1996-08-01

Effects of Mixture Preparation Characteristics on Four-Stroke Utility Engine Emissions and Performance 961738

A laboratory-based fuel mixture system capable of delivering a range of fuel/air mixtures has been used to observe the effects of differing mixture characteristics on engine combustion through measurement and analysis of incylinder pressure and exhaust emissions.
Fuel air mixtures studied can be classified into four different types: 1) Completely homogeneous fuel/air mixtures, where the fuel has been vaporized and mixed with the air prior to entrance into the normal engine induction system, 2) liquid fuel that is atomized and introduced with the air to the normal engine induction system, 3) liquid fuel that is atomized, and partially prevaporized but the air/fuel charge remains stratified up to introduction to the induction system, and 4) the standard fuel metering system. All tests reported here were conducted under wide open throttle conditions. A four-stroke, spark-ignited, single-cylinder, overhead valve-type engine was used for all tests. No attempt was made to operate the engine with speed governing, and the engine was operated without considering potential durability issues. No attempt was made to optimize engine performance at any condition, or with any particular fuel mixture.
One of the most significant effects of mixture preparation was on the combustion behavior. Using the cylinder pressure and calculated IMEP, the magnitude of cyclic variations could be examined. When operated with stoichiometric mixtures, it was found that the homogeneous mixture exhibited little variation with a COV of IMEP near 1 %, followed by an increase for the stratified cases which ranged from 2.5-9%, and the carburetor at 14%. Combustion quality results followed the same trends.
Exhaust emissions were collected across a wide range of A/F ratios, typically 11-18. It was found that the HC levels were similar for all of the mixture cases on the rich side of stoichiometric, but were greatly reduced with the homogeneous and stratified cases on the lean side due to the reduction of cyclic variation. NO emissions were similar on the rich and lean extremes, however, were approximately 50% higher for the homogeneous compared to the stock intake when near stoichiometric. The stratified cases were similar to one another and fell in between. CO levels were similar under rich operation, but were also reduced by increased combustion stability at stoichiometric and lean operation points.

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