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Within the Engine Combustion Network (ECN) research frame, the mixing process and the fuel mass concentration fields were investigated at spray G conditions and variants with optical diagnostics. Experiments were conducted in a high-temperature high-pressure constant-volume pre-combustion vessel. The target condition, called “Spray G”, which is representative of gasoline direct-injection engine conditions, uses well-defined ambient (573 K, 6 bar, 3.5 kg/m3, O2-free) and injector conditions (200 bar, eight-hole injector, 0.165 mm orifice diameter). Measurements were also conducted at 6 and 9 kg/m3 for temperatures of 700 and 800 K respectively. Two techniques were used to visualize the jet formation: p-difluorobenzene laser induced fluorescence (LIF) imaging and high-repetition-rate schlieren visualization. Images from both methods were compared in terms of jet penetration and size.

UV-chemiluminescence from the excited hydroxyl-radical (OH*) has been used as a marker for the high-temperature reacting zone in spark-ignited engines for quite some time. In research engines with large optical access, sensitive camera systems make it possible to obtain images of the flame that can be used for, e.g., determining the flame-front's propagation speed [Aleiferis et al., Combust. Flame 136 (2004) 283-302]. However, on one hand such optical engines are limited in their speed and load range, on the other, typical UV endoscopes make wide-field imaging at low light levels challenging. Here, a large-aperture UV endoscope is used to capture sequences of OH* chemiluminescence during early flame propagation in a nearly unmodified production engine. We compare three imaging systems: phase-locked single-shot imaging, phase-locked double-frame imaging, and “high-speed” cinematography at kHz repetition rates.

Within the Engine Combustion Network (ECN) spray combustion research frame, simultaneous line-of-sight laser extinction measurements and laser-induced incandescence (LII) imaging were performed to derive the soot volume fraction (fv). Experiments are conducted at engine-relevant high-temperature and high-pressure conditions in a constant-volume pre-combustion type vessel. The target condition, called "Spray A," uses well-defined ambient (900 K, 60 bar, 22.8 kg/m₃, 15% oxygen) and injector conditions (common rail, 1500 bar, KS1.5/86 nozzle, 0.090 mm orifice diameter, n-dodecane, 363 K). Extinction measurements are used to calibrate LII images for quantitative soot distribution measurements at cross sections intersecting the spray axis. LII images are taken after the start of injection where quasi-stationary combustion is already established.

Laser-based in-cylinder diagnostics are well established in engine research. The requirement of large-scale optical accesses, however, makes the application expensive and time consuming. It furthermore limits the engine operation range to low loads and speeds. We introduce laser excitation and imaging optics with a minimal outer diameter of 10 mm (imaging optic) respectively 9 mm (excitation optics). The imaging optics allow the observation of a 30×30 mm2 field with a working distance of 35-42 mm. In order to increase the optical performance diffractive elements are integrated. These elements provide great flexibility for the excitation beam shaping and help to reduce aberrations in the imaging system with a light throughput comparable to imaging setups with standard large-scale UV optics at the same image magnification. We present this miniaturized diagnostic technique based on fuel tracers for measuring fuel density, equivalence ratio and temperature in IC engines.

Laser-induced fluorescence (LIF) is frequently used for the investigation of mixing processes in internal engine combustion. Toluene is one of the main fluorescing compounds of commercial gasoline. Understanding its fluorescence properties is therefore crucial for the correct interpretation of signal intensities observed under engine (i.e. high temperature and high pressure) conditions. Toluene LIF signal has been investigated as a function of temperature and oxygen concentration in order to enable quantitative fuel tracer imaging. Signal behavior and interpretation for engine-related conditions is demonstrated based on a semi-empirical fluorescence model. Toluene as well as gasoline-LIF is strongly quenched by oxygen. It has therefore been suggested for a direct measurement of fuel/air equivalence ratios.

Multi-species laser based imaging measurements have been carried out in a reacting Diesel spray in order to provide a detailed data base for model development and validation. In a high-pressure high-temperature spray chamber the measurements addressed the fuel vapor concentration, ignition and flame development and the soot formation. The fuel vapor distribution was measured quantitatively by Rayleigh scattering and compared to measurements by tracer laser-induced fluorescence. Soot volume fractions were observed by laser-induced incandescence. Fuel vapor and soot distributions were measured simultaneously and provide insight in the ignition and pollutant formation process. Specific digital image processing algorithms were developed to correct for beam steering and laser attenuation.

In direct-injection gasoline (GDI) engines with charge stratification, minimizing engine-out nitrogen oxide (NOx) emission is crucial since exhaust-gas aftertreatment tolerates only limited amounts of NOx. Reduced NOx production directly lowers the frequency of energy-inefficient catalyst regeneration cycles. In this paper we investigate NO formation in a realistic GDI engine. Quantitative in-cylinder measurements of NO concentrations are carried out via laser-induced fluorescence imaging with excitation of NO (A-X(0,2) band at 248 nm), and subsequent fluorescence detection at 220-240 nm. Engine modifications were kept to a minimum in order to provide results that are representative of practical operating conditions. Optical access via a sapphire ring enabled identical engine geometry as a production line engine. The engine is operated with commercial gasoline (“Super-Plus”, RON 98).

The complexity of the mixture formation in direct injection engines requires - according to a suitable mixture transportation and vaporization of the fuel - detailed knowledge of the in-cylinder processes to reliably place an ignitable mixture at ignition timing near the spark plug for any speed and load. Two different optical measurement techniques were adapted to a single cylinder engine and the spray propagation was observed from the start of injection until ignition. 3-pentanone tracer-LIF signals (laser-induced fluorescence) and CN spark emission signals were detected simultaneously in order to get information about the local equivalence ratio at the spark plug and compare the two methods. While there is a good correlation for homogeneous operating conditions of the engine, the results diverge in the stratified mode.

The NO distribution in a directly-injected Diesel engine with realistic combustion chamber geometry was investigated with laser-induced fluorescence (LIF) imaging with KrF excimer laser excitation. The highest possible level of selectivity has been ensured using spectrally resolved LIF investigations inside the Diesel engine. To minimize interference from both, oxygen and polycyclic aromatic hydrocarbon (PAH) LIF the NO signal was detected around 237 nm, blue-shifted compared to the excitation wavelength resulting in a background contribution below 10% at the earliest detection timing possible in the engine under study (20°ca after top dead center, TDC). The in-cylinder NO LIF intensities were compared for different injection systems and operating conditions and correlated to variations in pressure traces and soot temperature measurements.

In an optical 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 (HCHO) 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. Despite the fact that a frequency-tripled Nd:YAG laser excites only weak transitions in the HCHO molecule, the high concentration (several thousands ppm) provide for sufficient signal intensity when detecting fluorescence above 395 nm. Using formaldehyde LIF, auto-ignition (occurring close to 356°ca) and the further development of combustion was observed.

The influence of exhaust gas recirculation (EGR) on the formation of nitric oxide (NO) was studied experimentally in a transparent gasoline direct injection engine by quantitative laser-induced fluorescence imaging. Spectral properties of the excited transition within the NO A2∑+-X2∏(0,2) band are well known from previous studies. The excitation scheme allows quantitative NO concentration measurements without detailed knowledge of the gas phase temperature. Good agreement was found with exhaust gas NOx chemi-luminescence (CLD) measurements. The experiments were carried out in an optically accessible gasoline engine featuring a direct injection cylinder head (BMW) and a Bosch injection system, based on a serial inline six-cylinder engine with an enlarged crankcase. The measurements were performed in the pentroof section of the combustion chamber.

The purpose of the 4-SPACE (4-Stroke Powered gasoline Auto-ignition Controlled combustion Engine) industrial research project is to research and develop an innovative controlled auto-ignition combustion process for lean burn automotive gasoline 4-stroke engines application. The engine concepts to be developed could have the potential to replace the existing stoichiometric / 3-way catalyst automotive spark ignition 4-stroke engines by offering the potential to meet the most stringent EURO 4 emissions limits in the year 2005 without requiring DeNOx catalyst technology. A reduction of fuel consumption and therefore of corresponding CO2 emissions of 15 to 20% in average urban conditions of use, is expected for the « 4-SPACE » lean burn 4-stroke engine with additional reduction of CO emissions.

The formation of nitric oxide in a transparent direct injection gasoline engine was studied experimentally using two different schemes of laser-induced fluorescence (LIF) with KrF excimer (248 nm) excitation. With detection of the fluorescence shifted towards the red, strong interference from fluorescence of partially burned fuel was found. With blue-shifted fluorescence, interference was minimized allowing selective detection of NO. Possibilities of quantifying NO fluorescence intensities in inhomogeneous combustion are discussed.