Characteristic time model development for direct injection diesel engines.

摘要

The development of a characteristic time model for NOx emissions from direct injection Diesel engines is presented. In this modeling technique, one objective is to evaluate the dominant fluid mechanic processes in the cylinder of the Diesel engine by using characteristic length and velocity scales to describe the flow variations. Ratios of these length to velocity scales form characteristic times, and thus the term characteristic time modeling describes this methodology.; Kinetic descriptions of the NOx formation/decomposition processes are developed which improve upon the existing models in the literature. A two-zone model is used which involves a seven reaction skeletal mechanism. In the first zone, a surrogate stoichiometric flame temperature surrounding a spray plume at start of combustion conditions describes the environment for NO formation. NO decomposition may occur in the second zone surrounding the stoichiometric region, depending on an end of combustion surrogate flame temperature computed from a fuel/air limited-pressure cycle analysis. Both Zeldovich and nitrous oxide mechanisms are considered in each zone and are required to adequately describe NO formation. However, at representative Diesel operating conditions, NO decomposition in zone 2 is dominated by the reverse nitrous oxide mechanism.; The proposed model is validated with data from a Ford 2.2L high speed direct injection Diesel engine with a common rail fuel injection system. Exhaust gas recirculation variations from zero to maximum level possible are performed for parametric variations of engine speed, injection pressure, and load. With the skeletal mechanism for NO chemistry and the zonal concept for NO formation and decomposition, the law of mass action is used to develop a characteristic time model for NOx emissions from Diesel engines. NO decomposition is predicted to occur for high engine load conditions.; Additional focus is on development of the fluid mechanic aspects of the model. A characteristic mixing time that is a function of Reynolds and Weber numbers and engine geometry is developed. It is then used in combination with the kinetic time in the form of a Damköhler number to correlate NOx emissions for the above parametric variations for conditions where NO decomposition does not occur.