An infrared laser absorption technique has been developed to measure in-cylinder concentrations of CO in an optical, automotive HCCI engine. The diagnostic employs a distributed-feedback, tunable diode laser selected to emit light at the R15 line of the first overtone of CO near 2.3 {micro}m. The collimated laser beam makes multiple passes through the cylinder to increase its path length and its sampling volume. High-frequency modulation of the laser output (wavelength modulation spectroscopy) further enhances the signal-to-noise ratio and detection limits of CO. The diagnostic has been tested in the motored and fired engine, exhibiting better than 200-ppm sensitivity for 50-cycle ensemble-average values of CO concentration with 1-ms time resolution. Fired results demonstrate the ability of the diagnostic to quantify CO production during negative valve overlap (NVO) for a range of fueling conditions.
Negative valve overlap (NVO) is a valve strategy employed to retain and recompress residual burned gases to assist HCCI combustion, particularly in the difficult regime of low-load operation. NVO allows the retention of large quantities of hot residual burned gases as well as the possibility of fuel addition for combustion control purposes. Reaction of fuel injected during NVO increases charge temperature, but in addition could produce reformed fuel species that may affect main combustion phasing. The strategy holds potential for controlling and extending low-load HCCI combustion. The goal of this work is to demonstrate the feasibility of applying two-wavelength PLIF of 3-pentanone to obtain simultaneous, in-cylinder temperature and composition images during different parts of the HCCI/NVO cycle. Measurements are recorded during the intake and main compression strokes, as well as during the more challenging periods of NVO recompression and re-expansion. To improve measurement quality, effects of diagnostic uncertainty and fluorescence interference are quantified. Temperature, fuel, and EGR images are captured for a range of NVO operating conditions, including main and NVO fuel-injection timings as well total load. The results demonstrate that the diagnostic is capable of providing information useful for the study of HCCI/NVO engine operation.
Many experimental efforts to track fuel-air-residual mixture preparation in internal combustion engines have employed laser induced fluorescence (LIF) of tracers. Acetone and 3-pentanone are often chosen as tracers because of their relatively strong LIF signal, weak quenching, and reasonable match to thermo-chemical properties of common fuels such as iso-octane. However, the addition of these tracers to fuel-air mixtures could affect combustion behavior. In this work, we assess these effects to better understand limitations of tracer-based engine measurements. The effects of tracer seeding on combustion phasing, duration, and variation are studied in an HCCI engine using a recompression strategy to accommodate single- and multi-stage-ignition fuels. Using direct-injected (DI) fuels iso-octane and n-heptane, comparisons are made of combustion performance with and without seeding of the intake air (air seeding, as opposed to the more common fuel seeding, is a variation of LIF used to measure residual-gas concentration). Chemical and premixing effects of tracer addition are distinguished by substituting equivalent amounts of fuel for the tracer. Chemical kinetic simulations of iso-octane and n-heptane oxidation help explain the experimentally determined trends. Results show that the phasing of iso-octane combustion can be significantly impacted by premixing effects because of the sensitivity of ignition to charge temperature. For n-heptane, the chemical effects of tracer addition are shown to be more pronounced because of impact on low-temperature heat release. Acetone retards the combustion for both single- and two- stage-ignition fuels, whereas 3-pentanone advances iso- octane combustion while retarding n-heptane. Overall, we found that the impact of tracer addition is modest for the chosen operating conditions since varying the intake temperature can easily compensate for it.