Search

While Homogeneous Charge Compression Ignition (HCCI) is a promising combustion mode with significant advantages in fuel economy improvement and emission reductions for vehicle engines, it is subject to a number of limitations, for example, hardware and control complexity, or NOx and NVH deterioration near its operating upper load boundary, diminishing its advantages. Conventional spark-ignition combustion mode is required for higher loads and speeds, thus the operating conditions near the HCCI boundaries and their corresponding alternatives in Sl mode must be studied carefully in order to identify practical strategies to minimise the impact of the combustion mode transition on the performance of the engine. This paper presents the results of an investigation of the combustion mode transitions between Sl and HCCI, using a combination of an engine cycle simulation code with a chemical kinetics based HCCI combustion code. It investigates and discusses the key issues concerning the combustion mode transition and its control. A new approach has been adopted in the modelling of the HCCI engine gas exchange and combustion, which allowed a parametric study of the transient process in the transition on a cycle-by-cycle basis and provided guidelines in the design of the engine hardware and its control strategies.

A single zone combustion model based on a chemical kinetic solver has been combined with a one-dimension thermo/gas dynamic engine simulation code to study the operating characteristics of a V6 engine In which Homogeneous Charge Compression Ignition (HCCI) operation (also referred to as 'Controlled Auto-ignition" CAI) is enabled by a cam profile switching (CPS) system with negative valve overlap. An operational window within which HCCI combustion is possible has been identified and the limit of HCCI operating region for varied valve lift possibilities is explored. The mechanisms and potential fuel economy improvements within the HCCI envelope are studied and modelled results compared against data from similar engines. It is shown that for the best fuel economy the valve timing strategy needs to be selected very carefully, despite the engine's capability to operate at a range of valve timing combinations. The heating effect of the exhaust gases is dominant when using less than approximately 55% residual gases. For higher EGR rates, the diluting effect coupled with reducing exhaust gas temperatures makes the ignition timing retarded. It is demonstrated that for HCCI operation the selection of valve lift for the CPS system is limited between the constraints of the breathing capacity with the lower valve lift and misfires with the higher one.

High-speed cine photography has been applied to an optically-accessed version of a modern 4-valve, pent-roof, SI engine in order to observe the behavior of fuel under cold starting and warm-up conditions. A wall film was formed on the combustion chamber surface in the vicinity of the valves when fuel was supplied by pintle and disk-type injectors. This wall film survived combustion but was drawn into the exhaust system when the exhaust valves opened. The use of an air-blast atomizer avoided the formation of this wall film, but some wetting of the piston was apparent, as it was with the other types of injector.