Demonstrating a SI-HCCI-SI Mode Change on a Volvo 5-Cylinder Electronic Valve Control Engine 2003-01-0753
Operating an engine in homogeneous charge compression ignition (HCCI) mode requires the air fuel mixture to be very lean or highly diluted with residuals. This is in order to slow the kinetics down and to avoid too rapid heat release. Consequently, the operational window for the engine in HCCI mode is not the same as for the engine operating in spark ignited (SI) mode. Homogeneous charge compression ignition engine mode, in this study, is accomplished by trapping residual mass using variable valve timing.
With the residual trapping method, the engine cannot be started in HCCI mode and due to the dilution, the engine in HCCI mode can only be operated in the part - load regime. Hence, a mode change between spark ignited and HCCI modes, and vice versa is required.
This study reports the development of a mode change strategy for a single cylinder camless engine, and its successful implementation in a camless multi cylinder engine. It was found that the operation in the first couple of cycles following a mode change deviate markedly from the steady state operation in the new mode, therefore the control parameters during the change have to be optimized. The influence of the residual mass fraction and the intake valve closing as control parameters are investigated. Both single cylinder and multi cylinder mode changes from un-throtteld SI combustion (generated with early intake valve closing) to HCCI combustion (generated with negative valve overlap) and vice versa were investigated.