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Multiple injection tests were carried out using a 1.2-liter 4-cylinder Ford DIATA (Direct Injection Aluminum Through-bolt Assembly) engine at various operating conditions. The results were simultaneous reductions in NOx and soot emissions. An engineering model based on characteristic times, with consideration of both thermal and N2O formation kinetics, is utilized to gain insight into the reasons for NOx reduction due to multiple injections. Stoichiometric combustion is assumed for NO formation. In this research, normalization and parametric studies are used to study the effects of injection timing, fuel quantity per injection pulse, and injection rate on NOx emissions. NO formation is reduced by modifications that lower stoichiometric flame temperature at the start of combustion of fuel injection pulses or decrease time spent by a fluid element in the stoichiometric zone.

Work by Plee, Ahmad, and coworkers in the 1980s [1, 2, 3, 4 and 5] showed that for changes in intake air state, Diesel NOx, soot, soluble organic fraction, and HC emissions could be correlated using the stoichiometric flame temperature calculated at SOC or peak pressure conditions. In the present work, similar flame temperature correlations are obtained for emissions from three test engines; a 1.2L high speed direct injection (HSDI) Diesel, a 2.4L HSDI Diesel, and a 2.34 L single cylinder direct injection (DI) Diesel engine, the first of which was tested using four alternative fuels. Use of the flame temperature correlations presented may reduce the number of engine tests required to evaluate the effects of EGR on emissions of NOx, particulate, and HC, even when alternative fuels are used.

A skeletal mechanism for NOx emissions is incorporated into a cycle simulation code for direct-injection (DI) Diesel engines. The skeletal mechanism consists of seven chemical reactions associated with the extended Zeldovich and N2O mechanisms. In combining the skeletal mechanism with the cycle simulation code, both a two- and a one-zone combustion model are examined. In the former, NO forms in zone 1, which is characterized by the stoichiometric flame temperature, and decomposes in zone 2, which is represented by the overall bulk cylinder temperature. For the one-zone combustion model, it is postulated that both the NO formation and decomposition processes are characterized by the stoichiometric flame temperature. The main objective of this work is to examine the relative contribution of the Zeldovich and N2O mechanisms to the NO formation and decomposition processes occurring during Diesel combustion.

An algebraic engineering model for emissions of nitric oxides (NOx) from direct injection (DI) Diesels, first suggested by Mellor et al. [1], is evaluated with results from engine tests involving the injection of pure nitric oxide (NO) into the intake air of a 2.4L high speed direct injection (HSDI) Diesel engine [2]. The model is based on a two-zone representation of the DI Diesel spray plume flame. As originally suggested by Mellor et al. [1], NO forms in zone 1, which is characterized by the adiabatic stoichiometric flame temperature at start of combustion (SOC), and decomposes in zone 2, which is characterized by the end of combustion (EOC) temperature. Engine-out NOx emissions are correlated using ratios of characteristic fluid mechanic mixing times to characteristic chemical kinetic times (Damköhler numbers). The kinetic times for NO formation and decomposition take into account both the extended Zeldovich and nitrous oxide mechanisms [1].

Recent measurements of NOx emissions from a 2.2L HSDI Diesel engine have suggested that NO decomposition may be important at high load [1]. In interpretation of these data, Mellor et al. [2] determined that the nitrous oxide and extended Zeldovich mechanisms are both important pathways for NO formation and decomposition. To further examine the importance of NO decomposition in Diesels, results from tests that involve the injection of pure NO into the intake air of a 2.4L HSDI Diesel are presented. The effects of engine speed and load on the relative importance of NO decomposition are directly discernable from graphs of engine–out NOx versus engine–in NO for speed and load sweeps. The importance of NO decomposition is found to increase with engine load, while engine speed exhibits a tradeoff. Furthermore, the results indicate that the reverse of the Zeldovich mechanism dominates the NO decomposition process.

Although alternative NOx control schemes, such as catalysis, are promising means of reducing emissions from Diesel engines, many such methods have yet to be developed into reliable and cost-effective solutions. Consequently, NOx reduction through in-cylinder techniques remains the most widely used approach in meeting current and future emissions standards. One such common technique is the use of an inert diluent, such as water/steam or exhaust gas recirculation (EGR), introduced into the combustion chamber to reduce the peak flame temperatures associated with NO formation. Here the role of water/steam in reducing NOx emissions is analyzed in depth. In particular, two methods of water injection are studied: stratified fuel-water-fuel injection and intake manifold fumigation. In each case, the NOx emissions are modeled using a two-zone characteristic time model (CTM) based on the dominant physical and chemical subprocesses occurring in the cylinder.

The specific aim is to validate an engineering model for direct injection (DI) Diesel engine emissions. Characteristic times describing the controlling fluid mechanics and chemical kinetics will be employed in the model to correlate both NOx and particulate emissions. Because the model equations are algebraic, they are suitable for implementation in a phenomenological cycle simulation program, or as an emissions model option in a computational fluid dynamics code. An original premise was that earlier work on global NO chemistry based on pollutant emissions dominated by diffusion flame contributions had adequately elucidated the kinetic aspects of the model. It is shown here that this approach is not valid for modern engines. Rather, an improved two-zone flame model for NO formation/decomposition is required. Mellor et al. [1] propose such a model, but include only qualitative preliminary model validation.

Most computational schemes and kinetic models for engine-out NOx emissions from Diesels are based on the Zeldovich or extended Zeldovich mechanism. However, at pressures typical of both the premixed and diffusion portions of the combustion process, the third-body reaction leading to the formation of N2O (O + N2 + M) becomes faster than the leading reaction in the Zeldovich mechanism (O + N2). As in gas turbines, particularly those involving lean-premixed combustor designs, NO formation in Diesels through the N2O mechanism can thus proceed more efficiently than through the traditional route. Decomposition of NO in the combustion products during the power stroke can also occur by both the reverse Zeldovich reactions and the second order step that produces N2O (2NO ® N2O + O). Based on these observations, a skeletal mechanism consisting of seven elementary reactions is used to develop a two-zone model for NOx emissions from direct injection (DI) Diesel engines.

Inert injection is an often-used technique to reduce NOx emissions from engines. Here the effects of a new Mitsubishi water injection system for a direct injection (DI) Diesel engine on exhaust emissions are examined. Stoichiometric flame temperature correlations of thermal NOx emissions for conventional gas turbine combustors provide an activation energy to form NO of approximately 135 kcal/g-mol, the value for the Zeldovich mechanism with O/O2 equilibrium. Two theoretical limiting temperatures determined to bracket NOx emissions data for gas turbines are computed for the Diesel engine considered here. At low water to fuel ratios, the reductions of NOx for the DI Diesel engine are less than predicted for uniform distribution of an inert throughout the charge, but as the water to fuel ratio is increased the reductions are bounded successfully by the limiting temperatures.