Special Issue on Innovative Technologies on Combustion of Biofuels in Engines: Issues and Challenges

The application of biofuels in the field of engine combustion develops as a really exciting episode in the history of technology. Initially, the field appeared to be essentially detached from combustion science. The emphasis was on the biological and agricultural upstream technologies that would produce the so-called renewable fuels, which would have a substantially reduced carbon footprint. Ideally, these would be drop-in fuels that would substitute currently used hydrocarbons and would burn nicely in engines that had been designed for hydrocarbon combustion. Implicitly, the wishful thinking existed that the relatively simple thermochemistry of combustion (after all, the main products are still ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$, right?) and the chaotic nature of intense turbulent motions would take care of the fuel energy conversion, if only biology could provide some form of fuel that would be produced from biological sources.

It was not long after the first applications that the naiveté of such an approach became evident. It was first demonstrated in the field of emissions. Because ethanol was the first biofuel, it came as no surprise that carbonaceous emissions were reduced, but the situation regarding ${\mathrm{NO}}_x$ was perplexing. Soon thereafter, the introduction of biodiesel raised intriguing combustion-related questions. It was not even clear what the appropriate basis for comparison was between logistic fuels and biofuels. Researchers who focused on fundamentals insisted on comparing emissions and flame structure under constant fundamental quantities, such as strain rate and overall equivalence ratio, whereas automotive engineers correctly pointed out that what mattered was a comparison for given power output of practical engines. As a result, a fruitful line of research into downstream technologies appeared that related to biofuel use.

Soon, this line of research expanded way beyond emission measurements. Physical chemists presented detailed mechanisms for the chemical kinetics of heavy alcohols and fatty-acid methyl esters. Engine research demonstrated the potentially powerful coupling of biofuel use with low-temperature-combustion technologies. The electrostatically manipulated sprays that had been long forgotten by the power-generation and automotive communities attracted new research interest, it was realized that the electric conductivity of some biofuels could be as much as four orders of magnitude higher than the one of hydrocarbons. Especially the last two technologies are examples of a paradigm inversion, which seems to be necessary for effective biofuel research.

Biofuels should not be expected to operate optimally in engines that have been designed for completely different fuels. Instead, it is an exciting challenge for the engine-combustion community to develop novel technologies that will harness the unique physical and chemical characteristics of these novel fuels. In fact, it has been suggested that the emergence of such technologies might generate a feedback to the upstream technologies and motivate chemical and biological processes, which will produce molecules by design that will have favorable combustion properties.

With this special issue, we aspire to offer to the readership of JEE a sample of this recent flurry of activity in the field of automotive use of biofuels. It is, of course, realized that the sample cannot be expected to exhaust every single aspect of biofuel combustion, but we believe that the readers will get a representative vista of the state of the art.

In particular, the special issue contains 15 papers and one Case Study. A first group of these contributions advances the technology of currently employed biofuels with an emphasis on biodiesel and biofuel gas. In several of those papers, detailed data are reported relating to the performance and emissions characteristics of several vegetable oils and their methyl-esters. Also, detailed ${\mathrm{NO}}_x$ measurements and computations are reported for transient operation as opposed to the steady-state analysis that is currently the usual, and the performance of the widely considered B-20 fuel is evaluated at in-cylinder conditions. A second group of papers discusses the application potential of novel biofuels such as wood gas, hydro-treated waste cooking oil, and valeric fuels, as well as the way biofuel combustion would behave during hydrogen or reformate addition. A third group of papers exposes the reader to the fundamental scientific and technological challenges that relate to automotive biofuel combustion. They span from advanced thermodynamics that address biofuel combustion from the second-law perspective to detailed determination of fundamental physical and chemical properties of biofuel containing blends, and the introduction of laser diagnostics to the study of in-cylinder biofuel combustion. In this part, particular emphasis is placed on bio-butanol, which is currently considered as a fuel that might combine production from renewable feedstock, sharing very similar properties with the conventional diesel fuel. The Case Study paper discusses cuphea oil biodiesel properties.

Our hope is that the special issue will act as a reference material for the state-of-the-art for practitioners of related research, an instrumental introduction to the related scientific and technological challenges for the uninitiated, and above all as a catalyst of further research in an exciting new branch of combustion science and technology.

We would like to thank all of the authors for their valuable contributions, and the referees for conducting thorough and detailed reviews that have raised even further the level of the contributed papers. We express our gratitude to the editor-in chief of the Journal, Dr. Chung-Li Tseng, for the opportunity and excellent cooperation to produce this special issue.