Long-term petroleum prices and increasingly-stringent emissions regulations are driving manufacturers and users alike to consider alternatives to diesel-fueled engines. Mixing controlled combustion of alcohol fuels, such as ethanol, has been identified as a promising technology based the low propensity for particulate and NOx production, but the higher heats of vaporization and auto-ignition temperatures of these fuels make their direct use in diesel engine architectures a challenge. However, because alcohol fuels do not form appreciable levels of soot even in mixing-controlled (MCCI) mode, and because stoichiometric air/fuel ratios (AFR) can be used to simplify NOx aftertreatment, engine design optimization efforts can be targeted to maximize thermal efficiency. Therefore, to realize the potential of alcohol-fueled combustion, engineering insight is required to understand how design parameters, such as increased engine insulation, piston bowl geometry, or spray targeting, should be optimally utilized. In this work, a computational fluid dynamics (CFD) modeling framework is developed and validated in order to identify pathways to improve the performance of an ethanol-fueled engine operating in an MCCI mode at a stoichiometric AFR. To evaluate the use of TBCs as an engine insulation method, a simplified 1-D conjugate heat transfer (CHT) modeling framework is employed. The CFD model is first validated against baseline engine data over selected inlet air heating temperatures for two piston bowl-injector configurations that define the extrema of the design space. The addition of the 1-D CHT model only increases the computational expense by 15% relative to traditional approaches, yet offers more accurate heat transfer predictions over constant temperature boundary conditions. The model is then used to explore the efficacy of injector orientations and piston bowl geometries in improving the indicated thermal efficiency of alcohol fueled compression ignition engines. Using a design of experiments approach, several candidate designs were identified that improved fuel-air mixing, shortened the combustion duration, and increased thermal efficiency. The most promising design was then fabricated and tested in a Caterpillar 1Y3700 Single Cylinder Oil Test Engine (SCOTE). The engine testing confirmed the findings from the CFD simulations, and found that the co-optimized injector and piston bowl design yielded over 2-percentage point increase in thermal efficiency at the same equivalence ratio (0.96) and over 6-percentage point increase at the same engine load (10.1 bar indicated mean effective pressure), while satisfying design constraints for peak pressure and maximum pressure rise rate.

Lean combustion in an internal combustion engine is a promising strategy to increase thermal efficiency by leveraging a more favorable specific heat ratio of the fresh mixture and simultaneously suppressing the heat losses to the cylinder wall. However, unstable ignition events and slow flame propagation at fuel-lean condition lead to high cycle-to-cycle variability and hence limit the high-efficiency engine operating range. Pre-chamber ignition is considered an effective concept to extend the lean operating limit, by providing spatially distributed ignition with multiple turbulent flame-jets and enabling faster combustion rate compared to the conventional spark ignition approach. From a numerical modeling perspective, to date, still the science base and available simulation tools are inadequate for understanding and predicting the combustion processes in pre-chamber ignited engines. In this paper, conceptually different RANS combustion models widely adopted in the engine modeling community were used to simulate the ignition and combustion processes in a medium-duty natural gas engine with a pre-chamber spark-ignition system. A flamelet-based turbulent combustion model, i.e., G-equation, and a multi-zone well-stirred reactor model were employed for the multi-dimensional study. Simulation results were compared with experimental data in terms of in-cylinder pressure and heat release rate. Finally, the analysis of the performance of the two models is carried out to highlight the strengths and limitations of the two formulations respectively.

A skeletal mechanism with 117 species and 472 reactions for a Diesel surrogate i.e., n-heptane, was developed. The detailed mechanism for n-heptane created by Lawrence Livermore National Laboratory (LLNL) was employed as the starting mechanism. The detailed mechanism was then reduced with an enhancement of the Direct Relation Graph (DRG) technique called Parallel DRG-with Error Propagation and Sensitivity Analysis (PDRGEPSA). The reduction was performed for pressures from 20 to 80 atm, equivalence ratios from 0.5 to 2, and an initial temperature range of 600–1200 K, covering the compression ignition (CI) engine conditions. Extensive validations were performed against both 0-D simulations with the detailed mechanism and experimental data for spatially homogeneous systems. In order to perform three-dimensional turbulent spray-combustion and engine simulations, the mechanism was integrated with the multi-zone model in the CONVERGE CFD software to accelerate the calculation of detailed chemical kinetics. The Engine Combustion Network (ECN) data from Sandia National Laboratory was used for validation purposes along with single-cylinder Caterpillar engine data. The skeletal mechanism was able to predict various combustion characteristics accurately such as ignition delay and flame lift-off length (LOL) under different ambient conditions. The performance of the multi-zone solver with respect to the full cell-by-cell chemistry solver (SAGE) is compared for the Caterpillar engine simulation and a good match is obtained with significant speed-up of computational time for the multi-zone solver.

There has been an extensive worldwide search for alternate fuels that fit with the existing infrastructure and would thus displace fossil-based resources. In metabolic engineering work at Argonne National Laboratory, strains of fuel have been designed that can be produced in large quantities by photosynthetic bacteria, eventually producing a heavy alcohol called phytol (C 20 H 40 O). Phytol’s physical and chemical properties (cetane number, heat of combustion, heat of vaporization, density, surface tension, vapor pressure, etc.) correspond in magnitude to those of diesel fuel, suggesting that phytol might be a good blending agent in compression ignition (CI) engine applications. The main reason for this study was to investigate the feasibility of using phytol as a blending agent with diesel; this was done by comparing the performance and emission characteristics of different blends of phytol (5%, 10%, 20% by volume) with diesel. The experimental research was performed on a single-cylinder engine under conventional operating conditions. Since phytol’s viscosity is much higher than that of diesel, higher-injection-pressure cases were investigated to ensure the delivery of fuel into the combustion chamber was sufficient. The influence of the fuel’s chemical composition on performance and emission characteristics was captured by doing an injection timing sweep. Combustion characteristics as shown in the cylinder pressure trace were comparable for the diesel and all the blends of phytol at each of the injection timings. The 5% and 10% blends show lower CO and similar NO x values. However, the 20% blend shows higher NO x and CO emissions, indicating that the chemical and physical properties have been altered substantially at this higher percentage. The combustion event was depicted by performing high-speed natural luminosity imaging using endoscopy. This revealed that the higher in-cylinder temperatures for the 20% blend are the cause for its higher NO x emissions. In addition, three-dimensional simulations of transient, turbulent nozzle flow were performed to compare the injection and cavitation characteristics of phytol and its blends. Specifically, area and discharge coefficients and mass flow rates of diesel and phytol blends were compared under corresponding engine operating conditions. The conclusion is that phytol may be a suitable blending agent with diesel fuel for CI applications.

Inner nozzle flow characteristics (e.g., cavitation, turbulence, injection velocity) are known to affect spray development and hence combustion and emissions. Our previous studies showed that petrodiesel and biodiesel (soybean-based fuels) had very different cavitation and turbulence characteristics, which caused differences in spray breakup, penetration, dispersion, etc. Specifically, the atomization characteristics of biodiesel were worse than those of diesel; they were a direct consequence of biodiesel’s reduced cavitation and turbulence levels at the nozzle exit. In this study, the nozzle flow characteristics of biodiesel (from different feedstocks like tallow, soy, rapeseed, cuphea, and hydrotreated vegetable oil [HVO]) were compared with those of diesel. The first step was to obtain data on the physical properties of these fuels (e.g., their density, viscosity, surface tension, vapor pressure) at different temperatures. At full-needle open position, the cavitation contours scaled with the vapor pressure and viscosity; hence, methyl esters such as soy (SME), rapeseed (RME), and tallow (TME) exhibited less cavitation. The nozzle discharge coefficient, exit velocity, turbulent kinetic energy, and dissipation rate at the orifice exit were also compared for these fuels. Transient effects due to needle movement upon the inception of cavitation were studied. The effects of different needle-lift profiles (pertaining to various load conditions) on the nozzle flow development of these fuels were also characterized. This study also provides data on the critical boundary conditions for spray simulations from using the Kelvin Helmholtz-aerodynamic cavitation turbulence (KH-ACT) model, which accounts for cavitation and turbulence-induced breakup in addition to aerodynamic breakup.

Combustion in direct-injection diesel engines occurs in a lifted, turbulent diffusion flame mode. Numerous studies indicate that the combustion and emissions in such engines are strongly influenced by the lifted flame characteristics, which are in turn determined by fuel and air mixing in the upstream region of the lifted flame, and consequently by the liquid breakup and spray development processes. From a numerical standpoint, these spray combustion processes depend heavily on the choice of underlying spray, combustion, and turbulence models. The present numerical study investigates the influence of different chemical kinetic mechanisms for diesel and biodiesel fuels, as well as Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) turbulence models on predicting flame lift-off lengths (LOLs) and ignition delays. Specifically, two chemical kinetic mechanisms for n-heptane (NHPT) and three for biodiesel surrogates are investigated. In addition, the RNG k-ε (RANS) model is compared to the Smagorinsky based LES turbulence model. Using adaptive grid resolution, minimum grid sizes of 250 μm and 125 μm were obtained for the RANS and LES cases respectively. Validations of these models were performed against experimental data from Sandia National Laboratories in a constant volume combustion chamber. Ignition delay and flame lift-off validations were performed at different ambient temperature conditions. The LES model predicts lower ignition delays and qualitatively better flame structures compared to the RNG k-ε model. The use of realistic chemistry and a ternary surrogate mixture, which consists of methyl decanoate, methyl 9-decenoate, and NHPT, results in better predicted LOLs and ignition delays. For diesel fuel though, only marginal improvements are observed by using larger size mechanisms. However, these improved predictions come at a significant increase in computational cost.

A skeletal mechanism with 118 species and 837 reactions was developed from a detailed LLNL mechanism for a biodiesel surrogate mixture with equal mole fraction of methyl decanoate, methy-9-decenoate and n-heptane, through subsequent applications of a revised method for directed relation graph (DRG), isomer lumping, and DRG-aided sensitivity analysis (DRGASA). The reduction was performed within a parameter range of pressure from 1 to 100atm, equivalence ratio from 0.5 to 2, and temperature above 1000K for both auto-ignition and perfect stirred reactors. The detailed mechanism consists of 3329 species and 10806 reactions. With the reduction in mechanism size by almost a factor of 30, the skeletal mechanism was shown to still feature a good accuracy for temperature higher than 1000K, both in predicting the system parameters, such as ignition delay and extinction time, and detailed species profiles, compared with the detailed mechanism. Numerical simulations in jet stirred reactors were also performed and compared with experimental measurements for rapeseed oil methyl ester. This skeletal mechanism can be used to model methyl esters from different feedstocks since it is a tri-component mixture.

The objective of this research was to experimentally evaluate the effects of two biodiesel fuels with different viscosities on fuel injection characteristics using a light-duty, common-rail, diesel injection system. A pure biodiesel (B100) and a 50/50 blend of pure biodiesel and refined, bleached, and deodorized vegetable oil (B50V50) were compared with a laboratory diesel fuel equivalent (D100). The fuel viscosity ranged from 2.6 cSt (D100) to 10.9 cSt (B50V50). Three injection pressures and two injector nozzle geometries and surface finishes were also investigated. Measurements of the injected fuel quantity showed that as fuel viscosity increased, the injected volume decreased and the variability in the injected volume tended to increase. This effect was more significant in an injector nozzle with converging, highly hydro-ground holes than one with straight, lightly hydroground holes. The rate-of-injection (ROI) data were quite similar for D100 and B100 when using the straight, lightly hydro-ground nozzle. There is a marked reduction in peak injection rate for the B100, compared to D100, when the highly hydro-ground nozzle was utilized. With both nozzles, the B50V50 blend produced narrower ROI curves with peak injection rates equal to or exceeding those of D100 fuel. For all three fuels, the start-of-injection delay increased as fuel viscosity increased. The end-of-injection time was very similar for D100 and B100 but was advanced for the B50V50 blend.

The long time challenge for diesel engine manufacturers has been to reduce both particulate matter (PM) and NO x emissions simultaneously without sacrificing engine performance. One technique for reducing PM has been to inject air or oxygen-enriched air directly into the combustion chamber. Previous studies using the KIVA-3V computational fluid dynamics (CFD) model have shown benefits and the importance of the gas injection’s characteristics on the technique’s effectiveness in reducing emissions. Using a Caterpillar 3401E single cylinder engine, an experimental investigation has been conducted to demonstrate the effectiveness of an oxygen-enriched air injection system and fuel injection timing retard in reducing the NO x and the PM emissions in terms of both the particulate size and concentration. The gaseous emissions were measured using a Pierburg AMA 2000 gaseous emissions bench which included a chemiluminescent analyzer for NO x volumetric measurements, non-dispersive infrared (NDIR) analyzer for CO and CO 2 measurements, and a parametric fuel cell for O 2 measurements. PM emissions (i.e., soot particle concentration and size distribution) were measured using a TSI Model 3936 Scanning Mobility Particle Sizer (SMPS). The experimental observations regarding the effects of oxygen-enriched air injection on NO x and PM emissions were in accord with the previously reported results for late-cycle gas injection from a KIVA-3V model. The air injection technique additionally provided a low level of oxygen-enrichment during the compression cycle, with results similar to previous intake air oxygen-enrichment studies. A simultaneous reduction of NO x and particulates was demonstrated when the fuel injection timing characteristics were optimized in conjunction with the oxygen-enriched air injection. The experimental PM emissions were analyzed for number and size distributions and also found to be consistent with previously reported trends.