A New Single-Zone Multi-Stage Scavenging Model for Real-Time Emissions Control in Two-Stroke Engines

Abstract

Gas exchange processes in two-stroke internal combustion engines, i.e. scavenging, remove exhaust gases from the combustion chamber and prepare the fuel-oxidizer mixture that undergoes combustion. A non-negligible fraction of the mixture trapped in the cylinder at the conclusion of scavenging is composed of residual gases from the previous cycle. This can cause significant changes to the combustion characteristics of the mixture by changing its composition and temperature, i.e. its thermodynamic state. Thus, it is vital to have accurate knowledge of the thermodynamic state of the post-scavenging mixture to be able to reliably predict and control engine performance, efficiency and emissions. Several simple-scavenging models can be found in the literature that — based on a variety of idealized interaction modes between incoming and cylinder gases — calculate the state of the trapped mixture. In this study, boundary conditions extracted from a validated 1-D predictive model of a single-cylinder two-stroke engine are used to gauge the performance of four simple scavenging models. It is discovered that the assumption of thermal homogeneity of the incoming and exiting gases is a major source of inaccuracy. A new non-isothermal multi-stage single-zone scavenging model is thus, proposed to address some of the shortcomings of the four models. The proposed model assumes that gas-exchange in cross-scavenged two-stroke engines takes place in three stages; an isentropic blowdown stage, followed by perfect-displacement and perfect-mixing stages. Significant improvements in the trapped mixture state estimates were observed as a result.

Experimental investigation of scavenging in two-stroke engines using continuous CO2 sampling

“…The second crest in the exhaust CO 2 wave probably represents a delayed perfect mixing type scavenging stage, which takes place when the mixture formed on the intake port side hemi-cylinder by the mixing of turbulent incoming gases and residual gases reaches the exhaust. These observations corroborate the choice of various scavenging stages for the scavenging model presented in Bajwa et al 9 A scavenging progression model is presented next to provide a mechanistic explanation for the foregoing gas-exchange observations. The bimodal exhaust CO 2 wave observed in the present study might be unique to cross-scavenged engines because a similar study conducted on a cylinder-headvalved two-stroke engine 33 did not show the second peak.…”

“…Previous work by Bajwa et al 9 using gas-dynamic based simulation models studied the shortcomings of some existing simple-scavenging models and proposed a new model that built upon their strengths while ensuring computational economy, which is vital for real-time engine control applications. The current paper is a continuation of the same research effort and hopes to increase our understanding of scavenging in cross-scavenged, two-stroke engines so that simple scavenging models can be made more phenomenological and less empirical in nature.…”

Development of a three-zone combustion model for stratified-charge spark-ignition engine

“…The three-zone combustion model takes into account the change in the volume of the CP, FAM zones and formation of the air zone volume when organizing SFAC in the above-piston volume (15). After fuel ignition (10 CAD BTDC), the FAM volume (21) decreases and the volume of the CP zone (20) increases according to the fuel burnup characteristic (28) and the change in the above-piston volume (23). The volume of the air zone (22) changes in proportion to the above-piston volume and is not mixed with the FAM volume.…”

“…The one-zone multistage model [20] for calculating the two-stroke engine specifies stages of scavenging processes, but does not take into account joint processes of mixture formation with DI in the cylinder and open valves.…”

Using gas dynamic models to improve exhaust system design for large-bore, two-stroke engines

“…Such transducers are rarely found in field engines (especially in the exhaust system) because they are costly and can only operate in a narrow pressure and temperature range. A more detailed description of the experimental setup can be found in Bajwa et al 32 The current model (Figure 3(b)) is an improved version of the model described in Bajwa et al 32,33 A brief description of the model is provided here for reference. The modeling domain includes the entire engine setup downstream of the scavenging chamber.…”