Future for IC Engines? Sustainable biofuels and advanced combustion converge to rescue a technology that has powered the economy for more than 100 years

Significance 

Homogeneous Charge Compression Ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer are compressed to the point of auto-ignition. This system has both very low engine output emissions and high efficiency and is a potential successor to the conventional combustion modes. Nonetheless, it has some critical drawbacks relating to the fact that it lacks controllability over the start and rate of combustion. This restricts HCCI to a narrow operating load range. Numerous solutions have been proposed, including other forms of Low Temperature Combustion (LTC) modes, which are all modified versions of HCCI and use equivalence ratio inhomogeneity to improve controllability. But these modes possess fundamental flaws that can lead to the production of particulate matter and nitrogen oxide emissions. In a bid to control the heat release process by increasing the inhomogeneity of the temperature distribution rather than the equivalence ratio, a recent publication has presented the Thermally Stratified Compression Ignition (TSCI). Basically, TSCI attempts to intentionally stratify the temperature distribution in the cylinder prior to ignition in a lean, premixed, compression-ignited combustion mode. This technique has been successful at improving the controllability and expanding the operating range of a premixed LTC mode, specifically when utilized with water injection. Unfortunately, certain commercial applications view the requirement of a separate water injector as a limitation.

Recently, Mozhgan Rahimi Boldaji (PhD candidate), Brian Gainey (PhD student), and Professor Benjamin Lawler from the Stony Brook University investigated the usability of the latent heat of vaporization of wet ethanol to introduce the forced thermal stratification of TSCI to control the combustion process. They were hoping to develop an alternative method for enabling TSCI, by using water-fuel mixture such as wet ethanol instead of direct water injection which requires two distinct injection systems. To actualize such idea and comprehend the proposed technique more fundamentally, a Computational Fluid Dynamics (CFD) model of the engine was built in CONVERGE CFD. Their work is currently published in the research journal, Applied Energy.

“Having a renewable and sustainable fuel for IC engines has always been a goal, but also a challenge. Our new proposed combustion technology not only pairs advanced high-efficiency, low emissions IC engines with biofuels, it also saves energy during fuel production by truncating the distillation process and eliminating the dehydration process.” Mozhgan Rahimi said in a statement to Advances in Engineering.

In brief, the researchers commenced the experimental work by first, constructing an engine model for the purpose of simulations. The model had a side-mounted direct injector which was used for the direct injection of the water-fuel mixture (wet ethanol in this study). Two separate injection was used to achieve TSCI. All in all, the model was validated and the simulations conducted. The simulations were aimed at unearthing the effects of ethanol-water blend ratio and the effects of split injection mass fraction on TSCI.

The authors observed that TSCI with split direct injection of wet ethanol had an overall lower heat release rate and longer burn duration compared to HCCI due to the higher levels of thermal stratification prior to ignition achieved through the evaporative cooling effect of the wet ethanol and its high latent heat of vaporization. Moreover, results of different split injection mass fractions indicated that injecting a higher mass fraction of wet ethanol in the second injection reduced the heat release rates further and had potential to increase the burn duration up to 68.8%.

“TSCI has very high efficiency and low pollutant emissions and can be paired with renewable biofuels. But being fuel-flexible is what makes TSCI distinctive compare to other advanced combustion modes. What we need is an engine that can operate on a wide range of fuels based on their availability around the world without requiring any hardware changes.”, Rahimi said.

In summary, the study by Stony Brook University scientists successfully reported CFD simulations for the novel TSCI concept. In general, wet ethanol was injected with different blend ratios of ethanol/water and various split injection mass fractions to study their effects on the combustion process. Altogether, the new system presents an alternative for the IC engine that can be implemented without requiring infrastructure or engine architecture changes.

Future for IC Engines? Sustainable biofuels and advanced combustion converge to rescue a technology that has powered the economy for more than 100 years - Advances in Engineering

About the author

Mozhgan Rahimi Boldaji is a PHD candidate in the Department of Mechanical Engineering at Stony Brook University. She is also a member of the Engine Combustion Research Group (ECRG) where she conducts various research projects in an effort to lower harmful pollutant emissions and increase efficiency of a new generation of engines for transportation and stationary power generation. In particular, her research objective focuses on a new advanced combustion mode, called Thermally Stratified Compression Ignition (TSCI), as a pathway to future clean and highly efficient engines. TSCI is a highly efficient and clean combustion mode that solve the problems of Homogeneous Charge Compression Ignition (HCCI) by using a direct injection event to control the energy release process. TSCI can be achieved using either the direct injection of water or a water-fuel mixture (such as wet ethanol).

During her Masters, Mozhgan focused on the first variant of TSCI, TSCI with water injection. She conducted multiple studies to assess TSCI with water injection more fundamentally and her recent Computational Fluid Dynamic (CFD) studies indicated that TSCI with water injection has an energy release rate at least 83% lower than HCCI, resulting in enhanced control over combustion. Before joining Stony Brook University, she received her Bachelor from Isfahan University of Technology.

The preliminary results of TSCI with water injection were very encouraging, leading to the idea of a second variant of TSCI proposed by ECRG group (Professor Benjamin Lawler, Mozhgan Rahimi Boldaji, and Brian Gainey) which uses direct injection of wet ethanol instead of direct water injection to control the combustion. The latent heat of vaporization of wet ethanol reduces the rate of energy release in TSCI to control the combustion process. Ethanol is a biofuel which is fairly ubiquitous around the world. The widely spread sources of ethanol along with its lower carbon dioxide (CO2) emissions make ethanol a viable alternative source of energy to fossil fuels. However, the production process of ethanol is very energy intensive, with 37% of the energy being used during distillation and dehydration processes.

This new proposed variant of TSCI provides the unique opportunity to stop the distillation process early and eliminate the dehydration process when producing the fuel and therefore save up to 33% of the energy required to produce ethanol. Therefore, the proposed technology would pair significant energy and economic savings during the production of ethanol with a specifically designed advanced combustion mode that is capable of simultaneously high efficiencies and low pollutant emissions.

Reference

Mozhgan Rahimi Boldaji, Brian Gainey, Benjamin Lawler. Thermally stratified compression ignition enabled by wet ethanol with a split injection strategy: A CFD simulation study. Applied Energy, volume 235 (2019) page 813–826.

Go To Applied Energy

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