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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.

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.

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.

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.

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.