Knocking Suppression using Stratified Stoichiometric Mixture in a DISI Engine

Knocking is the main obstacle of increasing compression ratio to improve the thermal efficiency of gasoline engines. In this paper, the concept of stratified stoichiometric mixture (SSM) was proposed to suppress knocking in gasoline engines. The rich mixture near the spark plug increases the speed of the flame propagation and the lean mixture in the end gas suppresses the auto ignition. The overall air/fuel ratio keeps stoichiometric to solve the emission problem using three way catalysts (TWC). Moreover, both the rich zone and lean zone lead to soot free combustion due to homogeneous mixture. The effect on the knocking of homogeneous and stratified mixture was studied in a direct injection spark ignition (DISI) engine using numerical simulation and experimental investigation respectively. The results show that SSM formed by the two-stage injection can suppress knocking significantly, and the knocking intensity decreases gradually as the second fuel mass increasing or the second injection timing retarding; the combustion rate increases and the combustion phase advances when the rich zone of the stratified mixture becomes homogeneous, while the combustion rate decreases, the combustion phase retards, the peak pressure and the pressure rise rate decreases when the rich zone of the stratified mixture is inhomogeneous. The stratified combustion with overall stoichiometric mixture increases the CO emission and depresses the NOx emission, and the emissions can be effectively cleaned by TWC. The two-stage injection leads to soot formation due to the diffused combustion and the over-rich combustion, while Two-Zone Homogeneous Charge may be an ideal concept to avoid it. Compared with conventional methods of knocking suppress, the stratified combustion with stoichiometric mixture can keep the power output in contrast with retarding the spark timing, and can improve fuel economy than the enriched mixture.