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The goal of this study is to address fundamental limitations to achieving diesel-like efficiencies in heavy duty on-highway natural gas (NG) engines. Engine knock and misfire are barriers to pathways leading to higher efficiency engines. This study explores enabling technologies for development of high efficiency stoichiometric, spark ignited, natural gas engines. These include design strategies for fast and stable combustion and higher dilution tolerance. Additionally, advanced control methodologies are implemented to maintain stable operation between knock and misfire limits. To implement controlled end-gas autoignition (C-EGAI) strategies a Combustion Intensity Metric (CIM) is used for ignition control with the use of a Woodward large engine control module (LECM). Tests were conducted using a single cylinder, variable compression ratio, cooperative fuel research (CFR) engine with baseline conditions of 900 RPM, engine load of 800 kPa indicated mean effective pressure (IMEP), and stoichiometric air/fuel ratio. Exhaust gas recirculation (EGR) tests were performed using a custom EGR system that simulates a high pressure EGR loop and can provide a range of EGR rates from 0 to 40%. The experimental measurements included the variance of EGR rate, compression ratio, engine speed, IMEP, and CIM. These five variables were optimized through a Modified BoxBenken design Surface Response Method (RSM), with brake efficiency as the merit function. A positive linear correlation between CIM and f-EGAI was identified. Consequently, CIM was used as the feedback control parameter for C-EGAI. As such, implementation of C-EGAI effectively allowed for the utilization of high EGR rates and CRs, controlling combustion between a narrower gap between knock and lean limits. The change from fixed to parametric ignition timing with CIM targeted select values of f-EGAI with an average coefficient of variance (COV) of peak pressure of 5.4. The RSM efficiency optimization concluded with operational conditions of 1080 RPM, 1150 kPa IMEP, 10.55:1 compression ratio, and 17.8% EGR rate with a brake efficiency of 21.3%. At this optimized point of peak performance, a f-EGAI for C-EGAI was observed at 34.1% heat release due to auto ignition, a knock onset crank angle value of 10.3° aTDC and ignition timing of −24.7° aTDC. This work has demonstrated that combustion at a fixed f-EGAI can be maintained through advanced ignition control of CIM without experiencing heavy knocking events.

Engine knock and misfire are barriers to pathways leading to high-efficiency Spark-Ignited (SI) Natural Gas engines. The general tendency to knock is highly dependent on engine operating conditions and the fuel reactivity. The problem is further complicated by low emission limits and the wide range of chemical reactivity in pipeline quality natural gas. Depending on the region and the source of the natural gas, its reactivity, described by its methane number (analogous to the octane number for liquid SI fuels) can span from 65–95. In order to realize diesel-like efficiencies, SI natural gas engines must be designed to operate at high BMEP near knock limits over a wide range of fuel reactivity. This requires a deep understanding regarding the combustion-engine interactions pertaining to flame propagation and end-gas autoignition (EGAI). However, EGAI, if controlled, provides an opportunity to increase SI natural gas engine efficiency by increasing combustion rate and the total burned fuel, mitigating the effects of the slow flame speeds of natural gas fuels which generally reduce BMEP and increase unburned hydrocarbon emissions. For this reason, in order to study EGAI phenomenon, the present work highlights multi-dimensional computational fluid dynamics (CFD) models of the Cooperative Fuel Research (CFR) engine. The CFR engine models are used to investigate fuel-engine interactions that lead to EGAI with natural gas, including effects of fuel reactivity, engine operating parameters, and exhaust gas recirculation (EGR). A Three-Pressure Analysis, performed with GT-Power, was used to estimate initial and boundary conditions for the three-dimensional CFD model. CONVERGE CFD v2.4 was used for the three-dimensional CFD modeling where the level set G-Equation model and SAGE detailed chemical kinetics solver were used. An assessment of the different modeling approaches is also provided to evaluate their limitations, advantages and disadvantages, and for which situations they are most applicable. Model validation was performed with experimental data taken with a CFR engine over varying compression ratio, CA50, EGR fraction, and IMEP and shows good agreement in Peak Cylinder Pressure (PCP), PCP crank angle, and the location of the 10%, 50%, and 90% mass fraction burned (CA10, CA50, and CA90, respectively). The models can predict the onset crank angle and pressure rise rate for light, medium, and heavy EGAI under a variety of fuel reactivities and engine operating conditions.

To evaluate the effect of exhaust gas recirculation (EGR) and variable fuel reactivity on knock and misfire in spark ignited national gas engines, experiments were conducted in a rapid compression machine to measure homogeneous ignition delay, flame propagation rate, and end-gas autoignition fraction for stoichiometric natural gas/oxidizer/EGR blends. Natural gas with a range of chemical reactivity was simulated using mixtures of CH4, C2H6, and C3H8. Reactive exhaust gas recirculation (R-EGR) gases were simulated with mixtures of Ar, CO2, CO, and NO and non-reactive exhaust gas recirculation gases (NR-EGR) were simulated with mixtures of AR and CO2. Homogeneous ignition delay period, flame propagation rate and end-gas autoignition fraction were measured at compressed pressures and temperatures of 30.2 to 34.0 bar and 667 to 980 K, respectively. Flame propagation rate decreased with both R-EGR and NR-EGR substitution. The substitution of R-EGR increased the end-gas autoignition fraction, whereas NR-EGR substitution decreased the end-gas autoignition fraction. The results indicate that the presence of the reactive species NO in the R-EGR has a strong impact on end-gas autoignition fraction. An 82-species reduced chemical kinetic mechanism was also developed that reproduces measured homogeneous ignition delay period with a total average relative error of 11.0%.

Microalgae are a promising feedstock for biofuel production, but difficulties associated with harvesting suspended cultures contribute to the high costs of algal feedstock production. Ultrasonic harvesting has been identified as a potential low-cost technique, but limited data are available on the response of microalgae cells in the presence of an acoustic field. The acoustic radiation force acting on a cell depends upon cell size and the acoustic contrast factor (ACF) of the cell in the media. The ACF depends upon the density and compressibility of the cell and the media. Cell size and ACF were measured for Microchloropsis gaditana, Nannochloropsis oculata, Phaeodactylum tricornutum, and Chlamydomonas reinhardtii. The average ACFs, which were determined by measuring the densities and sound velocities of suspensions containing varying concentrations of cells in growth media, were 0.04 (range = 0.03–0.05) for M. gaditana, 0.02 (range = 0.01–0.04) for N. oculata, 0.05 (range = 0.04–0.07) for P. tricornutum, and 0.05 (range = 0.049–0.053) for C. reinhardtii. The ratio of the acoustic radiation force to the drag force would be highest for C. reinhardtii cells due to their larger effective radius (5.6 μm compared to 1.9–2.7 μm for the other species). The effective ACF of C. reinhardtii was also evaluated by recording the motion of cells in the presence of an acoustic field, using particle tracking velocimetry, and then modeling the recorded motion using COMSOL Multiphysics software. The result (ACF = 0.04) demonstrated agreement with the density/sound velocity meter method. Experiments with starch null sta6 mutant C. reinhardtii cells demonstrated that the effective ACF can transition from positive to zero and eventually become negative as microalgae cells accumulate lipids. The dynamic nature of the ACF represents an opportunity and a challenge for acoustic harvesting of algal cells.

Knock in spark-ignited (SI) engines is initiated by autoignition of the unburned gasses upstream of spark-ignited, propagating, turbulent premixed flames. Knock propensity of fuel/air mixtures is typically quantified using research octane number (RON), motor octane number (MON), or methane number (MN; for gaseous fuels), which are measured using single-cylinder, variable compression ratio engines. In this study, knock propensity of SI fuels was quantified via observations of end-gas autoignition (EGAI) in unburned gasses upstream of laser-ignited, premixed flames at elevated pressures and temperatures in a rapid compression machine. Stoichiometric primary reference fuel (PRF; n-heptane/isooctane) blends of varying reactivity (50 ≤ PRF ≤ 100) were ignited using an Nd:YAG laser over a range of temperatures and pressures, all in excess of 545 K and 16.1 bar. Laser ignition produced outwardly-propagating premixed flames. High-speed pressure measurements and schlieren images indicated the presence of EGAI. The fraction of the total heat release attributed to EGAI (i.e., EGAI fraction) varied with fuel reactivity (i.e., octane number) and the time-integrated temperature of the end-gas prior to ignition. Flame propagation rates, which were measured using schlieren images, were only weakly correlated with octane number but were affected by turbulence caused by variation in piston timing. Under conditions of low turbulence, measured flame propagation rates approached one-dimensional premixed laminar flame speed computations performed at the same conditions. Experiments were simulated with a three-dimensional CONVERGE™ model using reduced chemical kinetics (121 species, 538 reactions). The simulations accurately captured the measured flame propagation rates, as well as the variation in EGAI fraction with fuel reactivity and time-integrated end-gas temperature. The simulations also revealed low-temperature heat release as well as formaldehyde and hydrogen peroxide formation in the end-gas upstream of the propagating flame, which increased the temperature and degree of chain branching in the end-gas, ultimately leading to EGAI.

Presented herein is a fundamental study of laser ignition of methane/air mixtures at temperatures and pressures representative of an internal combustion engine. An Nd:YAG laser operating at λ = 1064 nm was used to ignite methane/air mixtures at equivalence ratios of 0.4 ≤ Φ ≤ 1 in a Rapid Compression Machine (RCM). Experiments were conducted to study the lean limit, minimum spark energy (MSE), and minimum ignition energy (MIE). The results show that laser ignition exhibits a stochastic behavior which must be interpreted statistically. A 90% probability of occurrence was used to evaluate the MSE and MIE, which resulted in MSE 90 =2.3 mJ and MIE 90 =7.2 mJ at an equivalence ratio Φ = 0.4 at compressed pressure and temperature of P comp = 29 bar and T comp = 750 K, respectively. The lean limit was characterized based on the fraction of chemical energy converted into thermal energy, which was determined by calculating the apparent rate of heat release as derived from RCM high speed pressure data. A lean limit for 90% chemical energy conversion was found to correspond to an equivalence ratio of 0.47 ( T comp = 782 K). Schlieren photography was employed as a diagnostics tool to visualize the flame initiation and propagation inside the RCM.

We demonstrate fiber mode-locked dual frequency comb spectroscopy for broadband, high resolution measurements in a rapid compression machine (RCM). We apply an apodization technique to improve the short-term signal-to-noise-ratio (SNR), which enables broadband spectroscopy at combustion-relevant timescales. We measure the absorption on 24345 individual wavelength elements (comb teeth) between 5967 and 6133 cm-1 at 704 microsecond time resolution during a 12-ms compression of a CH4-N2 mixture. We discuss the effect of the apodization technique on the absorption spectra, and apply an identical effect to the spectral model during fitting to recover the mixture temperature. The fitted temperature is compared against an adiabatic model, and found to be in good agreement with expected trends. This work demonstrates the potential of DCS to be used as an in situ diagnostic tool for broadband, high resolution, measurements in engine-like environments.

Effective assessment of nanoparticle exposures requires accurate characterization of the aerosol. Of increasing concern is personal exposure to engineered nanoparticles that are specifically designed for use in the nanotechnology sector. This manuscript describes the operation and use of a personal sampler that utilizes thermophoretic force to collect nanoparticles onto a standard TEM (transmission electron microscope) grid. After collection, nanoparticles on the TEM grid are analyzed with an electron microscope, and the resultant data used to determine the characteristics of the nanoparticle aerosol sampled. Laboratory experiments were conducted to determine the inlet losses and collection efficiency of the thermophoretic sampler for particles between 20 and 600 nm in diameter. These results are used together with theory for thermophoretic velocity to form a transfer function that relates the properties of the collected particles to the properties of the sampled aerosol. The transfer function utilizes a ...

We present here a study of laser ignition in a rapid compression machine (RCM). Laser induced ignition of natural gas-air mixtures at various test conditions was investigated. An Nd:YAG laser (1064 nm, 12 ns pulse duration) was employed as the laser source. Methaneair mixtures ranging from stoichiometric conditions to equivalence ratio of 0.4 were ignited using the laser as part of a lean limit study where the fuel-energy was kept constant for all cases. A study of minimum laser spark energy (MSE) and minimum ignition energy (MIE) was also conducted. We show that both MSE and MIE exhibit a stochastic behavior and, as a consequence, results can only be interpreted statistically. For our conditions, we found MSE at 90 % probability of MSE90=2.3 mJ. The MIE study was conducted at an equivalence ratio of φ=0.4 yielding an MIE (90% probability) of MIE90=7.2 mJ.

Previous attempts to model natural gas/diesel reactivity controlled compression ignition (RCCI) engines using single fuel component chemical kinetics have demonstrated difficulties with reproducing the gradual increase in combustion rate observed experimentally. This study investigates whether employing a multi-component vaporisation and chemical kinetics model for diesel fuel can yield closer agreement with experimental combustion rates. Multi-dimensional CFD simulations are compared against an injection timing sweep from a GM 1.9 L diesel engine modified with port injected natural gas. Using the multi-component model for both diesel vaporisation and diesel chemical kinetics resulted in a closer match with experimental heat release rate than using single component diesel chemical kinetics. However, the overly fast combustion rates at ignition could not be completely eliminated. In addition, a parameter study revealed that the simulation results are strongly sensitive to the ratio of components in the diesel fuel surrogate, the injected mass, and the injection velocity.

Results are presented from experiments on the combustion of freely floated n-heptane droplets in helium-oxygen environments conducted in Spacelab onboard the Space Shuttle Columbia during the first launch (STS-83) of the Microgravity Science Laboratory mission in April 1997. During this shortened flight, a total of eight droplets were burned successfully in nominally 300 K oxygen-helium atmospheres having oxygen mole fractions of 25, 30, and 35% at a total pressure of 1 atm. Initial droplet sizes ranged from about 2 to 4 mm. The results demonstrated both radiative and diffusive flame extinction during burning, whereas droplet surface regression followed the d-square law. The full range of possible droplet-burning behaviors was thus observed. The results provide information for testing future theoretical and computational predictions of burning rates, soot and flame characteristics, and extinction conditions.

Scaling behavior of burning rate constant with initial droplet diameter is investigated for a single component, sooting fuel under going spherically symmetric combustion in microgravity. Three different regions were identified: the D2-law region where the burning rate constant is independent of initial droplet size, the sooting region, and the non-luminous radiative loss region. In the last two regions the burning rate constant is shown to decrease as D0 -1/4 where D0 is the initial droplet size. This decrease is primarily due to temperature dependent property variation. An estimate for the critical diameter that divides the first two regions is developed using a semi-empirical formulation for sooting.