Nitric Oxide Emissions From Stratified Charge Engines: Prediction and Control

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Abstract
A model is developed for calculating NO emissions for stratified charge engines in which the fuel is completely vaporized prior to combustion. The validity of the model is limited to single chamber geometry as it does not treat bulk gas motion typical of prechamber or divided chamber designs. The parameters of the model are fuel type, humidity of inlet air, overall equivalence ratio, stratification function, per cent EGR, intake manifold pressure, intake manifold temperature, compression ratio, piston connecting rod to crank radius ratio, RPM, and position and duration of combustion in the cycle. Stratification in which the first elements to burn are rich and the last elements to burn are lean is most effective relative to a uniform air-fuel mixture at overall air-fuel ratios near stoichiometric. Under these conditions almost all elements in the uniform charge are being “replaced” by elements of lesser NO-producing air-fuel ratios. On the overall rich side smaller reductions in NO level are possible with rich to lean stratification. However, if the stratification were poorly controlled and lean to rich stratification were to result, large increases in NO emissions would be expected. On the lean side stratification is not advisable since very close control of the air-fuel distribution is required to prevent increases in NO concentration. NO control achieved via stratification results in a loss in power and efficiency since the rich elements contain high concentrations of CO whose energy of oxidation is not converted to work if mixing takes place late in the expansion stroke. The effect of mixing the stratified burned gases on efficiency and NO control indicates the existence of an optimal crank angle for dissipating the stratification. Most NO activity terminates within 30° after the end of the combustion process at all levels of EGR including no EGR. Stratification beyond this point maintains high CO levels in rich regions reducing the cycle work with no further reduction in NO concentration. On a specific basis NO control decreases beyond this point. The greatest reduction in NO emissions at overall air-fuel ratios near stoichiometric results from step distribution functions in which the first half of the charge is rich and the latter half lean as opposed to a continuous progression. This is accompanied by a considerable loss in power and efficiency for the reasons cited above. For engines operating with stratification and EGR, increasing the compression ratio and retarding the combustion interval leaves the efficiency of the cycle unaltered but lowers specific NO emissions. For stratified charge engines operating with injection, this is a real possibility due to lower octane requirements.

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