Publications

Negative valve overlap (NVO) is an operating mode that enables efficient, low-temperature gasoline combustion in automotive engines. In addition to retaining a large fraction of residuals, NVO operation also enables partial fuel injection during the recompression period as a means of enhancing and controlling main combustion. Thermal effects of NVO fueling on main combustion are well understood, but chemical effects of the products of NVO reactions remain uncertain. To address this topic, we have fabricated a dump valve that extracts a large fraction of cylinder charge at intake valve closing (IVC), yielding a representative sample of NVO products mixed with intake air. Sample composition is determined by gas chromatography. Results from a sweep of NVO start-of-injection (SOI) timings show that concentrations of the reactive species acetylene and hydrogen rise to several hundred parts-per-million as NVO SOI is retarded toward top center of NVO. Since experiments have previously demonstrated that low concentrations of acetylene seeded into the intake flow advance combustion phasing, the current results support the conclusion that NVO fueling can chemically enhance main combustion. This conclusion is further strengthened by a one-dimensional chemical kinetics simulation of main combustion that uses measured compositions as initial conditions at IVC. Comparing early and late NVO SOI, the model predicts that the hydrogen and acetylene produced by late NVO injection significantly advance the phasing of main combustion. © 2014 SAE International.

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.

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Prior HCCI optical engine experiments utilizing laser-induced fluorescence (LIF) measurements of stratified fuel-air mixtures have demonstrated the utility of probability density function (PDF) statistics for correlating mixture preparation with combustion. However, PDF statistics neglect all spatial details of in-cylinder fuel distribution. The current computational paper examines the effects of spatial fuel distribution on combustion using a novel combination of a 3-D CFD model with a 1-D linear-eddy model of turbulent mixing. In the simulations, the spatial coarseness of initial fuel distribution prior to the start of heat release is varied while keeping PDF statistics constant. Several cases are run, and as the initial mixture is made coarser, combustion phasing monotonically advances due to high local equivalence ratios that persist longer. The effect of turbulent mixing is more complex. For the case where the length scale of the initial distribution matches the integral length scale of turbulence, turbulent mixing leads to moderation of peak heat-release rate. The randomness of turbulence is captured in the simulation, and for the above case, cycle-to-cycle variation of the combustion is evident. In contrast, when the initial fuel distribution is significantly finer or coarser than the turbulence length scale, turbulent mixing does not affect combustion for two different reasons. For fine distributions, molecular diffusion alone homogenizes the fuel-air mixture prior to ignition, so turbulence adds nothing. For initial distributions that are coarse compared to the turbulence length scale, diffusion and turbulence are both ineffective at mixing, so again turbulence has a minimal effect on combustion.

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A review is given of contemporary research on the hydrogen-fueled internal combustion engine. The emphasis is on light- to medium-duty engine research. We first describe hydrogen-engine fundamentals by examining the engine-specific properties of hydrogen and surveying the existing literature. Here it will be shown that, due to low volumetric efficiencies and frequent preignition combustion events, the power densities of premixed or port-fuel-injected hydrogen engines are diminished relative to gasoline-fueled engines. Significant progress has been made in the development of advanced hydrogen engines with improved power densities. We discuss several examples and their salient features. Finally, we consider the overall progress made and provide suggestions for future work. © 2006 International Association for Hydrogen Energy.

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Optical engines are often skip-fired to maintain optical components at acceptable temperatures and to reduce window fouling. Although many different skip-fired sequences are possible, if exhaust emissions data are required, the skip-firing sequence ought to consist of a single fired cycle followed by a series of motored cycles (referred to here as singleton skip-firing). This paper compares a singleton skip-firing sequence with continuous firing at the same inlet conditions, and shows that combustion performance trends with equivalence ratio are similar. However, as expected, reactant temperatures are lower with skip-firing, resulting in retarded combustion phasing, and lower pressures and combustion efficiency. LIF practitioners often employ a homogeneous charge of known composition to create calibration images for converting raw signal to equivalence ratio. Homogeneous in-cylinder mixtures are typically obtained by premixing fuel and air upstream of the engine; however, premixing usually precludes skip-firing. Data are presented demonstrating that using continuously-fired operation to calibrate skip-fired data leads to over-prediction of local equivalence ratio. This is due to a combination of lower reactant temperatures for skip- versus continuous-fired operation, and a fluorescence yield that decreases with temperature. It is further demonstrated that early direct injection can be used as an alternative approach to provide calibration images. The influence of hardware modifications made to optical engines on performance is also examined. Copyright © 2005 SAE International.