Abstract
There are only so many technologies and devices that have the same type of impact as that of the internal combustion (IC) engine. Its ubiquitous nature pervades our everyday life, many times without us even realizing it. Whether it be the spark-ignited engine driving our vehicle, the compression-ignition engine hauling food to our local grocery store, the jet engine we hear flying 38,000 feet overhead, or the gas turbine powering the laptop screen from which we read this article, internal combustion engines are quite literally intricately and irreplaceably woven into our daily lives. The internal combustion has taken on many different forms throughout its long, greater than 150-year history, but combustion has always been one of its few constants. Indeed, combustion is even in its name and helps differentiate it from other thermodynamic work devices such heat engines and fuel cells.
Introduction
There are only so many technologies and devices that have the same type of impact as that of the internal combustion (IC) engine. Its ubiquitous nature pervades our everyday life, many times without us even realizing it. Whether it be the spark-ignited engine driving our vehicle, the compression-ignition engine hauling food to our local grocery store, the jet engine we hear flying 38,000 feet overhead, or the gas turbine powering the laptop screen from which we read this article, internal combustion engines are quite literally intricately and irreplaceably woven into our daily lives. The internal combustion has taken on many different forms throughout its long, greater than 150-year history, but combustion has always been one of its few constants. Indeed, combustion is even in its name and helps differentiate it from other thermodynamic work devices such heat engines and fuel cells.
In spite of combustion’s constancy in the internal combustion engine, the nature of this combustion has always been of interest, of fervent research, of subject of study, and of key importance to the internal combustion engine’s ability to deliver power efficiently and cleanly. Indeed, to this day, there is much to learn about combustion within the internal combustion engine and how this combustion may be exploited better to achieve goals for healthy benefit to society.
A key advancement to this exercise of improving our knowledge and use of combustion has been experimental visualization of flames, species concentrations, fluid motions, and other gradients that occur during combustion within an internal combustion engine. Tremendous progress has been made to precisely measure features of in-cylinder or in-burner reactions that not only aid our understanding of the phenomena taking place but also equip confidence in the use of simulation-based tools that further lend insight where experimentation cannot (at least at the present).
This chapter features some of the more recent major advancements in optical or visual measurements being made of combustion as used in internal combustion engines. I am gratefully indebted to the authors’ whose work follows for their dedicated contribution to this chapter. Roughly half the submissions involve internal combustion engines of the reciprocating piston type, including those from Drs. Busch, Ciatti, Mueller, Musculus, and Sjoberg. The remaining half involve visualization of the combustion present in open internal combustion engines, such as jet engines and gas turbines, and include the contributions by Drs. Han, Kero, Stohr, and Versailles. What is most intriguing about these visualizations is not so much the qualitative features one can extract – which by themselves are incredibly useful particularly from an educator’s perspective – but the quantitative features that are used to make impactful conclusions that change the course of research or execute substantial design decisions leading to a better engineered product.
3.1 Single-Cylinder Version of a Cummins Six-Cylinder N-14 Highway Truck Engine
Fuel injection, combustion, and pollutant formation processes are studied in an optically accessible, single-cylinder version of the Cummins six-cylinder N-14 highway truck engine. Windows in the cylinder walls provide access for laser illumination, while windows in the extended piston and cylinder head provide imaging access to the combustion bowl and “squish” region above the piston. Using lenses, high-energy pulsed laser beams are formed into thin sheets to probe the cross-section of the in-cylinder jets, which are imaged by intensified cameras.
3.2 Mixing-Controlled Combustion in a Heavy-Duty Compression-Ignition Engine
This figure shows an in-cylinder image of natural luminosity (NL, left), an in-cylinder image of electronically excited hydroxyl radical chemiluminescence (OH* CL, center), and the apparent heat-release rate (AHRR) and the estimated equivalence ratio at the flame lift-off length (ϕ(H), right) during conventional, mixing-controlled combustion in a heavy-duty diesel engine. The images were acquired at 5.0 crank-angle degrees (CAD) after top-dead-center by viewing the combustion through a fused-silica window in the bowl of the piston. The piston bowl diameter is indicated by the larger white circle in each image, and the location of the fuel injector tip is indicated by the red dot at the center of each image. The NL signal is dominated by incandescence from hot soot, with the six saturated (white) regions corresponding to soot from each of the six fuel sprays. Liquid fuel in the six fuel sprays is also visible as gray lines emanating from the injector tip due to NL signal from hot soot being elastically scattered from the fuel droplets to the camera. The NL and OH* CL cameras detected photons with wavelengths in the range of 380–1000 nm and 308 ± 10 nm, respectively. The OH* CL image shows regions where high-temperature chemical reactions are occurring. The distance from the injector orifice exit to the most upstream extent of this region is known as the flame liftoff length, and the liftoff length for each spray is shown with a yellow bar. The estimated ϕ(H) values shown in the plot on the right are > 2, indicating that soot should be present, which is consistent with the hot soot evident in the NL image. The AHRR plot shows a typical premixed-burn spike followed by a period of mixing-controlled heat release, which is relatively brief at this 6 bar gross indicated mean effective pressure operating condition. Other engine operating conditions are: 1,500 rpm speed, 240 MPa injection pressure, −5 CAD start of combustion timing, 21% intake-oxygen mole fraction, 30°C intake manifold temperature, and 85°C coolant temperature. The red vertical line on the AHRR figure indicates the timing that corresponds to the images shown, which were acquired when 70% of the cumulative fuel mass was burned. The fuel is a commercial #2 diesel fuel.
Reference
3.3 Visible Combustion Emissions in a Swirl-Supported, Light-Duty Diesel Engine
This series of images taken with a high-speed camera show visible and near-infrared combustion emissions inside a swirl-supported, optical diesel research engine. The images are monochromatic, but are shown with a false color scaling to help distinguish regions of high intensity (yellow and white) from regions of low intensity (dark-red and orange). Images are taken through the engine’s transparent piston, looking up at the cylinder head, as shown by the cross-sectional view. The thick white circle represents the edge of the field of view, which is several millimeters smaller than the cylinder bore.
Figure 3.4 (560 µs after the start of injection – ASOI): Seven jets of liquid fuel issuing from the centrally mounted fuel injector are dimly illuminated by the hot, glowing soot particles that form in regions where fuel has mixed with hot air and begun to burn. The swirling air in the cylinder has transported the fuel–air mixture so that the soot clouds are now displaced from the liquid jets in a clockwise direction. The presence of these glowing soot particles indicates hot combustion of rich mixtures. The piston is just starting to move downward at the beginning of the expansion stroke.
Figure 3.5 (760 µs ASOI): combustion continues as the outwardly propagating sprays impinge on the piston bowl rim. Part of the combusting mixture is deflected down into the piston bowl, and part of the mixture continues to propagate outward into the region between the piston and the cylinder head (this is called the squish region).
Figure 3.6 (960 µs ASOI): The injection of liquid fuel is nearing completion. Mixture in the bowl follows the contour of the wall and is redirected inward and upward along the central “pip” while also being transported by the clockwise swirl. This three-dimensional flow pattern is often referred to as a toroidal vortex. Outward propagation continues into the squish region.
Figure 3.7 (1,360 µs ASOI): The last fuel to be injected mixes poorly, and soot forms as it burns near the center of the cylinder. The toroidal vortex in the bowl supports the mixing of partially burned fuel and air, as the rate of fuel energy conversion to heat is near its maximum. Combusting mixture in the squish region continues to propagate outward.
Figure 3.8 (1,760 µs ASOI): Regions of hot soot are consumed by combustion reactions in the squish region. Soot intensities in the bowl are near their maximum values. The inward motion associated with the toroidal vortex in the bowl begins to decay, but swirling motion continues.
Figure 3.9 (3,320 µs ASOI): Late in the combustion event, most of the hot soot is observed in the bowl, and the downward motion of the piston acts to expand and cool the cylinder contents. Oxidation of soot requires high temperatures, as well as mixing and reaction with an oxidizer, so turbulent mixing processes and chemical kinetics play important roles in this stage of combustion.
3.4 Sequential Images of Gasoline Compression Ignition Inside an Engine
High-speed imaging captured this sequence of the auto-ignition and combustion progression of gasoline used in a compression ignition engine. The work is important to defining the compression ignition characteristics of gasoline rather than standard octane ratings empirically defined for spark ignition, stoichiometric engines. The images are compared with advanced engine simulations to better understand the mechanism of gasoline auto-ignition.
3.5 Spray–Swirl Interactions Stabilize Stratified-Charge SI Operation by Reducing Flow Variability near the Spark
Research at Sandia National Laboratories has clarified the mechanism through which intake-generated swirl promotes stable combustion in stratified-charge spark-ignition (SI) engines operated on gasoline-type fuels. Using an optically accessible engine, the in-cylinder flow was examined with particle image velocimetry (PIV) for both swirling and non-swirling flows at an engine speed of 2,000 rpm. The measurements show that swirl makes the flow patterns of individual cycles more similar to the ensemble-averaged cycle, as exemplified in Figure 3.11. This effect is quantified in Figure 3.12 using a “flow similarity” parameter (Rp). With in-cylinder swirl, Rp remains consistently high for all examined cycles, indicating a high similarity between the ensemble-averaged flow and each individual cycle. Combustion data indicate a low IMEP variability of 1.4% for operation with swirl. In contrast, for operation without swirl, the average Rp is lower, and some cycles have very low Rp values. This leads to an unacceptably high IMEP varibility of 3.5%.
