Spectral Analysis and Chemiluminescence Imaging of Hydrogen Addition to HSDI Diesel Combustion Under Conventional and Low-Temperature Conditions

Late-injection low-temperature diesel combustion is found to further reduce NOx and soot simultaneously. The combustion phenomena and detail chemical kinetics are studied with high speed spray/combustion images and time-resolved spectroscopy analysis in a rapid compression machine (RCM) with a small bowl combustion chamber. High swirl and high EGR condition can be achieved in the RCM; variable injection pressure and injection timing is supplied by the high-pressure common-rail fuel injection system.

Effect of small amount of premix hydrogen gas on diesel combustion is also studied in the RCM. A hydrogen injector is located in the upstream of air inlet for delivery small amount and premixed hydrogen gas into cylinder just before the compression stroke. The ignition delay is studied both from the pressure curves and the chemiluminescence images. High speed color images of spray and combustion show not only the high sooting diesel spray impinging on the chamber wall and interacting with very strong swirling air, but also the color temperature and volume fraction of soot. Hydrogen addition of 10% of energy released for low temperature combustion is shown to further reduced soot concentration, but hydrogen at 15% shows higher soot temperature and concentration. Small amount of hydrogen does not results in increasing peak pressure and fast burning rate because of higher EGR up to 50%.

Time resolved spectrum of chemiluminescence from cool flame, premixed combustion, and diffusion flame mainly show the time evolution of OH radicals. With small amount of hydrogen up to 15% of energy released, the OH spectrum is detected easily started from premix combustion period till early of the diffusion combustion period. However, the strong intensity of OH emission is very sensitive to EGR concentration and equivalence ratio due to changes of oxygen concentration. Temperature derivation from rotational spectral lines of OH spectrum is also employed for cases with strong OH emission.