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 +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.