Modelling Study of Combustion and Gas Exchange in a HCCI (CAI) Engine 2002-01-0114

The main obstacle for the development of Homogeneous Charge Compression Ignition (HCCI) engines is the control of auto-ignition timing, and one key is to control the trapped gas temperature so as to enable the autoignition at the end of compression stroke. Using special valve mechanisms, very high residual gas mass fraction can be achieved to raise the charge temperature. Gas exchange process hence plays a crucial role in such HCCI engines because of its strong interaction with combustion. The modification of the gas exchange process in a 4-stroke automotive engine for HCCI combustion is not straightforward, since the engine must be able to operate across a considerably wide range of speeds and loads. Intake air temperatures and the valve mechanism need to be controlled in order to deliver optimal engine performance and fuel economy.

This paper presents a modelling study of the combustion and gas exchange in a HCCI engine. In the simulation, a single zone combustion model based on detailed chemical kinetics is combined with an in-house thermodynamic engine simulation program. The characteristics of HCCI combustion in the engine were investigated, including the development of pressure, temperature and species concentration. It was found that formaldehyde (CH₂O) concentration is related to ignition delay whereas hydroxyl (OH) concentration rapidly increases after the start of the main combustion stage. Effects of intake and exhaust valve timing on EGR rate, load, fuel economy, trapped gas temperature and temperature at the end of compression were studied. The characteristics predicted by the simulation are in good agreement with the available experimental data. It is shown that the gas exchange of the HCCI engine adopting a special valve mechanism effectively controls the gas temperature at the end of compression hence the auto-ignition process. In addition, this gas exchange results in a low pumping loss, which makes a significant contribution to fuel economy improvement at part load.