Towards a Better Understanding of Controlled Auto-Ignition (CAI™) Combustion Process From 2-Stroke Engine Results Analyses 2001-01-1859
Owing to its inherent high internal residual gas rate in partial load operation, the 2-stroke engine has been the first application to take benefit of the unconventional CAI™ (Controlled Auto-Ignition) combustion process. For a long time, the objective of the different research works on 2-stroke engines optimization was to eliminate its two main drawbacks leading to high emissions of unburned hydrocarbons and a poor fuel efficiency. The first one is the unstable running operation combined with incomplete combustion, especially at light load, The second one is fuel short circuit at medium and full load. From the end of seventies, an approach developed by Onishi from Nippon Clean Engine was to take benefit of an high amount of hot internal residual gases to help auto-ignition of the fresh charge. This solution has been further developed up to the industrialization on 2-stroke engines.
More recently, within the framework of the European 4-SPACE program driven by IFP, the CAI™ combustion process has been successfully applied to a 4-stroke engine, demonstrating its potential for near zero NOx emission levels combined with a significant fuel saving and thus reduction of CO2 emissions. This application was directly derived from 2-stroke engine analyses which can be considered as an useful tool to better understand this combustion process.
This paper presents several approaches to analyze this unconventional combustion process by combining 2-stroke engine trials on a dyno test bench with more advanced research and development tools such as 3D CFD and optical diagnostics. The fluid dynamical behavior in the cylinder during the scavenge process and the compression stroke was studied using 3D CFD and showed that the mixing level between the fresh charge and the internal residual gases is a key parameter to obtain and to control CAI™.
The experiments performed on an optically accessible engine allowed to make direct visualization of combustion and to show the outline of the zones of combustion from an initiation and development point of view. Results obtained by a Mie scattering technique allowed us to distinguish the distribution of fresh and residual gases during the scavenging process and the compression stroke in motored conditions. The application of this optical technique combined with direct combustion vizualisation gives more detailed information on the physical process, especially when studying the effects of certain parameters as the engine speed. The post processing of the images allowed notably to estimate the number of auto-ignition sites and the propagation mode of the combustion.