Using Extreme Compression to Promote Fuel Reformation within a Reacting Jet: The Path Towards a Sootless Diesel Engine

Direct injection Diesel-style engines emit solid-phase carbonaceous particulate matter (i.e. soot) at levels that negatively impact both climate change and personal health. Although advances in injection technology have increased atomization and mixing of the injected fuel jet, which reduces local equivalence ratios and decreases soot formation, aftertreatment is still required. This in turn has associated efficiency and cost penalties. Our research is aimed at eliminating cylinder-out soot particles entirely. Extremely high compression of air has the potential to provide an environment where in-plume fuel reformation can occur. It is well understood that soot formation is kinetically controlled. By achieving extremely high temperatures within the fuel jet, it is possible to reach the equilibrium state of the mixture locally before getting kinetically constrained and building significant populations of soot precursor species. By adding a moderator species to the fuel, control over atom ratios – and thus composition of the equilibrium state – may be adjusted such that the resulting reformed fuel will then combust without forming soot. To date, numerical tools have been built that allow analysis of chemical kinetics of crucial soot precursor species, namely Polycyclic Aromatic Hydrocarbons (PAHs). Some initial experimental investigations have been carried out with direct-injected, ethanol/water mixtures. Optical access and a high frame-rate camera allows for visible detection of luminous soot formation. Work is underway to systematically investigate variations on mixture composition and initial air temperature (at start of injection) in order to observe and measure the effects on soot formation within the reacting jet. Numerical modeling and analysis is carried out to understand and explain experimental results. Introduction The primary goal of this research is to eliminate in-plume, jet-core soot production by using extremely high temperature air and modifying the local atom ratios with a moderator species added to the fuel. High temperature is achieved by using a piston-cylinder device that is capable of a geometric compression ratio of 100:1. By preheating the air, on the order of 100 o C or more, the air temperature at the Start of Injection (SOI) can be upwards of 2000 K. Since chemical kinetic rates are exponentially dependent on temperature, this extremely high air temperature drastically decreases the time to reach equilibrium. And with the use of a moderator species, the equilibrium state can be manipulated such that the reformed-fuel species are non-sooting: primarily carbon monoxide and hydrogen. The importance of approaching equilibrium comes from the fact that soot formation is a kinetically constrained process. Through the chemical pathways that are taken from fuel/air reactants to products, a bottle neck exists under typical Diesel engine operating conditions whereby a large population of PAHs are formed and subsequently nucleate a condensed phase. These nanoparticles then grow in size and agglomerate, and ultimately form particulate matter that is emitted from the engine. Initial numerical analyses have been made that indicate the propensity for various premixed, rich mixtures to form soot. Chemical kinetic computations are made using a mechanism that has been developed with the specific inclusion of PAH gas-phase species and their respective radical species [1]. In parallel, experimental work is being done to obtain images of the injection and combustion of ethanol/water mixtures in the extreme compression device. These images provide a clear, qualitative indication of where and when soot is present due to the visible luminosity of soot particles. Ethanol has been chosen as the starting point for fuel since it is lightly sooting and miscible with water, a simple moderator species (and O-atom donor). Beyond this, we have plans to work with higher alcohols, such as propanol and butanol, combined with water or other moderators. Background The Significance of Soot Particulate matter emissions are of critical importance to both global climate change and human health. Studies have shown that human exposure to carbon-based ambient aerosols increases the odds of developing pulmonary and cardiac problems which can lead to lung and heart diseases, and in the long-term may be fatal [2]. With regards to climate effects, carbon aerosols have the ability to absorb and scatter solar radiation. Recent modeling efforts have indicated the propensity for carbon particulates to increase the global mean surface air temperature by 0.2 to 0.37 K per year, and the radiative forcing is estimated to be 70% that of CO2 in the atmosphere [3]. The largest effects occur in regions with high fossil fuel emissions and burning of biomass. Clearly there is an urgent need to curb carbon particulate emissions. Current Approaches to Mitigating Diesel Soot Formation Within a direct-injection Diesel engine, soot formation occurs in regions of locally high equivalence ratios and favorable temperatures. There exist both lowand high-temperature limits, although a change in the combustion strategy is required to move beyond these limits. Other research and development efforts in the field have moved towards the direction of staying below the low temperature limit, and this has the additional benefit of avoiding high thermal NOx production. Figure 1 below shows a graph of equivalence ratio versus temperature, indicating how a typical Diesel engine fuel jet occupies several regions. Figure 1: Local equivalence ratio versus temperature, with reference to regions that describe a conventional Diesel fuel jet [4]. The method by which a Low Temperature Combustion (LTC) strategy is implemented usually involves the use of Exhaust Gas Recirculation (EGR). This has the disadvantage of reducing engine load capability. Our approach of using an injected fuel/moderator mixture and going to extremely high temperatures is aimed at keeping engine load capabilities at their highest while simultaneously avoiding particulate emissions. Using an Appropriate Chemical Mechanism Much work within the combustion community has been done investigating the development of chemical mechanisms that can predict both the nucleation and growth of a condensed phase. The type of species called Polycyclic Aromatic Hydrocarbons (PAHs) has been identified experimentally as being critical to initiating soot inception. A chemical mechanism has been developed by Blanquart [1] that includes up to 4-ringed aromatic species, inclusive of many aromatic radicals. It was designed to validate the combustion of large hydrocarbon fuels, namely n-heptane and iso-octane, although a number of smaller hydrocarbon species have been validated against experimental data as well. It contains ~1650 elementary reactions and ~150 species, and includes both transport as well as thermodynamic data. This mechanism is being used to study gas-phase kinetics, in particular the temporal growth of PAH species, in order to predict soot formation tendencies. Equilibrium and Forming a Condensed Phase Within a direct-injected jet the formation of soot is kinetically controlled; that is, there are regions within the fuel jet where specific species are formed, and given little oxygen and favorable temperatures, a condensed phase is formed. In other words, equilibrium is not reached. The goal of this research is to both control the equilibrium state and to reach it, all within the reacting jet. It is thus informative to have a clear idea of what thermodynamic equilibrium indicates about the product composition of a particular mixture. The following graph shows the equilibrium composition for various n-heptane/air mixtures, where n-heptane is chosen as a diesel fuel surrogate. The C/O atom ratio is varied by adjusting the fuel/air equivalence ratio. The vertical dashed line indicates where the stoichiometric condition exists. It is noted that nitrogen chemistry is not included in the aforementioned chemical mechanism. Figure 2: Thermodynamic equilibrium composition as a function of overall C/O atom ratio for n-heptane/air (21% oxygen, 79% nitrogen) mixtures. 0.2 0.4 0.6 0.8 1 1.2 1.4 10 -4 10 -3 10 -2 10 -1 10 0