Experimental and modeling studies of hydrocarbon emissions from automotive and small utility engines.

摘要

Alternative strategies for reducing hydrocarbon emissions from automotive and small utility engines were assessed using experimental and modeling studies. For the experimental study, a small utility engine test facility, and an automotive engine test facility were designed and constructed. Nineteen small utility engines, selected by the U.S. EPA, were tested at various air/fuel ratios under steady-state and transient operations. The effects of combustion chamber design, carburetor design, lean burning, and fuel composition on performance, hydrocarbon and carbon monoxide emissions were studied. A Ford 2.0L Zetec engine was also tested under different operating conditions.; For the modeling study, a hydrocarbon emissions model was developed. The model accounts for unburned hydrocarbon emissions from oil film absorption and desorption, crevice flows, and post-flame and exhaust port oxidation. The correlation between engine-out hydrocarbon emissions, exhaust temperature, and engine speed was also investigated and a semi-empirical formula was derived. The model was then implemented within a thermodynamic, spark-ignition engine cycle simulation code. Model calibration and validation with experiment were carried out systematically on baseline engines. Comparisons between model predictions and experiment of small utility engines showed good agreement at high and medium loads but significant differences at idle. Nevertheless, the model predicted satisfactory the SAE J1088 A Cycle-Weighted hydrocarbon emissions. This is of practical significance since the index of cycle-weighted emissions is the sole regulatory indicator. For the automotive engines, the model yielded reasonable agreement with experiment. This is caused by the different designs between small and automotive engines. To improve model accuracy for automotive engines, some sophisticated submodels are needed.; With the assistance of model prediction and experiment, alternative emissions control strategies were studied extensively. Those included lean burning, in-cylinder fuel injection, catalytic conversion, acceleration pumps, oxygenated fuels, and air injection. It was found that since small utility engines were designed to run fuel rich to obtain satisfactory cooling and since their size, weight and cost were very critical, any single emission control method could not yield satisfactory results. A compromise has to be reached between emissions, performance, durability, size, weight, and cost for a given engine design. Finally, several effective strategies were recommended.