A laboratory-based fuel mixture preparation system has been developed that is capable of generating a wide range of fuel/air mixtures, including production of a premixed, prevaporized homogeneous charge, beginning with liquid gasoline fuel. This system has been developed to allow the study of the effects of fuel/air mixture preparation characteristics on engine combustion, in-cylinder pressure, and exhaust emissions.
For the study to be described here, engine combustion behavior and emissions measurements were obtained for a wide range of A/F's with the fuel mixture preparation being produced in one case, by the stock carburetor operating with fixed throttle position, and the other case, with the custom-built system producing a homogeneous mixture (HM.) A four-stroke, spark-ignited, single-cylinder, overhead valve-type utility engine was used for all tests. No attempt was made to operate the engine with speed governing, and several results were obtained with operation lean of stoichiometric, without considering the potential negative effects on engine durability. No attempt was made to optimize engine performance at any operating condition, or fuel mixture preparation method. Finally, there was a difference in charge temperatures used in each of the two cases, because of the need to operate at higher charge temperatures when using a homogeneous mixture than when metering with the carburetor.
Two fixed throttle conditions were tested: approximately 10 % rated load, and 50 % rated load with the A/F near stoichiometric. Trends observed were as follows: Hydrocarbon emission trends were similar for the two methods for a range of A/F from 11.5 to 16. Effects of increased cyclic variation were observed with carburetion at leaner A/F. Induction of a homogeneous prevaporized mixture produced the “classic” dependence of NO emissions levels with the peak near stochiometric, while in general trends were similar for the two methods of fuel metering. Trends for CO were also alike.
Cylinder pressure analysis demonstrated the effect homogeneous mixture preparation had in the reduction of cyclic variation. Heat release analysis showed that an increase in intake charge temperature necessary to ensure fuel vaporization resulted in faster burns when operating with the homogeneous mixture.