heptane), a stratified flame propagating from a fuel-rich mixture to a lean mixture has a higher flame propagation speed (up to 50%) compared to a corresponding homogeneous mixture. Lighter species such as hydrogen (H2) and hydrogen radical (H) generated in the burnt products of rich mixtures preferentially diffuse towards the flame front, i.e., a spontaneous fuel reforming process. The increased amount of the reactive species enhances the local flame reactivity, leading to an increased burning rate. This finding also indicates the necessity of considering detailed chemistry and transport processes when simulating combustion of inhomogeneous mixtures. An experimental replica of the simulation setup has also been established to validate the simulation results. Using a soap bubble inside a pressure chamber, two different mixtures are placed inside and outside the bubble. Experiments with this setup showed that the rich-to-lean flames indeed have higher propagation speeds compared to the homogeneous cases. This improved understanding provides useful guidelines to design and optimize in-cylinder fuel injection and ignition to improve the fuel economy and reduce NOx and soot emissions.

End-gas autoignition

End-gas autoignition

Car manufacturers are increasingly investing in smaller but highly turbocharged engines to reduce engine weight and improve performance and efficiency. The development is hampered, however, by a new engine knocking mode, called super knock, which produces catastrophically high peak pressures and associated oscillations. An investigation on the relationship between pre-mature ignitions and super knock was carried out at the Clean Combustion Research Center (CCRC) at King Abdullah University of Science and Technology (KAUST). Through artificially introducing an ignition event ahead of the normal ignition, a strong correlation between naturally occurring pre-mature ignition events and super knock was confirmed. This finding pinpointed the root cause of super knock and helped the research team to focus on investigating possible pre-mature ignition mechanisms including the catalytic effects of lubricant oil droplets and metal deposits or early heat release from hot surfaces within the cylinder.

ICE chemical reactors

Internal combustion engines as chemical reactors

As chemical reactors, reciprocating engines have a range of advantages that have been historically overlooked: 1) a wide range of thermodynamic and chemical kinetic conditions (e.g., temperature, pressure, and their unique time histories) are easily achieved and accurately controlled by advanced cam/crankshaft technologies. 2) Catalysis can be incorporated into the piston and chamber design. 3) Reagent delivery is readily available through intake/exhaust and in-cylinder injection systems. 4) Modern engine diagnostics techniques offer access to a variety of in-cylinder parameters. 5) The expansion phase provides another pathway for product collection through phase separation: for example, during ammonia synthesis, the traditional expansive condensing system can be simply replaced by the expansion and subsequent liquid collection. Many multi-billion sectors can be significantly disrupted if reciprocating engines are proven to be an efficient and cost-effective chemical synthesis reactor. Two top process candidates are dry reforming of natural gas for carbon capture, and Haber-Bosch process for ammonia synthesis, both of which are being investigated using computational fluid dynamics (CFD) tools for proof of concept.