Laser diagnostics in Combustion: Investigation of Auto-ignition
Controlled Auto Ignition (CAI) is one candidate to meet pollution standards for future cars . In contrast to lean burn SI engines Controlled Auto Ignition has the potential to reduce pollutant formation without exhaust gas after-treatment while increasing fuel economy. Different HCCI processes (often also called Homogeneous Charge Compression Ignition, HCCI) have been studied over the past several years [1-4], but few engines were made commercially available. Both, homogeneous and stratified concepts have been investigated . However, stable operation is problematic under variable operating conditions. All current concepts rely on the presence of residual gases to increase the unburned gas temperature and enhance the presence of residual radicals that initiate the combustion process. New concepts are needed to control the concentration and the in-cylinder distribution of residual gases necessary for different operating conditions of the engine. For the observation of fuel and temperature distribution prior to ignition, and the observation of the combustion process in terms of flame propagation and pollutant formation laser-based imaging techniques have proven to be helpful tools. Whilst extensive studies have been performed in SI and Diesel engines, relatively little experimental data is available about HCCI combustion. The processes involved in the auto-ignition of hydrocarbon fuels, however, imply that some special requirements be met. With most fuels, especially low-octane number fuels, an extended pre-combustion phase ("cool flame") is observed prior the development of the hot flame [3,4]. During this phase endothermic reactions cause the formation of high levels of partially-oxidized hydrocarbon compounds like aldehydes. These compounds have been used earlier to study knock in SI engines . During auto-ignition, depending on the fuel, formaldehyde (CH2O) can be present at concentrations up to the percent level [4,7]. Since HCHO is completely burned after the "hot" flame initiation, its gradients clearly mark the position of the hot reaction zone.
In an optically accessible 4-stroke engine laser-induced fluorescence (LIF) imaging measurements of fuel tracer (3-pentanone) and formaldehyde were performed during the compression stroke and combustion. Formaldehyde (CH2O) is intermediately present at high concentrations within the cool flame and is burned later on when the "hot" combustion proceeds. It can be used as an internally generated tracer to observe the boundaries of the hot combustion zones. Using formaldehyde LIF, auto-ignition (occurring close to 356°ca) and the further development of combustion was observed. Combustion was completed within the field of view by 360°ca in most of the observed engine cycles. 3-pentanone, which has been used frequently for fuel-concentration imaging in spark ignited engines, was of limited use in the HCCI engine due to laser and signal attenuation prior to top dead center. The attenuation occurs due to tracer destruction and UV absorption.
Figure 1: Schematic of the optically accessible engine.
Figure 2: Progression of the "hot combustion" in the HCCI engine.
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