Starting to Unpick the Unique Air–Fuel Mixing Dynamics in the Recuperated Split Cycle Engine

In this work air fuel mixing and combustion dynamics in the recuperated split cycle engine (RSCE) are investigated through new theoretical analysis and complementary optical experiments of the flow field. First, a brief introduction to the basic working principles of the RSCE cycle will be presented, followed by recent test bed results relevant to pressure traces and soot emissions. These results prompted fundamental questioning of the air-fuel mixing and combustion dynamics taking place. Hypotheses of the mixing process are then presented, with differences to that of a conventional Diesel engine highlighted. Moreover, the links of the reduced emissions, air transfer processes and enhanced atomisation are explored. Initial experimental results and Schlieren images of the air flow through the poppet valves in a flow rig are reported. The Schlieren images display shockwave and Mach disk phenomena. Demonstrating supersonic air flow in the chamber is consistent with complementary CFD work. The results from the initial experiment alone are inconclusive to suggest which of the three suggested mixing mechanism hypotheses are dominating the air–fuel dynamics in the RSCE. However, one major conclusion of this work is the proof for the presence of shockwave phenomena which are atypical of conventional engines

High-Speed Thermographic Analysis of Diesel Injector Nozzle Tip Temperature

The temperature of fuel injectors can affect the flow inside nozzles and the subsequent spray and liquid films on the injector tips. These processes are known to impact fuel mixing, combustion and the formation of deposits that can cause engines to go off calibration. However, there is a lack of experimental data for the transient evolution of nozzle temperature throughout engine cycles and the effect of operating conditions on injector tip temperature. Although some measurements of engine surface temperature exist, they have relatively low temporal resolutions and cannot be applied to production injectors due to the requirement for a specialist coating which can interfere with the orifice geometry. To address this knowledge gap, we have developed a high-speed infrared imaging approach to measure the temperature of the nozzle surface inside an optical diesel engine. We investigated ways of increasing the emissivity of the nozzle surface with minimal intrusion by applying thin carbon coatings. We compare our measurements with those from a production injector that was instrumented with internal thermocouples. Our steady-state off-engine investigation showed that nozzle surface temperature measured by infrared imaging could yield data at 1200 fps with uncertainties of #x0002B;20K to -1K compared to simultaneous thermocouple measurements. We applied this approach to an optical diesel engine to investigate the effect of injection duration and increased swirl ratio on injector nozzle temperature and surface homogeneity.</p

A phenomenological model for near-nozzle fluid processes:Identification and qualitative characterisations

It is well established that emissions and inefficiencies primarily arise from localised fuel rich regions, which do not undergo complete combustion. In order to achieve significant reductions in NOx and soot, modern engines employ multiple injection and flow rate profiling strategies. However, during the end of each injection event, the needle restricts the internal flow of fuel, rapidly reducing the inertia of the emerging spray. Atomisation is inhibited and large fluid structures are released into the cylinder. Once the flow subsides, fuel films remain on the nozzle surface well into the cycle. Fuel residing in the nozzle cavities emerges as the cycles progresses, amalgamating with the spray deposited films. The surface-bound fuel presents an ideal environment for deposit forming reactions and a medium for precursors to adhere onto. In order to better understand these processes we performed measurements of injection transients inside an optical reciprocating rapid compression machine, using high-speed long-distance microscopy to obtain detailed characterisations of fuel films on the surface of an injector. We summarise our observations into a new phenomenological model which describes the uncontrolled release of fuel over an entire engine cycle. This model identifies the micro-scale processes that lead to the ejection, accumulation and vaporisation of fuel in-between injection events. Characterisation of these critical, transient processes can support mitigation strategies that inhibit pollutants and the formation of injector deposits.</p