Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Knocking combustion in spark-ignition engines

The kinetic modeling of the pyrolysis and combustion of liquid transportation fuels is a very complex task for two different reasons: the challenging characterization of the complex mixture of several hydrocarbon isomers and the complexity of the oxidation mechanisms of large hydrocarbon and oxygenated molecules. While surrogate mixtures of reference components allow to tackle the first difficulty, the complex behavior of the oxidation mechanisms is mostly overcome by reducing the total number of involved species by adopting a lumping approach. After a first investigation of the different liquid fuels (gasoline, kerosene, and diesel fuels), a short discussion on the lumping techniques allows to highlight the advantages of this approach. The lumped POLIMI pyrolysis and oxidation mechanism of hydrocarbon and oxygenated fuels is then used for generating several skeletal mechanisms for typical surrogate mixtures, moving from pure n-heptane up to heavy diesel fuels. These skeletal models are simply reduced with a reaction flux analysis, and they involve between 100 and 200 species. While these sizes already allow detailed computational fluid dynamics (CFD) calculations in internal combustion engines, further reduction phases are necessary when the interest is toward more complex CFD computations. To maintain the standard structure of the skeletal mechanisms, successive reduction phases are not considered. Moreover, new regulations pushed toward a greater use of renewable fuels. For these reasons, the skeletal models are also extended to biogasolines including methanol, ethanol, and n-butanol. Similarly, skeletal models of diesel and biodiesel fuels, including methyl esters, are also provided. Several comparisons with experimental data and complete validations in the operating range of internal combustion engines are also reported. The whole set of comparisons with experimental data obtained in a wide range of conditions not only validate the reduced models of specific transportation fuels but also the complete kinetic scheme POLIMI_1404.

Reduced Kinetic Schemes of Complex Reaction Systems: Fossil and Biomass‐Derived Transportation Fuels

There is an immediate need to drastically reduce the emissions associated with global fossil fuel consumption in order to limit climate change. However, carbon-based materials, chemicals, and transportation fuels are predominantly made from fossil sources and currently there is no alternative source available to adequately displace them. Gas-fermenting microorganisms that fix carbon dioxide (CO2) and carbon monoxide (CO) can break this dependence as they are capable of converting gaseous carbon to fuels and chemicals. As such, the technology can utilize a wide range of feedstocks including gasified organic matter of any sort (e.g., municipal solid waste, industrial waste, biomass, and agricultural waste residues) or industrial off-gases (e.g., from steel mills or processing plants). Gas fermentation has matured to the point that large-scale production of ethanol from gas has been demonstrated by two companies. This review gives an overview of the gas fermentation process, focusing specifically on anaerobic acetogens. Applications of synthetic biology and coupling gas fermentation to additional processes are discussed in detail. Both of these strategies, demonstrated at bench-scale, have abundant potential to rapidly expand the commercial product spectrum of gas fermentation and further improve efficiencies and yields.

/pdf/gas-fermentation-a-flexible-platform-for-commercial-scale-4m7a4ek8u8.pdf

Gas Fermentation—A Flexible Platform for Commercial Scale Production of Low-Carbon-Fuels and Chemicals from Waste and Renewable Feedstocks

Earlier studies of pre-ignitions at hot surfaces are first reviewed. The concept of a critical radius of a hot pocket of gas, closely related to the laminar flame thickness, that is necessary to initiate a propagating flame, has been used successfully to predict relative tendencies of different fuel–air mixtures to pre-ignite. As the mixture is compressed, the thickness of potential laminar flames decreases, and when this becomes of the order of the thermal sheath thickness at the hottest surface, pre-ignition can occur there, creating a propagating flame. Measured engine pre-ignition ratings are shown to correlate well with laminar flame thicknesses. Predictions are made concerning the effects of changes in intake temperature and pressure on the pre-ignition of different fuels.A growing current concern is occasional gas-phase, autoignitive, pre-ignitions that can occur in turbo-charged engines, giving rise to very severe autoignition and knock. It is concluded from the evidence of engine pressure records...

Pre-ignition and 'super-knock' in turbo-charged spark-ignition engines

Optimization of in-cylinder air—fuel mixture preparation in port fuel injected engines during all phases of operation is critical for maximizing engine performance while minimizing harmful emissions. In this study, a co-operative fuels research (CFR) gasoline engine is used to evaluate torque and measure in-cylinder and exhaust CO, CO2, and unburned hydrocarbons under various fuelling and spark conditions during cranking and startup phases. Fast flame ionization detectors and non-dispersive infrared fast CO and CO2 detectors are used as the principal diagnostics. Measured component trends are shown with detailed explanations. CFR engine exhaust unburned hydrocarbon levels are shown to be 3 to 7 times higher for startup than for steady state operation depending on spark timing and fuelling. Torque is found to be relatively insensitive to fuelling within 5 per cent of stoichiometric. Additionally, in-cylinder and exhaust CO levels are strong functions of startup enrichment. Exhaust CO, much easier t...

Simultaneous in-cylinder and exhaust cycle resolved non-dispersive infrared and flame ionization detector experimental sampling results during cranking and startup in a port fuel injection co-operation fuel research engine


 In the past, the Navy has investigated new renewable diesel engine fuels in comparison to conventional petroleum-based fuels using some previously published combustion metrics based on ignition delay, maximum rate of heat release and combustion phasing. These metrics quantify new fuel combustion changes using a conventional heat release analysis as compared to the base fuel. When combustion changes exceed established levels, fuel may be deemed marginal or unacceptable. In this study, these metrics are applied to Primary Reference diesel Fuel (PRF) mixtures in the range from 35 to 65. As expected, significant combustion variations are observed with lengthening ignition delay and increasing maximum heat release rate resulting from lower Cetane Number (CN) fuel. In this work, these steady-state metrics were then applied to predict the worsening of cold start engine performance. It is seen that the acceptable (‘green’) range of combustion change variations provides for acceptable cold engine starting performance. As the steady-state combustion changes become moderately more significant (‘yellow’), the cold start performance degrades. With very substantial steady-state combustion metric changes (‘red’), the cold start nature becomes unacceptable. This study shows that excellent correlation between steady-state engine performance and cold start behavior is possible.

The Relationship Between Steady-State Cetane Combustion Changes and Cold Start Engine Performance

Fuel Accounting Analysis during Cranking and Startup using Simultaneous In-Cylinder and Exhaust Fast FID and NDIR Detectors

Air-fuel mixture preparation is particularly challenging during cold engine throttle transients due to poor fuel vaporization and transport delays in port fuel injected (PFI) engines. In this study, a PFI cooperative fuels research engine is used to evaluate torque and to measure in-cylinder and exhaust CO, CO2, and unburned hydrocarbons during throttle transients at various early stages of engine warmup. Fast flame ionization detectors and nondispersive infrared fast CO and CO2 detectors are used to provide a detailed cycle-by-cycle analysis. Ttorque after cold throttle transients is found to be comparable to steady-state torque due to allowable spark advance. However, cold transients produce up to four times the unburned hydrocarbons when compared to steady-state operation. Finally, the x-tau fuel control model is evaluated in this challenging operating regime and is found to provide poor transient fuel control due to excessive fueling.

Cold Engine Transient Fuel Control Experiments in a Port Fuel Injected CFR Engine

/pdf/partially-premixed-combustion-application-for-diesel-power-4bqftpo2of.pdf

Partially Premixed Combustion Application for Diesel Power Improvement

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