Measurement of Air-Fuel Mixing in a Diesel Spray at Engine Relevant Conditions Using UV-VIS DBI Diagnostic

Due to the non-premixed nature of diesel combustion, mixing prior to the reaction zone has proven to be one of the primary factors in emissions formation. Therefore, the advancement of diagnostics used to measure mixing fields in diesel applications is imperative for a greater understanding of how in-cylinder emissions mitigation techniques operate. Towards this goal, we have recently demonstrated the use of a high-speed two-wavelength extinction imaging measurement, UV-VIS DBI, for time-resolved measurements of mixing in a diesel spray. This diagnostic operates by back-lighting the spray with ultra-violet and visible illumination. The visible illumination is selected at a non-absorbing wavelength, such that the visible light is only attenuated by liquid droplet scattering, enabling discrete detection of the liquid-vapor mixture and pure vapor phases of the spray. For this work, Ultraviolet and visible light are generated using a ND:YAG pumped frequency-doubled tunable dye laser operating at 9.9 kHz . The simultaneous UV-Visible illumination is used to back-illuminate a vaporizing diesel spray, and the resulting extinction of each signal is recorded by a pair of high-speed cameras. Using an aromatic tracer (naphthalene, BP = 218 °C) in a base fuel of dodecane (BP = 215–217 °C), the UV illumination (280 nm) is absorbed along the illumination path through the spray, yielding a projected image of line-of-sight optical depth that is proportional to the projected fuel vapor concentration in the pure vapor region of the spray. In this paper, a new method of determining the absorption coefficient for the pure-vapor phase of the spray will be discussed, along with showing how an Inverse-Abel transform can be used to compute planar concentration data from the projected concentration data yielded by the diagnostic. This diagnostic and data processing is applied to diesel sprays from two Bosch CRI3-20 ks1.5 single-orifice injectors (140 μm and 90 μm orifice diameters) injecting into a nonreacting high-pressure and temperature nitrogen environment using a constant-flow, optically-accessible spray chamber operating at 60 bar and 900 K. The mixing data produced agrees well with previously existing mixing data, which further instills confidence in the diagnostic, and gives the diesel combustion community access to mixing field data for a 140 μm orifice diameter injector at a 60 bar and 900 K condition.

CFD simulation-based predesign of an advanced gas-diesel combustion concept

The greenhouse gas saving potential of using gaseous fuels with high methane content (e.g. natural gas) in internal combustion engines instead of conventional liquid fossil fuels (e.g. petrol, diesel) is considerable due to the comparatively low emission of carbon dioxide resulting from the low C/H ratio of methane. However, to fully exploit this potential, it is of utmost importance to keep methane slip at a very low level. In contrast to mixture aspirated gas engines and diesel-gas engines, the gas-diesel combustion concept avoids methane slip nearly completely since the gaseous fuel is directly injected into the combustion chamber at the end of the high-pressure phase of the engine cycle, resulting in mixing-controlled combustion with low emission of unburned hydrocarbons. An advanced high-speed large engine concept based on the gas-diesel combustion process was developed. An effective and reliable virtual design methodology was applied during the development of the concept. The methodology comprehensively combines 3D CFD and 1D simulation tools in the combustion concept predesign phase with experiments on a single-cylinder research engine in the concept validation phase. A major challenge in the virtual design of this dual fuel combustion process is the large number of degrees of freedom that result in particular from the use of a fully flexible combined gas/diesel injector. This paper describes in detail the role of 3D CFD simulation in this approach, which allows precise prediction of the optimal geometries and operating strategies for high-efficiency and low-emission engine operation.