Studies of the Combustion Process with Simultaneous Formaldehyde and OH PLIF in a Direct-Injected HCCI Engine(HCCI, Combustion Processes II)

Abstract

To run a Diesel engine in Homogeneous Charge Compression Ignition (HCCI) mode has proved to be a highly promising approach towards reduced engine-out emissions of NO_X and particulate matter. A crucial issue when utilizing HCCI is the degree of charge homogeneity that is required to achieve the desired low temperature combustion. A very well mixed charge can be created through the use of traditional port injection. This approach would most often result in low emissions of NO_X and soot. However, this strategy might also see a penalty in the form of high levels of unburned hydrocarbons due to incomplete combustion, especially under low load conditions. A proposed solution to this is to utilize stratified charge in the lower load range. The creation of a stratified charge imposes no major problems in modem DI engines. The important parameter is the degree of stratification that can be tolerated. If the charge gets too highly stratified, the combustion will become more diesel-like with dramatically increased levels of NO_X and soot as a result. This paper presents simultaneous laser based measurements of formaldehyde and OH-radical distributions in an HCCI engine. Formaldehyde is formed as an intermediate species when combusting hydrocarbons. The formation occurs through low temperature reactions in an early phase of the combustion process. Later in the process formaldehyde is being consumed. Formaldehyde is, therefore, used as indicator of the first stage of combustion and a marker of zones with low-temperature reactions. The OH radical is formed as an intermediate during the high temperature reactions, and is used as a marker of zones where the combustion is ongoing. The purpose of the investigation was to study how the combustion process is affected by the change in homogeneity that arises from early and late injection, respectively. A 0.5 liter single-cylinder optical engine equipped with a DI common rail fuel system was operated with a number of different injection timings, resulting in various levels of charge stratification. A blend of iso-octane and n-heptane was used as fuel. The measurement technique used was planar laser-induced fluorescence where formaldehyde was excited at 355nm and OH at 283nm. Two separate ICCD units were used to detect the resulting fluorescence from formaldehyde and OH. Measurement series covering the process from the start of injection until late in the expansion stroke is presented for different injection timings as well as pressure traces and emission analysis. A homogeneity index is calculated and used to compare the level of homogeneity resulting from injection timings. From early injection until about 50CAD BTDC the time, between onset of low temperature reactions and start of the high temperature reactions, is long enough for the formaldehyde to form an almost homogeneous distribution before it is being consumed. For later injection timings the high temperature reactions starts before this mixing is completed and therefore the formaldehyde distribution is not longer homogeneous and the combustion is more diesel like.