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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.

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.

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.