Fuels and energetic materials enhancements have been focus of several studies in the past few years, as the sole development of the engines (design, structure of the combustion chambers and stronger materials) leads to a limited efficiency. One of the most effective ways of enhancing the energetic capabilities of a fuel is via enrichment with a highly reactive species, such as hydrogen and ozone, as these materials significantly increase the reaction mechanism kinetics, enabling the occurrence of faster reactions and increasing the temperature of the chamber. The present chapter aims to summarize some of the applications of fuel enrichment and the so far achieved results, focusing in internal combustion engines and systems for automotive, energy-generation or aerospace applications.
TopIntroduction
During the past decades, propulsive systems have gone through significant modifications and enhancements, aiming higher efficiencies, lower pollutant emissions and lower fuel consumption. The problem resides in the technical limits of these modifications, i.e. physical changes in the system (e.g. dimensions, design, composite materials etc.) shall not impact so much without modifying the reactant mixture (fuel/oxidizer) in parallel. On the other hand, application of other fuel species may increase the power yield of the engine, but generate even worse pollutant emissions (such as halogenated species). In the hypothesis of using new fuels or formulations, the emissions generated must be severely controlled.
Despite all of the precautions taken according to the Kyoto Protocol United Nations Framework Convention on Climate Change, an increase of 27% was observed in CO2 emissions from 1990 to 2004, and transport related CO2 emissions increased by 37% (Sopena et al., 2010). Therefore, enhancements of the current combustion processes require immediate attention for new technologies and improvements.
One of the most promising ideas to fill that purpose is the use of additives for enhancing the completion of the combustion process. These additives may be oxidizer or fuel species. As an example, the sole use of higher O2 concentrations causes an increase of ~30% (2200 K to 2800 K) in the temperature of the chamber and in water final content, i.e. improves the combustion process (Figure 1) (Beltrame et al., 2001). This result was achieved experimentally and numerically with a diffusion flame of methane with against current, with oxidant composed by a mixture of air and oxygen (68% enrichment). As the peaks approach the nozzle, it is stated that the combustion is faster (lower duration) and more complete, which generate less intermediates (highly pollutant species) like CO and unburned hydrocarbons (the reactants are completely consumed before the stagnation plane). The observed increase in the combustion chamber temperature enables the use of lean regime (lower concentration of fuels species – lower fuel consumption).
Figure 1. Computed temperature and major species profiles for: (a) methane/air flame, (b) methane/68% O2 flame (Sopena et al., 2010)
During the past few years, low molecular weight (and high reactive) fuel species have been studied in the presence of different fuels and engines, as an attempt of achieving better efficiencies and conditions. Therefore, this chapter will cover briefly the combustion of several formulations and systems, as well as some enhancement studies and forthcoming technologies.
TopHidrogen Enrichment
The most common species used as additive or secondary fuel for combustion improvement is hydrogen. In small to moderate amounts, H2 is able to reduce pre-ignitions (due to its high-octane rating), lower NOx emissions, reduce energy losses and increase power output of IC engines (Akansu et al., 2007; Ma et al., 2008; Bauer and Forest, 2001; Kahraman et al., 2009). Figure 2 below, adapted from the work of Greenwood et al. (2014), presents a comparison between H2 benefits and challenges, when used as secondary fuel in IC engines.
Figure 2. H2 benefits and challenges
One of the mentioned challenges of hydrogen regards its low density, which leads to a storage issue, as there are no safe and efficient methods available to all demands. Also, storing H2 as cryogenic liquid, compressed gas or dissolved in metal hydrides significantly increases the weight of the vehicles, the overall price (Fontana et al., 2002; Whiete et al., 2006) and the risks of explosion due to an easy ignition (Abdel-Aal et al., 2005).