Investigation of Boundary Layer Behaviour in HCCI Combustion using Chemiluminescence Imaging

A five-cylinder diesel engine, converted to a single cylinder operated optical engine is run in Homogeneous Charge Compression Ignition (HCCI) mode. A blend of iso-octane and n-heptane is used as fuel. An experimental study of the horizontal boundary layer between the main combustion and the non-reacting surface of the combustion chamber is conducted as a function of speed, load, swirl and injection strategy. The combustion behaviour is monitored by chemiluminescence measurements. For all cases an interval from -10 to 16 crank angles after top dead center (CAD ATDC) in steps of one CAD are studied. One image-intensified camera observes the boundary layer up close from the side through a quartz cylinder liner while a second camera has a more global view from below to see more large scale structure of the combustion. The averaged chemiluminescence intensity from the HCCI combustion is seen to scale well with the rate of heat release. A boundary layer is defined and studied in detail between the main combustion volume and the piston crown surface as a function of crank angle. The boundary layer is found to be in the range from 2 to 4 mm for all cases by the definition used; however, the location for the measurements becomes more and more important as combustion becomes more inhomogeneous. To get accurate calculations, the level of noise must also be considered and definitions of boundary layer thickness should not be made at to low chemiluminescence intensity

Laser Diagnostics of HCCI and Partially Premixed Combustion

The work presented in this thesis deals with measuring in-cylinder combustion species and events using different laser-based diagnostic methods. A variety of engine operating modes, like HCCI and partially premixed diesel combustion, have been investigated. In the very first measurements, in-cylinder flow-fields were compared to CFD and steady-state blow rig results. After that, the ignition and combustion process of HCCI was investigated using laser-induced fluorescence (LIF) of formaldehyde and hydroxyl. In the low-temperature reactions that precede the main combustion, formaldehyde (HCOH) was seen to form homogeneously over the viewed area. Hydroxyl, OH, is formed in the high temperature regions that mark the main combustion; it was determined that OH is formed in areas from which formaldehyde had disappeared. By using different start of injection timings, different degrees of homogeneity could be obtained for HCCI combustion and the effects of this were examined using the above mentioned laser-technique. The engine-out NOx level was monitored to see what in-homogeneity level could be tolerated before getting too much NOx. Going from early injections towards late, a distinct change in the homogeneity was seen with injection at 70 CAD and around 50 CAD NOx levels started to increase. Later, LIF measurements were performed on combustion modes other than HCCI, these studies also included the use of exhaust gas recirculation (EGR). The modes that were examined and compared with port and DI HCCI were UNIBUS combustion using two fuel injection events and low-temperature diesel combustion with one injection 8 crank angles before top dead centre. The feasibility of using formaldehyde, or other partially-oxidized fuel elements, as a naturally occurring fuel tracer were investigated by comparing the distribution of those species with that of the common fuel tracer toluene. The distributions of the two species are similar to each other in HCCI meaning that formaldehyde could be a tracer candidate. In low-temperature diesel, it seems like a good tracer; however more research is needed on the impact of polyaromatic hydrocarbons. In the last part, high-dilution low-temperature diesel combustion was studied and during this time the first planar flow measurements in a firing diesel engine were obtained. Furthermore, LIF measurements of partially-oxidized fuel and LII of soot were performed. The initial distribution of partially-oxidized fuel was found to correlate well with regions of heat release that were identified from the flow field divergence. In the later stages of combustion, soot and partially-oxidized fuel were found to be concentrated in the cylinder centre and no bulk flow exist, at least not for our piston design, that transport this fluid to regions where oxidation could take place. Partially-oxidized fuel was also found in the squish region which can be a source of emissions of CO and unburned fuel. It was also seen that single-cycle measurements show good similarity to the mean results. In the very last study that is presented, the flow structures of two low-temperature diesel operating conditions are compared at two swirl ratios. Differences are seen in the reverse-squish flow as well as in the fluid motion near the bowl rim. The main influence of decreasing the swirl was that the fluid motion at the bowl rim was altered

Low NOx and Low Smoke Operation of a Diesel Engine Using Gasolinelike Fuels

Much of the technology in advanced diesel engines, such as high injection pressures, is aimed at overcoming the short ignition delay of conventional diesel fuels to promote premixed combustion in order to reduce NOx and smoke. Previous work in a 2 l single-cylinder diesel engine with a compression ratio of 14 has demonstrated that gasoline fuel, because of its high ignition delay, is very beneficial for premixed compression-ignition compared with a conventional diesel fuel. We have now done similar studies in a smaller-0.537 l-single-cylinder diesel engine with a compression ratio of 15.8. The engine was run on three fuels of very different auto-ignition quality-a typical European diesel fuel with a cetane number (CN) of 56, a typical European gasoline of 95 RON and 85 MON with an estimated CN of 16 and another gasoline of 84 RON and 78 MON (estimated CN of 21). The previous results with gasoline were obtained only at 1200 rpm-here we compare the fuels also at 2000 rpm and 3000 rpm. At 1200 rpm, at low loads (similar to 4 bars indicated mean effective pressure (IMEP)) when smoke is negligible, NOx levels below 0.4 g/kWh can be easily attained with gasoline without using exhaust gas recirculation (EGR), while this is not possible with the 56 CN European diesel. At these loads, the maximum pressure-rise rate is also significantly lower for gasoline. At 2000 rpm, with 2 bars absolute intake pressure, NOx can be reduced below 0.4 g/kW h with negligible smoke (FSN < 0.1) with gasoline between 10 bars and 12 bars IMEP using sufficient EGR, while this is not possible with the diesel fuel. At 3000 rpm, with the intake pressure at 2.4 bars absolute, NOx of 0.4 g/kW h with negligible smoke was attainable with gasoline at 13 bars IMEP. Hydrocarbon and CO emissions are higher for gasoline and will require after-treatment. High peak heat release rates can be alleviated using multiple injections. Large amounts of gasoline, unlike diesel, can be injected very early in the cycle without causing heat release during the compression stroke and this enables the heat release profile to be shaped. [DOI: 10.1115/1.4000602

The influence of fuel injection and heat release on bulk flow structures in a direct-injection, swirl-supported diesel engine

Particle image velocimetry is applied to measure the vertical (r-z) plane flow structures in a light-duty direct-injection diesel engine with a realistic piston geometry. The measurements are corrected for optical distortions due to the curved piston bowl walls and the cylindrical liner. Mean flow fields are presented and contrasted for operation both with and without fuel injection and combustion. For operation with combustion, the two-dimensional divergence of the measured mean velocity fields is employed as a qualitative indicator of the locations of mean heat release. In agreement with numerical simulations, dual-vortex, vertical plane mean flow structures that may enhance mixing rates are formed approximately mid-way through the combustion event. Late in the cycle a toroidal vortex forms outside the bowl mouth. Imaging studies suggest that soot and partially oxidized fuel trapped within this vortex are slow to mix with surrounding fluid; moreover, the vortex impedes mixing of fluid exiting the bowl with air within the squish volume

Surrogate fuels for premixed combustion in compression ignition engines

Simple surrogate fuels are needed to model practical fuels, which are complex mixtures of hydrocarbons. The surrogate fuel should match the combustion and emissions behaviour of the target fuel as much as possible. This paper presents experimental results using a wide range of fuels in both the gasoline and diesel auto-ignition range, but of different volatilities and compositions, in a single cylinder diesel engine. Premixed combustion in a compression ignition engine is defined, in this paper, to occur when the injection event is clearly separated from the combustion and the engine-out smoke is very low - below 0.05 FSN (filter smoke number). Under such circumstances, if the combustion phasing is matched for two fuels at a given operating condition and injection timing, the emissions are also comparable regardless of the differences in composition and volatility. For the experimental conditions considered, combustion phasing at a given operating condition and injection timing depends only on the octane index (OI), OI = (1-K)RON + KMON, where RON and MON are research and motor octane numbers and K is an empirical constant that depends on operating conditions. A mixture of iso-octane, n-heptane and toluene can be found to match the RON and MON of any practical gasoline and will be a very good surrogate for the gasoline since it will have the same OI. If the compression ratio is greater than 14, practical diesel fuels, with DCN (derived cetane number) between 40 and 60, will have comparable ignition delays to n-heptane, which is an adequate surrogate for such fuels. However, premixed combustion can be attained only at much lower loads at a given speed with diesel fuels compared to gasolines

Autoignition quality of gasoline fuels in partially premixed combustion in diesel engines

A single-cylinder diesel engine has been run on gasolines of different octane numbers and on model fuels, mixtures of iso-octane, n-heptane and toluene, at different operating conditions. The autoignition quality of the fuel is best described by an Octane Index, OI = (1 - K) . RON + K . MON for fuels in the gasoline autoignition range where RON and MON are, respectively, the Research and Motor Octane numbers and K is an empirical constant which is measured to be negative. Hence for a given RON, a non-paraffinic fuel, of lower MON, will have higher OI and more resistance to autoignition. For a given operating condition, ignition delay increases non-linearly with OI and changes little over the autoignition range of practical diesel fuels. Heat release following the autoignition is influenced by the stratification which will increase as the time between the end of injection and start of combustion decreases and combustion phasing parameters such as Combustion Delay, the difference between the 50% burn time and the start of injection, become less correlated with fuel autoignition quality. Higher ignition delays facilitate premixed combustion in the diesel engine. If two fuels have similar combustion phasing at the same injection timing, their emissions performance is also similar. Hence a good surrogate for gasoline in partially premixed compression ignition engines is a mixture of toluene, iso-octane and n-heptane with the same RON and MON. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Studies of the Combustion Process with Simultaneous OH- and Formaldehyde-PLIF in a Direct-Injected HCCI engine

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 NOx 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 NOx 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 modern 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 NOx 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

Combined measurements of flow structure, partially oxidized fuel, and soot in a high-speed, direct-injection diesel engine

The evolution of bulk flow structures and their influence on the spatial distribution of heat release zones and of partially oxidized fuel and particulate matter (soot) is examined experimentally in a swirl-supported, direct-injection diesel engine. Vector fields describing the bulk flow structures are measured with particle image velocimetry (PIV), while complementary scalar field measurements of partially oxidized fuel and soot are obtained in the same vertical plane using broadband laser-induced fluorescence (LIF) and laser-induced incandescence (LII) techniques, respectively. The two-dimensional divergence of the mean velocity fields is also employed to provide information on the mean locations of heat release. Measurements are performed at a highly dilute, 12% O-2, operating condition characteristic of low-NO,, low-temperature diesel combustion systems. The spatial distributions of unburned fuel rapidly develop a structure characterized by two separate zones of high fuel concentration, an inner zone in the cylinder center and an outer zone in the squish volume. Single-cycle measurements show that this two-zone structure is present on an individual cycle basis, and is not an artifact of averaging distinct, single-zone distributions. For this engine build, the mean flow structures developed do not actively promote mixing of either zone, although bulk flow structures in the upper-central region of the cylinder vary significantly on a cycle-by-cycle basis. The measured spatial distributions of particulates indicate that particulates are formed primarily in the inner zone-and remain un-oxidized late in the cycle. Published by Elsevier Inc. on behalf of The Combustion Institute

Numerical and Experimental Investigation of Turbulent Flows in a Diesel Engine

This paper presents a study of the turbulence field in an optical diesel engine operated under motored conditions using both large eddy simulation (LES) and Particle Image Velocimetry (PIV). The study was performed in a laboratory optical diesel engine based on a recent production engine from VOLVO Car. PIV is used to study the flow field in the cylinder, particularly inside the piston bowl that is also optical accessible. LES is used to investigate in detail the structure of the turbulence, the vortex cores, and the temperature field in the entire engine, all within a single engine cycle. The LES results are compared with the PIV measurements in a 40 x 28 mm domain ranging from the nozzle tip to the cylinder wall. The LES grid consists of 1283 cells. The grid dynamically adjusts itself as the piston moves in the cylinder so that the engine cylinder, including the piston bowl, is described by the grid. In the intake phase the large-scale swirling and tumbling flow streams are shown to be responsible for the generation of large-scale vortex pipes which break down to small-scale turbulent eddies. In the later phase of compression turbulence is mainly produced in the engine bowl. The bore wall and the piston bowl wall heat the fluid near the walls. Turbulence and the large-scale coherent vortex shedding due to the Kelvin-Helmholtz instability are responsible for the enhanced heat transfer between the bulk flow and the walls. A temperature inhomogeneity of about 50 - 60 K can be generated in the cylinder