Stanislav V. Bohac

Lean and Rich Premixed Compression Ignition Combustion in a Light-Duty Diesel Engine

ABSTRACT Lean premixed compression ignition low-temperature combustion promises to simultaneously reduce NO x and PM emissions while suffering a moderate penalty in fuel consumption. Similarly, opportunities exist to develop rich combustion strategies which can provide the necessary exhaust constituents for aggressive regeneration of a lean NO x trap (LNT). The current work highlights the development of lean and rich premixed compression ignition combustion strategies. It is shown that the lean premixed compression ignition combustion strategy successfully operates with low NO x and smoke, at the expense of a 5% increase in fuel consumption over conventional diesel operation. The rich premixed compression ignition combustion strategy similarly operates with low NO x and smoke, and produces enough CO (up to 5% by volume in exhaust) for aggressive regeneration of an LNT. INTRODUCTION In his lecture given to the Society of Cassell on June 16, 1897, Rudolph Diesel stipulated as a third condition of his rational heat motor that fuel must be introduced gradually so as to maintain an isothermal combustion process [1]. Hence, from the early days of diesel engine development, it appeared that diffusion burn combustion would dominate. Nevertheless, this developmental road progressively changed direction as awareness of vehicle emissions and their impact on the atmosphere surfaced [2]. As researchers learned more about diesel engine combustion, it became increasingly clear that the diffusion burn portion was largely responsible for its soot emission [3]. Therefore, the desire to overturn Diesel’s condition of isothermal combustion developed, and attention shifted to premixed combustion modes [4]. Today, the development of combustion strategies resembling homogenous charge compression ignition strategies is vigorously pursued. The promise of simultaneously reduced NO

Nitrogen front evolution in purged polymer electrolyte membrane fuel cell with dead-ended anode

In this paper, we model and experimentally verify the evolution of liquid water and nitrogen fronts along the length of the anode channel in a proton exchange membrane fuel cell operating with a dead-ended anode that is fed by dry hydrogen. The accumulation of inert nitrogen and liquid water in the anode causes a voltage drop, which is recoverable by purging the anode. Experiments were designed to clarify the effect of N2 blanketing, water plugging of the channels, and flooding of the gas diffusion layer. The observation of each phenomenon is facilitated by simultaneous gas chromatography measurements on samples extracted from the anode channel to measure the nitrogen content and neutron imaging to measure the liquid water distribution. A model of the accumulation is presented, which describes the dynamic evolution of a N2 blanketing front in the anode channel leading to the development of a hydrogen starved region. The prediction of the voltage drop between purge cycles during nonwater plugging channel conditions is shown. The model is capable of describing both the two-sloped behavior of the voltage decay and the time at which the steeper slope begins by capturing the effect of H2 concentration loss and the area of the H2 starved region along the anode channel.

Effect of fuel cetane number on a premixed diesel combustion mode

Abstract The ability of premixed low-temperature diesel combustion to deliver low particulate matter (PM) and NO x emissions is dependent on achieving optimal combustion phasing. Small deviations in combustion phasing can shift the combustion to less optimal modes, yielding increased emissions, increased noise, and poor stability. This paper demonstrates how variations in fuel cetane number affect the detailed combustion behaviour of a direct-injection, diesel-fuelled, premixed combustion mode. Testing was conducted under light load conditions on a modern single-cylinder engine, fuelled with a range of ultra-low sulphur fuels with cetane numbers ranging from 42 to 53. Fuel cetane number is found to affect ignition delay and, accordingly, combustion phasing. Gaseous emissions are a function of combustion phasing and exhaust gas recirculation (EGR) quantity, but are not directly tied to fuel cetane number. Fuel cetane number is merely one of many different engine parameters that shift combustion phasing. Furthermore, the operating range is constrained by the changes in cetane number: no injection timings yield acceptable combustion across the whole spread of tested cetane numbers. However, in terms of combustion phasing, the operating range is consistent, independent of fuel cetane number.

SOURCES OF HYDROCARBON EMISSIONS FROM LOW-TEMPERATURE PREMIXED COMPRESSION IGNITION COMBUSTION FROM A COMMON RAIL DIRECT INJECTION DIESEL ENGINE

Low-temperature premixed compression ignition combustion (PCI) discussed in this study is achieved via late injection timing (close to top dead center) and heavy exhaust gas recirculation (EGR) using ultra low sulfur Swedish diesel fuel (sulfur content less than 15 ppm). PCI obtains a simultaneous decrease in particulate matter (PM) and oxides of nitrogen (NOx), as where injection timing is retarded, as opposed to conventional combustion, where a PM-NOx trade-off is observed. In PCI, hydrocarbon (HC) and carbon monoxide (CO) are increased, and must be removed using aftertreatment. In order to understand the sources of HC from the PCI regime, gas chromatography with a flame ionization detector is employed to perform exhaust HC speciation at three EGR rates and three injection timings. Volatile HC, semi-volatile HC, and CO increase as the injection timing is retarded or EGR is increased. Retarded injection timing or increased EGR reduces peak cylinder bulk temperature and thereby increases the yield of CO and volatile HC (mostly C1–C3). Retarded injection timing or increased EGR also increases ignition delay, which increases over-mixing and causes more mixture to become so lean that combustion ceases and the output of semi-volatile HC species (mostly unburned C10–C12 fuel) is increased. Volatile HC species are increased more than semi-volatile HC species, which results in a shift to a lighter exhaust HC mixture as injection timing is retarded or EGR is increased.

The Fuel Mix Limits and Efficiency of a Stoichiometric, Ammonia, and Gasoline Dual Fueled Spark Ignition Engine

An overall stoichiometric mixture of air, gaseous ammonia, and gasoline was metered into a single cylinder, variable compression ratio, supercharged cooperative fuel research (CFR) engine at varying ratios of gasoline to ammonia. The engine was operated such that the combustion was knock-free with minimal roughness for all loads ranging from idle up to a maximum load in the supercharge regime. For a given load, speed, and compression ratio, there was a range of ratios of gasoline to ammonia for which knockfree, smooth firing was obtained. This range was investigated at its rough limit and also at its maximum brake torque (MBT) knock limit. If too much ammonia was used, then the engine fired with an excessive roughness. If too much gasoline was used, then knock-free combustion could not be obtained while the maximum brake torque spark timing was maintained. Stoichiometric operation on gasoline alone is also presented, for comparison. It was found that a significant fraction of the gasoline used in spark ignition engines could be replaced with ammonia. Operation on about 100% gasoline was required at idle. However, a fuel mix comprising 70% ammonia/30% gasoline on an energy basis could be used at normally aspirated, wide open throttle. Even greater ammonia to gasoline ratios were permitted for supercharged operation. The use of ammonia with gasoline allowed knock-free operation with MBT spark timing at higher compression ratios and higher loads than could be obtained with the use of gasoline alone. DOI: 10.1115/1.2898837

Condensational Growth of Particulate Matter from Partially Premixed Low Temperature Combustion of Biodiesel in a Compression Ignition Engine

Condensational growth is not typically assumed to be significant compared with adsorption for conversion of unburned hydrocarbons in the exhaust of diesel engines to the particulate phase. However, when partially premixed low temperature combustion (LTC) modes designed to simultaneously reduce soot and NO X emissions are implemented, unburned hydrocarbon (UHC) concentrations in the exhaust are an order of magnitude higher than for conventional combustion modes, increasing the likelihood of gas to particle conversion by condensation. In this work, two LTC operating conditions are compared with conventional diesel combustion using a multi-cylinder direct-injection diesel engine using low-sulfur fuel, a soy-based biodiesel and a 50% by volume biodiesel blend. Gaseous emissions of unburned hydrocarbons were measured and particulate samples were taken using a partial-flow dilution tunnel. Gravimetric analysis of the collected filters, Soxhlet extraction of particulate and speciation using GC-FID was performed for all operating conditions. Elemental carbon (EC) emissions were measured using a thermal optical analyzer and particle size distribution was analyzed using a differential mobility spectrometer. For increasing biodiesel concentration in the fuel, mass emissions of both EC and UHC decreased for all combustion modes compared with petroleum diesel. However, for biodiesel use in LTC modes of operation, particulate mass significantly increased following exhaust dilution. Low vapor pressure methyl esters found in the exhaust of biodiesel LTC increases heterogeneous condensation onto soot particles in the exhaust compared with unburned species from petroleum diesel fuel operation. A model estimating this condensation mechanism accurately predicts the experimental findings of increased mass of particulate for biodiesel operation.

Comparison of Filter Smoke Number and Elemental Carbon Mass From Partially Premixed Low Temperature Combustion in a Direct-Injection Diesel Engine

Partially premixed low temperature combustion (LTC) is an established advanced engine strategy that enables the simultaneous reduction of soot and NOX emissions in diesel engines. Measuring extremely low levels of soot emissions achievable with LTC modes using a filter smoke meter requires large sample volumes and repeated measurements to achieve the desired data precision and accuracy. Even taking such measures, doubt exists as to whether filter smoke number (FSN) accurately represents the actual smoke emissions emitted from such low soot conditions. The use of alternative fuels such as biodiesel also compounds efforts to accurately report soot emissions since the reflectivity of high levels of organic matter found on the particulate matter collected may result in erroneous readings from the optical detector. Using FSN, it is desired to report mass emissions of soot using empirical correlations derived for use with petroleum diesel fuels and conventional modes of combustion. The work presented in this paper compares the experimental results of well known formulae for calculating mass of soot using FSN and elemental carbon mass using thermal optical analysis (TOA) over a range of operating conditions and fuels from a four cylinder direct injection passenger car diesel engine. The data show that the mass of soot emitted by the engine can be accurately predicted with the smoke meter method utilizing a 3000 ml sample volume over a range of FSN from 0.02 to 1.5. Soot mass exhaust concentration calculated from FSN using the best of the literature expressions and that from the TOA taken over all conditions correlated linearly with a slope of 0.99 and R2 value of 0.94. A primary implication of the work is that the level of confidence in reporting soot mass based on FSN for low soot formation regimes like LTC is improved for both petroleum diesel and biodiesel fuels.Copyright

Robust optimization of an automobile valvetrain using a multiobjective genetic algorithm

A robust optimization of an automobile valvetrain is presented where the variation of engine performances due to the component dimensional variations is minimized subject to the constraints on mean engine performances. The dimensional variations of valvetrain components are statistically characterized based on the measurements of the actual components. Monte Carlo simulation is used on a neural network model built from an integrated high fidelity valvetrain- engine model, to obtain the mean and standard deviation of horsepower, torque and fuel consumption. Assuming the component production cost is inversely proportional to the coefficient of variation of its dimensions, a multi-objective optimization problem minimizing the variation in engine performances and the total production cost of components is solved by a multi-objective genetic algorithm (MOGA). The comparisons using the newly developed Pareto front quality index (PFQI) indicate that MOGA generates the Pareto fronts of substantially higher quality, than SQP with varying weights on the objectives. The current design of the valvetrain is compared with two alternative designs on the obtained Pareto front, which suggested potential improvements.

Method and Detailed Analysis of Individual Hydrocarbon Species From Diesel Combustion Modes and Diesel Oxidation Catalyst

An undiluted exhaust hydrocarbon (HC) speciation method, using flame ionization detector (FID) gas chromatographs (GC), is developed to investigate HC species from conventional and low-temperature premixed charge compression ignition (PCI) combustion, from pre- and post-diesel oxidation catalyst (DOC) exhaust. This paper expands on previously reported work by describing in detail the method and effectiveness of undiluted diesel exhaust speciation and providing a more detailed analysis of individual HC species for conventional and PCI diesel combustion processes. The details provided regarding the effectiveness of the undiluted diesel exhaust speciation method include the use of a fuel response factor (RF) for HC species quantification and demonstration of its linearity, detection limit, accuracy and precision. The listing of individual HC species provides not only the information needed to design surrogate exhaust mixtures used in reactor tests and modeling studies, but also sheds light on PCI combustion and DOC characteristics. Significantly increased engine-out concentrations of acetylene, benzene and toluene support the theory that net soot reduction associated with PCI combustion occurs due to the reduction of soot formation from soot precursors. DOC oxidation behavior differs depending on the combustion characteristics, which change exhaust species and temperature.Copyright

Speciated Hydrocarbon Emissions from an Automotive Diesel Engine and DOC Utilizing Conventional and PCI Combustion

Premixed compression ignition low-temperature diesel combustion (PCI) can simultaneously reduce particulate matter (PM) and oxides of nitrogen (NOx). Carbon monoxide (CO) and total hydrocarbon (THC) emissions increase relative to conventional diesel combustion, however, which may necessitate the use of a diesel oxidation catalyst (DOC). For a better understanding of conventional and PCI combustion, and the operation of a platinum-based production DOC, engine-out and DOC-out exhaust hydrocarbons are speciated using gas chromatography. As combustion mode is changed from lean conventional to lean PCI to rich PCI, engine-out CO and THC emissions increase significantly. The relative contributions of individual species also change; increasing methane/THC, acetylene/THC and CO/THC ratios indicate a richer combustion zone and a reduction in engine-out hydrocarbon incremental reactivity. The DOC is most effective in oxidizing CO, followed by acetylene and olefins, aromatics, non-methane paraffins and methane. DOC conversion efficiency of CO and THC is high for lean conventional and lean PCI but very low for rich PCI. The high CO/O2 ratio of rich PCI is believed to cause nearly all of the catalyst’s active sites to be filled with CO, essentially disabling it until the CO/O2 ratio is reduced. Lean PCI DOC-out exhaust has the lowest combination of NOx, PM, CO and THC emissions, and the lowest atmospheric ozone forming potential.